We systematically investigated the effects of hydrogen and deformation temperature on the micro-damage evolution and fracture behavior of the TRIP-aided bainitic ferrite steel using slow strain rate tensile tests and microstructure examinations. For hydrogen-uncharged condition, the micro-damage evolution behavior was not dependent on temperature, except for −100 °C. The exceptional behavior at −100 °C was attributed to the occurrence of brittle fracture. Hydrogen uptake significantly increased the number density of micro-damage in all deformation temperatures. The fracture mode of hydrogen-charged specimens changed from quasi-cleavage and ductile fractures to cleavage, quasi-cleavage, and intergranular fractures with decreasing deformation temperature. Quasi-cleavage fracture features were attributed to the crack growth process consisting of repetition of small crack initiation/blunting (or void initiation) and crack coalescence, whereas the cleavage and intergranular surfaces with smooth facets were due to the occurrence of hydrogen-reduction cohesive strength of cleavage planes and prior austenite grain boundaries.

The environment-assisted cracking behavior of AZ31 magnesium alloy was investigated using tensile tests at cathodic and corrosion potentials in a Na2B4O7∙10H2O solution containing NH4SCN and in air. Although the mechanical properties of the AZ31 with an initial strain rate of 10−4 s−1 were independent of the environments, those degraded in the solution with an initial strain rate of 10−6 s−1. The total elongation became smaller at higher potentials. The average hydrogen absorption rate increased with a positive potential shift. These facts imply that environment-assisted cracking became more severe under relatively high potential owing to corrosion accompanied by enhanced hydrogen absorption.

Microstructural paths of hydrogen-assisted fatigue crack growth (HAFCG) in tempered martensitic steels were investigated relying on martensite boundary characteristics. Factors determining the HAFCG paths were tensile strength (TS)-dependent. HAFCG paths occurred preferentially along prior austenite grain boundaries with a brittle trend in 1025-MPa-TS steel. However, 811-MPa-TS steel showed HAFCG predominantly along block boundaries with large plasticity evolution, indicating that the mechanism of HAFCG in lower-TS steel required the assistance of plasticity accumulation during cyclic loadings. Graphical abstract: [Figure not available: see fulltext.].

We investigate the effect of post oxidizing treatment (POT) on the structural, optical, and passivation performances of the titanium oxide coated crystalline Si (c-Si) heterostructures prepared by the solution process. The 2.7-fold effective lifetime is achieved after POT using the mixture of H2O2 and H3PO4 solutions, indicating passivation performance of the TiOx/c-Si heterostructures is improved by POT. From spectroscopic ellipsometry, thermal desorption spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses, the TiOx film, and TiOx/c-Si heterointerface are oxidized, suggesting the reduction of oxygen vacancies in the TiOx film and formation of the O-terminated c-Si surface after POT. Furthermore, XPS revealed that POx is formed after POT, a possible source of positive fixed charges. The improved passivation level of the TiOx/c-Si heterostructures after POT is caused by the oxidation of the c-Si surface and the formation of POx.

In the present study, the temperature dependence of microstructure and damage evolution in the medium Mn steel was investigated by tensile testing and microstructure examinations to explain the damage and fracture mechanisms. The flow behavior with serrations and work hardening rate were significantly affected by the deformation temperatures ranging from 173 K to 373 K (- 100 degrees C to 100 degrees C). Moreover, ultimate tensile strength, uniform elongation and total elongation showed significant temperature dependence. Deformation-induced martensite acted as the initiation site of damage. Accordingly, the critical strain for damage initiation and associated damage initiation probability decreased with decreasing deformation temperature due to the combined effect of the increase in flow stress and the decrease in the retained austenite stability. Furthermore, the decrease in deformation temperature strongly deteriorated the micro-damage arrestability of ferrite, which was attributed to increasing flow stress or stress intensity factor for brittle cracking after martensite cracking, resulting in fracture mode transition from ductile to quasi-cleavage fractures.

The Cr effects on hydrogen embrittlement behaviors in pure Ni, Ni20Cr, and Ni44Cr alloys with similar grain sizes were investigated using tensile tests under electrochemical hydrogen charging and microstructure observations. The relative elongation (defined as elongation under hydrogen charging divided by elongation in air) in the pure Ni was higher and lower than those of the Ni20Cr and Ni44Cr alloys, respectively. The behaviors of the hydrogen-charged specimens were as follows. The hydrogen embrittlement susceptibility nonmonotonically varied with an increase in the amount of Cr substitution. The fracture surfaces of the pure Ni and Ni20Cr alloy showed intergranular fractures, and that of the Ni44Cr alloy showed a fully ductile feature. Post-mortem electron backscattered diffraction analyses revealed that grain reference orientation deviation (GROD) values, which correspond to local plasticity-induced lattice distortions, were high around grain boundary triple junctions, and the maximum value in the pure Ni at the grain boundary triple junction (18°) was nearly the same as that of the Ni20Cr alloy (17°), although the total elongation of the pure Ni was twice that of the Ni20Cr alloy. This result indicated that Cr addition promoted the plasticity-induced local stress evolution associated with dislocation pile-up. Moreover, the maximum GROD value of the pure Ni near the fracture surface was 56°, which is considered to be the critical level for plasticity-induced cracking in pure Ni. Interestingly, the Ni44Cr alloy showed a similar GROD value (55°) even in the uniformly deformed portion of the fractured specimen, while showing ductile fracture and no cracks in the fractured specimen. This result indicated that the Ni44Cr alloy had higher grain boundary strength than that of the pure Ni even after hydrogen charging.

Hydride formation and associated phase transformations have been extensively studied in pure Fe. However, the understanding of hydrogenation in Fe-based alloy systems is still lacking. Such an investigation is particularly important in the case of austenitic (face-centered cubic) steels given that hydrogen has been reported to alter the austenite phase stability. In the current work, we investigate the phase transformations associated with depressurization of non-hydrogenated and hydrogenated Fe–Mn–Si–Cr alloy (FMS) (Fe-28.84Mn-7.04Cr-6.14Si, in mass %) using in-situ x-ray diffraction technique. Additionally, we study the change in microstructure using electron backscatter diffraction (EBSD) and electron channeling contrast imagining (ECCI).

The effects of mechanical loading on hydrogen uptake were investigated by thermal desorption experiments under various combinations of hydrogen charging and loading conditions. The diffusible hydrogen content increased with increasing elastic and plastic strains. The increase in diffusible hydrogen content per elastic strain was larger than that per plastic strain.

In the present work, we clarify the fracture mechanisms in a dual-phase (DP) steel consisting of ∼75% martensite and exhibiting tensile strength of 1.5 GPa. Generally, the DP steels are considered resistant to brittle fracture due to the crack-arresting behavior of the ferrite. However, contrary to conventional wisdom, we report a predominant brittle fracture in the DP steel and clarify the associated damage mechanisms. Crack initiation occurs via martensite cracking along the prior austenite grain boundaries. The crack propagation in ferrite primarily occurs via {100} brittle cleavage cracking. Occasional void coalescence is also observed in ferrite. The brittle fracture observed in martensite is a mixed mode fracture consisting of both intergranular fracture and transgranular cleavage cracking along {110} planes. Furthermore, it is observed that the morphology of the martensite crack (sharp and blunt crack) influences ferrite cleavage cracking. The ferrite damage mechanisms associated with the sharp and blunt crack have been elucidated. Finally, the micro mechanism for the occasional ductile ferrite fracture has been explained.

The color change of opal photonic crystal films (OPCFs) due to deformation was quantitatively evaluated using digital image correlation (DIC) analysis. OPCFs were pasted on specimens of three different gauge geometries, and random patterns were formed on the opposite side of each specimen for DIC analysis. To assess the applicability of using OPCFs-based strain characterization for analyzing steel structural components and associated metallurgical analyses, smooth, width-gradient, and holed specimens were prepared in this study. As deformation increased in the smooth specimen, the color of the OPCFs changed significantly. The color change in the OPCFs could be quantitatively converted into strain values through Hue value analysis. Heterogeneous strain distributions could also be quantitatively analyzed using OPCFs-based analysis at the submillimeter or millimeter scale. When the strain gradient is too high, for example, near a stress concentration site such as a hole, local peeling of the OPCFs away from the specimen surface can occur. Consequently, for quantitative characterization, we must take proper care when measuring this upper limit of the "strain gradient" as well as strain, which would depend on the adhesion and surface condition of the specimen.

The distribution of hydrogen entering an Fe sheet under a droplet of NaCl solution was successfully visualized using a hydrogenochromic sensor consisting of a polyaniline layer and a Ni intermediate layer. At the initial stage of corrosion, the hydrogen entry was barely confirmed. The hydrogen entry was observed as the corrosion proceeded, and the preferential hydrogen entry site corresponded to the rust-formed area. It was postulated that the hydrogen entry is promoted by the decrease in pH due to the hydrolysis reactions of Fe ions under the rust. The hydrogenochromic sensor paves the way for the visualization of the hydrogen entry into metals under corrosion conditions.

We investigated the effect of hydrogen on the resistance to mechanically long fatigue crack growth in an equiatomic Fe–Cr–Ni–Mn–Co high-entropy alloy using compact tension (CT) tests at a frequency of 1 Hz and room temperature (20 °C). The CT test specimens were hydrogen charged using 100 MPa of hydrogen gas prior to testing. Fatigue crack growth progressed three times faster after exposure to hydrogen gas. The major change caused by hydrogen charging was the occurrence of intergranular crack growth. Striation features were observed in the intergranular fracture region, indicating that intergranular crack growth occurred via a plasticity-driven cycle-by-cycle mechanism. More specifically, we propose that the dislocation emission from the grain boundaries at a crack tip controls the intergranular fatigue crack growth.

Intergranular fracture is the primary fracture mode when a significant amount of hydrogen is introduced into lath martensitic steels. When the introduced hydrogen content is reduced, the contribution of plasticity in the intergranular fracture is significant. Specifically, the crack path is still along prior austenite grain boundaries, but there are some signs of activation of the ductile mechanism (i.e., the tear ridges) on the fracture surface. The plasticity-assisted fracture mode is classified as an intergranular-like fracture. To characterize the difference between intergranular and intergranular-like fractures, these two hydrogen-assisted fracture modes were analyzed in terms of crack path, crack morphology, and local plasticity evolutions. Intergranular fracture occurred in the macroscopically elastic regime with a significant portion of smooth regions on the fracture surface. The intergranular crack growth was discontinuous at the grain size scale, and the preferential microstructure preventing the crack growth was triple junctions of the prior austenite grain boundary. For intergranular-like fracture, plasticity significantly evolved during crack growth, which caused nano-voids in the vicinity of the crack tip. Coalescence of the crack and nano-voids resulted in intergranular-like crack growth along prior austenite grain boundaries. The coalescence with nano-voids caused tear ridges on the intergranular-like fracture surface.

Mechanical properties of TRIP-aided steel with bainitic ferrite matrix in the presence of hydrogen were investigated at the deformation temperature range of −100 to 100 °C. In hydrogen-uncharged condition, the yield strength (YS) first increased and then remained almost unchanged, while the highest uniform elongation (UEl) was observed at a deformation temperature of 21 °C. Hydrogen charging slightly increased the YS and deteriorated the UEl at all the temperature ranges. In the hydrogen-uncharged and hydrogen-charged specimens, the transformation ratio of retained austenite increased with a decrease in the deformation temperature; however, the hydrogen charging decreased the transformation ratio. The fracture behavior drastically changed when hydrogen was introduced, owing to which deformation-induced martensitic transformation occurred and hydrogen played dual roles in accelerating brittle-like cracking.

The origins of the superior work hardening capability of medium manganese (M-Mn) and conventional transformation-induced plasticity-aided bainitic ferrite (TBF) steels of similar tensile strength and elongation are comparatively investigated via synchrotron X-ray diffraction measurements. The M-Mn steel undergoes preferential plastic deformation in austenite; its superior work hardening capability and associated uniform elongation are attributed to the high rates of martensitic transformation and dislocation accumulation per strain in the retained austenite. By contrast, the excellent work hardening behavior and uniform elongation of the TBF steel are attributed to the sustained transformability until the occurrence of a large strain and significant stress partitioning between the face-centered cubic (FCC) and body-centered cubic (BCC) phases due to the high austenite phase stability and high resistance to slip deformation of austenite.

The transformation and stabilization behaviors of the austenite (γ) phase during bainite transformation in the early stages of austempering treatment of low-alloy transformation-induced plasticity-aided bainitic ferrite (TBF) steel were investigated by in-situ neutron diffraction measurement and microstructure observation. Neutron diffraction peak profile analysis with a short time range resolution showed the changes in the phase fractions and the lattice constants of the α and γ phases in TBF steel during rapid cooling and austempering treatment. The γ phase with a low carbon concentration (γ(low-C)-Fe) immediately disappeared before the austempering time of 300 s. This indicates a remarkable increase in the lattice constant of the γ with a high carbon concentration (γ(high-C)-Fe) because of the bainite transformation of γ(low-C)-Fe and carbon partitioning to γ(high-C)-Fe. When the austempering time increased from 300 s to 3600 s, the volume fraction of γ(high-C)-Fe monotonically decreased and the lattice constant was hardly changed.

The effects of grain size on the hydrogen embrittlement susceptibility of pure Ni and Ni-20Cr alloy were investigated. The hydrogen embrittlement susceptibility was evaluated through tensile testing under electrochemical hydrogen charging. Relative elongation, defined as the elongation under hydrogen charging divided by elongation in air, increased with increasing grain size in pure Ni (the grain size was in the range of 11-22 µm). In contrast, the relative elongation of the Ni-20Cr alloy increased with decreasing grain size from 13 to 1.8 µm. Correspondingly, the intergranular fracture was suppressed by grain coarsening in pure Ni and grain refinement in the Ni-20Cr alloy. In addition, the intergranular fracture surface in pure Ni exhibited curved slip lines and that in the Ni-20Cr alloy exhibited straight line marks. These fractographic features imply that the mechanisms of the hydrogen-assisted intergranular crack growth in pure Ni and Ni-20Cr alloy were different, which could be attributed to the difference in their stacking fault energies.

The effects of thermomechanical processing on the microstructure and hydrogen embrittlement properties of ultrahigh-strength, low-alloy, transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steels were investigated to apply to automobile forging parts such as engine and drivetrain parts. The hydrogen embrittlement properties were evaluated by conducting conventional tensile tests after hydrogen charging and constant load four-point bending tests with hydrogen charging. The 0.4 mass%C-TBF steel achieved refinement of the microstructure, improved retained austenite characteristics, and strengthening, owing to thermomechanical processing. This might be attributed to dynamic and static recrystallizations during thermomechanical processing in TBF steels. Moreover, the hydrogen embrittlement resistances were improved by the thermomechanical processing in TBF steels. This might be caused by the refinement of the microstructure, an increase in the stability of the retained austenite, and low hydrogen absorption of the thermomechanically processed TBF steels.

Hydrogen is one of the key elements for the economy based on renewable and sustainable resources. Because a very low concentration of hydrogen in metallic structural materials often causes hydrogen embrittlement and decreases their mechanical property, there is a great interest in the development of an inexpensive and versatile sensor for hydrogen in metals with high sensitivity and high spatial resolution. Here, real-time visualization of atomic state hydrogen in a metal has been successfully achieved using a hydrogenochromic sensor consisting of a conventionally fabricated polyaniline layer and an interfacial Ni layer plated on the metal surface. The color change of the polyaniline layer due to its hydrogenation reaction can visualize the distribution of atomic state hydrogen in metals. Because the Ni interfacial layer functions as a catalyst for the hydrogenation reaction, the high sensitivity of ≈2 µmol m–2 and high spatial resolution of hundreds of micrometers are realized. This study opens a gate to a general approach for visualizing atomic state hydrogen in metals using polymers. In addition to the easy fabrication process and simple analytical method, the high sensitivity and high spatial resolution make the hydrogenochromic sensor promising for studying hydrogen behavior in metals.

Effect of retained austenite characteristics on V-bending in ultrahigh-strength TRIP-aided steel sheets with bainitic ferrite matrix (TBF steel) was investigated for automotive applications. V-bending tests were performed on a servohydraulic testing machine at a processing speed of 1 mm/min using a 88-degree V-punch (2.0 mm in punch radius), 88-degree V-die (12 mm in die gap, 0.8 mm in die shoulder radius), and a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness). The results are summarized as follows.(1) The 0.2C-1.5Si-1.5Mn (mass%) TBF steel sheets were able to perform V-bending by strain-induced martensitic transformation of TRIP effect. On the other hand, ferrite-martensite dual-phase (MDP0) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack in tension area.(2) The TBF375 steel sheet that produced by heat treatment of annealing at 1 173 K (900 degrees C) for 1 200 s followed by austempering at 648 K (375 degrees C) for 200 s, of 1 100 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Delta theta = theta(1) - theta(2)), in which the theta(1) and the theta(2) were a bending angle on loading and a bending angle after unloading respectively, of more than 2-degree by controlling a displacement of punch bottom dead center.

This study investigated the hydrogen embrittlement of a high strength Zr or Cr- added aluminum alloy specimens containing coherent Al3Zr or incoherent Al18Mg2Cr3 dispersoids, respectively. The elastic interaction between coherency strain and hydrogen leads to the accumulation of hydrogen at Al3Zr dispersoids and an additional peak in the thermal desorption spectra. For Al18Mg2Cr3 dispersoids, the elastic strain induced by lattice misfit decreases by the introduction of the misfit dislocations leading to hydrogen trapping by misfit dislocations. The obtained results firstly revealed that the presence of coherent Al3Zr dispersoids in the matrix results in a superior hydrogen embrittlement resistance of the alloy.

The pre-deformation effects of hydrogen embrittlement on nitrogen-doped duplex stainless steel were investigated using tensile testing. Hydrogen pre-charging and pre-straining were adopted, which increased the yield strength, but decreased the ductility. Silver decoration revealed that diffusible hydrogen was preferentially located in the ferrite or grain boundaries of the undeformed steel, whereas the 22% pre-strained steel had diffusible hydrogen in austenite. When deformed, the preferential plastic deformation path of the steel was in the ferrite; hence, the apparent diffusivity in ferrite decreased, while the austenite had a relatively high apparent hydrogen flux. Pre-straining suppressed macroscopic hydrogen diffusion and local diffusion in ferrite and austenite by increasing the dislocation density. The degree of the suppression of the local diffusion was more significant in ferrite than austenite. The changes in dislocation and hydrogen behaviors associated with pre-straining altered the cracking sites in the austenite and ferrite, and decreased hydrogen embrittlement susceptibility.

<title>Abstract</title>The use of hydrogen in iron and steel has the potential to improve mechanical properties via altering the phase stability and dislocation behavior. When hydrogen is introduced under several gigapascals, a stoichiometric composition of hydrogen can be introduced for steel compositions. In this study, a face-centered cubic (fcc) stainless steel was hydrogenated under several gigapascals. When the steel was not hydrogenated, the microstructure after depressurization was an fcc with a hexagonal close-packed (hcp) structure. In contrast, the hydrogenation treatment resulted in a fine lath body-centered cubic (bcc) structure arising from diffusionless transformation. In particular, the bcc phase formed through the following transformation sequence: fcc → hcp → dhcp (double hexagonal close-packed phase) → bcc. That is, the use of hydrogenation treatment realized fine microstructure evolution through a new type of diffusionless transformation sequence, which is expected to be used in future alloy design strategies for developing high-strength steels.

Effect of hydrogen on spot-welded tensile properties in ultrahigh-strength TRIP-aided martensitic steel (TM steel) sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 1 mm/min (strain rate of 2.8x10(-4)/s), using base metal and spot-welded specimens with or without hydrogen charging.The results are as follows.(1) The difference between the tensile strength (TS) of 1 532 MPa for base metal specimen without hydrogen charging and the maximum stress (TS-H) of 1 126 MPa for the base metal specimen with hydrogen charging (Delta TS-H=TS-TS-H) in the TM steel was smaller than that of hot stampted steel (HS1 steel) and superior to that of HS1 steel. On the other hand, the TS-H of 725 MPa for the base metal specimen with hydrogen charging was halved in comparison with the TS of 1 438 MPa for base metal specimen without hydrogen charging in the HS1 steel. It is considered that this was because the retained austenite suppressed the strength reduction due to the hydrogen embrittlement of the TM steel.(2) The amount of hydrogen decreased in the order of the HS1 steel, the TM steel, and the tempered martensitic steel (HS7 steel), and the HS1 steel was the highest. This is thought to be due to the high dislocation density of the HS1 steel.(3) The difference between the maximum stress (TS-W) of the spot-welded specimen without hydrogen charging and the maximum stress (TS-WH) of the spot-welded specimen with hydrogen charging (Delta TS-WH=TS-W -TS-WH) in the TM steel and that of the HS1 steel were similar. It was considered that this is partly due to the effect of the stress concentration on heat affected zone (HAZ) softening of the hardness distribution of the spot-weld.

Phase transformation in hydrogenated iron during depressurization from several gigapascals was investigated through in-situ synchrotron radiation X-ray diffraction and post-mortem electron backscatter diffraction measurements. The hydrogenated iron under 8.6 GPa at 293 K showed a double hexagonal close-packed (dhcp) structure, and it gradually transformed into a body-centered cubic (bcc) structure with decreasing pressure. The final crystal structure consisted entirely of a bcc phase. The structural change from dhcp to bcc structure was diffusionless-type phase transformation. The bcc phase showed lath morphology and could grow during aging under a constant pressure of 1.9 GPa, which indicated that it was bainitic-type transformation that required hydrogen diffusion or desorption. (C) 2021 The Author(s). Published by Elsevier B.V.

The effect of the annealing temperature after cold rolling on hydrogen embrittlement resistance was investigated with a face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy using tensile testing under electrochemical hydrogen charging. Decreasing annealing temperature from 800 °C to 750 °C decreased grain sizes from 3.2 to 2.1 μm, and resulted in the σ phase formation. Interestingly, the specimen annealed at 800 °C, which had coarser grains, showed a lower hydrogen embrittlement susceptibility than the specimen annealed at 750 °C, although hydrogen-assisted intergranular fracture was observed in both annealing conditions. Because the interface between the FCC matrix and σ was more susceptible to hydrogen than the grain boundary, the presence of the matrix/σ interface significantly assisted hydrogen-induced mechanical degradation. In terms of intergranular cracking, crack growth occurred via small crack initiation near a larger crack tip and subsequent crack coalescence, which has been observed in various steels and FCC alloys that contained hydrogen.

This study presents the degree of promotion of deformation-induced γ−ε martensitic transformation by hydrogen charging and the associated fracture behavior at various strain rates ranging from 10−5 to 10−2 s−1. A decrease in the strain rate from 10−2 to 10−5 s−1 promotes the deformation-induced γ−ε martensitic transformation regardless of hydrogen charging (65%→82% in area fraction for uncharged specimens, 68%→84% for hydrogen-charged specimens). Hydrogen charging, which provides 11.7 mass ppm hydrogen concentration, further promotes the γ−ε martensitic transformation. However, the degree of promotion of the transformation by hydrogen is insensitive to the strain rate. Corresponding to the promotion of the γ−ε martensitic transformation, the hydrogen embrittlement susceptibility increases with a decrease in the strain rate. For instance, the elongation of the hydrogen-charged specimens decreases from 36 to 32% by decreasing strain rate from 10−2 to 10−5 s−1. Hydrogen uptake deteriorates the resistance to crack initiation and propagation. Furthermore, the primary effect of the decrease in strain rate on hydrogen embrittlement is the acceleration of the crack propagation. In addition to the promotion of γ−ε martensitic transformation, a decrease in the strain rate in the presence of hydrogen may cause hydrogen localization at the crack tip, which assists brittle-like martensite cracking.

In this study, the effects of austempering treatment on the microstructure, mechanical properties, and fracture mechanisms of Fe-0.4C-1.5Si-1.5Mn transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel were investigated. When the austempering time increased from 0 to 3600 s, the volume fraction of retained austenite and its carbon concentration initially increased and then decreased. The increase in carbon concentration during austempering may be attributed to the formation of fine bainitic ferrite, representing the matrix and in turn the formation of retained austenite with a film-type morphology. When the austempering time was increased from 0 to 1000 s, the TBF steel showed excellent mechanical properties such as high yield and tensile strengths, and uniform and total elongations. Meanwhile, void density increased significantly in the initial stages and then slightly decreased; in contrast, the trend observed for mean crack length was opposite to that of void density leading to the change in the fracture mode of the TBF steels from intergranular to quasi-cleavage and dimple fractures. It could be inferred that the observed improvement in mechanical properties, especially the good ductility of TBF steels, at the optimal austempering time is due to the suppression of crack propagation and crack blunting to form voids owing to the effective deformation-induced transformation of metastable retained austenite.

Effects of silicon and manganese contents on V-bending in high -strength TRIP-aided dual -phase (TDP) steel sheets with polygonal ferrite matrix were investigated for automotive applications. V-bending test was performed on a hydraulic testing machine at a processing speed of 1 mm/min, using a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness), 88-degree punch (2.0 mm in punch radius), and 88-degree die (12 mm in die width, 0.8 mm in die radius). The main results are as follows.(1) The 0.2C-(1.0-2.5)Si-(1.0-2.0)Mn, mass% TDP steel sheets were able to perform V-bending by strain induced martensitic transformation of TRIP effect. On the other hand, ferrite-martensite dual -phase (MDPO) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack on an outer surface.(2) The 0.2C- 2.5Si-1.5Mn, mass% TDP-G steel sheet of 980 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Delta theta = theta(1) - theta(2)), in which the 81 and the 82 are a bending angle on loading and a bending angle after unloading respectively, of more than 2 -degree by controlling a displacement of punch bottom dead center.

Hydrogen blister (HB) and hydrogen-induced cracking (HIC) of pure iron at various charging times were investigated. The results show that an increase in charging time leads to the increased HB height/width and HIC length/depth, and a change in hydrogen damage morphology. Subsurface HIC initiates and propagates along the grain boundaries, which makes the surface bend outwards to form blister on blister features, suggesting grain boundary cracking-induced blister-blister interaction mechanism. Crystallographic analysis demonstrates that adjacent grain boundary oriented-{100}//ND and {110}//ND exhibits the highest susceptibility to HIC, which offers a pathway to design hydrogen-resistant structural steels through the control of grain boundary.

To develop ultrahigh-strength steels for automotive impact safety parts, the effects of the microstructure and nitrogen content on the fatigue properties of ultrahigh-strength low alloy transformation-induced plasticity (TRIP)-aided steels with martensite (TM), bainitic ferrite-martensite (TBM), and bainitic ferrite (TBF) matrices were investigated. Compared to TBF steels, both the TM and TBM steels achieved high tensile strength, of more than 980 MPa, and excellent fatigue properties. This results from the suppression of crack propagation due to the effective TRIP of the relatively stable interlath retained austenite and the increase in tensile and yield strengths attributed to the low isothermal transformation treatment. The fatigue strengths of the ultrahigh-strength low alloy TRIP-aided steels were slightly increased by the addition of 100 ppm of nitrogen. The increase in fatigue strength of TM, TBM, and TBF steels with 100 ppm of nitrogen was caused by the fine and uniform martensite and bainitic ferrite matrices and retained austenite, along with the increase in carbon concentration in the retained austenite due to the precipitation of AlN.

Effect of carbon content on V-bending in high-strength TRIP-aided dual-phase (TDP) steel sheets with polygonal ferrite matrix was investigated for automobile applications. V-bending test was performed on a hydraulic testing machine at a processing speed of 1 mm/min, using a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness). The main results are as follows. (1) The (0.1-0.4)C-1.5Si-1.5Mn, mass% TDP steel sheets were able to perform V-bending by straininduced martensitic transformation of TRIP effect. On the other hand, ferrite-martensite dual-phase (MDP0) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack in tension region. (2) The 0.4C-1.5Si-1.5Mn, mass% TDP4 steel sheet of 1 100 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (=2?1), in which the 1 and the 2 are a bending angle on loading and a bending angle after unloading respectively, of more than 2-degree by controlling a displacement of punch bottom dead center.

The effects of hydrogen on the ductility loss and fracture behavior of a nitrogen-doped duplex stainless steel were investigated via tensile testing after electrochemical hydrogen pre-charging. Post-mortem microstructure analyses were performed to characterize the microstructural damage evolution and fracture surface. Hydrogen charging of the steel resulted in quasi-cleavage and intergranular fractures, which were associated with transgranular austenite cracking and ferrite/austenite interface cracking, respectively. The crucial factors that resulted in brittle-like cracking were deformation twinning in austenite and nitrogen-related solution hardening.

The effects of stress and plastic strain distributions on the hydrogen embrittlement fracture of the U-bent martensitic steel sheet specimen were investigated. The hydrogen embrittlement testing of the U-bent specimen was performed. Fracture morphology mainly consisting of intergranular fracture was found inside the hydrogen charged U-bent specimen, which indicated that the crack initiation took place in the interior, and shear lips were found near both surfaces of the U-bent sheet. The synchrotron X-ray diffraction measurement and the finite element simulation were utilized to analyze the stress and plastic strain distributions in the thickness direction of the U-bent specimen. The elastic strain distributions obtained by the measurement showed a good agreement with the simulation. The crack initiation site of the hydrogen-charged U-bent specimen was considered to be correspondent with the region where the tensile stress was the highest, suggesting that the maximum tensile stress predominantly determine the crack initiation.

To evaluate the susceptibility of steels to hydrogen embrittlement, it must be hydrogen charged and have its hydrogen content controlled before any mechanical testing. In this study, hydrogen permeation tests of an iron sheet were performed during potentiostatic hydrogen charging in various solutions containing ammonium thiocyanate to obtain a guideline for efficient hydrogen charging for a wide range of hydrogen contents. As the polarization potential shifted in the negative direction, the hydrogen permeation current increased before becoming almost constant. In all cases, besides when an aqueous sodium hydroxide solution was employed, the hydrogen permeation current increased due to the addition of ammonium thiocyanate. The effect of adding ammonium thiocyanate was enhanced as the aqueous solution pH was decreased. The hydrogen permeation current under various hydrogen charging conditions obtained in this study can be used as a reference for hydrogen charging of steels.

From the aspect of crack/void initiation and growth characteristics, the effects of pre-strain on the hydrogen embrittlement resistance of a transformation-induced plasticity-aided bainitic ferrite steel were examined. The hydrogen uptake in the specimens without pre-strain caused degradation of crack growth resistance, but the crack initiation probability did not change significantly. It is noteworthy that the degree of degradation was independent of the hydrogen content in the present hydrogen charging condition. Pre-straining to 3% and 6% improved the crack growth resistance of the hydrogen-charged specimens because of a reduction in the probability of austenite presence at the crack tip. Furthermore, a high level of pre-strain provided high hydrogen concentration and resulted in strain-age-hardening, which caused an acceleration of quasi-cleavage fracture, an increase in yield strength, and a stress/strain concentration associated with Lüders deformation. These factors diminished the crack initiation resistance and crack growth resistance.

Here, we present a review of the hydrogen embrittlement behavior of face-centered cubic (FCC) alloys with short-range order (SRO) of solute atoms. In this paper, three types of FCC alloys are introduced: Fe–Mn–C austenitic steels, high-nitrogen steels, and CoCrFeMnNi high-entropy alloys. The Fe–Mn–C austenitic steels show dynamic strain aging associated with Mn–C SRO, which causes deformation localization and acceleration of premature fracture even without hydrogen effects. The disadvantageous effect of dynamic strain aging on ductility, which is associated with the deformation localization, amplify plasticity-assisted hydrogen embrittlement. Cr–N and Co–Cr–Ni SRO effects in high-nitrogen austenitic steels and high-entropy alloys enhance the dislocation planarity, which causes stress concentration in the grain interior and near the grain boundaries. The stress concentration coupled with hydrogen effects causes quasi-cleavage and intergranular fractures.

The impact of hydrogen desorption on the electrical properties of TiO on crystalline silicon (c-Si) with SiO interlayers is studied for the development of high-performance TiO carrier-selective contacts. Compared with the TiO /c-Si heterocontacts, a lower surface recombination velocity of 9.6 cm/s and lower contact resistivity of 7.1 mΩ cm are obtained by using SiO interlayers formed by mixture (often called SC2). The hydrogen desorption peaks arising from silicon dihydride (α1) and silicon monohydride (α2) on the c-Si surface of the as-deposited samples are observed. The α1 peak pressure of as-deposited heterocontacts with SiO interlayers is lower than that of heterocontacts without a SiO interlayer. Furthermore, the hydrogen desorption energies are found to be 1.76 and 2.13 eV for the TiO /c-Si and TiO /SC2-SiO /c-Si heterocontacts, respectively. Therefore, the excellent passivation of the TiO /SC2-SiO /c-Si heterocontacts is ascribed to the relatively high rupture energy of bonding between Si and H atoms. x y x x y x y x x y x y 2

Hydrogen assisted cracking on hemispherically-stretch-formed specimens of transformation induced plasticity-aided martensitic steel was investigated. Hydrogen charging induced cracking around the foot of the impression formed on the steel sheet, and the cracks propagated along the radial direction toward the hillside and the plains. Distributions of stress, plastic strain and volume fraction of retained austenite were analyzed employing the energy-dispersive X-ray diffraction method utilizing the synchrotron X-ray radiation at SPring-8. It was notable that the crack initiation took place in the region where the measured tensile stress was the highest. Influences of plastic strain and resulted martensitic transformation were also suggested.

The role of mill scale on the corrosion behavior of steel rebars in mortar was studied using Hyperbaric-Oxygen Accelerated Corrosion Tests (HOACT) and polarization measurements. Defects of mill scale deteriorated passivity of rebars when mortar and saturated Ca(OH) solution contained chloride. Localized corrosion beneath mill scale with defects was responsible for the acceleration of depassivation. SEM observations and Raman spectroscopy measurements revealed that a Fe O layer initially formed under the mill scale and further corrosion resulted in migration of Fe ions through the defects and formation of a rust layer mainly composed of α- and γ-FeOOH on the mill scale. 2 3 4

We investigated the effects of hydrogen on ε-martensite-related damage evolution (crack/void initiation and growth) in Fe-Mn-Si-base austenitic steel using tensile tests after gaseous hydrogen charging at 100 MPa. Specifically, we evaluated the quantitative hydrogen effects on ε-martensite fraction and associated damage evolution with different strains and strain rates. Hydrogen charging increased the probability of ε-martensite-related damage initiation and deteriorated micro-damage arrestability, which decreased elongation. The primary factor causing the detrimental hydrogen effects on resistance to damage evolution was the promotion of deformation-induced γ-ε martensitic transformation. An increasing strain rate from 10 to 10 s suppressed the γ-ε martensitic transformation and correspondingly increased elongation. Interestingly, the ε-martensite fraction near the fracture surface did not change with increasing strain rate, but the area fraction of the brittle-like fracture region decreased. This fact implied that the brittle-like fracture at the low strain rate, which had a longer time for damage growth, was assisted by stress-driven hydrogen diffusion near the crack/void tips. −4 −2 −1

An opal photonic crystal film was applied to characterize local strain evolution associated with Lürders band propagation in an annealed low carbon steel. A local change in color of the opal film was observed, which corresponded to the propagation of the Lürders band. In particular, we carried out two tensile experiments for line and area analyses of RGB (Red-Green-Blue) values of the opal films pasted on the specimens. Both of the experiments clearly exhibited a quantitative correspondence between color variation and local strain evolution, namely, the present study demonstrated the potential of the opal films to analyze heterogeneous strain evolution in steels.

The effects of Mn content and grain size on the hydrogen embrittlement resistance of face-centered cubic (FCC) CrCoFeMnNi high-entropy alloys were examined using tensile testing at an initial strain rate of 10  s . The most important finding of this study was that the combined effect of Mn removal and grain refinement provided nearly complete resistance to hydrogen embrittlement with over 800 MPa tensile strength in FCC high-entropy alloys, even after 100 MPa hydrogen gas charging. −4 −1

In this study, we characterized the details of the pre-strain effects on the microstructure, deformation/fracture behavior, and hydrogen content, with respect to the true fracture strength in the TRIP-aided bainitic ferrite steel. Three types of hydrogen embrittlement behavior were distinguished, based on pre-strain and hydrogen content. Pre-strain reduces the fraction of retained austenite, which in turn decreases the hydrogen embrittlement susceptibility when the hydrogen content is low. However, further pre-straining increases dislocation density, which has three main effects: an increase in hydrogen content, work hardening, and strain-age hardening. The increase in the hydrogen content that exceeds 4 mass ppm has been found to decrease the true fracture strength from approximately 1.5 to 1.2 GPa. The work hardening and strain-age hardening were found to increase until the critical fracture stress was achieved with respect to the strain; this led to a reduction in the elongation, particularly when the hydrogen content was high.

Hydrogen embrittlement of high strength steel is seriously concerned and hydrogen visualization techniques in metals are strongly required to investigate the mechanism of hydrogen embrittlement. In this study, an in situ technique for hydrogen visualization in metals using aqua(2,2′-bipyridine-6,6′-dionato)(pentamethylcyclopentadienyl)iridium(III) (hereafter, Ir complex) has been developed. Hydrogen permeated through palladium-plated iron reacted with the Ir complex leading to the color change of the Ir complex. Furthermore, the flux of hydrogen could be quantitatively obtained from the changes in the RGB color values of the Ir complex. These results indicate that the Ir complex can be used for in-situ hydrogen visualization in metals.

This paper marks the first report on the fact that interstitial hydrogen improves the shape recovery property of an Fe-based face-centered cubic alloy, regardless of pre-strain level. The maximum shape recovery strain when using hydrogen was increased from 2.8 to 3.4 pct at an approximately 7 pct pre-strain. The enhancement of the shape memory effect by hydrogen stems from (1) increasing deformation-induced martensite fraction, (2) thinning of martensite plates, and (3) increasing the number of martensite plates.

In the study, the pre-strain effect on hydrogen embrittlement property of the ultra-high-strength transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel was investigated towards application for automobile frame parts. Specifically, 3–10% tensile pre-strain suppressed hydrogen-induced mechanical degradation relative to total elongation (pre-strain + elongation after hydrogen charging) while 12–15% pre-strained specimen did not exhibit elongation after hydrogen charging. The advantageous effect of the 3–10% pre-strain was attributed to the suppression of crack initiation related to retained austenite. Specifically, the TRIP by pre-straining decreased the volume fraction of retained austenite before hydrogen charging, thereby reducing existing probabilities of preferential crack initiation sites and propagation paths. Conversely, high pre-strain such as 12–15% does not effectively work due to work hardening resulting in increases in hydrogen embrittlement susceptibility and a significant increase in hydrogen content due to the multiplication of dislocations.

This study aims at understanding the change of the hydrogen embrittlement mechanism with respect to hydrogen content of a precipitation-strengthened Fe–Ni–Cr-based steel. Hydrogen was electrochemically introduced with different current densities. The hydrogen-charging deteriorated crack initiation and propagation resistances as well as the crack tip blunting capability. Further, with increasing hydrogen content, the primary cracking sites changed from coarse carbides to slip bands, and then to grain boundaries. Moreover, because the crack initiation probability increased and the resistance to trans-granular crack propagation decreased with hydrogen content, the crack coalescence associated with slip localization occurred more frequently.

Effect of carbon content on V-bending in high-strength TRIP-aided dual-phase (TDP) steel sheets with polygonal ferrite matrix was investigated for automobile applications. V-bending test was performed on a hydraulic testing machine at a processing speed of 1 mm/min, using a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness). The main results are as follows. (1) The (0.1-0.4)C-1.5Si-1.5Mn, mass% TDP steel sheets were able to perform V-bending by strain-induced martensitic transformation of TRIP effect. On the other hand, ferrite- martensite dual-phase (MDP0) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack in tension region. (2) The 0.4C-1.5Si-1.5Mn,mass% TDP4 steel sheet of 1100 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Δθ=θ -θ ), in which the θ and the θ are a bending angle on loading and a bending angle after unloading respectively, of more than 2-degree by controlling a displacement of punch bottom dead center. 2 1 1 2

Effects of strain rate and hydrogen on crack propagation from a notch were investigated using a Fe-33Mn-1.1C steel by tension tests conducted at a cross head displacement speeds of 10 and 10 mm/s. Decreasing cross head displacement speed reduced the elongation by promoting intergranular crack initiation at the notch tip, whereas the crack propagation path was unaffected by the strain rate. Intergranular cracking in the studied steel was mainly caused by plasticity-driven mechanism of dynamic strain aging (DSA) and plasticity-driven damage along grain boundaries. With the introduction of hydrogen, decrease in yield strength due to cracking at the notch tip before yielding as well as reduction in elongation were observed. Coexistence of several hydrogen embrittlement mechanisms, such as hydrogen enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) were observed at and further away from the notch tip resulting in hydrogen assisted intergranular fracture and cracking which was the key reason behind the ductility reduction. −2 −4

To develop ultra high-strength cold stamping steels for automobile frame parts, the effects of alloying elements on hydrogen embrittlement properties of ultra high-strength low alloy transformation induced plasticity (TRIP)-aided steels with a martensite matrix (TM steels) were investigated using the four-point bending test and conventional strain rate tensile test (CSRT). Hydrogen embrittlement properties of the TM steels were improved by the alloying addition. Particularly, 1.0 mass% chromium added TM steel indicated excellent hydrogen embrittlement resistance. This effect was attributed to (1) the decrease in the diffusible hydrogen concentration at the uniform and fine prior austenite grain and packet, block, and lath boundaries; (2) the suppression of hydrogen trapping at martensite matrix/cementite interfaces owing to the suppression of precipitation of cementite at the coarse martensite lath matrix; and (3) the suppression of the hydrogen diffusion to the crack initiation sites owing to the high stability of retained austenite because of the existence of retained austenite in a large amount of the martensite– austenite constituent (M–A) phase in the TM steels containing 1.0 mass% chromium.

© 2019 Hydrogen Energy Publications LLC Hydrogen desorption behavior associated with γ-α′ and γ-ε martensitic transformation during tensile tests is investigated in four kinds of alloys with different austenite stabilities. Remarkable deformation-induced hydrogen desorption is detected not only as a result of the γ-α′ martensitic transformation, but also because of the γ-ε martensitic transformation, and the amount of desorbed hydrogen from transformed α′ martensite is more than that of transformed ε martensite. This suggests that the solubility of hydrogen in the ε phase is higher than that in the α′ martensite but lower than that in austenite.

© 2019 Hydrogen Energy Publications LLC The effects of martensitic transformation on microstructural hydrogen distribution in a multiphase transformation-induced plasticity (TRIP) steel were investigated using silver decoration before and after cooling and cryogenic thermal desorption spectroscopy (C-TDS) during cooling. Transformation-induced hydrogen effusion occurred; however, there was a significant difference between the temperature of the peak hydrogen desorption rate and the start temperature of the martensitic transformation, in contrast to the findings of a previous study on fully austenitic steels. Furthermore, the silver-decoration experiment clarified that martensitic transformation caused a significant increase in local hydrogen flux and accordingly enhanced the heterogeneity of the hydrogen distribution, which originates from the transformation-induced hydrogen effusion observed in the C-TDS analysis.

© 2019 Hydrogen embrittlement of Inconel 718 alloy was investigated. Multi-scale observation technique were employed, comprising slow strain rate tensile tests, scanning electron microscopy and transmission electron microscopy analysis. The results demonstrate that hydrogen charging deteriorates mechanical properties of the alloy. Inconel 718 alloy shows partial Portevin-Le Chatelier (PLC) effect at room temperature when hydrogen charging current density is 220 mA cm−2 and 590 mA cm−2. Moreover, plastic deformation features with dislocation cells are detected in hydrogen-induced brittle zone. Thus, it is concluded that dragging effect of hydrogen atoms on dislocations contributes to PLC effect.

© 2019, The Minerals, Metals & Materials Society and ASM International. We investigated the strain rate dependency of the hydrogen-induced mechanical degradation of Fe-33Mn-1.1C steel at 303 K within the strain rate range of 10−2 to 10−5 s−1. In the presence of hydrogen, lowering the strain rate monotonically decreased the work hardening rate, elongation, and tensile strength and increased the yield strength. Lowering the strain rate simultaneously enhanced the plasticity-related effects of hydrogen and carbon, leading to the observed degradation of the ductility.

© 2018 Elsevier B.V. Hydrogen embrittlement behaviors of PH17-4 steel and PH13-8Mo steel used as steam turbine last stage blades were investigated by means of slow strain rate tensile tests and hydrogen permeation tests. The results indicate that PH13-8Mo steel exhibits higher strength level mainly due to higher precipitate strengthening and lath strengthening compared with PH17-4 steel in the absence of hydrogen. Moreover, pre-hydrogen charging does not affect tensile strength of PH17-4 steel, but decreases tensile strength of PH13-8Mo steel. The plasticity of PH17-4 steel and PH13-8Mo steel is degraded after hydrogen charging and the deterioration is accompanied with considerable change in fracture mode. The loss of plasticity of PH13-8Mo steel is higher than that of PH17-4 steel, indicating that PH13-8Mo steel exhibits lower resistance to hydrogen embrittlement, which is mainly related to higher hydrogen diffusion behaviors and higher strength level of PH13-8Mo steel. Additionally, PH17-4 steel exhibits lower apparent hydrogen diffusion coefficient and higher apparent hydrogen solubility in comparison with those of PH13-8Mo steel, which implies that incoherent Cu-rich precipitates in PH17-4 steel possess higher capability to trap hydrogen atoms than that of coherent NiAl precipitates in PH13-8Mo steel.

© 2019 Iron and Steel Institute of Japan. All rights reserved. Synopsis : An evaluate method for hydrogen embrittlement property of high strength steel sheet has been proposed in this study. To take into consideration of the effect of plastic strain in addition to the effects of applied/residual stress and diffusible hydrogen, U-bend specimens have been adopted because steel sheets for automobiles are usually used after press forming into various parts. After U-shape bending, the specimen was loaded using a bolt. The proposed evaluation method is based on the measurement of critical hydrogen content or critical hydrogen charging condition for hydrogen embrittlement fracture at given stress and strain conditions. The hydrogen charging current density was increased in step-wise manner until cracking was observed, and cracking was detected by optical observation and by monitoring voltage between the sample and a counter electrode. The critical hydrogen contents for specimens with varied applied stress were obtained by means of thermal desorption spectroscopy. For the critical hydrogen content, both the hydrogen contents in strained portion of the specimen and no-strained portion were measured. The former is affected by introduced dislocations caused by straining and the latter is thought to be proportional to the hydrogen fugacity. Both critical hydrogen contents tended to be decreased slightly when the applied stress was relatively high.

© 2018 Hydrogen Energy Publications LLC Hydrogen embrittlement of a nickel-based superalloy IN718 was investigated using slow strain rate tensile tests. Post-mortem observation of fractured samples was performed to explore hydrogen-assisted failure mechanisms of the alloy. The results reveal that hydrogen charging reduces yield strength, tensile strength, fracture strain and work hardening rate. With increasing current density, yield strength and tensile strength reduce linearly and fracture strain decreases exponentially. Furthermore, the crack initiation and propagation in hydrogen-charged region depends on the distribution of δ phase in the alloy. For needle-shaped δ phase within the grains, the nucleation of voids takes place at the intersections between dislocation slip bands and δ phase due to the hydrogen-enhanced localized plasticity (HELP)-assisted shear localization and possible hydrogen agglomeration. For δ phase along the grain boundaries, the impingement of slip bands and local hydrogen accumulation at γ-matrix/δ phase interfaces as well as hydrogen-enhanced decohesion (HEDE)-assisted decohesion lead to the void nucleation at the interfaces. Because of the decoration of δ phase at the grain boundaries, hydrogen-assisted cracking preferentially propagates along the grain boundaries. It is hence suggested that the synergistic interplay of HELP mechanism and HEDE mechanism can be used to explain the embrittlement of the alloy.

© 2018 Hydrogen Energy Publications LLC The microstructural and crystallographic study of hydrogen-assisted cracking in high strength PSB1080 steel was conducted. The results indicate that the mechanical properties of PSB1080 steel are seriously deteriorated in the presence of hydrogen, which is ascribed to the coupling effect of hydrogen-assisted cracking from the O–Al–Si–Ca inclusion and accelerated phase transformation from the austenite to the martensite due to hydrogen. The hydrogen uncharged sample exhibits dimple fracture pattern, whereas the fracture surface of hydrogen charged sample consists of three zones, i.e., quasi-cleavage zone, a mixed zone of quasi-cleavage and dimple as well as dimple zone. Crystallographic orientation analysis beneath the three zones demonstrates that the proportion of low angle grain boundary is the highest, followed by high angle grain boundary and then medium angle grain boundary, and the high Kernel Average Misorientation region facilitates hydrogen-assisted crack propagation. Additionally, the grains oriented with {001}//ND, {110}//ND, {123}//ND exhibit the high possibility of hydrogen-assisted cracking. This suggests that these oriented grain textures should be reduced to design the resistance-hydrogen embrittlement alloys.

© 2018 Elsevier B.V. We investigated the effects of electrochemical hydrogen charging on the mechanical properties of a Fe-33Mn-1.1C austenitic steel with high carbon concentration and relatively high stacking fault energy. Hydrogen pre-charging increased the yield strength and degraded the elongation and work-hardening capability. The increase in yield strength is a result of the solution hardening of hydrogen. A reduction in the cross-sectional area by subcrack formation is the primary factor causing reduction in work-hardening ability. Fracture modes were detected to be both intergranular and transgranular regionally. Neither intergranular nor transgranular cracking modes are related to deformation twinning or simple decohesion in contrast to conventional Fe-Mn-C twinning-induced plasticity steels. The hydrogen-assisted crack initiation and subsequent propagation are attributed to plasticity-dominated mechanisms associated with strain localization. The occurrence of dynamic strain aging by the high carbon content and ease of cross slip owing to the high stacking fault energy can cause strain/damage localization, which assists hydrogen embrittlement associated with the hydrogen-enhanced localized plasticity mechanism.

© The Author(s) 2018. Five Fe-33Mn-xC steels, referred to as 0 C, 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels according to their carbon content in mass%, were prepared to clarify the effect of interstitial carbon on the dissolution behavior of steel. The 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels indicated a fully austenitic structure with no carbide precipitate. The lattice parameters of the 0.6 C, 0.8 C, and 1.1 C steels calculated from the γ(111) and γ(200) diffraction peaks increased by up to around 0.8% over that of the 0.3 C steel, suggesting that the added carbon was present as interstitial carbon in the steels. The 0.6 C, 0.8 C, and 1.1 C steels were passivated during the anodic polarization measurements in 0.1 M Na2SO4 solution at pH 12.0, whereas the 0 C and 0.3 C steels actively dissolved. The anodic polarization measurements in a buffer solution at pH 10.0 demonstrated a lower dissolution current density for the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels with higher amounts of interstitial carbon. The dissolution current density at 0.3 V vs. Ag/AgCl (3.33 M KCl) of the 1.1 C steel was reduced to approximately 1 × 10-2 A m-2, which was one hundredth that of the 0.3 C steel. The dissolution current density of the steels was not inhibited by the presence of 0.1 M CO32- ions, which is an expected dissolution product of interstitial carbon, implying that the interstitial carbon improved the electrochemical property of the steels themselves. The work function of the 1.1 C steel, which showed improved corrosion resistance with interstitial carbon, was 0.12 eV lower than that of the 0 C steel. The peak positions of the Fe 2p3/2 and Mn 2p3/2 spectra of the 1.1 C steel indicated the binding energies were approximately 0.1 eV and 0.2 eV higher than those of the 0 C steel. This can likely be attributed to the partial chemical bonding of interstitial carbon to iron and manganese, respectively.

© The Author(s) 2018. The effects of oxygen pressure on the corrosion of iron embedded in mortar with various Cl− contents were examined by using a novel hyperbaric-oxygen accelerated corrosion test (HOACT). Exposure and electrochemical tests of a pure iron sample embedded in mortar with a cover thickness of 5 mm were performed in HOACT condition with pressurized 100% oxygen gas and with relative humidity of 95%. For comparison, these tests were also performed in ambient air with the same relative humidity. Oxygen reduction current density of iron embedded in mortar increased with an increase in oxygen pressure, suggesting that the corrosion of the embedded iron is expected to be accelerated with oxygen gas pressure assuming an oxygen reduction limited corrosion current density. Iron corrosion in mortar without Cl− was minimal regardless of oxygen gas pressure. In contrast, the corrosion of iron in mortar with Cl− was accelerated with increasing the oxygen gas pressure up to a certain pressure as expected, and the thickness of the rust formed on the iron surface increased with Cl− concentration under each oxygen gas pressure, indicating that Cl− is necessary to initiate corrosion which is enhanced in the pressurized oxygen gas. However, when the oxygen pressure exceeded a certain level, the corrosion was suppressed. Electrochemical impedance spectroscopy measurement using a sensor consisting of a pair of carbon steel electrodes in mortar showed that the charge transfer resistance of the carbon steel under excessively high oxygen gas pressure became high in comparison to that at lower oxygen pressure, indicating the suppression of corrosion initiation is due to formation of a protective passive film under the excessive supply of oxygen. Consequently, the pressurized oxygen gas accelerates the corrosion of iron in mortar with Cl−; however, excess oxygen suppresses corrosion. The optimum condition to efficiently accelerate corrosion of iron in mortar was evaluated.

© 2018 The Japan Institute of Metals and Materials. A novel accelerated corrosion test method which enhances oxygen supply has been proposed for reinforcing steel in concrete in this study. Oxygen reduction current density (ORCD) was measured by means of potentiodynamic polarization test for an iron specimen embedded in cement paste or mortar in a saturated Ca(OH)2 solution in ambient air. The ORCD decreased with an increase in cover thickness and the current density was reciprocally proportional to the cover thickness from 1mm to 10 mm, suggesting that diffusion limited oxygen reduction can be accelerated by reducing the cover thickness below 10 mm. The oxygen supply to iron surface in cement paste or mortar was enhanced by pressurized oxygen gas using a newly developed hyperbaric-oxygen accelerated corrosion test container. Iron specimens with 5mm cement paste and mortar covers showed almost 25 times higher ORCD in 0.5 MPa oxygen gas than that in ambient air, respectively. The iron specimens covered with 5mm of cement paste or mortar containing chloride ion were immersed in a saline solution and exposed to 0.5 MPa oxygen gas in the container for 30 days. The thickness of the rust layer formed for 30-days was in good agreement with that estimated from the ORCD obtained in 0.5 MPa oxygen gas, indicating that the corrosion was accelerated in proportion to the oxygen (partial) pressure. Furthermore, the rust formed in pressurized oxygen gas showed similar characteristics to that formed in a practical service environment. Thus, the hyperbaric-oxygen is beneficial and effective to validly accelerate the corrosion of reinforcing steel in concrete.

© 2018 ISIJ. The effect of strain rate on the hydrogen embrittlement property of an ultra high-strength TRIP-aided bainitic ferrite (TBF) steel with bainitic ferrite matrix was investigated to clarify the correlation between the transformation behavior of retained austenite and the hydrogen embrittlement fracture behavior of the TBF steel. Tensile tests were carried out at the strain rates between 5.56 × 10−6 and 2.78 × 10−2/s without and with hydrogen charging. Hydrogen analysis after tensile tests was conducted by using thermal desorption spectroscopy (TDS). Fracture strain decreased with decreasing the strain rate due to the hydrogen absorption to the TBF steel although fracture strain without hydrogen charging slightly increased with decreasing the strain rate. However, it was observed that transformation behavior of retained austenite was hardly changed by the hydrogen absorption and the change in the strain rate. When tensile test was carried out to the TBF steel at the slow strain rate with hydrogen charging, fracture surface of quasi cleavage fracture containing flat facet, which was fractured transformed martensite, was obtained and the crack perpendicular to the tensile direction was observed near transformed martensite. It was considered that the decrease in the resistance to hydrogen embrittlement of the TBF steel tensile tested at the slow strain rate was attributed to the initiation of flat facet and the hydrogen concentration at the crack tip due to the hydrogen diffusion from transformed martensite during tensile testing at slow strain rate.

© 2018 The Authors. Published by Elsevier B.V. Hydrogen embrittlement properties of nitrogen-added TRIP-aided martensitic steels for improving the fuel efficiency of vehicles due to the weight reduction and the impact safety were evaluated by using conventional strain rate tensile tests. Decrease in the total elongation due to hydrogen of a 100-ppm-nitrogen-added TRIP-aided martensitic steel was small in comparison with that of the TRIP-aided martensitic steel without nitrogen. This was attributed to refinement of martensite lath matrix. On the other hand, hydrogen charging decreased the total elongation of a 200-ppm-nitrogen-added TRIP-aided martensitic steel, and decrease in the total elongation of the 200-ppm-nitrogen-added TRIP-aided martensitic steel was more than that of the 100-ppm-nitrogen-added TRIP-aided martensitic steel. This might be caused by the promotion of crack initiation at AlN and AlN/matrix interfaces induced by trapped hydrogen.

© 2017 Acta Materialia Inc. Fe-33Mn-xC (x = 0, 0.3, 0.6, 0.8, and 1.1 mass%) fully austenitic steels showed ductility degradation owing to dynamic strain aging (DSA). The elongation increased with increasing carbon concentration at a strain rate of 10− 2 s− 1. However, in the steels with carbon contents of 0.6%, 0.8%, and 1.1%, the elongation decreased with increasing carbon concentration at a strain rate of 10− 5 s− 1 where the DAS effect is distinct. Although all specimens showed ductile fracture with the formation of dimples, the work hardening-true stress relation of the Fe-33Mn-1.1C steel demonstrated fracture before satisfying Considère's criterion even at high strain rates.

我々の研究グループでは、モルタルに埋設した鉄試料に高圧酸素を供給することで腐食を促進させる高酸素腐食促進試験を開発した。この方法は、腐食の過程を発生と進展に分類すると腐食の進展の促進を行う方法である。本研究では、モルタル埋設鉄の腐食発生を促進する方法を検討し、高酸素腐食促進試験と組み合わせることでさらなる腐食促進を達成した。

© 2017 Elsevier Ltd Effect of heat treatment on hydrogen-assisted fracture behavior of PH13-8Mo steel was investigated. The results show that hydrogen embrittlement susceptibility of the specimens aged at 600 °C is the lowest compared with that of the specimens aged at 430 °C and 540 °C at the same aging time. With an increase in aging time, the susceptibility to hydrogen embrittlement increases for specimens aged at 430 °C while it decreases for specimens aged at 540 °C and 600 °C. Hydrogen-assisted cracks initiate and propagate along lath and prior austenite grain boundaries. In addition, the quantitative relationship between hydrogen embrittlement index and fracture mode is given.

© 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement. This paper is part of a thematic issue on Hydrogen in Metallic Alloys.

© 2017 Hydrogen embrittlement properties were investigated for 1.8-GPa-class ultra-high strength low-alloy steels by means of slow-strain-rate test of the pre-hydrogen-charged notched specimens, accelerated atmospheric corrosion test, and thermal desorption spectrometry. A Mo-bearing steel with a chemical composition of Fe-0.4C-2Si-1Cr-1Mo (mass%) was quenched and tempered at 773 K for 1 h and then deformed by multi-pass caliber rolling with a cumulative rolling reduction of 76% at 773 K to create an ultrafine elongated grain structure with a strong <110>//rolling direction fiber texture. The warm tempformed (TF) sample was subsequently annealed for 1 h to clarify the hydrogen trapping effect of nanoscale carbides relative to additive Mo. When the TF sample was annealed at 843 K (TFA sample), the hydrogen absorption capacity was enhanced significantly through the formation of nanoscale Mo-rich precipitates in the matrix of ultrafine elongated grains. A high potential for hydrogen embrittlement resistance in an atmospheric corrosion environment was demonstrated in both the TF and TFA samples with an ultra-high tensile strength of 1.8 GPa. The TF and TFA samples were much less susceptible to hydrogen embrittlement as compared to the tempered martensitic samples at an ultra-high tensile strength of 1.8 GPa. The hydrogen trapping states and the high resistance to hydrogen embrittlement in the TF and TFA samples are discussed in association with the anisotropic, ultrafine grained structures with the nanoscale Mo-rich precipitates.

© 2017 Hydrogen Energy Publications LLC Hydrogen and fuels derived from it will serve as the energy carriers of the future. The associated rapidly growing demand for hydrogen energy-related infrastructure materials has stimulated multiple engineering and scientific studies on the hydrogen embrittlement resistance of various groups of high performance alloys. Among these, high-Mn steels have received special attention owing to their excellent strength – ductility – cost relationship. However, hydrogen-induced delayed fracture has been reported to occur in deep-drawn cup specimens of some of these alloys. Driven by this challenge we present here an overview of the hydrogen embrittlement research carried out on high-Mn steels. The hydrogen embrittlement susceptibility of high-Mn steels is particularly sensitive to their chemical composition since the various alloying elements simultaneously affect the material's stacking fault energy, phase stability, hydrogen uptake behavior, surface oxide scales and interstitial diffusivity, all of which affect the hydrogen embrittlement susceptibility. Here, we discuss the contribution of each of these factors to the hydrogen embrittlement susceptibility of these steels and discuss pathways how certain embrittlement mechanisms can be hampered or even inhibited. Examples of positive effects of hydrogen on the tensile ductility are also introduced.

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The hydrogen distribution in an austenite–martensite dual-phase steel was investigated using silver decoration and scanning Kelvin probe force microscopy. The silver decoration technique optimized for spacial resolution reveals interfacial segregation of hydrogen along the plate-type martensite–martensite grain boundaries. In addition, the scanning Kelvin probe force microscopy kinetically elucidates that hydrogen preferentially diffused out from the martensite–martensite grain boundaries. These preferential sites of hydrogen desorption correspond to the regions of hydrogen-assisted damage.

<p>Hydrogen entry into a high strength steel with tribocorrosion in acidic solutions was studied in the present study and it was compared with that in a neutral solution. The samples of SCM 435 high strength steel were subjected to tribocorrosion using a pin-on-disk type wear equipment and to static corrosion in neutral and acidic solutions containing Cl^-. The hydrogen content entered into the steel was measured using hydrogen thermal desorption analysis. In a neutral solution, the hydrogen content in the tribocorroded sample was larger than that in the corroded sample without wear. In the acidic solution of pH 2.7, the hydrogen content in the tribocorroded sample was also larger than that in the corroded sample without wear in the case of 6 and 12 h duration time. On the other hand, the hydrogen content in the corroded sample without wear became larger than that in the tribocorroded sample after 18 h duration time. In the measurement of pH changes of the solution for the duration of 48 h tribocorrosion test and static corrosion test, it is revealed that the pH of acidic solution during tribocorrosion increased from about 2.7 to 6.0, though the pH of the acidic solution during static corrosion test revealed little variation. Because H⁺ consumption by hydrogen generation in cathodic reaction was enhanced with tribocorrosion in acidic solution. The concentration of H⁺ decreased gradually and pH became higher. As a result, corrosion of the steel and hydrogen generation were suppressed and hydrogen entry into the sample was diminished.</p>

© 2017 The Authors. Published by Elsevier Ltd. To clarify the effects of hydrogen on the deformation properties of TBF steel at low strain rate, the tensile properties of hydrogen-charged TBF steel were investigated by using slow strain rate technique (SSRT). Total elongation of TBF steel decreased due to the hydrogen absorption. The hydrogen absorption changed fracture surfaces of TBF steel from dimple fracture to quasi cleavage fracture. In addition, hydrogen-assisted cracks perpendicular to the tensile direction were observed at the prior austenitic grain, the packet and the bainitic ferrite lath boundaries although voids occurred at the lath boundary in non-hydrogen-charged TBF steel. In the case of the hydrogen-assisted cracks in hydrogen-charged TBF steel, transformed martensite is located near cracks. Transformation of retained austenite to martensite in the TBF steel was slightly accelerated during tensile deformation when the TBF steel was charged with hydrogen. When tensile strain was applied to the hydrogen-charged TBF steel, hydrogen was evolved at the high temperature region of peak corresponding to vacancy clusters generated by the intersection of dislocation in the hydrogen evolution curve. Thus, deterioration of ductility of hydrogen-charged TBF steel was caused by the acceleration of transformation of retained austenite because of the hydrogen absorption and the promotion of crack initiation at the prior austenitic grain, packet and lath boundaries due to the hydrogen diffusion from transformed martensite.

コンクリート中の鉄筋は表面に保護性の酸化皮膜である不働態を形成するため良好な耐食性を有しているが、コンクリート中への塩化物イオンや二酸化炭素の侵入により不働態は破壊され鉄筋が腐食しコンクリートの劣化につながる。自然界では数十年から数百年かかるといわれている鉄筋の腐食を実験室レベルで再現するために、短期間でコンクリート中の鉄筋を腐食させる加速試験法を考案した。現在、コンクリート中と比較して約20倍の加速に成功しており、生成した鉄さびの組成も自然界で生成されるものと同等である。

© 2016 Elsevier B.V. Hydrogen desorption associated with thermally induced α′ and ϵ martensitic transformations was characterized by cryogenic thermal desorption spectroscopy (C-TDS) during cooling. The utilization of a cooling process during C-TDS measurements revealed that the dominant factors in the hydrogen desorption related to the α′ and ϵ martensitic transformations were local changes in diffusivity and motions of transformation dislocations, respectively.

© 2016 by De Gruyter. Electrochemical hydrogen permeation tests using pure Fe sheet specimens were performed under various galvanostatic hydrogen charging conditions commonly applied to tensile test specimens used for evaluation of hydrogen embrittlement property to obtain a guideline for efficient hydrogen charging method for a wide range of hydrogen content. Aqueous NaCl solutions containing varied concentrations of ammonium thiocyanate and a 0.1-m NaOH solution were used as electrolytes for relatively high and low hydrogen content, respectively. Addition of ammonium thiocyanate changes the mechanism of hydrogen evolution reaction and enhances hydrogen entry. By changing the concentration of ammonium thiocyanate and by changing cathodic current density, a wide range of hydrogen content can be covered. The hydrogen content in an AISI 4135 steel estimated from hydrogen permeation current density obtained for a pure Fe specimen was in good agreement with the hydrogen content experimentally measured by hydrogen thermal desorption analysis for the AISI 4135 steel. The hydrogen permeation current density data in various hydrogen charging conditions can be used as a reference for hydrogen charging of specimens.

© 2015 Taylor & Francis. For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1 1 0}M localized slip and {1 1 2}M twin and (2) cracking of martensite-martensite grain interfaces. The former resulted in nanovoids along the {1 1 2}M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking.

© 2016 ISIJ. Electrochemical hydrogen permeation tests were carried out under controlled temperature and humidity using pure Fe sheet specimens outdoor-exposed at Beijing and Chongqing, China and Okinawa, Japan in order to understand the effect of environmental factors on hydrogen uptake behavior. The maximum hydrogen permeation current densities of the exposed specimens were in the order of Beijing Chongqing Okinawa, while the order of the degree of corrosion of the specimens was Okinawa Chongqing Beijing. X-ray fluorescence analysis showed that the surface concentrations of sulfur on the Beijing- and Chongqing-exposed specimens were higher than that of the Okinawa-exposed specimens, whereas chlorine concentrations of the Beijing and Okinawa-exposed specimens were higher than that of the Chongqingexposed specimen. Nitrate concentrations of Beijing- and Chongqing-exposed specimens evaluated using nitrate test strips were obviously higher than that of the Okinawa-exposed specimens. It is suggested that air pollutants such as SO2 and NO2 and particulate matters containing inorganic acid ions, likely surfate and nitrate ions, and possibly organic acids contribute to acidification of rust layer leading to the enhanced hydrogen entry.

© 2016 ISIJ. Resistance to hydrogen embrittlement of low alloy steels was evaluated based on their critical hydrogen content and critical stress. Constant load test (CLT), Slow Strain Rate Test (SSRT) and Conventional Strain Rate Test (CSRT) were carried out using JIS-SCM435 and V-added steels in six laboratories. It was confirmed that the same test results were obtained in different laboratories under the same test conditions. Furthermore, the relationships between diffusible hydrogen content and nominal fracture stress obtained by means of CLT and SSRT were similar to each other. In CSRT, the nominal fracture stress was higher than that in CLT and SSRT under the same absorbed hydrogen content in the specimens. In SSRT and CSRT, fracture surfaces showed Quasi-cleavage mode under small hydrogen content, while they showed Inter-granular fracture under large hydrogen content. In order to compare the three methods considering the concentration of hydrogen in stress field, locally accumulated hydrogen content under the same fracture stress is calculated. The locally accumulated hydrogen under the same applied stress, in other words, critical hydrogen content to hydrogen embrittlement, is the following order; SSRT < CLT < CSRT in JIS-SCM435, and CSRT < CLT ≓ SSRT in V-added steels.

Hydrogen entry into AISI 4135 high strength steel in tribocorrosion environment was studied in the present study. The samples were corroded in tribocorrosion environment using a pin-on-disk type wear gear equipped with a solution cell and in static corrosion environment, and the hydrogen content entered into the steel was measured using hydrogen thermal desorption analysis. The hydrogen content in tribocorroded sample was larger than that in corroded sample without wear. From the cathodic polarization curves and surface morphology, it is revealed that the main factor which increases the amount of entered hydrogen is solution flow. The solution flow enhances the supply of oxygen, and the difference of flow rate forms anodic and cathodic areas on the samples. Solution flow also enhances anodic dissolution and increases hydrogen generation reaction. As a result, hydrogen entry into the high strength steel is enhanced in tribocorrosion environment.

Electrochemical hydrogen permeation tests were carried out under potentiostatic hydrogen charging conditions in various solutions, and the hydrogen entry behavior was compared with that of galvanostatic method. The relationship between hydrogen permeation current density and potential was similar in potentiostatic and galvanostatic methods and wide range of hydrogen concentration could be covered as well as galvanostatic method by varying NH4SCN content and potential. However, slight differences in hydrogen permeation current density and efficiency of hydrogen entry were seen between the methods, and the stability of hydrogen entry of potentiostatic method was slightly higher than that of galvanostatic method. In both methods, hydrogen permeation current density tended to increase with time in a NH4SCN containing solution suggesting that it is recommended to measure resulted hydrogen content by means of thermal desorption analysis for accuracy, though the relationship between hydrogen permeation current density and potential can be used as a rough standard.

© 2015 The Japan Institute of Metals and Materials. The relation between hydrogen distribution and metallographic microstructure was investigated by means of Ag decoration technique for a SUS304 austenitic stainless steel, and martensitic and spheroidized SCM440 steels precharged with hydrogen. Preferential distribution of Ag particles was seen on the slip lines of the deformed and hydrogen-charged SUS304 stainless steel, suggesting that the slip lines act as hydrogen trap sites. The martensitic SCM440 steel showed almost no selective Ag deposition, indicative of apparently homogeneous distribution of hydrogen. This is probably because the distribution of dislocations with relatively high concentration and the fine structure including lath boundaries etc. acting as hydrogen traps is homogeneous. The spheroidized SCM440 steel showed almost no Ag deposition on the coarse cementite particles and the Ag particle distribution on the other areas did not show clear selectivity. This result suggests that the hydrogen does not diffuse through the cementite particles. The smallest Ag particle size observed by means of atomic force microscope was in the order of 10 nm. Though the minimum size of the Ag particle does not necessarily indicate the resolution of the hydrogen visualization, Ag decoration technique is useful to observe the hydrogen distribution.

© 2015 The Electrochemical Society. The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1-2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption.

Hydrogen embrittlement affects high-strength ferrite/martensite dual-phase (DP) steels. The associated micromechanisms which lead to failure have not been fully clarified yet. Here we present a quantitative micromechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy-based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both with and without hydrogen precharging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack-arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Al effects on strain aging and resistance against hydrogen embrittlement were examined in Fe-18Mn-0.6C-based twinning-induced plasticity steels deformed at different strain rates. The Fe-18Mn-0.6C steel showed hydrogen-induced fracture when it had been pre-deformed at a strain rate of 1.7×10-6s-1. The hydrogen-induced fracture was suppressed by increasing strain rate and increasing Al content. From the viewpoint of material strengthening by strain aging, we found two important factors improving the resistance to the hydrogen embrittlement; (1) suppression of dynamic strain aging by increasing strain rate and Al content, and (2) suppression of static strain aging under loading by the Al addition.

Hydrogen embrittlement of a Fe-18Mn-0.6C-1.5Al steel was observed in tensile deformation during cathodic hydrogen charging. The fracture mode was quasi-cleavage fracture. The relationship between diffusible hydrogen content and fracture stress was arranged by the power law like that for ferritic and Al-free TWIP steels. The Al addition did not affect the magnitude of the degradation of hydrogen embrittlement property at the same current density in TWIP steels. However, the Al-added steel showed a suppression of hydrogen entry and a larger total elongation in comparison to those of the Al-free TWIP steel in the same environment, although the Al addition decreased fracture stress. The larger elongation is one of the reasons for why the Al addition improves the hydrogen embrittlement property of cup specimens.

We investigated the factors affecting static strain aging under stress in a Fe-22Mn-0.6C twinning-induced plasticity steel at room temperature. The magnitude of strengthening by the static strain aging was estimated by tensile strain holding and subsequent re-loading. Strain holding time, pre-strain, strain rate, external stress, and diffusible hydrogen content were varied to clarify their effects on static strain aging, and the present static strain aging was found to be affected by all of these factors. In this paper, we show the phenomenological laws of the relationship among the factors and the stress increase due to the static strain aging.

The relation between hydrogen distribution and metallographic microstructure was investigated by means of Ag decoration technique for a SUS304 austenitic stainless steel, and martensitic and spheroidized SCM440 steels precharged with hydrogen. Preferential distribution of Ag particles was seen on the slip lines of the deformed and hydrogen-charged SUS304 stainless steel, suggesting that the slip lines act as hydrogen trap sites. The martensitic SCM440 steel showed almost no selective Ag deposition, indicative of apparently homogeneous distribution of hydrogen. This is probably because the distribution of dislocations with relatively high concentration and the fine structure including lath boundaries etc. acting as hydrogen traps is homogeneous. The spheroidized SCM440 steel showed almost no Ag deposition on the coarse cementite particles and the Ag particle distribution on the other areas did not show clear selectivity. This result suggests that the hydrogen does not diffuse through the cementite partioles. The smallest Ag particle size observed by means of atomic force microscope was in the order of 10 nm. Though the minimum size of the Ag particle does not necessarily indicate the resolution of the hydrogen visualization. Ag decoration techniaue is useiui to opserve tne nvarogen aistriDution. © 2013 The Japan Institute of Metals and Materials.

Hydrogen embrittlement properties were examined in a single crystalline type 316 austenitic stainless steel. Tensile tests were conducted along the 〈1. 1. 1〉 and 〈0. 0. 1〉 directions under hydrogen charging. Hydrogen-assisted {1. 1. 1} quasi-cleavage fracture was observed in both tensile orientations. The degradation of fracture stress and elongation, hydrogen uptake, and hydrogen-induced fracture surface were dependent on the tensile orientation. The tensile orientation dependence of the hydrogen embrittlement properties was shown to result from the deformation twinning behavior. In addition, Ag decoration technique clarified that hydrogen localizes on regions where hydrogen-assisted quasi-cleavage fracture appeared. © 2013 Elsevier Ltd.

We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Fretting fatigue behaviour of Ni-free high-nitrogen steel (HNS) with a yield strength of about 800 MPa, which was prepared by nitrogen gas pressurized electroslag remelting, was studied in air and in phosphate-buffered saline (PBS(-)). For comparison, fretting fatigue behaviour of cold-rolled SUS316L steel (SUS316L(CR)) with similar yield strength was examined. The plain fatigue limit of HNS was slightly lower than that of SUS316L(CR) although the former had a higher tensile strength than the latter. The fretting fatigue limit of HNS was higher than that of SUS316L(CR) both in air and in PBS(-). A decrease in fatigue limit of HNS by fretting was significantly smaller than that of SUS316L(CR) in both environments, indicating that HNS has better fretting fatigue resistance than SUS316L(CR). The decrease in fatigue limit by fretting is discussed taking into account the effect of friction stress due to fretting and the additional influences of wear, tribocorrosion and plastic deformation in the fretted area. © 2013 National Institute for Materials Science.

Hydrogen embrittlement of high-strength steels was investigated by using slow strain rate test (SSRT) of circumferentially notched round bar specimens after hydrogen precharging. On top of that, cyclic corrosion tests (CCT) and outdoor exposure tests were conducted prior to SSRT to take into account the effect of hydrogen uptake under atmospheric corrosion for the evaluation of the susceptibility of high-strength steels. Our studies of hydrogen embrittle properties of high-strength steels with 1100 to 1500 MPa of tensile strength and a prototype ultrahigh-strength steel with 1760 MPa containing hydrogen traps using those methods are reviewed in this article. A power law relationship between notch tensile strength of hydrogen-precharged specimens and diffusible hydrogen content has been found. It has also been found that the local stress and the local hydrogen concentration are controlling factors of fracture. The results obtained by using SSRT after CCT and outdoor exposure test were in good agreement with the hydrogen embrittlement fracture property obtained by means of long-term exposure tests of bolts made of the high-strength steels. © 2012 The Minerals, Metals & Materials Society and ASM International.

The local concentration of hydrogen absorbed from the environment in the section of high strength steels, where the hydrostatic stress is the highest under loading and where the initiation of delayed fracture takes place, was investigated stochastically. For the investigation, the results of statistical analysis of critical hydrogen concentration for delayed fracture obtained by using conventional strain rate tests and the ratio of fracture of exposed high strength bolts were used. The probability distribution of the local hydrogen was successfully evaluated and the correspondent environmental pH was estimated. The pH value simulating the maximum hydrogen entry caused by atmospheric corrosion in Okinawa was slightly lower than 2.

Synopsis : Electrochemical hydrogen permeation tests of pure Fe sheets rusted by cyclic corrosion test (CCT) and atmospheric exposure were carried out under controlled temperature and humidity to investigate the influence of atmospheric corrosion on the hydrogen entry behavior. The hydrogen entry into the Fe specimens rusted by CCT increased under wet condition, and the hydrogen entry was increased with the CCT cycle number. During drying process after the wetting, hydrogen entry was further enhanced and a peak of hydrogen current was observed. The peak hydrogen permeation current tended to increase with the growth of rust layer, and the peak value of the hydrogen permeation current became remarkably higher than that at the highest humidity when the rust layer was relatively thick. Similar enhancement of hydrogen entry into an outdoor-exposed specimen was also observed during drying. Drying process after CCT resulted in an increase in hydrogen content of 5 mm-thick steel specimens measured by means of thermal desorption analysis, indicating the enhancement of hydrogen entry during drying process and showing a good agreement with the electrochemical hydrogen permeation test results. It is required to take into consideration the enhanced hydrogen entry to estimate concentration of hydrogen from the environment.

Al effects on strain aging and resistance against hydrogen embrittlement were examined in Fe-18Mn- 0.6C-based twinning-induced plasticity steels deformed at different strain rates. These steels showed a hydrogen-induced fracture when they were pre-deformed at a strain rate of 1.7×10-6 s -1. This fracture was suppressed by increasing the strain rate and Al content. The two important factors for improving the resistance to hydrogen embrittlement from the viewpoint of material strengthening by strain aging were found to be (1) the suppression of dynamic strain aging by increasing the strain rate and Al content, and (2) the suppression of static strain aging under loading by the Al addition. © 2013 ISIJ.

We investigated the factors affecting static strain aging under stress in a Fe-22Mn-0.6C twinninginduced plasticity steel at room temperature. The magnitude of strengthening by the static strain aging was estimated by tensile strain holding and subsequent re-loading. Strain holding time, pre-strain, strain rate, external stress, and diffusible hydrogen content were varied to clarify their effects on static strain aging, and the present static strain aging was found to be affected by all of these factors. In this paper, we show the phenomenological laws of the relationship among the factors and the stress increase due to the static strain aging. © 2013 ISIJ.

Electrochemical hydrogen permeation tests of pure Fe sheets rusted by cyclic corrosion test (CCT) and atmospheric exposure were carried out under controlled temperature and humidity to investigate the influence of atmospheric corrosion on the hydrogen entry behavior. The hydrogen entry into the Fe specimens rusted by CCT increased under wet condition, and the hydrogen entry was increased with the CCT cycle number. During drying process after the wetting, hydrogen entry was further enhanced and a peak of hydrogen current was observed. The peak hydrogen permeation current tended to increase with the growth of rust layer, and the peak value of the hydrogen permeation current became remarkably higher than that at the highest humidity when the rust layer was relatively thick. Similar enhancement of hydrogen entry into an outdoor-exposed specimen was also observed during drying. Drying process after CCT resulted in an increase in hydrogen content of 5 mm-thick steel specimens measured by means of thermal desorption analysis, indicating the enhancement of hydrogen entry during drying process and showing a good agreement with the electrochemical hydrogen permeation test results. It is required to take into consideration the enhanced hydrogen entry to estimate concentration of hydrogen from the environment. © 2013 ISIJ.

Extrapolation of world energy consumption from 1990 to 2010 indicates the complete exhaustion of world reserves of oil, natural gas, uranium and coal by 2040, 2043, 2046 and 2053, respectively. For the survival of all people in the whole world, intermittent and fluctuating electricity generated from renewable energy should be supplied in the form of usable fuel to all people in the whole world. We have been working on research and development of global carbon dioxide recycling for the use of renewable energy in the form of methane via electrolytic hydrogen generation using carbon dioxide as the feedstock. We created energy-saving cathodes for hydrogen production, anodes for oxygen evolution without chlorine formation in seawater electrolysis, and catalysts for methanation of carbon dioxide and built pilot plants of industrial scale. Recent advances in materials are described. Industrial applications are in progress. © Warszawa - Krakow 2013.

The hydrogen embrittlement of a Fe-18Mn-0.6C austenitic steel (wt.%) was examined using tensile tests under hydrogen charging at various current densities. The tensile properties deteriorated due to the occurrence of intergranular fracture above a specific current density. The work hardening behavior was not affected by the hydrogen charging, indicating that the embrittlement was independent of the change in behavior of slip deformation, martensitic transformation, and twinning deformation. The relationship between the fracture stresses for the intergranular fracture and the diffusible hydrogen content of the austenitic steel was approximated to the power law similarly to ferritic high strength steels. © 2012 Elsevier Ltd.

Hydrogen-induced delayed fracture under loading was investigated in a Fe-22Mn-0.6C twinning-induced plasticity steel that had been pre-deformed at various strain rates. Hydrogen-induced delayed fracture was suppressed by increasing the strain rate of the pre-deformation. In this study on the strain-rate effect, factors affecting the delayed fracture were found to be the negative strain-rate sensitivity of flow stress, stress drop caused by the relaxation phenomenon, and the increase in material strength due to strain aging. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A deformation of a tempered martensitic structure (i.e., tempforming) at 773 K (500°C) was applied to a 0.6 pct C-2 pct Si-1 pct Cr steel. The hydrogen embrittlement (HE) property of the tempformed (TF) steel was investigated by a slow strain rate test (SSRT) and an accelerated atmospheric corrosion test (AACT). Hydrogen content within the samples after SSRT andAACT was measured by thermal desorption spectrometry (TDS). The tempforming at 773 K (500°C) using multipass caliber rolling with an accumulative are reduction of 76 pct resulted in the evolution of an ultrafine elongated grain (UFEG) structure with a strong h110i//rolling direction (RD) fiber deformation texture and a dispersion of spheroidized cementite particles. The SSRT of the pre-hydrogen-charged notched specimens and the AACT demonstrated that the TF sample had superior potential for HE resistance to the conventional quenched and tempered (QT) sample at a tensile strength of 1500 MPa. The TDS analysis also indicated that the hydrogen might be mainly trapped by reversible trapping sites such as grain boundaries and dislocations in the TF sample, and the hydrogen trapping states of the TF sample were similar to those of the QT sample. The QT sample exhibited hydrogen-induced intergranular fracture along the boundaries of coarse prioraustenite grains. In contrast, the hydrogen-induced cracking occurred in association with the UFEG structure in the TF sample, leading to the higher HE resistance of the TF sample. © 2011 The Minerals, Metals & Materials Society and ASM International.

Hydrogen embrittlement was observed in an Fe-18Mn-1.2C (wt.%) steel. The tensile ductility was drastically reduced by hydrogen charging during tensile testing. The fracture mode was mainly intergranular fracture, though transgranular fracture was also partially observed. The transgranular fracture occurred parallel to the primary and secondary deformation twin boundaries, as confirmed by electron backscattering diffraction analysis and orientation-optimized electron channeling contrast imaging. The microstructural observations indicate that cracks are initiated at grain boundaries and twin boundaries. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Experimental studies attempting to evaluate the delayed fracture property of high strength bolt steels by using slow strain rate tests (SSRT) of circumferentially notched round bar specimens are reviewed in this paper. The relationship between the notch tensile strength (NTS) of the hydrogen-precharged specimens and the hydrogen content was approximated by power law relationship. It has been found that the local stress and the local diffusible hydrogen concentration control the occurrence of delayed fracture. To take into account the effect of hydrogen uptake from the environment, the notched bar specimens were subjected to cyclic corrosion test (CCT) and outdoor exposure, and their NTSs were measured using SSRT. The susceptibility of high strength steels to delayed fracture estimated from the decrease of NTS was correspondent to the fracture ratio of high strength bolts of the steels obtained by outdoor exposure tests, and was successfully evaluated with consideration of hydrogen uptake. Hydrogen entry behavior under CCT was monitored by electrochemical hydrogen permeation test, and continuous increase in hydrogen entry with growth of rust layer was observed. It is suggested that the "delay" of delayed fracture is the time required for the enhancement of the hydrogen entry efficiency. © 2011 ISIJ.

Hydrogen embrittlement of a Fe-18Mn-0.6C-1.5Al steel was observed in tensile deformation during cathodic hydrogen charging. The fracture mode was quasi-cleavage fracture. The relationship between diffusible hydrogen content and fracture stress was arranged by the power law like that for ferritic and Al-free TWIP steels. The Al addition did not affect the magnitude of the degradation of hydrogen embrittlement property at the same current density in TWIP steels. However, the Al-added steel showed a suppression of hydrogen entry and a larger total elongation in comparison to those of the Al-free TWIP steel in the same environment, although the Al addition decreased fracture stress. The larger elongation is one of the reasons for why the Al addition improves the hydrogen embrittlement property of cup specimens. © 2012 ISIJ.

The influence of hydrogen entry on ductility was evaluated in a ternary twinning-induced plasticity (TWIP) steel with a composition of Fe-18Mn-0.6C in wt.% using tensile tests. The samples with a thickness of 1.2. mm were charged with hydrogen galvanostatically during the tensile tests. Significant hydrogen content was introduced by the hydrogen-charging. The total elongation was significantly deteriorated from approx. 60% to 30% by the hydrogen-charging. A clear intergranular fracture surface was observed in a vicinity of the sample surface in the hydrogen-charged samples. © 2011 Elsevier Ld.

Corrosion behaviour of sputter-deposited Mg-Zr alloys was examined in a borate buffer solution of pH 8.7. XRD measurements showed that the alloys were supersaturated with Zr. The addition of 29. at.% Zr or more was very effective in increasing the corrosion resistance of Mg by more than four orders of magnitude. XPS analysis of spontaneously formed passive films revealed that the passive film consisted of double oxyhydroxide composed of enriched tetravalent Zr and divalent Mg cations. The enrichment of Zr cations in the passive film is responsible for the enhanced corrosion resistance of Mg-Zr alloys. © 2011 Elsevier Ltd.

Constant load tests of circumferentially notched round bar specimens of high strength steels after cyclic corrosion test and outdoor exposure have been performed to demonstrate that delayed fracture occurs when the hydrogen content from the environment, H E , exceeds the critical hydrogen content for delayed fracture, H C . During the constant load tests the humidity around the specimen was increased in stepwise manner to increase hydrogen entry. After fracture the specimen was kept at the humidity long enough to homogenize hydrogen in the specimen and to obtain more quantitative hydrogen content by thermal desorption analysis. H E of the fractured specimens was higher than H C , and H E of the specimens not fractured was lower than H C . This result confirms that the balance between H C and H E determines the occurrence of delayed fracture and that hydrogen-content-based evaluation of susceptibility to delayed fracture is reasonable. To certify the increase of H E with increase in humidity, electrochemical hydrogen permeation test was carried out. The hydrogen permeation current density was increased especially at 98%RH. Enhancement of hydrogen entry with increase in CCT number was also shown by the test. © 2011 Elsevier B.V. All rights reserved.

Electrochemical hydrogen permeation tests of Fe sheets under two cyclic corrosion test (CCT) conditions were performed to understand hydrogen entry behavior under atmospheric corrosions. Hydrogen entry into 1300 MPa-class high strength steels under two CCT conditions was also investigated using thermal desorption analysis. One CCT consisted of salt spray, dry and wet stages (Salt Spray CCT; SSCCT), and the other consisted of dry and wet stages after NaCl deposition (Dry-Wet CCT; DWCCT). The corrosion rates of Fe and the steels were almost constant under SSCCT and they decreased under DWCCT with time. Nevertheless, both CCTs resulted in increases in hydrogen permeation current and diffusible hydrogen content with time indicating enhancement of hydrogen entry. Corrosion current monitored by means of an atmospheric corrosion monitoring sensor consisting of Fe anode and Ag cathode decreased obviously under dry stage of the CCTs, whereas hydrogen permeation was high at the beginning of the dry stage. The discrepancy between hydrogen entry and corrosion rate indicates that the hydrogen entry is not directly controlled by corrosion rate. Increase in acidity of underlying rust layer with growth of rust layer monitored using a W/WO3 electrode is considered to be one of the factors affecting the hydrogen entry efficiency. © 2010 Elsevier Ltd All rights reserved.

Delayed fracture properties of AISI 4135 high strength steels with 1490 and 1310. MPa of tensile strength, represented as B15 and B13, respectively, have been studied by means of slow strain rate test (SSRT) of notched bar specimens after outdoor exposure at rural and coastal areas. The exposed specimens were kept at humid medium before SSRT to reproduce active hydrogen entry influenced by the rust layer and to homogenize hydrogen distribution. The influences of exposure site and exposure time on fracture stress have been investigated. The susceptibility of B15 to delayed fracture was obviously higher than that of B13. © 2010 Elsevier Ltd.

Hydrogen entry into high strength steels by atmospheric corrosion has been investigated to evaluate their susceptibility to hydrogen embrittlement. High strength steels were corroded by dry/wet cyclic corrosion after NaCl deposition. The maximum diffusible hydrogen concentration around the surface was successfully obtained by means of thermal desorption analysis after keeping the specimens at high humidity to reproduce enhanced hydrogen entry influenced by the rust layer and to homogenize the hydrogen distribution. Despite decrease in corrosion rate, hydrogen content in specimens did not decrease. Decrease of pH in inner rust layer is responsible for the enhanced hydrogen entry into steel. © 2010 Elsevier Ltd. All rights reserved.

To evaluate susceptibilities of high strength steels to delayed fracture, slow strain rate tests (SSRT) of notched bar specimens of AISI 4135 with tensile strengths of 1300 and 1500 MPa and boron-bearing steel with 1300 MPa have been performed after cyclic corrosion test (CCT). During SSRT the humidity around the specimen was kept high to keep absorbed diffusible hydrogen. The fracture stresses of AISI 4135 steels decreased with increment of diffusible hydrogen content which increased with CCT cycles. Their delayed fracture susceptibilities could be successfully evaluated in consideration of both influence of hydrogen content on mechanical property and hydrogen entry. © 2010 Elsevier Ltd. All rights reserved.

The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels. © 2010 National Institute for Materials Science.

Effects of severe plastic deformation on the corrosion behaviors of Al alloys containing precipitates have been investigated. Al and its alloys were severely deformed by equal-channel angular pressing (ECAP) processes and the corrosion behaviors of the Al alloys were evaluated by means of potentiodynamic polarization in a neutral buffer solution containing 0.002 M chloride ion. Introduction of huge plastic deformation to both of Al-5.4 wt% Ni and Al-5 wt% Cu alloys increased pitting potential. In contrast, ECAP treatment of 4N pure Al resulted in a decrease in open circuit potential, slight increase of passive current and shift of pitting potential to the negative direction. The influence of the change in microstructures caused by severe plastic deformation was investigated. © 2007 Springer-Verlag.

Delayed fracture properties of martensitic high strength steels have been evaluated by measuring the change in fracture stress of circumferentially notched bar specimens pre-charged with hydrogen by means of slow strain rate test (SSRT). The fracture stress of a steel with higher strength level showed sharper decrease indicating higher susceptibility to delayed fracture. It has been found that the decrease of fracture stress with hydrogen concentration follows a power law relationship when intergranular fracture mode is observed. Another finding is that the local stress and the local hydrogen concentration estimated by finite element analysis are determining delayed fracture. Hydrogen entry into steels caused by atmospheric corrosion has been evaluated as well by using cyclic corrosion test (CCT) simulating atmospheric corrosion. The circumferentially notched bar specimens were exposed to the CCT condition and SSRT was performed at humid environment to investigate both hydrogen entry and its influence on the decrease of fracture stress in the environment. The relation between fracture stress and the content of hydrogen in corrosive environment was compared with the relation of hydrogen pre-charged specimens. The SSRT combined with CCT was proposed as evaluation method considering both the effect of hydrogen in metals and hydrogen entry into metals. Copyright © 2009 ASM International® All rights reserved.

A conventional strain rate technique (CSRT) to evaluate the delayed fracture characteristics of high strength steels has been proposed. The critical "maximum stress-diffusible hydrogen concentration" at the delayed fracture initiation point near the notch tip is thought to be a material constant, which was originally demonstrated using the SSRT (slow strain rate technique) test method. The SSRT method takes hours to complete the test and uses a special test machine, which causes difficulty and complication. Therefore, a simple and conventional test technique, CSRT test method for delayed fracture was investigated. The crosshead speed is around 1 mm/min, so that the stress induced diffusion of hydrogen is negligible. The results obtained are as follows. (1) Since the stress induced hydrogen diffusion does not take place during the CSRT test, it is necessary to introduce the amount of hydrogen in the specimen, corresponding to the accumulated hydrogen at the vicinity of the notch tip region in the SSRT test. The electrochemical hydrogen charging conditions were established to introduce a wide range of hydrogen contents into the specimens. (2) A unique relationship between the maximum stress at the vicinity of the notch tip and hydrogen contents was obtained irrespective of the notch configuration using the CSRT test and FEM stress analysis. Therefore, it can be said that this relation is the material constants for delayed fracture.

The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel. © 2007.

In this study, an Al-7 wt% Si-1.5 wt% Cu alloy was subjected to severe plastic deformation (SPD) by an equal-channel angular pressing (ECAP) technique. The ECAP process was repetitively carried out up to 8 passes using a strain introduction method of route Bc, at a temperature of 25 °C and a pressing rate of 0.33 mm s-1. Microstructures of the samples before and after ECAP were observed by a scanning electron microscopy (SEM). Electrochemical properties of the Al-Si-Cu alloy fabricated by ECAP have been investigated in a borate-boric acid buffer solution containing Cl- ions at pH 8.3 and 25 °C by potentiodynamic polarization test. Corrosion pits on the sample surface after anodic polarization were investigated by means of SEM. The anodic polarization showed that as-cast Al-Si-Cu alloy with plate-shaped Si particles has poor resistance against pitting corrosion comparing to quenched sample without ECAP. Pitting potentials of ECAPed Al-Si-Cu alloy samples were higher than that of the sample without ECAP. In the Al-Si-Cu alloy, the corrosion pits were found in the region of Si particles and the size of pits formed on the ECAPed samples became smaller than that without ECAP. It is considered that the improvement of the pitting resistance of ECAPed Al-Si-Cu alloy is due to homogenous distribution of spherical Si particles generated during ECAP process.

Passivity of alloys containing corrosion-resistant elements were reviewed. Chromium and valve metals except aluminum form stable oxyhydroxide films even in aggressive hydrochloric acids. Molybdenum forms a passive MoO2 film in the active region of stainless steels and hence decreases the active dissolution current. In the passive region of transition metals and valve metals, molybdenum is generally in the transpassive state and dissolved. However, if the outer oxyhydroxide film is stable the inner MoO2 film is protected by the outer oxyhydroxide film and the MoO2 film acts as the effective barrier against diffusion of matters through the film. Thus the passive current density of 30Cr-2Mo ferritic stainless steel is more than two orders of magnitude lower than that of 30Cr steel without molybdenum in 1 M HCl. © 2006 Elsevier Ltd. All rights reserved.

Slurry impingement is a type of tribocorrosion resulting from simultaneous mechanical and electrochemical surface degradation. In order to understand the elementary process of slurry impingement and the effect of mechanical damage on passive films, electrochemical responses associated with passive film breakdown by particle impacts and following repassivation were measured during slurry impingement on Al and Al-1wt%Si alloy microelectrodes. Measurements were made using a slurry jet system which consists of a pump, a microelectrode, a potentiostat suited for measurements of fast transients of small currents and a high frequency data acquisition system. Current transients corresponding to separated single particle impacts have been successfully measured. The current transient sharply rises and gradually decays following a high field model of oxide growth. It is shown that pure Al repassivates faster after particle impact than the Al-1 wt% Si alloy. This type of erodent particle had an influence on the apparent repassivation rate of these electrodes, and both Al and Al-1 wt% Si alloy showed slower current decay after the impact of angular SiC particle than after the impact of spheroidal zirconia particle. The SiC particles made deep scars and scratches, and the zirconia particles made shallow depressions. © 2006 IOP Publishing Ltd.

The critical hydrogen concentration for hydrogen induced delayed fracture of the AISI 4135 steel at 1320 and 1450 MPa has been determined by constant load tests in combination with numerical calculations, and thus the concept of a critical hydrogen concentration has been verified. The time to fracture was obtained for circumferentially notched round bar specimens under a constant load after electrochemically pre-charged with various hydrogen contents. A numerical model was then developed for calculating the accumulated hydrogen concentration in the vicinity of the notch root, taking into account the driving effect of the hydrostatic stress on hydrogen transport. The results showed that the delayed fracture of the steel occurred when a critical hydrogen concentration at the location of the stress peak was reached by accumulation, and that the time to fracture was related to the stress-driven hydrogen accumulation process. The critical hydrogen concentration was dependent not only on the strength level, but also on the stress concentration factor of the specimens. © 2005 Elsevier Ltd. All rights reserved.

Hydrogen embrittlement susceptibility of a high strength steel SCM 435 was investigated by means of slow strain rate tests. It is shown that the SCM 435 steel has high susceptibility to hydrogen embritdement when quenched and tempered to a tensile strength of 1450 MPa. The notch tensile strength of hydrogen-precharged specimens is lower than that of the uncharged specimen, and it decreases in a power law manner with the increase of diffusible hydrogen content. Results on notched specimens with different stress concentration factors indicate that hydrogen-induced fracture of the steel is dependent on the peak value of the locally accumulated hydrogen concentration and the peak value of the maximum principal stress.

The notch tensile strength of a boron bearing steel at 1305 MPa has been investigated by means of slow strain rate tests after electrochemical hydrogen charging. Results show that the notch tensile strength of the steel decreased with increasing diffusible hydrogen content and the decrease in the notch tensile strength was more pronounced for specimens at a higher stress concentration factor. Finite element analysis results show that the dependence of the notch tensile strength on stress concentration factor cannot be accounted for by the local peak stress and local peak hydrogen concentration; however, the equivalent plastic strain at the notch root at an applied stress equal to the notch tensile strength was independent of stress concentration factor. The equivalent plastic strain at the notch root can be used as a fracture criterion for hydrogen embrittlement of the steel. © 2006 Institute of Materials, Minerals and Mining.

Crevice corrosion experiments on pure iron were earned out in a 0.5 M acetate buffer with varied chloride concentrations. Changes in resultant currents and morphology due to crevice attack were explained by IR potential drop mechanisms. The specimens experienced potential drop inside the crevice, which resulted in the formation of passive, active, and hydrogen evolution regions. The passive region did not exist in the electrolyte containing 0.05 M and 0.5 M chloride. Hydrogen evolution, which occurred inside the crevice was measured on rear side of the specimen using hydrogen permeation test. The results suggest that the hydrogen produced inside the crevice is measurable using a permeation test. The entry of diffusible hydrogen showed a significant increase with the addition of chloride into the acetate buffer.

We examine the hydrogen embrittlement susceptibility of a high-strength AISI 4135 steel by means of a slow strain-rate test (SSRT) using notched round bar specimens. Hydrogen was introduced into the specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS). It was found that the maximum tensile stress decreased in a power law manner with increasing diffusible hydrogen content. Finite element method (FEM) calculations demonstrated that the peak value of the maximum principal stress and the peak value of the locally accumulated hydrogen concentration at the maximum tensile stress were in good agreement with one power law relationship for the specimens with different stress concentration factors.

The influence of microstructure on the electrochemical properties of an Al-5 wt% Cu binary alloy which was severely deformed by equal-channel angular pressing (ECAP) has been investigated in a borate-boric acid buffer solution containing Cl- ions at pH 8.3 and 25°C by potentiodynamic polarization test. The anodic polarization results showed that pitting potentials of ECAPed Al-Cu alloy samples were higher than that of the sample without ECAP and increased with repetitive ECAP passes. The pitting corrosion attack of Al-Cu alloy samples took place in the surroundings of Al2Cu (9) phase. It is considered that the grain refinement of 9 phase and the solid solution generated in Al matrix during ECAP process are responsible for the improvement of the corrosion resistance of ECAPed Al-Cu alloy.

Pure iron exposed to acetate buffer showed both metal dissolution and hydrogen evolution inside the crevice region. Potential drop occurred inside the crevice led to both active dissolution and cathodic reactions. Hydrogen ingress into the specimen was monitored on the rear side using electrochemical hydrogen permeation technique. The results showed that magnitude of hydrogen entered inside the crevice could be detected by the permeation technique. Change in the applied potential significantly influenced the amount of permeated hydrogen. The results are correlated potential drop occurred inside the crevice, which influenced the magnitude of diffusible hydrogen. © 2006 ISIJ.

Iron exposed to acetate buffer in the presence of a crevice assembly showed metal dissolution and hydrogen evolution. Permeated hydrogen was successfully mapped on the rear surface of the crevice using the silver decoration technique. Magnitude of silver deposit varied with the change in the applied potential, with inhomogeneous distribution. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The notch tensile strength of quenched and tempered AISI 4135 steel after hydrogen charging decreases with decreasing crosshead speed. Numerical simulation results show that the accumulated hydrogen concentration in the vicinity of the notch root increases with decreasing crosshead speed, resulting in the decrease in the notch tensile strength. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The quantitative relationship between notch tensile strength and diffusible hydrogen content has been investigated for the AISI 4135 steel at 1320 MPa. The notch tensile strength was obtained by means of a slow strain rate test on circumferentially notched round bar specimens with stress concentration factors of 2.1, 3.3 and 4.9 after hydrogen charging, and the diffusible hydrogen content was then measured by thermal desorption spectrometry analysis. The diffusible hydrogen has been found to decrease the notch tensile strength in a power law manner, and the decrease is more prominent at a higher stress concentration factor. The finite element analysis results of stress and hydrogen distributions in the vicinity of the notch root have shown that the local fracture stress decreases with increasing local hydrogen concentration as the diffusible hydrogen content or stress concentration factor increases, resulting in the decrease in the notch tensile strength. © 2005 Elsevier B.V. All rights reserved.

The maximum tensile stress of hydrogen pre-charged specimens during slow strain rate test decreased in a power law manner with increasing diffusible hydrogen content in the presence of intergranular fracture. The lower hydrogen degradation susceptibility at a lower strength level is due to delayed onset of intergranular fracture. ©2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Current transients of a titanium microelectrode corresponding to an impingement of a single zirconia particle were measured in a buffer solution. It was found that the repassivation after a particle impact followed high field model of oxide growth. The apparent repassivation rate evaluated by the relation between the charge and the peak height of a current transient showed dependence on applied potential, and the apparent repassivation rate increased with increase of potential. The influence of potential applied passivation prior to particle impingement on the apparent repassivation rate was more predominant than that of the potential applied during impingement of relatively large particle. This indicates that the apparent repassivation rate does not represent the actual repassivation kinetics. It is suggested that the passive film formed prior to a particle impact and repassivation give rise to resistance against surface deformation and film breakdown, and that it affects the apparent repassivation behavior. copyright The Electrochemical Society.

Crevice corrosion was investigated in pure iron exposed to a strong acetate buffer solution. The specimens experienced the potential drop inside the crevice, which resulted in the formation of passive, active, and hydrogen evolution regions. The distribution of hydrogen evolution occurring inside the crevice was mapped using a silver decoration technique applied on the other side of the specimen. The results suggest that the entry of absorbed hydrogen show a variation in the intensity across the crevice. The crevice mouth and bottom portions showed the least hydrogen entry, whereas the intermediate portion yielded the significant amount which resulted in the accumulation of silver deposits. © 2005 The Electrochemical Society. All rights reserved.

The release of chromate ions from chromate conversion coatings (CCCs) on Al alloys was studied, and the effect of aging of CCCs on the chromate release kinetics was investigated. Chromate release from CCCs into aqueous solutions was monitored by measuring the change in the chromate concentration in solution using UV-visible spectroscopy. Heat-treatment of the CCC greatly reduced the chromate release rate. The chromate release rate also decreased with increasing aging time at room temperature. A diffusion-control model was proposed based on the notion that the CCC in an aqueous solution is a porous, two-phase structure consisting of a solid phase with adsorbed Cr(VI) species that is in local Langmuir-type equilibrium with an interpenetrating solution phase. This model results in a concentration gradient of soluble Cr(VI) in the solution phase of the CCC as chromate is released. The concentration and diffusion coefficients of soluble Cr(VI) in CCC were estimated. The estimated diffusion coefficient tended to decrease with aging time, suggesting that the CCC is modified with aging time. © 2003 The Electrochemical Society. All rights reserved.

The distribution of hydrogen occluded in high strength bolts tightened beyond the yield strength and exposed for 9 years has been investigated to establish an evaluation method for delayed fracture property. Hydrogen thermal desorption analysis was employed for determining hydrogen content in specimens cut out from the exposed bolts. Diffusive hydrogen causing delayed fracture is not uniform in concentration within the bolt and enriched in the threaded portion where diffusive hydrogen concentration was about three times that in the body portion. It is reasonable that diffusive hydrogen intrudes according to corrosion reaction, but no clear correlation between rusting and hydrogen concentration was recognized. From evaluation of the role of stress and plastic strain on hydrogen concentration, it is concluded that high concentration of diffusive hydrogen in the threaded portion was mainly caused by plastic straining.

CO2 emissions, which induce global warming, increase with the development of economic activity. It is impossible to decrease the CO2 emissions by suppression of the economic activity. Global CO2 recycling can solve this problem. The global CO2 recycling consists of three district: The electricity is generated by solar cells on deserts. At desert coasts, the electricity is used for H2 production by seawater electrolysis and H2 is used for CH4 production by the reaction with CO2. CH4 which is the main component of liquefied natural gas is liquefied and transported to energy consuming districts where CO2 is recovered, liquefied and transported to the desert coasts. A CO2 recycling plant for substantiation of our idea has been built on the roof of the Institute for Materials Research in 1996. Key materials necessary for the global CO2 recycling are the anode and cathode for seawater electrolysis and the catalyst for CO2 conversion. All of them have been tailored by us. They have very high activity and selectivity for necessary reactions in addition to excellent durability. A pilot plant consisting of minimum units in an industrial scale is going to be built in three years. © 2001 Elsevier Science Ltd. All rights reserved.

CO2 emission increase inducing global warming occurs mostly with the growth of the economic activity. Global CO2 recycling can prevent global warming and supply abundant renewable energy. Global CO2 recycling consists of three district: The electricity is generated by solar cells on deserts. At coasts close to the deserts, the electricity is used for hydrogen production by seawater electrolysis and hydrogen is used for methane production by the reaction with CO2. Methane (CH4) is liquefied and transported to energy consuming districts where after CH4 is used as a fuel CO2 is recovered, liquefied and transported to the coasts close to the deserts. Key materials necessary for the global CO2 recycling are the anode and cathode for seawater electrolysis and the catalyst for CO2 conversion. All of them have been tailored by us. Amorphous and nanocrystalline nickel alloys are active cathodes for hydrogen production in seawater electrolysis. Anodically deposited nanocrystalline Mn-Mo and Mn-W oxides are the unique substance which can evolve oxygen with 100% efficiency without evolving chlorine in seawater electrolysis. Amorphous Ni-Zr alloys are excellent precursors of catalysts for conversion of CO2 into CH4 by the reaction with hydrogen at 1 atm. A prototype CO2 recycling plant to supply clean energy preventing global warming has been built on the roof of our Institute (IMR) in 1996 using these key materials and has been operating successfully. © 2001 Elsevier Science B.V. All rights reserved.

Alloying additions of Ni to amorphous Al-Ti alloy and Mo to Al-Cr alloys and simultaneous addition of Ti and Cr to Al have been performed to improve the corrosion resistance of amorphous Al-Ti and Al-Cr alloy, which are not passivated in 1 M HCl. Mg addition to Al-Ti alloys was also performed expecting enrichment of Ti in the surface film caused by rapid dissolution of alloying constituents unnecessary for passivation. The amorphous Al-Ti-Ni, Al-Cr-Ti and Al-Cr-Mo alloys were passivated spontaneously even in 1 M HCl solution and showed higher pitting potentials than those of Al-Ti and AI-Cr binary alloys. The addition of relatively small amount of Mg to Al-Ti alloys increased the pitting potential of Al-Ti alloys. X-ray photoelectron spectroscopy (XPS) measurement allowed to correlate the protective ability of the passive films to the stability of Cr or Ti enriched surface layer of the passive films.

Angle-resolved x-ray photoelectron spectroscopy (ARXPS) in combination with the generalized Fick's first law was applied to determine diffusion coefficients and mobilities of cations in passive films formed by anodic polarization on amorphous Al-Cr-Mo and Al-Ti-Mg alloys. The depth profiles of cations in the surface films were fitted to a depth distribution model expected from Fick's first law. From the fitting parameters, the diffusion coefficients and mobilities of Al, Cr and Ti cations in anodic films formed on Al-Cr-Mo and Al-Ti-Mg alloys were determined. Diffusion coefficients of cation M in the anodic film, D(M), were as follows: D(Al) = 4.02×10-19 cm2 s-1 and D(Cr) = 6.51×10-19 cm2 s-1 for Al-36Cr-9Mo Alloy; and D(Al) = 3.18×10-19 and D(Ti) = 2.36×10-19 cm2 s-1 for Al-36Ti-7Mg Alloy. The mobility ratios were μCr/μAl = 1.62 and μTi/μAl = 0.74, although both Cr and Ti were enriched in the surface region of the anodic films.

The release of soluble CrVI species by a chromate conversion coating (CCC) was monitored quantitatively by ultraviolet-visible spectroscopy. By careful selection of measurement wavelength (339 nm), the CrVI concentration could be determined without regard to solution pH or CrVI speciation. The CrVI concentration in solution over a CCC reached an equilibrium value that depended on pH, ionic strength, and the ratio of the CCC surface area to the solution volume (A/V). In separate experiments, the adsorption of CrVI by synthetic CrIII hydroxide to form a CrIII-CrVI mixed oxide was observed, and also led to an equilibrium concentration of CrVI in solution. The equilibrium CrVI concentration was determined for a variety of A/V values on both AA1100 and AA2024-T3 aluminum alloys. The results are inconsistent with release mechanisms based on the solubility of a CrVI salt in the solution or depletion of CrVI from the CCC. However, the observed concentrations are consistent with a mechanism similar to a Langmuirian adsorption-desorption equilibrium of CrVI on a porous, insoluble CrIII hydroxide matrix. The CrIII hydroxide matrix has a finite number of CrVI binding sites and exhibits a nonlinear relationship between solution and solid CrVI concentrations governed by an equation similar to a Langmuir adsorption isotherm. The proposed model incorporates reversible adsorption and desorption of CrVI, with adsorption favored at low pH during formation of the CCC, and desorption favored in field conditions. The model quantitatively predicts the observed concentrations after determining the binding constant from fits to the data. The model explains the capacity of a CCC to release active CrVI corrosion inhibitor and provides strong evidence that CrVI storage in a CCC involves reversible formation of a CrVI-O-CrIII mixed oxide.

The nanocrystalline heterogeneity has been introduced into sputter-deposited Cr-Nb alloys by heat treatment in order to clarify the effect of alloy heterogeneity on the corrosion resistance of the alloys. The as-deposited amorphous Cr-Nb alloys are spontaneously passive in 6 and 12 M HCl and show higher corrosion resistance than chromium and niobium. The heat treatment leads to the formation of two phase mixtures of bcc Cr and Cr2Nb, Cr2Nb and niobium-rich amorphous phase, Cr2Nb and bcc Nb, and chromium-rich amorphous phase and bcc Nb, depending on the alloy composition and the heat treatment conditions. It has been found that the Cr2Nb phase has high corrosion resistance, comparable to that of the amorphous alloys. The formation of nanocrystalline bcc Cr or bcc Nb phases results in the increase in the initial passive current density. However, the long time potentiostatic polarization has revealed that the steady state anodic current density of the heat-treated alloys is similar to that of the as-deposited alloys as a consequence of preferential dissolution of less corrosion-resistant phases. Accordingly, the high corrosion resistance of the amorphous Cr-Nb alloys is sustained even after the introduction of nanocrystalline heterogeneity. (C) 2000 Elsevier Science Ltd. All rights reserved.

In order to control the oxidation products of amorphous Ni-Zr and Ni-Zr-rare earth element alloys, which have an important role in the catalytic activities of the amorphous alloy-derived catalysts, their oxidation behavior was examined as a function of nickel and samarium contents. The faster oxidation occurs for the nickel-rich Ni-Zr binary allays in comparison with the zirconium-rich alloys. The addition of samarium further accelerates the oxidation, due to the formation of the poor protective oxide scale. The oxides formed from amorphous Ni-Zr and Ni-Zr-Sm alloys an nano-grained zirconia as well as NiO. The zirconia consists of metastable tetragonal and monoclinic phases. The relative amount of the tetragonal phase increases with an increase in the alloy nickel content. The suppression of grain growth of zirconia by the fine dispersion of NiO results in the formation of the metastable tetragonal phase. The addition of samarium to the Ni-Zr alloys further promotes the formation of the tetragonal phase as a result of the stabilization of the tetragonal phase by doping of samarium ions.

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) in combination with the generalized Fick's first law was applied to determine diffusion coefficients and mobilities of cations in passive films formed by anodic polarization on amorphous Al-Cr-Mo and Al-Ti-Mg alloys. The depth profiles of cations in the surface films were fitted to a depth distribution model expected from the Fick's first law. From the fitting parameters, diffusion coefficients and mobilities of Al, Cr and Ti cations were obtained.

Dissolution kinetics for pits and crevices in aluminum and the effect of dichromate ions on the dissolution kinetics were investigated by using artificial crevice electrodes. The aluminum artificial crevice electrodes were potentiostatically polarized over a range of potential in 0.1 M NaCl solution with and without dichromate ions. The anodic dissolution charge, and cathodic charges for the hydrogen and dichromate reduction reactions, were measured. The addition of dichromate ions did not suppress the active dissolution. This indicates that the mechanism of localized corrosion inhibition by dichromates is something other than anodic inhibition of Al dissolution in the pit or crevice environment. The relative amount of local cathodic reactions on Al was increased by the addition of dichromate because of the dichromate reduction. The initial dissolution of aluminum in a crevice was ohmic controlled. From the change in the dissolution current with time, the conductivity of the crevice and potential at the bottom of crevice were estimated. The conductivity and the bottom potential decreased with the ratio of cathodic charge of hydrogen evolution to anodic dissolution charge. The conductivity in the crevice and thus the dissolution current seem to be controlled by hydrogen evolution and only indirectly by dichromate concentration.

Amorphous Fe-Cr-Ni-Ta alloys containing 7-75 at.% tantalum were successfully prepared by the D.C. magnetron sputtering method by using a target of commercial Type 304 stainless steel and high purity tantalum discs. The corrosion behavior of the Fe-Cr-Ni-Ta alloys in 12 M HCl at 30°C was investigated by immersion tests, electrochemical measurements and XPS analysis, and compared with that of Cr-Ta binary alloys. The corrosion rate of the sputter-deposited Fe-Cr-Ni-Ta alloys containing more than 19 at.% tantalum was more than six orders of magnitude lower than that of bulk Type 304 stainless steel and even lower than that of sputter-deposited tantalum metal, although the corrosion rate of the alloys was higher than that of the Cr-Ta alloys. The open circuit potentials of the Fe-Cr-Ni-Ta alloys are located in the passive regions of both chromium and tantalum, and all the alloys are spontaneously passivated. XPS analysis revealed that the passive films formed on the alloys by immersion in 12 M HCl were significantly rich in tantalum and chromium cations. The films were deficient in iron cation and the formation of homogeneous double oxyhydroxides containing mainly chromium and tantalum should be responsible for the high corrosion resistance of the alloys.

The effect of heat treatment, inducing the formation of nanocrystalline two phase alloys on the corrosion resistance of sputter-deposited Cr-Ti alloys containing 22, 48, 60, and 72 at.% titanium has been studied in 6 M HCl. The as-deposited homogeneous Cr-Ti alloys are spontaneously passive in 6 M HCl, except Cr-72Ti alloy, although both chromium and titanium metals undergo active dissolution in this environment. The air-formed surface films on the alloys containing sufficient amounts of both chromium and titanium are highly protective, being responsible for spontaneous passivation. The heat treatment of the alloys leading to the formation of hcp titanium or bcc chromium with a grain size of 20-25 nm as well as Cr2Ti is detrimental for the high corrosion resistance of these alloys. However, when Cr2Ti and bcc solid solution of titanium containing ~14 at.% chromium are formed, the high corrosion resistance of the as-deposited alloys is sustained. The high corrosion resistance of the sputter-deposited Cr-Ti alloys is largely reduced by the formation of less corrosion-resistant hcp titanium or bcc chromium phases, but is maintained when only the phases containing certain amounts of both elements are formed by heat treatment.

Al-Nb-Mo ternary alloys of about 5 μm thickness have been deposited on quartz substrates using a sputter deposition technique, and their sulfidation and oxidation behavior has been compared with the Al-Nb, Al-Mo and Nb-Mo binary alloys. The sulfidation of the ternary alloys follows two-stage parabolic kinetics at temperatures of 1073-1273 K in sulfur pressure of 103 Pa. The steady-state sulfidation is significantly slower than the initial sulfidation. An interesting fact is that the steady-state sulfidation rates of the ternary alloys are further lower than those of the Al-Nb and Al-Mo binary alloys. In particular, an increase in niobium content in the ternary alloys leads to a decrease in the sulfidation rate. Similarly to the Al-Nb and Al-Mo binary alloys, apparently two-layered scales are formed on the ternary alloys, consisting of an outer Al2S3 layer and an inner refractory metal sulfide layer. The detailed analysis of the inner layer reveals that the inner barrier layer is not a single layer but consisting of an outer thin NbS2 layer and inner major layer of MoS2 containing Nb3S4. The formation of the inner barrier layer consisting of two layers in which diffusing matters and directions are different is attributed to the higher sulfidation resistance of the ternary alloys than those of Al-Nb and Al-Mo binary alloys. The steady-state sulfidation rates of the ternary alloys are also lower than those of the Nb-Mo binary alloys, because the two layered niobium sulfide-molybdenum sulfide doped with Al3+ ions is more protective than the individual NbS2 and MoS2 sulfides doped with Al3+ ions. The oxidation resistance of the aluminum-rich Al-Nb-Mo alloys is better than the Al-Nb and Al-Mo alloys, although the resistance of the former alloys is still not sufficient.

New series of nanocrystalline and amorphous single phase binary Mn-Zr and Mn-Cr and ternary Mn-Zr-Cr manganese-base alloys are successfully prepared by DC magnetron sputtering. Electrochemical measurements in combination with XPS analysis are used for examining the effect of single and binary additions of zirconium and chromium metals on the passivity and breakdown of manganese-base alloys in borate-boric acid solutions of pH 8.4 containing 0.01-1 M NaCl. Significant improvement in the resistance to passivity breakdown is observed when chromium and zirconium are added simultaneously to manganese instead of single addition of zirconium or chromium metals. The beneficial effect of simultaneous addition of zirconium and chromium is based mainly on the synergistic interaction between chromium and zirconium cations in the homogeneous double oxyhydroxide passive films formed on the alloy by open circuit immersion.

Nickel catalysts supported on nano-grained oxides have been prepared from amorphous Ni-Zr-Sm and Ni-Zr-Mm (Mm: misch metal) alloys and crystalline Ni-Sm and Ni-Mm alloys. These catalysts show higher catalytic activity for methanation of carbon dioxide than a conventionally prepared zirconia supported nickel catalyst. The catalytic activity of Ni-Zr-5 at% Sm catalysts increases with increase in nickel content, and is higher than the samarium-free Ni-Zr catalysts containing the same amount of nickel. The stabilization of tetragonal zirconia and the increase in the number of active surface nickel sites by addition of samarium to the nickel-rich catalysts leads to enhancement of catalytic activity. In the Ni-Zr-5 at% Mm catalysts, only the activity of the catalyst containing 60 at% nickel is enhanced in comparison with misch metal-free Ni-Zr catalysts. It is also found that Ni-Sm and Ni-Mm catalysts show activities as high as that of Ni-Zr catalyst, suggesting that samarium and misch metal oxides also act as good catalyst supports for methanation catalysts.

MnO2-type manganese-molybdenum oxide electrodes with extremely high oxygen evolution efficiency in chloride-containing solutions have been prepared by anodic deposition on IrO2-coated titanium substrates. The anodic activity and the durability of electrodes have been examined in a 0.5 M NaCl solution at pH 12 and 30°C. For all prepared compositions, the manganese-molybdenum oxide electrodes show an oxygen evolution efficiency of almost 100%, when adequately thick oxides are deposited on the substrate. The galvanostatic polarization reveals that an increase in molybdenum content results in an increase in overpotential at current densities lower than 100 Am-2. However, at a current density of 1000 Am-2, often used for practical electrolysis, the overpotential is almost independent of the molybdenum content. The manganese-molybdenum oxide electrodes have significantly higher durability than the manganese oxide electrodes. During electrolysis at 1000 Am-2, the manganese-molybdenum oxide electrodes show only a slight decrease in oxygen evolution efficiency. Surface observation indicates that the slight decrease in the oxygen evolution efficiency results from the fact that the oxide layer is partly peeled off after electrolysis, even though a thin manganese-molybdenum oxide layer remains in the peeled-off regions.

CO2 emissions which induce global warming, increase with the growth of the economic activity. It is, therefore, impossible to decrease emissions only by energy savings and by improvements of the energy efficiency. Global CO2 recycling can solve this problem and supply abundant renewable energy. Global CO2 recycling consists of three districts: (i) in deserts, all necessary electricities are generated by solar cells; (ii) on coasts close to the deserts, the electricity is used for production of H2 by seawater electrolysis, H2 is converted to CH4 by the reaction with CO2 and liquefied CH4 is transported to energy consuming districts; (iii) at energy consuming district, after CH4 is used as a fuel, CO2 is recovered, liquefied and transported to the coasts close to the deserts. A CO2 recycling plant for substantiation of our idea has been built on the roof of our Institute (IMR) in 1996, using key materials tailored by us. The key materials necessary for global CO2 recycling are the anode and cathode for seawater electrolysis and the catalyst for CO2 methanation. Since the quantities of CO2 to be converted far exceed an industrial level, the system must be very simple and the rate of conversion must be very fast. These requirements are satisfied in our global CO2 recycling system. When global CO2 recycling is conducted on a large scale, the energies and costs required to form liquefied CH4 in our global CO2 recycling system are almost the same as those for production of LNG from natural gas wells. A project for field experimenting the global CO2 recycling using pilot plants in Egypt has been planned in cooperation with Egyptian scientists, engineers and industries. © 1999 Elsevier Science S.A.

Amorphous Fe-Cr-Ni-Nb alloys containing 9-68 at.% niobium were successfully prepared by the D.C. magnetron sputtering method using targets consisting of a commercial Type 304 stainless steel disc and high purity niobium discs. The corrosion rate of the sputter-deposited Fe-Cr-Ni-Nb alloys containing more than 19 at.% niobium in 6 M HCl at 30°C was more than four orders of magnitude lower than that of bulk Type 304 stainless steel and almost the same as that of sputter-deposited niobium metal, although the corrosion rate of the alloys was higher than that of the binary Cr-Nb alloys. The corrosion potential of the alloys was higher than that of niobium and all the Fe-Cr-Ni-Nb alloys were spontaneously passivated in 6 M HCl at 30°C. XPS analysis revealed that the spontaneous passive films formed on the alloys after immersion in 6 M HCl for 48 h were rich in chromium and niobium cations and deficient in iron cation. The formation of the homogeneous double oxyhydroxide films containing chromium, niobium and iron cations is responsible for the high corrosion resistance of the alloys.

This work attempts to understand the ion-release behavior from chromate conversion coatings (CCCs) on Al alloys, and also aims to investigate the effect of aging of CCC on the release kinetics of chromate ions. The release of chromate ions from CCCs was studied by measuring the change in the concentration of chromates in solutions using UV adsorption spectroscopy. Heat treatment greatly reduced the ion-release rate. The release rate of chromates decreased with increasing RT aging time at room temperature. A diffusion-control model was proposed based on the notion of a concentration gradient of soluble Cr(VI) in CCCs. The concentration and diffusion coefficient of soluble Cr(VI) in CCC were estimated. The estimated diffusion coefficient tended to decrease with aging time suggesting that the CCC is modified with aging time.

The change in the corrosion resistance in concentrated hydrochloric acids with precipitation of nanocrystalline phases into sputter-deposited homogeneous amorphous or nanocrystalline Zr-Cr and Nb-Cr alloys was studied. In general, phases consisting of chromium and other effective elements are spontaneously passive and the corrosion resistance increases with increasing chromium content of the phases, but the bcc chromium phase itself is actively dissolved in concentrated hydrochloric acids. When less chromium-containing phases of about 20nm in diameter are precipitated, the corrosion resistance of the alloys increases because a more protective passive film with a higher chromium content can cover entire surface including the surface of the precipitates as a result of an increase in the chromium content of the matrix. However, if the average size of the precipitates exceeds about 20 nm, the corrosion resistance decreases because the chromium-enriched passive film cannot cover the surface of the grown less chromium precipitates. By contrast, the precipitation of the bcc chromium phase decreases the corrosion resistance, because of lowering of the passivating ability of the matrix regardless of the size of the precipitates, as a result of a decrease in the chromium content of the matrix.

Manganese-molybdenum oxide electrodes were prepared by anodic deposition on an IrO2-coated titanium substrate at a constant current density of 600 A m-2 from baths containing 0.2 M MnSO4 and 0-0.1 M Na2MoO4 at 90 °C and pH 0.5. These electrodes were characterized for oxygen evolution in the electrolysis at 1,000 A m-2 in 0.5 M NaCl solution at 30 °C and pH 8 or 12. The most active and stable oxygen evolving anode exhibited 100% efficiency for oxygen evolution, and an efficiency of 98.5% for over 1,500 h at pH 12 and of 96.5% for over 2,800 h at pH 8 of continuous electrolysis. X-ray diffraction measurement and XPS analysis indicated that the deposits consist of a nanocrystalline single γ-MnO2 type phase, and manganese and molybdenum in the deposits are in the Mn4+ and Mo6+ states. The electrochemical studies showed that the manganese-molybdenum oxide electrodes drastically reduced the electrocatalytic activity for chlorine evolution to the undetectable level, resulting in 100% efficiency for oxygen evolution, although the addition of molybdenum slightly increased the oxygen overpotential.

Nano-crystalline single phase bcc solid solution of sputter-deposited tungsten-rich W-Nb alloys are passivated spontaneously in all hydrochloric acid solutions examined, and their corrosion rates are dependent upon the concentration of HCl solutions at 30°C. The corrosion rate of the tungsten-rich W-Nb alloys in 12 M HCl is lower than that in 6 M HCl, although the initial corrosion rate measured for less than about 24 h in 12 M HCl is higher than that in 6 M HCl. XPS and AFM results reveal that the more aggressive nature of 12 M than 6 M HCl solution enhances fast formation of more stable and protective passive films, consisting of tungsten-rich double oxyhydroxide of tungsten and niobium ions due to fast dissolution of niobium. This is responsible for higher corrosion resistance in 12 M HCl than that in 6 M HCl solution at 30°C.

Binary Co-Mo and Co-Al, and ternary Co-Mo-Al alloy electrodes for hydrogen evolution were prepared by using sputter deposition on a nickel substrate with good adhesion. The hydrogen evolution reaction (HER) of these alloy electrodes in the deaerated 1 kmol·m-3 NaOH at 303 K is enhanced significantly compared with pure cobalt electrode. The effective surface area of these alloy electrodes estimated from ac impedance technique is largely increased by leaching of aluminum into alkaline solution. The electrocatalytic activity sequence is Co-Mo-Al>Co-Mo>Co-Al. The maximum activity is obtained for Co-10Mo-58Al alloy with a hydrogen overpotential as low as 110 mV at 103 A·m-2. The enhancement of HER is attributed to both the synergistic effect of alloy constituents and the large effective surface area.

The effect alloying tantalum and chromium on the corrosion resistance of amorphous manganese-base alloys was studied by weight-loss, electrochemical and XPS techniques. Amorphous alloys were formed in a wide composition range by the addition of tantalum to manganese-chromium alloys. The corrosion rates of Mn-Ta-Cr alloys in 1 M H2SO4 open to air at 30°C were about four and six orders of magnitude lower than those of Mn-Ta and Mn-Cr alloys containing the same amount of manganese, respectively, and decreased with increasing alloy chromium content. Thus, additions of chromium and tantalum to manganese were synergistically effective in enhancing the corrosion resistance of Mn-Ta-Cr alloys. The beneficial effect of chromium in the presence of tantalum is explained in terms of enhancement passivity and cathodic reactions of the alloys.

Amorphous Mn-Nb alloys are prepared by D.C. magnetron sputter deposition method in a wide composition range. The effects of niobium addition on the corrosion behaviour of manganese are evaluated in a borate buffer solution of pH 8.4 containing 0.1 M NaCl by using weight-loss measurement, electrochemical and XPS techniques. A clear improvement of the corrosion resistance of these alloys compared with electro-deposited manganese is found. The alloys containing 24 at.% or more Nb are spontaneously passive. Their corrosion rates are about three to four orders of magnitude lower that that of manganese. The alloys containing 40 at.% or more Nb show almost the same corrosion rate as that of niobium. XPS analysis reveals that niobium is concentrated in air-formed films, and the films formed after open circuit immersion are further enriched in niobium. The improvement of the corrosion resistance by alloying with niobium is explained in terms of the formation of niobium-rich passive films composed of double oxyhydroxides of Mn2+ and Nb5+ ions.

The sputter-deposited Cr-Ta alloys show extremely high corrosion resistance in 12 M HCl. The open circuit potentials of the Cr-Ta alloys are located in the passive regions of both chromium and tantalum, and all Cr-Ta alloys are spontaneously passivated. XPS analysis indicates that the composition and thickness of the air-formed film are the same as those of spontaneously passivated film, and the composition of the passive films becomes constant after prolonged immersion. The film consists of a double oxyhydroxide of chromium and tantalum cations. The formation of the homogeneous double oxyhydroxide film consisting of both chromium and tantalum cations by air oxidation is responsible for the extremely high corrosion resistance of the Cr-Ta alloys in comparison with the corrosion resistance of chromium and tantalum metals.

The sputter-deposited Cr-Ta alloys show extremely high corrosion resistance in 12M HCl. The open circuit potentials of the Cr-Ta alloys are located in the passive regions of both chromium and tantalum, and all Cr-Ta alloys are spontaneously passivated. XPS analysis indicates that the composition and thickness of the air-formed film are the same as those of spontaneously passivated film, and the composition of the passive films becomes constant after prolonged immersion. The film consists of a double oxyhydroxide of chromium and tantalum cations. The formation of the homogeneous double oxyhydroxide film consisting of both chromium and tantalum cations by air oxidation is responsible for the extremely high corrosion resistance of the Cr-Ta alloys in comparison with the corrosion resistance of chromium and tantalum metals. (C) 1998 Elsevier Science Ltd. Ali rights reserved.

The oxygen evolution efficiency in seawater electrolysis has been examined for manganese oxide and manganese-tungsten oxide electrodes anodically deposited on iridium oxide-coated titanium substrates as a function of pH of the deposition electrolytes and tungsten content in the oxides. The oxygen evolution efficiency in 0.5 M NaCl solution at 30°C on the MnO2 electrodes increases with a decrease in pH of the deposition electrolyte. The tungsten-containing electrodes show significantly higher efficiency for oxygen evolution. The tungsten addition enhances oxygen evolution and suppresses chlorine evolution, and hence 99.6% efficiency for oxygen evolution was obtained at 200 Am-2 on the oxide with the molar ratio W6+/(Mn4+ + W6+) of 0.16. The anodically deposited manganese oxide consists of nanocrystalline γ-MnO2. The manganese-tungsten oxides are also composed of a single orthorhombic nanocrystalline γ-MnO2-type phase, in which Mn4+ and W6+ ions are homogeneously distributed, even when cationic fraction of tungsten in the oxide is 0.16. The Tafel slopes of the MnO2 and (Mn-W)OX electrodes for oxygen evolution are both higher than those for chlorine evolution. The MnO2 and (Mn-W)OX electrodes anodically deposited at lower pH and the (Mn-W)OX electrodes with higher tungsten content have smaller grain size and higher surface roughness. The surface roughening of the oxides leading to the decrease in the electrode potential under the galvanostatic condition accelerates the oxygen evolution reaction and suppresses chlorine evolution. © 1998 Elsevier Science Ltd. All rights reserved.

The corrosion behavior of sputter-deposited Cr-Nb alloys in 12 M HCI solution was investigted by electrochemical and XPS methods. All of the Cr-NB alloys are spontaneously passivated in 12 M HCI solution open to air at 30°C. The open circuit potentials for chromium-rich alloys containing 42 at% or less niobium are located in the passive region of chromium and niobium, but those for niobium-rich alloys containing 52 at% or more niobuim are located in the active region of chromium. XPS analysis reveals that the passive film consists of a doubel oxyhydroxide of chromium and niobium. The formation of the passive film consisting of double oxyhydroxide of chromium and niobium is responsible for the high corrosion resistance of the Cr-Nb alloys. The passive films formed on the chromium-rich alloys are slightly rich in chromium, and are not appreciably different from the air-formed films before immersion. Angle-resolved XPS analysis indicates no concentration gradient of Cr3- and Nb5+ ions in depth of the passive films on the chromium-rich alloys. On the other hand, the air-formed fil on the niobium-rich alloys is not stable in 12 M HCI and converted to the passive film in which niobium is slightly enriched.

A series of nanocrystalline or amorphous single solid solutions of W-Ta alloys are prepared by D.C. magnetron sputtering. The passivation behavior of these alloys is studied by immersion test, electrochemical measurements and X-ray photoelectron spectroscopy (XPS) analyhsis. The W-Ta alloys are passivated sponstaneously and show significantly high corrosion resistance in 12 M HCI at 30°C. Their corrosion rates are about two orders of magnitude lower than that of sputter-deposited tungsten and are lower than that of sputter-deposited tantalum. XPS analysis shows that tantalum is concentrated in both the air-formed films and the passive films formed spontaneously on the alloys after long time immersion in 12 M HCI. The surface films are composed of double oxhydroxides of Ta5+ and W4+ ions. The formation of spontaneous passive film composed of a double oxyhydroxide of tungsten and tantalum ions is responsible for high corrosion resistance of the W-Ta alloys in concentrated hydrochloric acid solution.

The effect of heat treatment on the corrosion resistance of sputter-deposited aluminum-chromium alloys containing 16-51 at% chromium has been studied in 0.1 and 0.5 M HCl. Structural relaxation decreases the corrosion rate of Al-16Cr alloy on which the passive film cannot be formed, since the corrosion rate is controlled by the reactivity of the alloy surface. The corrosion rate of spontaneously passive Al-35Cr alloy increases by crystallization. The chromium enrichment of the matrix, as a result of precipitation of a nanocrystalline aluminum-rich phase, results in enhancement of the corrosion resistance. However, if the size of the less corrosion-resistant aluminum-rich grains exceeds a critical limit of approximately 20 nm, the corrosion resistance decreases.

Nanocrystalline, single bcc solid solutions of W-Cr alloys have been prepared by DC magnetron sputtering over a wide composition range. The passivation behavior of these alloys was studied by electrochemical measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The W-Cr alloys are passivated spontaneously and showed significantly high corrosion resistance in 12 M HCl solution at 30°C. Their corrosion rates are about one order of magnitude lower than that of sputter-deposited tungsten and about five orders of magnitude lower than that of chromium metal even after prolonged immersion. XPS analysis showed that tungsten is enriched in the spontaneously passivated films formed on the alloys after long immersion time, and the passive films on the W-Cr alloys are found to be composed of double oxyhydroxide of tungsten and chromium ions. Angle-resolved XPS measurements reveal that tungsten and chromium ions are homogeneously distributed in the spontaneously passivated films. The synergistic effect of tungsten and chromium in forming the double oxhydroxide is responsible for the higher corrosion resistance of the W-Cr alloys than of the alloy component metals.

Manganese-tungsten oxide electrodes have high efficiency for oxygen evolution in seawater electrolysis. Manganese oxide, tungsten oxide and manganese-tungsten oxides were coated on the IrO2-coated titanium (IrO2/Ti electrode) by thermal decomposition. The oxygen evolution efficiencies at 200 A · m-2 of the manganese oxide and tungsten oxide electrodes in 0.5 M NaCl solution at pH 8 and 30°C were 70 and 68%, respectively. The efficiency for oxygen evolution is enhanced by small amounts of mutual additions into manganese oxide or tungsten oxide. In particular, the efficiencies of α-Mn2O3-type oxide with 15 mol% W6+ and WO3-type oxide with 10 mol% Mn2+ go up to over 92 and 86%, respectively. Further increase in additives leads to the formation of MnWO4 and to a significant decrease in the oxygen evolution efficiency.

Amorphous Mn-Zr alloys were prepared by DC magnetron sputtering in a wide composition range. The electrochemical behavior in borate-boric acid solutions of pH 8.4 containing 0.01-1M NaCl was investigated in combination with XPS analysis. All alloys were spontaneously passive. Their pitting potential increased with increasing zirconium content, and the alloys with 75-87 at.% zirconium possess higher pitting resistance than zirconium. The high passivating ability was attributed to the formation of a homogeneous double oxyhydroxide film composed of major Zr4+ and minor Mn2+. The formation of zirconium-enriched passive film results from preferential oxidation of zirconium with a consequent enrichment of manganese in the exterior of the underlying alloy surface. The formation of a zirconium oxyhydroxide film containing a few percent of manganese cations on zirconium-rich alloys is responsible for a significantly thinner film thickness and higher pitting resistance in comparison with zirconium.

CO2 emissions, which induce global warming, increase with the development of economic activity. It is impossible to ask the CO2 emission decrease based on suppression of the economic activity. Global CO2 recycling can solve this problem. The global CO2 recycling consists of three district: The electricity is generated by solar cells on deserts. At coasts close to the deserts, the electricity transmitted from the deserts is used for H-2 production by seawater electrolysis and H-2 is used for CH4 production by the reaction with CO2. CH4 is liquefied and transported to energy consuming districts where, after CH4 is used as a fuel, CO2 is recovered, liquefied and transported to the coast close to the deserts. Since 90% of city gases in Japan is liquefied natural gas (LNG) that consists mostly of CH4, CH4 produced in the global CO2 recycling on be used immediately as the city gas. A CO2 recycling plant for substantiation of our idea has been built on the roof of our Institute (IMR) in 1996. Rey materials necessary for the global CO2 recycling are the anode and cathode for seawater electrolysis and the catalyst for CO2 conversion. All of them have been tailored by us. Since quantities of CO2 to be converted far exceed an ordinary industrial level, the system must be very simple, the rate of conversion must be very fast and precious metals must not be used for the system. All these requirements are satisfied in the global CO2 recycling. When the global CO2 recycling is conducted in a large scale, the energies and costs required to form liquefied CH4 in the global CO2 recycling are almost the same as those for production of LNG from natural gas wells. The project to examine feasibility of the global CO2 recycling has been planned in Egypt in cooperation of Egyptian and Japanese scientists, engineers and industries.

Sputter-deposited Mo-Nb alloys are spontaneously passive in 12 M HCl and show significantly higher corrosion resistance than molybdenum and niobium. The air-formed film is slightly rich in niobium. Immediately after immersion, the molybdenum content in the film begins to increase and finally the molybdenum-enriched film is formed on the alloys except the alloys with 75 at% or higher niobium. After immersion for 50 min no concentration gradient in depth is observed, while immersion for 168 h leads to a niobium enrichment in the exterior of the film mostly due to transpassive dissolution of molybdenum. Nevertheless, the Mo content significantly increases in the interior of the film, indicating an increase in the protectiveness of the passive film. Synergistic effect of molybdenum and niobium is responsible for the high corrosion resistance. The niobium enrichment in the exterior of the film prevents transpassive dissolution of molybdenum and the increase in the Mo4+ content in the interior of the film increases the protectiveness of the passive film.

Cr-Ti, Cr-Zr, Cr-Nb, Cr-Ta, Mo-Cr, Mo-Ti, Mo-Zr, Mo-Nb, Mo-Ta, W-Cr, W-Ti, W-Zr, W-Nb and W-Ta alloys prepared by sputter deposition are extremely corrosion-resistant due to spontaneous passivation in 6 and 12 M HCl at 30 degrees C. They are composed of either amorphous single phase or nanocrystalline single solid solution. Spontaneously passivated films are composed of double oxyhydroxides of both alloy component cations, which have higher corrosion resistance than that of passive oxyhydroxide films of alloy component metals. Molybdenum suffers transpassive dissolution at open circuit potentials of the alloys. This results in concentration gradient in the film, while homogeneous passive firms are formed on chromium- and tungsten-valve metal alloys. Corrosion-resistant coatings on inner walls of narrow tubes are successfully performed by magnetron sputtering.

Sputter-deposited amorphous Cr-60Zr and Cr-67Zr alloys are spontaneously passive in 6 M HCl and sputter-deposited amorphous Al-Cr alloys are spontaneously passive in 0.1 M HCl. They suffer pitting by anodic polarization. The change in the pitting susceptibility by introduction of heterogeneity to the homogeneous amorphous alloys by crystallization heat treatment at 400-700°C for 30 min is studied. Heat treatment of the Cr-Zr alloys results in two stage crystallization, (i) precipitation of hcpzirconium phase and (ii) massive transformation from amorphous phase to intermetallic compound, Cr2Zr. Since the precipitation of the hep zirconium phase depends on the zirconium content of alloys, the average size of the hep zirconium phase in the Cr-60Zr alloy specimens heated to 700°C is less than 20 nm, while heating of the Cr-67Zr alloy to 600°C results in precipitation of the hep zirconium phase larger than 20 nm in diameter. Heat treatment of Al-43Cr alloy leads to formation of a two phase mixture of Al17Cr9 and bcc Cr(Al). The size of nanocrystalline two phase mixture is about 15 nm when heated at 550°C but becomes 20-30 nm by heating at 600°C The heat treatment gives rise to the ennoblement of pitting potential in spite of the formation of nanocrystalline phases. This is due to the fact that the formation of the hep zirconium or Al17Cr9 phase leads to an increase in the chromium content of the matrix phase which is able to form thin, protective chromium-rich passive films covering entire heterogenous alloy surfaces. However, when the average size of the less corrosion-resistant phases exceeds 20 nm, the protective chromium-rich passive films cannot completely cover the less corrosion-resistant phases and the pitting resistance decreases.

Nano-grained Ni/ZrO2 and Ni/ZrO2-Sm2O3 catalysts were prepared from amorphous Ni-Zr and Ni-Zr-Sm alloys by oxidation-reduction treatment. Their catalytic activity for methanation of carbon dioxide was examined as a function of precursor alloy composition and temperature. The addition of samarium is effective in enhancing the activity of the nickel-rich catalysts, but not effective for the zirconium-rich catalysts. The surface area and hydrogen uptake of the nickel-rich catalysts are increased by the samarium addition. In addition, tetragonal zirconia, the formation of which is beneficial to the catalytic activity, is stabilized and formed predominantly by the addition of samarium to the nickel-rich catalysts, although monoclinic zirconia is also formed in the zirconium-rich catalysts. As a consequence, the higher conversion of carbon dioxide is obtained on the Ni-Zr-Sm catalysts with relatively high nickel contents. © 1998 Elsevier Science B.V. All rights reserved.

The nano-grained Ni/ZrO2 catalysts containing rare earth element oxides were prepared by oxidation-reduction pretreatment of amorphous Ni-(40-x) at% Zr-x at% rare earth element (Y, Ce and Sm; x=1-10) alloy precursors. The conversion of carbon dioxide on the catalysts containing 1 at% rare earth elements was almost the same as that on the rare earth element-free catalyst, but the addition of 5 at% or more rare earth elements increased remarkably the conversion at 473 K. In contrast to the formation of monoclinic and tetragonal ZrO2 during pretreatment of amorphous Ni-Zr alloys containing 1 at% rare earth elements, tetragonal ZrO2, which is generally stable only at high temperatures, was predominantly formed during the pretreatment of the catalysts containing 5 at% or more rare earth elements. The surface area of the catalysts increased with the content of rare earth element. Thus, the increase in the surface area and stabilization of tetragonal ZrO2 seem to be responsible for the improvement of catalytic activity of the Ni-Zr alloy-derived catalysts by the addition of rare earth elements. © 1998 Elsevier Science B.V. All rights reserved.

Sputter-deposited amorphous Cr-60Zr and Cr-67Zr alloys are spontaneously passive in 6 M HCl but suffer pitting by anodic polarization. The change in pitting susceptibility by the introduction of heterogeneity to the homogeneous amorphous alloys by crystallization heat treatment is studied. Heat treatment results in two-stage crystallization: (i) precipitation of hcp zirconium ranging in average size from 8 to 20 nm and (ii) massive transformation from amorphous phase to intermetallic compound, Cr2Zr. The heat treatment gives rise to the ennoblement of pitting potential in spite of the formation of nanocrystalline phases. This is due to the fact that the formation of the hep zirconium phase leads to an increase in the chromium content of the matrix phase which is able to form thin, protective chromium-rich passive films covering the entire heterogenous alloy surface. When the average size of the less corrosion-resistant hep zirconium precipitates exceeds a critical size, 20 nm, the protective chromium-rich passive films cannot completely cover the precipitates and the pitting resistance decreases. © 1997 Elsevier Science Ltd.

Amorphous W-Nb alloys containing 52-78 at% niobium have been prepared successfully by DC magnetron sputtering. The W-Nb alloys are passivated spontaneously and show high corrosion resistance in both 6 and 12 M HCl solutions at 30°C. In particular, their corrosion rates in 12 M HCl solution are about one order of magnitude lower than those of tungsten and niobium. Analysis by X-ray photoelectron spectroscopy (XPS) shows that tungsten is slightly enriched in the spontaneously passive films formed on the alloys after immersion for a long time in HCl solutions at 30°C, and the passive films are composed of double oxyhydroxides of tungsten and niobium ions. Angle-resolved XPS measurements reveal that tungsten and niobium ions are homogeneously distributed throughout the passive films. The formation of the spontaneous passive film composed of the homogeneous double oxyhydroxide of tungsten and niobium ions is responsible for the high corrosion resistance of the W-Nb alloys. © 1997 Elsevier Science Ltd.

Finely grained Ni/ZrO2 catalysts were prepared from amorphous Ni-Zr alloy precursors by oxidation and subsequent reduction pretreatment, and the catalytic activity for CO2 methanation was examined as a function of precursor alloy composition and temperature. The catalysts thus prepared produce exclusively methane, apart from water as a by-product. The conversion of CO2 increases with temperature in the range of 373-573 K. Among the catalysts examined, the maximum methanation rate is obtained on the catalysts prepared from the amorphous alloy precursors containing 40 and 50 at% zirconium. Further, the methanation rates of all the catalysts prepared from the amorphous alloy precursors are higher than that of a 3 at% Ni/ZrO2 catalyst prepared by wet impregnation. The number of surface nickel atoms, determined by hydrogen chemisorption, increases with zirconium content in the catalysts, while, interestingly, the turnover number decreases with increasing zirconium content. In the catalysts prepared from the amorphous alloys, two types of zirconia are present: metastable tetragonal and stable monoclinic zirconia. The former zirconia phase is present predominantly in the catalyst prepared from the Ni-30 at% Zr alloy, but the relative amount of this oxide phase, with respect to the total amounts of zirconia, gradually decreases with an increase in zirconium content of alloys. Thus, the higher turnover number of the catalysts with higher nickel content can be attributed to nickel supported on metastable tetragonal zirconia. Increasing nickel content of the precursor alloys leads to an increase in tetragonal zirconia and to a decrease in the number of surface nickel atoms on the catalysts. This is responsible for the fact that the maximum conversion appears at medium contents of zirconium in the precursor alloys. © 1997 Elsevier Science B.V.

Manganese oxide electrodes with some additives enhance oxygen evolution efficiency in seawater electrolysis. Electrodes were prepared by a thermal decomposition method. IrO2-coated titanium (IrO2/Ti electrode) was used as the substrate on which manganese oxide (MnOX/IrO2/Ti) and oxide mixtures of manganese and iridium, ruthenium, platinum, iron, cobalt, nickel, tin, lanthanum, cerium or molybdenum ((Mn-M)OX/IrO2/Ti electrode, M: additives) were coated. The oxygen evolution efficiency of the MnOX/IrO2/Ti electrode was 68-70%. The addition of small amounts of nickel, cobalt, iron or tin enhanced the oxygen evolution efficiency. However the addition of excess amounts of these elements and additions of noble metals, cerium or lanthanum were detrimental for the oxygen evolution. Among the additives examined, molybdenum was the most effective additional element to increase the oxygen evolution efficiency. The addition of a small amount of molybdenum leads to a remarkable increase in the oxygen evolution efficiency up to 91%. The formation of a single phase Mn2O3 with molybdenum ions seems to be responsible for the high efficiency for oxygen evolution.

Angle-resolved X-ray photo-electron spectroscopy (ARXPS) measurements were performed in order to characterize the in-depth structures of the passive film and underlying alloy below the passive film for sputter-deposited corrosion-resistant amorphous aluminum alloys. Titanium cation is enriched throughout the passive film formed on an amorphous Al-36Ti-7Mg alloy, while chromium cation is enriched in the exterior of the passive film formed on an amorphous Al-36Cr-9Mo alloy and is deficient in the interior. The analyses of the in-depth distribution of constituents in the passive films indicate that the field-assisted outward migration of chromium cation in the passive film is faster than that of aluminum cations though the corrosion resistance of chromium is apparently higher than that of aluminum in 1 M HCI. The protective ability of the passive film is attributed to the stability of the chromium-enriched surface layer of the passive film. The outward migration of titanium cation is slower than that of aluminum. The investigation of the structure of the underlying alloy below the passive film shows that chromium and titanium are enriched in the underlying alloy surface and the composition gradually approaches the alloy composition with depth. The thickness of the chromium-or titanium-enriched region estimated is less than 1 nm. © 1997 Elsevier Science Ltd.

X-ray photo-electron spectroscopy (XPS) has been used to examine spontaneously passivated films formed on sputter-deposited Cr-Zr alloys in 6 M HCl solution open to air at 30°C, for a better understanding of the high corrosion resistance of these alloys. The open circuit potentials of the Cr-Zr alloys are located in the passive regions of both chromium and zirconium, and all of the Cr-Zr alloys examined are spontaneously passivated. An increase in chromium content of the alloys enhances the cathodic activity for oxygen reduction and content of the alloys enhances the cathodic activity for oxygen reduction and decreases the anodic current density with a consequent ennoblement of the open circuit potential. XPS analysis indicates that the air-formed films on these alloys are composed of homogeneous double oxyhydroxide consisting of both chromium and zirconium ions. The cationic composition of the film is almost the same as the alloy composition although slight enrichment of zirconium occurs. The air-formed film is protective enough to lead to spontaneous passivation of these alloys. The immersion for a long period of time results in a slight deficiency in zirconium in the exterior of the passive film as a result of gradual dissolution of zirconium. At the same time, the total amount of chromium in the passive film increases. The passive film consists of double oxyhydroxide of chromium and zirconium, in which the cationic composition is almost the same as the alloy composition. The formation of the passive film consisting of double oxyhydroxide of chromium and zirconium is responsible for the higher corrosion resistance of these Cr-Zr alloys in comparison with chromium and zirconium metals. © 1997 Elsevier Science Ltd.

Sputter deposition technique has been applied to the preparation of amorphous alloy precursors of catalysts on a fine oxide powder, in order to overcome the low surface area of the catalysts prepared from melt-spun amorphous alloy precursors. Amorphous Ni-Ta-Pd alloys have been sputter-deposited onto the γ-alumina powder with a high surface area, and then pre-oxidized at 1023 K in a 0.5% NO atmosphere. During the preoxidation, the amorphous alloys have been converted to palladium catalysts supported on the NiTa2O6 double oxide. The BET areas of the catalysts thus prepared (sputter-deposited catalysts) are approximately 50 times higher than those prepared from a melt-spun Ni-40Ta-1Pd alloy precursor (melt-spun catalyst). The catalytic activity of the sputter-deposited catalysts for NO decomposition becomes about twice as high as that of the melt-spun catalyst. Furthermore, the selectivity of nitrogen formation is also improved compared with the melt-spun catalyst. Accordingly, the application of sputtering technique is quite suited for preparing amorphous alloy catalyst precursors with a high surface area.

Binary Ni-Mo and Ni-W alloy coatings with good adhesion to nickel substrate are successfully prepared by d.c. magnetron sputter deposition method. These alloy electrodes are found to be active hydrogen evolution electrocatalysts in 1 M NaOH solution at 30°C. Ni-Mo alloy electrodes exhibit the highest activity, which is higher than that of smooth platinum electrode. Leaching treatment in hot concentrated caustic solution for Ni-Mo alloys significantly enhances the activity. © 1997 Elsevier Science S.A.

The corrosion behavior of amorphous Al-Ti-Ni, Al-Cr-Ti and Al-Cr-Mo alloys in 1 M HCl and that of amorphous Al-Ti-Mg alloys in chloride containing neutral solution is investigated. The amorphous Al-Ti-Ni, Al-Ti-Cr and Al-Cr-Mo alloys are passivated spontaneously even in 1 M HCl and show lower corrosion rates and higher pitting potentials in comparison with Al-Ti and Al-Cr binary alloys. Al-Ti-Mg alloys containing relatively low concentration of magnesium possess a better resistance against pitting corrosion. X-ray photoelectron spectroscopy measurement reveals that titanium and chromium are enriched in the passive films formed on Al-Ti-Ni, Al-Ti-Mg and Al-Cr-Mo alloys, respectively. Angle-resolved XPS measurements were performed in order to characterize the in-depth distribution of constituents in the passive films formed on amorphous Al-Ti-Ni, Al-Ti-Mg and Al-Cr-Mo alloys in the passive films of which, passivating elements are enriched. © 1997 Elsevier Science S.A.

Cr-Nb and Cr-Mo alloys have been sputter-deposited on to quartz substrate, and their sulfidation and oxidation behavior has been studied as a function of temperature and alloy composition in He-S2 and Ar-O2 atmospheres. The sulfidation of these alloys follows a parabolic rate law, being diffusion controlled. The sulfidation rates of Cr-Nb alloys decrease with increasing niobium content in the alloy, and the sulfidation resistance of the high niobium alloys is comparable with that of niobium. The sulfidation resistance of Cr-Mo alloys is independent of alloy composition, being comprable with that of molybdenum. The sulfide scales formed on these alloys consist of two layers, comprising an outer chromium sulfide layer and an inner layer composed mainly of refractory metal sulfides. The formation of the refractory metal sulfide scales is responsible for the high sulfidation resistance to sulfide corrosion. Under the oxidation condition the Cr-Mo alloys are rapidly oxidized due to the formation of volatile molybdenum oxide. The oxidation of Cr-Nb alloys proceeds accompanying partial breakdown and the restoration of the scales, but the average oxidation rates are almost the same as their sulfidation rates. Consequently, the Cr-Nb alloys possess high resistance to both sulfidation and oxidation at high temperatures. © 1997 Elsevier Science S.A.

Among sputter-deposited Cr-Ti alloys, 30-65 at% Ti alloys are amorphized. The Cr-Ti alloys with 22 at% or less Ti are bcc single phase alloys. Regardless of the crystallinity, the open circuit potentials of sputter-deposited Cr-Ti alloys are located in the passive regions of both titanium and chromium in 6 M HCl solution open to air at 30°C, and all Cr-Ti alloys are spontaneously passivated. X-ray photo-electron spectroscopy (XPS) analysis reveals that the spontaneously formed passive film as well as air-formed film is slightly rich in titanium ions mainly because of preferential oxidation of titanium. According to angle-resolved XPS measurement, no concentration gradient of Cr3+ and Ti4+ ions has been detected in depth of the passive film. Analysis of binding energies of core electrons of Cr3+ and Ti4+ cations shows that Cr3+ and Ti4+ cations are located very closely in the film so as to show the electronic interaction, and the film is not composed of a heterogeneous mixture of chromium and titanium oxyhydroxides but of homogeneous double oxyhydroxide consisting of Cr3+ and Ti4+ cations. The formation of the homogeneous double oxyhydroxide film containing both Cr3+ and Ti4+ cations is responsible for extremely higher corrosion resistance of the homogeneous single phase Cr-Ti alloys in 6 M HCl open to air at 30°C in comparison with the corrosion resistance of chromium and titanium metals. © 1997 Elsevier Science Ltd. All rights reserved.

The effects of hydrofluoric acid (HF) treatment and bulk structure of precursor alloys on the catalytic decomposition of nitrogen monoxide have been investigated. Alloy catalysts were prepared from Ni-40 valve metal (Ta, Nb, Ti and Zr)-1Pd alloys by the HF treatment and subsequent pre-oxidation at 750 °C. The tantalum-containing catalyst without previous HF treatment shows the highest activity among the catalysts examined, while the preparation of the active catalysts from other amorphous alloys requires HF treatment for surface roughening and for surface enrichment of palladium prior to the pre-oxidation treatment. The catalyst prepared from the amorphous Ni-40Ta-1Pd alloy is superior to the conventionally prepared Pd/Al2O3 catalyst, in spite of the fact that the BET surface area of the latter catalyst is two orders of magnitude higher than that of the Ni-Ta-Pd catalyst. The catalyst prepared from the amorphous Ni-40Ta-Pd alloy shows a better performance for the NO decomposition in comparison with the catalyst prepared from the crystalline counterpart, because the amorphous precursor becomes a more irregular and more microporous catalyst.

On the basis of tailoring of amorphous alloy electrodes for seawater electrolysis to form H2 and amorphous alloy catalysts for conversion of CO2 to CH4, we are proposing global CO2 recycling: At deserts; power generation by solar energy, at coasts close to the deserts; production of H2 by electrolysis of seawater, production of CH4 by the reaction of H2 and CO2 transported, and at energy consuming districts; combustion of CH4, recovery of CO2 and transportation of liquefied CO2 to the coast close to the deserts. Since Egyptian scientists agree with us to do collaboration, the energy balance and the amount of reduction of CO2 emission in the global CO2 recycling between Middle East and Japan are estimated for the operation of a 1 GW CH4-combustion power plant. The energy consumed in a year up to liquefaction of CH4 including that corresponding to the repayment of solar power plant is almost the same as that spent up to obtaining LNG. The energy necessary for the global CO2 recycling is only 8.7% higher than the energy necessary for LNG combustion for power generation without control of CO2 emission. The extra energy is for recovery, liquefaction and transportation of CO2. The reduction of CO2 emission by the global CO2 recycling is 79% of CO2 emission from an LNG combustion power plant, that is, 2.62 Mtons/year.

Open circuit immersion immediately after potentiostatic polarization is carried out to investigate the effect of molybdenum addition on the stability of the passive films formed on amorphous Fe-SCr-13P-7C alloys by polarization in de-aerated 1 M HCl at -100 mV(SCE), which is in the passive region of both chromium and molybdenum. The open circuit potentials of both molybdenum-free and -containing alloys decrease with immersion time after polarization, apart from the passive potential region of chromium but in the passive region of molybdenum. The passive film formed on molybdenum-free alloy by polarization for 10 min and 2.5 h is disrupted by open circuit immersion for 30 min, and the alloy was corroded seriously, whereas molybdenum-containing alloys passivated by polarization for 10 min remains in the passive state during subsequent immersion for 5 h. Regardless of polarization time and molybdenum addition, the cationic fraction of chromium in the surface film decreases continuously with immersion. Open circuit immersion leads to an increase in the concentration of molybdenum ion in the film on the alloys polarized previously for 10 min and a remarkable enrichment of molybdenum ion occurs after prolonged immersion, showing the formation and stabilization of molybdenum-enriched passive film. On the other hand, the enrichment of molybdenum ion during immersion is more difficult in the film on the alloys polarized previously for 2.5 h than that on the alloys polarized previously for 10 min. © 1997 Elsevier Science Ltd. All rights reserved.

Phosphorus contained in amorphous metal-metalloid alloys is often beneficial in enhancing the corrosion resistance particularly in strong acids. On the basis of investigations conducted so far, the beneficial role of phosphorus is comprehensively summarized. Immersion of amorphous metal-phosphorus alloys containing passivating elements leads to the formation of elemental phosphorus layer on the topmost surface of the alloy as a result of initial alloy dissolution. The elemental phosphorus layer has high cathodic activity for oxygen and proton reduction and acts as the diffusion barrier against alloy dissolution. These beneficial effects ennoble the open circuit potential, and when the open circuit potential attains the passive region of the alloy spontaneous passivation occurs. An increase in the protectiveness of the passive film by ageing further ennobles the open circuit potential. At the high open circuit potential elemental phosphorus is no longer stable and dissolves into the solution. The passive film thus formed generally consists of passive oxyhydroxide and the phosphorus content in the film is rather low.

Amorphous Mn-Ti alloys were successfully prepared by DC magnetron sputter-deposition in a wide composition range. Corrosion experiments and anodic potentiodynamic polarization measurements performed in 0.5 M NaCl at pH 5.5 and 11.5, open to air at 30°C, show improved the corrosion resistance of the alloys compared with manganese. Alloying of 12 at.% or more titanium to manganese was very effective in increasing the corrosion resistance in aerated alkaline solution containing chloride ions. X-ray photo-electron spectroscopic analysis of spontaneously passivated films formed in alkaline medium revealed that titanium ions were enriched in the passive films. The passive films formed on Mn-Ti alloys at pH 11.5 were composed of a homogeneous double oxyhydoxide of tetravalent titanium and divalent manganese. It is concluded that the presence of oxidized manganese and titanium in the form of a homogeneous double oxyhydroxide is responsible for the corrosion resistance of the Mn-Ti alloys. © 1997 Elsevier Science Ltd. All rights reserved.

Amorphous W-Zr alloys containing 23-76 at% zirconium have been successfully prepared by DC magnetron sputtering. The W-Zr alloys are spontaneously passive and show significantly high corrosion resistance in 6 and 12 M HCl solutions. In particular, their corrosion resistance in 12 M HCl is higher than that of the alloy constituents. The pitting resistance of zirconium is greatly improved by alloying with tungsten. XPS analysis showed that zirconium is enriched in both the air-formed films and the passive films in HCl solutions. From angular-dependent XPS measurements, the passive films on W-Zr alloys are found to be composed of the double oxyhydroxide of tungsten and zirconium ions, although there are some concentration gradients of tungsten and zirconium ions. The formation of the homogeneous double oxyhydroxide of tungsten and zirconium ions acts synergistically in improving the corrosion resistance of W-Zr alloys in HCl solutions open to air at 30°C. © 1997 Elsevier Science Ltd. All rights reserved.

The corrosion behavior of Ni-10Ta-20P, Ni-10Ta and Ni-20P alloys in 12 M HCl solution open to air at 30°C was investigated in order to clarify the roles of tantalum and phosphorus. Additions of 10 at% tantalum and 20 at% phosphorus to the nickel alloy result in excellent corrosion resistance and in the accumulation of tantalum and phosphorus in the surface film on this alloy owing to dissolution of the nickel from just after immersion, the highly protective passive film formed being stable during prolonged immersion. By contrast, the binary nickel alloy with only a tantalum addition is not passivated and dissolves actively, forming a thick corrosion product film. The nickel alloy containing only phosphorus is not corrosion-resistant, forming a nickel phosphate film, although the film is more stable than that formed on the nickel alloy containing only tantalum. © 1997 Elsevier Science Ltd. All rights reserved.

Amorphous Ni-40Ta-Pd alloys are known as the precursor of the catalysts for the decomposition of nitrogen monoxide. Since the catalytic activity appears after oxidation, the relationship between the activity and structure was investigated after oxidation in air at 750°C for different periods of time. The structure developed by oxidation was characterized with special interest in the dispersion state of palladium phase in the matrix oxides by means of SEM/EDX, EPMA and TEM/EDX. At the early stage of the oxidation, NiO, Ta2O5 and PdO are first formed. The oxides consist of two layers; comprising an outer NiO layer and an inner layer containing Ni, Ta and Pd ions. The outer NiO is developed by the outward diffusion of nickel ion during oxidation. Prolonged oxidation leads to the development of porosity of the specimen and to the formation of double NiTa2O6 oxide by the reaction of NiO with Ta2O5 in the inner layer. The inner layer formed after the complete oxidation for the amorphous Ni-40Ta-Pd alloy has sponge-like morphology, and consists of PdO, NiO and NiTa2O6. The addition of palladium prevents the diffusion of nickel to the surface, leading to the decrease in the thickness of the surface NiO layer and to the decrease in the grain size of matrix oxides. No change in the supporting oxides was observed but PdO is decomposed to Pd after the decomposition of NO gas. Palladium particles of 50-100 nm in diameter were dispersed homogeneously in the matrix oxide. The high catalytic activity for the decomposition of NO gas is attributable to the palladium dispersed on the matrix oxides with fine grain size and porous structure developed by the oxidation of amorphous Ni-Ta-Pd alloys.

The detrimental effect of active carbon on dew point corrosion by carbon-containing sulfuric acids was investigated using Ni-Cr-Mo alloys in 60 and 80% H2SO4 with and without active carbon at 120° C. The addition of active carbon leads to an increase in the dissolution rate of the alloys over a wide potential rage from -900 to 1200 mV vs. SCE, along with an increase in current densities of both anodic and cathodic reactions. Active carbon decomposes adsorbed molecular Sulfuric acid into active oxygen and sulfur dioxide, the latter of which is identified by gas chromatography. The reaction of active carbon with sulfuric acid also forms sulfonic acids. These reactions lead to a decrease in the intrinsic concentration of sulfuric acid and an increase in the corrosiveness in addition to acceleration of cathodic reaction. Spontaneous passivation in carbon-containing 80% H2SO4 occurs by the formation of a remarkably chromium-enriched passive film containing about 20 cationic % of molybdenum.

In order to prevent dew point corrosion by sulfuric acid, the roles of chromium and molybdenum in enhancing the corrosion resistance in 60 and 80% H2SO4, at 120°C were investigated. Particular attention was paid to the addition of active carbon to H2SO4. The chromium contents of alloys were 16 and 20% and the molybdenum contents were 17, 20, 24 and 25%. In carbon-free H2SO4 increasing molybdenum content gives rise to a decrease in corrosion rate and Ni-20Cr-24Mo alloy shows high corrosion resistance by spontaneous passivation forming a remarkably molybdenum-enriched passive film. In carbon-containing H2SO4 increasing chromium content is effective in decreasing the corrosion rate. Three Ni-20Cr-Mo alloys are spontaneously passive in 80% H2SO4 with carbon. Because the carbon addition increases the open circuit potential exceeding the stability limit of tetravalent molybdenum as a result of enhancement of the cathodic activity, the spontaneously passivated film consists of remarkably concentrated chromium ions along with some molybdenum ions.

Utilizing the fact that the equilibrium potential of oxygen evolution is lower than that of chlorine evolution, oxygen evolution in seawater electrolysis was enhanced by decreasing the polarization potential under galvanostatic conditions through increasing the effective surface area of manganese oxide electrodes. Electrodes were prepared by a thermal decomposition method. IrO2-coated titanium (IrO2/Ti electrode) was used as the substrate on which manganese oxide was coated (MnOx/ IrO2/Ti electrode). Subsequently, oxide mixtures of manganese and zinc were coated (MnOx-ZnO/ MnOx/IrO2/Ti electrode). The effective surface area of the MnOx ZnO/MnOx/IrO2/Ti electrodes was increased by selective dissolution of zinc (leaching) into hot 6 M KOH. The oxygen evolution efficiency of the MnOx/IrO2/Ti electrode was 68-70%. Leaching of zinc from the MnOx-ZnO/ MnOx/IrO2/Ti electrodes with 25 mol % or less zinc led to a significant increase in the oxygen evolution efficiency. The maximum efficiency attained was 86% after leaching of zinc from the MnOx-25 mol %ZnO/MnOx/IrO2/Ti electrode. However, large amounts of zinc addition, such as 40 mol % or more are detrimental because of a decrease in the oxygen evolution efficiency. This is due to the formation of a double oxide, ZnMnO3, which is hardly dissolved in hot 6 M KOH.

Amorphous Mn-Ta alloys were successfully prepared by DC magnetron sputter deposition in a wide composition range. Their corrosion behaviour was investigated in 0.5 M NaCl solution of pH 11.5 at 30°C by electrochemical measurements and XPS analysis. The corrosion rates of these alloys were significantly lower than that of manganese metal, and decreased with increasing alloy tantalum content. The alloys containing 30 at.% or more tantalum were spontaneously passive and showed a wide passive region exceeding the potential of transpassive region of manganese. XPS analysis revealed that the spontaneously passivated films were not composed of a mixture of manganese and tantalum oxyhydroxide and consisted of a double oxyhydroxide containing Mn2+ and Ta5+ cations. The passive films are enriched in tantalum ions in almost the entire film except the top-most surface where manganese hydroxide is precipitated. The passive double oxyhydroxide film with an almost fixed composition grows with anodic polarization potential, revealing that both tantalum and manganese are effective in forming the barrier type oxyhydroxide film. The formation of tantalum-enriched double oxyhydroxide film is responsible for the high corrosion resistance of the alloys. © 1997 Elsevier Science Ltd.

The effect of open circuit immersion on the passivation behavior of amorphous Fe-8Cr-Mo-13P-7C alloys with 0, 2 and 6 at.% molybdenum by potentiostatic polarization in de-aerated 1 M HCl was investigated. The polarization was made at 400 mV (SCE) which is in the passive region of chromium but in the transpassive region of molybdenum, whereas the open circuit potential of these alloys is in the passive region of molybdenum but in the active region of chromium. Open circuit immersion of molybdenum-bearing alloys results in the formation of molybdenum-enriched passive film, but passivation at 400 mV is based on the formation of chromium-enriched passive film and the molybdenum-enriched film formed by open circuit immersion is converted to the chromium-enriched film at 400 mV. Polarization at 400 mV immediately after immersion leads to rapid enrichment of chromium to the level of 50% cations within 100 s, but polarization after open circuit immersion of molybdenum-bearing alloys for 10 min results in a gradual increase in chromium content to the level of 50% of cations in 5 h. The current density of molybdenum-bearing alloys after open circuit immersion is always higher than that of the alloys polarized immediately after immersion. Open circuit immersion of molybdenum-free alloy for 10 min gives rise to serious damage of the alloy surface and hence prevents passivation even if prolonged polarization is carried out. Copyright © 1996 Elsevier Science Ltd.

Mo-Ti alloys composed of a single bcc solid solution have been prepared by a sputter deposition technique. The corrosion resistance of the Mo-Ti alloys in 12 M HCl is extremely high, being superior to that of molybdenum and titanium metals. The Mo-Ti alloys are spontaneously passivated in 12 M HCl. The corrosion potential of these alloys is slightly lower than that of molybdenum. The spontaneously passivated film on the alloys is significantly enriched in tetravalent molybdenum, as a consequence of preferential dissolution of titanium. The angle-resolved XPS analysis reveals that there is a compositional gradient in the passive film; molybdenum ions are concentrated in the exterior of the passive films while the concentration of titanium increases in the interior of the films. Copyright © 1996 Elsevier Science Ltd.

Single bec phase Mo-Nb alloys are successfully prepared by sputter deposition. These alloys are spontaneously passivated and show significantly high corrosion resistance in 12 M HCl. Their corrosion rates are lower than those of molybdenum and niobium. XPS analysis revealed that the passive films formed on Mo-Nb alloys by open circuit immersion in 12 M HCl are composed of outer niobium-enriched oxyhydroxide and inner molybdenum-enriched oxyhyroxide with a concentration gradient. The concentration gradient in the passive film on Mo-Nb stems from the fact that the corrosion potential of these alloys is close to or rather exceeds the potential for the transpassive dissolution of molybdenum. Copyright © 1996 Elsevier Science Ltd.

Active catalysts for the direct decomposition of nitrogen monoxide were prepared from amorphous Ni-40Ta-Pd alloys by HF-treatment and subsequent pre- oxidation. The pre-oxidized catalyst consists of NiO, Ta2O5 and PdO, and PdO is decomposed to fcc Pd by heating at temperatures higher than 600°C. At further higher temperatures NiO and Ta2O5 are transformed to a very fine grained double oxide NiTa2O6. The catalysts thus formed consist of three layers and show high catalytic activity for the decomposition of nitrogen monoxide in a wide temperature range from 550 to 850°C. The catalytic behavior is affected by the structural change in the catalyst. With the transformation from NiO and Ta2O5 to NiTa2O6, the catalytic activity and the nitrogen formation selectivity increase significantly. TEM observation of ultramicrotomed cross-sections revealed that finely dispersed palladium supported on very fine grained NiTa2O6 is formed in the interface between the outer and intermediate layers. High catalytic activities and high nitrogen formation selectivities of the Ni-40Ta-Pd alloys are attributable to the formation of the NiTa2O6-supported palladium catalyst.

The effect of immersion prior to polarization on the potentiostatic polarization behavior of amorphous Fe-8Cr-xMo-13P-7C (x = 0, 2, 4 and 6 at%) alloys and the composition of the surface film produced by potentiostatic polarization at a low potential in the passive region in de-aerated 1 M HCl is investigated. Passivation of amorphous Fe-8Cr-13P-7C alloy becomes difficult with immersion prior to potentiostatic polarization and the specimen immersed for 10 min in de-aerated 1 M HCl does not passivate even after potentiostatic polarization of several hours. By contrast, the extension of pre-immersion time for molybdenum-containing alloys results in a decrease in the current density necessary for passivation. Although chromium ions accumulate rapidly in the surface film on the molybdenum-free alloy polarized immediately after immersion, the chromium concentration in the film does not increase by prolonged polarization. The chromium concentration in the surface film on the molybdenum-free alloy immersed previously for 10 min increases continuously with polarization time up to a very high value, which is much higher than that in the surface film on molybdenum-bearing alloys. Chromium is enriched in the surface film on molybdenum-bearing alloys throughout the polarization experiment of 5 h. Preliminary immersion of molybdenum-bearing alloys decreases the enrichment of chromium in the passive film formed by polarization. Molybdenum is deficient in the air-formed film but is concentrated in the films on molybdenum-containing alloys by pre-immersion, and is enriched remarkably in the film after prolonged polarization. Copyright © 1996 Published by Elsevier Science Ltd.

The corrosion behavior of sputter-deposited amorphous Al-Cr-Mo alloys in 1 M HCl solution was investigated by electrochemical measurements. Particular attention was paid to the effects of alloying elements on the surface film composition related to the corrosion behavior. The amorphous Al-Cr-Mo alloys are passivated spontaneously even in 1 M HCl, while the open circuit potential of Al-Cr binary alloys are in the active region even if a large amount of chromium is added. The addition of molybdenum to Al-Cr alloys leads to an enrichment of chromium in the surface film formed by anodic polarization with a consequent decrease in the passive current density and prevention of pitting corrosion, though the excess addition of molybdenum raises the anodic current density because the protective ability of the surface film is lowered by transpassive dissolution of molybdenum. The formation of the chromium enriched passive film suppresses the deterioration of the passive film due to transpassive dissolution of molybdenum. Thus, additions of chromium and molybdenum to aluminum are synergistically effective in improving the passivity of aluminum in a wide potential region. The cationic fraction of molybdenum in the spontaneously passivated film is almost the same as the atomic fraction of molybdenum in the bulk alloy, and molybdenum becomes deficient in the passive film formed by anodic polarization. It is therefore considered that the addition of molybdenum improves the passivating ability not by the formation of the molybdenum enriched passive film, but by assisting the formation of the chromium enriched passive film. Copyright © 1996 Elsevier Science Ltd.

The addition of a large amount of phosphorus, such as 20 at%, to arc-melted crystalline and rapidly quenched amorphous Ni-10Ta alloys greatly enhances the corrosion resistance in 12 M HCl at 30°C. The corrosion rate of the rapidly quenched amorphous Ni-10Ta-20P alloy after immersion for 168 h is about one order of magnitude lower than that of the arc-melted crystalline counterpart. The open circuit potentials of both alloys are initially ennobled by accumulation of elemental phosphorus having a high activity for the cathodic oxygen reduction. The open circuit potential is further ennobled by spontaneous passivation. The ennoblement of the open circuit potential of the amorphous alloy is faster than that of the crystalline counterpart, because of faster passivation as a result of the formation of a tantalum-enriched passive film. Copyright © 1996 Elsevier Science Ltd.

Angle-resolved X-ray photo-electron spectroscopy (ARXPS) measurements were performed in order to characterize the in-depth distribution of constituents in the passive film formed on amorphous Al-36Cr-9Mo alloy in 1 M HCl. The result showed that chromium cations were enriched in the exterior of the passive film and aluminum cations were rich in the interior. A model of in-depth distribution of constituents in the passive film considering migration and diffusion of cations was proposed. Experimental data by ARXPS were in agreement with the exponential concentration in-depth distribution of cations of the passive film. The analysis indicated that the migration and diffusion of chromium cations in the passive film were faster than those of aluminum cations. The enrichment of chromium at the outermost surface of the passive film was attributed to the faster dissolution of aluminum cations into the solution in comparison with that of chromium cations. Copyright © 1996 Elsevier Science Ltd.

The corrosion behavior of sputter-deposited Mo-Ta alloys in 12 M HCl at 30°C was investigated by electrochemical measurements and XPS analysis. The Mo-Ta alloys are composed of a single bcc solid solution. These alloys are spontaneously passive in 12 M HCl. The corrosion rate of low tantalum alloys is lower than that of molybdenum and comparable to that of tantalum. Their spontaneously passive film is rich in molybdenum. High tantalum alloys with 43 at% or more tantalum show a lower corrosion rate than tantalum and form spontaneously the tantalum-enriched passive film. Regardless of the concentrating cations in the spontaneously passivated film the concentration of tantalum decreases from the exterior to the interior of the film.

The corrosion resistance of arc-melted Ni-10Ta-P alloys containing 0, 10 and 20 at% phosphorus in 12 M HCl solution at 30°C was investigated. The alloys containing 0 and 10 at% phosphorus suffer severe corrosion. The addition of 20 at% phosphorus to crystalline Ni-10Ta alloy results in a three-orders-of-magnitude decrease in the corrosion rate. The open circuit potentials of the Ni-10Ta alloys containing O and 10 at% phosphorus stay almost constant in the active region of nickel, while the open circuit potential of the Ni-10Ta-20P alloy increases almost linearly in the initial 2 h. The Ni-10Ta alloy consists of intermetallic Ni8Ta and immersion in 12 M HCl results in faceting dissolution. Ni-10Ta-10P alloy is composed of major Ni8Ta and Ni3P phases and minor Ni2Ta and Ni2P phases. Immersion of Ni-10Ta-10P alloy leads to preferential dissolution of the Ni8Ta phase and to continuous thickening of the corrosion product film consisting mostly of tantalum as cations. Ni-10Ta-20P alloy consists of Ni2Ta, Ni3P, Ni2P and NiP phases. Immersion of Ni-10Ta-20P alloy gives rise to initial increase in elemental phosphorus on the surface as a result of selective dissolution of nickel and selective oxidation of tantalum. The formation of elemental phosphorus with a high cathodic activity is responsible for the initial ennoblement of the open circuit potential and for the formation of the passive film in which tantalum is highly concentrated. The higher corrosion resistance of Ni-10Ta-20P alloy than Ni-10Ta-10P alloy is attributable to the formation of the Ni2Ta phase with a higher tantalum content than the Ni8Ta phase which is the readily corroded major intermetallic phase in the Ni-10Ta-10P alloy.

Tailoring corrosion-resistant new alloys has recently been performed mostly by sputter deposition technique. This technique is suitable to form a single phase solid solution even when the boiling point of a component is lower than the melting point of the rest of components and/or when a component is immiscible to another component in the liquid state. Sputterdeposited chromium alloys with valve metals, such as titanium, zirconium, niobium and tantalum are composed of amorphous single phase in wide composition ranges. Molybdenum-valve metal alloys consist mostly of fine-grained bcc single phases. These alloys have very high corrosion resistance in concentrated hydrochloric acids which is higher than that of alloy constituting elements. The corrosion resistance increases with increasing chromium content of chromium-valve metal alloys, while the corrosion resistance decreases with increasing molybdenum content of molybdenum-valve metal alloys. Their corrosion resistance is based on spontaneous passivation. The passive films formed on amorphous chromium-valve metal alloys consist of double oxyhydroxides of chromic ion and valve metal cations. The passive films formed on molybdenum-valve metal alloys are heterogeneous. The exterior of the films is rich in valve metal cations and the interior is rich in letra valent molybdenum.

Molybdenum additions have been made to enhance the passivity of Al-Cr alloys prepared by a sputter-deposition method. The corrosion behavior of amorphous Al-Cr-Mo alloys in 1 M HCl solution has been investigated by corrosion tests and electrochemical measurements. The addition of molybdenum to Al-Cr alloys decreases the anodic current density and makes the open circuit potential more noble. The amorphous Al-Cr-Mo alloys are passivated spontaneously even in 1 M HCl, while the open circuit potentials of Al-Cr binary alloys are in the active region even if a large amount of chromium is added. The addition of molybdenum to Al-Cr alloys, therefore, decreases the corrosion rate. The molybdenum addition is also effective in making the pitting potential more noble. The passive current density of Al-Cr-Mo alloys is lower than the anodic current density of sputter-deposited amorphous Al-Mo alloys. Consequently, the addition of molybdenum to Al-Cr alloys is effective in enhancing the corrosion resistance.

The change in the corrosion behavior of amorphous Fe-8Cr-xMo-13P-7C (x=0, 2, 4, 6) alloys in de-aerated 1 M HCl with molybdenum content and pretreatment has been studied. Amorphous Fe-8Cr-13P-7C alloy shows active dissolution in de-aerated 1 M HCl. This alloy becomes corrosion-resistant with the addition of molybdenum. X-ray photo-electron spectroscopic analysis reveals that the chromium content of the films decreases continuously with immersion time despite an initial sharp increase in chromium concentration in the surface film. No enrichment of chromium is detected in the surface films on molybdenum-free and molybdenum-bearing alloys except for 6 at% molybdenum alloy after prolonged immersion. Molybdenum is deficient in the air-formed film but is concentrated in the films on molybdenum-containing alloys throughout the immersion experiment for 20 h. Molybdenum oxyhydroxide, rather than chromium oxyhydroxide, is considered to be responsible for the formation of the passive film on molybdenum-bearing alloys immersed in de-aerated 1 M HCl. The addition of 6 at% molybdenum is effective in maintaining metallic luster for prolonged immersion due to the formation of the passive film in which molybdenum and to some extent chromium are concentrated. Phosphorus is little involved in the air-formed films on all alloys but accumulates by immersion in the passive films on molybdenum-containing alloys up to its bulk composition, whereas the phosphorus content of the surface film on the molybdenum-free alloy is low. Air exposure for 24 h prior to polarization or immersion raises the initial open circuit potential of specimens and retards the enrichment of molybdenum in the passive film on the 6 at% molybdenum alloy in de-aerated 1 M HCl.

Effective electrodes for electrolysis of seawater for production of hydrogen (H2) ~without releasing chlorine into atmosphere and catalysts for production of methane (CH4~) by the reaction of carbon dioxide (CO2) with H2 have been tailored. Using these novel materials a CO2 recycling plant for substantiation of our proposal has been built on the roof of Institute for Materials Research, Tohoku University. The CO2 recycling plant consists of a desert, a coast close to the desert and an energy consuming district. At the desert electricity is generated by solar cell operation and transmitted to the coast close to the desert. At the coast H2 is produced by electrolysis of seawater using the electricity, and then CH4 is formed by the reaction of H2 and CO2. CH4 is transported to the energy consuming district. At the energy consuming district combustion of CH4 is carried out not by air but by O2 and CO2 is recovered after removing H2O from the exhaust gas composed only of CO2 and H2O. The recovered CO2 is sent again to the coast close to the desert for reproduction of CH4. The CO2 recycling plant has substantiated that the solar energy at the desert can be used by energy consumers in the form of CH4 without emitting CO2 into atmosphere.

Enantioselective synthesis of (-)-sedacryptine, a piperidine alkaloid, has been achieved via the double intramolecular conjugate additions of carbamate group and its absolute configuration was determined.

Ni-Mo-B amorphous ultra-fine particles were prepared by a chemical reduction method using KBH4 as a reducing agent. The concentration of molybdenum contained in the ultra-fine particles is about 10 at% when 30∼ 40 mol% of metal ions in the solution are molybdenum. The molybdenum concentration in the particles becomes the maximum value of about 30 at% in the aqueous solution where 90 mol% of metal ions are molybdenum. The structure is amorphous for all samples and the particles diameter is 20-50 nm. The XPS Mo3d spectrum consists of Mo0, Mo4+, Mo5+ and Mo6+ as-precipitated sample. The simultaneous determination of the composition of the surface film and underlying alloy indicates the enrichment of molybdenum in the surface oxide film. Nickel and boron are also composed of the metallic and oxidized states, the latter of which is associated with the surface oxide film. The XPS analysis clearly reveals that molybdenum can be reduced to the metallic state by reductive coprecipitation with nickel and boron.

The sulfidation and oxidation behavior of sputter-deposited Al-Mo, Al-Nb, Al-Ta and Al-W binary alloys has been investigated under isothermal-isobarometric conditions at the temperature ranging from 1073 to 1273 K. High temperature sulfidation was carried out in He-Sa atmosphere with sulfur vapor pressure kept constant at 1 kPa, and high temperature oxidation was carried out in Ar-O2 atmosphere with oxygen pressure fixed to 20 kPa. In sulfidation tests, these alloys showed superior high temperature sulfidation resistance to the known metallic materials, and the sulfidation rates of these alloys were comparable to those of refractory metals. The sulfide scales of these alloys were composed of bilayer, and it was revealed that the formation of the inner protective sulfide scale of refractory metal was responsible for the lower sulfidation rate of these alloys. The oxidation rates of these alloys were much lower than that of corresponding refractory metals, but higher than those of typical alumina-forming alloys because of the formation of refractory metal oxides or their double oxides with aluminum. keywords.

The addition of 30 at% or more tantalum to Ni-5P alloy greatly enhances the corrosion resistance in 12 M HC1 at 30°G The corrosion rate of amorphous Ni-40Ta-5P alloy is about three orders of magnitude lower than that of Ni-5P alloy. The addition of sufficient amounts of tantalum to Ni-SP alloys leads to an ennoblement of the open circuit potential and to spontaneous passivation. The addition of a large amount of tantalum to the Ni-Ta alloy contained a small amount of phosphorus, such as 5 at%, enhances the formation of the tantalum enriched passive film which prevents oxidation of phosphorus.

Amorphous WTi alloys were successfully prepared by DC magnetron sputtering in a wide composition range. The sputter deposited WTi alloys thus prepared were spontaneously passive, and their corrosion rates were lower than tungsten and about two orders of magnitude lower than titanium in 6 M HCl solution open to air at 30 °C. XPS analysis showed that tungsten was enriched in both the passive films and underlying alloy surfaces in 6 M HCl solution. The passive films on the WTi alloys were composed of a double oxyhydroxide of tetravalent tungsten and titanium ions. The formation of the double oxyhydroxide film seems responsible for high corrosion resistance of the alloys in comparison with alloying constituents in 6 M HCl at 30 °C. © 1995.

Cr-Mo alloys prepared by sputter deposition are composed of a single BCC solid solution. The corrosion behavior of binary Cr-Mo alloys in 12 M HCl solution was studied by electrochemical measurements and X-ray photo-electron spectroscopy (XPS) analysis. The sputter-deposited Cr-Mo alloys are spontaneously passive in 12 M HCl solution. Their corrosion resistance is significantly high and increases with increasing alloy molybdenum content. XPS analysis reveals that the spontaneously passivated film on the alloys with < 50 at% molybdenum is rich in chromium while that on the alloys with > 50 at% molybdenum is rich in molybdenum. The angle-dependent XPS analysis shows that the film is not homogeneous but that the chromium content increases from the interior to the exterior of the film, and that the film changes from the interior oxide to the exterior oxyhydroxide. The outer chromium-enriched oxyhydroxide on the high chromium alloys is responsible for spontaneous passivation because of a high activity for cathodic oxygen and proton reductions. © 1995.

Active electrode materials for a new zinc electrowinning process, in which the thermodynamic cell voltage is about half that of the conventional process by replacing oxygen evolution by anodic oxidation of SO2 produced in the zinc smelting process have been studied. Immersion in HF solution and subsequent cyclic voltammetry (CV) in sulfuric acid are known to be effective surface activation treatments of the amorphous alloy electrodes. The galvanostatic cathodic reduction (CR) treatment was applied to obtain further activation for sulfite oxidation for HF- and CV-treated electrodes prepared from amorphous nickel-valve metal-platinum group metal alloys. This treatment has been found to be effective in enhancing the activity. Among the amorphous Ni-40Nb alloys containing platinum group elements, the platinum-containing electrode showed the highest catalytic activity, which was higher than that of platinized platinum. Furthermore, the electrocatalytic activities of CR-treated electrodes prepared from amorphous alloys containing platinum and rhodium, and platinum and ruthenium were higher than that of the electrode containing only platinum. According to XPS analysis of the amorphous Ni-40Nb-1Pt-1Ru alloy specimen the enrichment of platinum and ruthenium occurred by CV treatment, and a small amount of oxidized platinum and ruthenium species remained on the electrode surfaces, but most of them were cathodically reduced to the metallic state by CR treatment. High catalytic activities for sulfite oxidation can be attributed to the metallic state of platinum and ruthenium contained in the alloy electrodes, even though the activity of these electrocatalysts is higher than that of pure Pt or Ru. © 1995 Chapman & Hall.

The formation of a single amorphous Fe-Cr-P phase was strongly related to the amount of phosphorus addition. The structure of the melt-spun Fe-8Cr-P alloys containing 16.5 at% or more phosphorus was confirmed to be amorphous by X-ray diffraction (XRD). However, high-resolution transmission electron microscopy (TEM) revealed that a microcrystalline bcc Fe phase was dispersed in the amorphous matrix of the Fe-8Cr-16.5P and Fe-8Cr-18P alloys. The corrosion behavior of Fe-Cr-P alloys is significantly sensitive to the presence of microcrystallites in the amorphous matrix. A thick corroded layer was formed on the Fe-8Cr-16.5P and Fe-8Cr-18P alloys by immersion in 9M H2SO4. A TEM image for the thick corroded layer of the Fe-8Cr-16.5P alloy showed that the microcrystalline bcc Fe phase was preferentially dissolved away in the solution and that the amorphous Fe-Cr-P phase remained on the surface of the alloy without dissolution. Accordingly, the thick corroded layer corresponds to the amorphous Fe-Cr-P phase which is covered with the passive film. The addition of 20 at% phosphorus is necessary to form an almost complete amorphous structure and to provide the high corrosion resistance even in 9M H2SO4. © 1995.

Amorphous Fe-8Cr-13P-7C alloy is corroded severely in 1 M HCl open to air. The addition of more than 2 at% molybdenum is effective in promoting spontaneous passivation. According to XPS analysis, chromium accumulates continuously in the surface film on molybdenum-containing alloys throughout the immersion experiment for 20 h, whereas for the molybdenum-free alloy an initial slight increase in chromium concentration in the surface film is followed by an almost continuous decrease. Molybdenum and phosphorus are deficient in the air-formed film but become concentrated in the film with immersion time. The Mo4+ species which is the constituent of the passive film on molybdenum metal is increased. Air exposure for 30 min prior to the polarization or immersion experiment in 1 M HCl raises the open circuit potential of specimens and retards the enrichment of chromium in the passive film on molybdenum-bearing alloys. © 1995.

The corrosion behavior of Ni-10Ta-P alloys in 12 M HCl solution was investigated. An increase in the alloy phosphorus content improves the corrosion resistance of the Ni- 10Ta-P alloys. Ni-10Ta-15P and Ni-10Ta-20P alloys with sufficient phosphorus additions are spontaneously passivated due to the formation of a stable passive film in which tantalum ions are accumulated, comprising more than 70% of cations just after immersion. The composition and thickness of the film on these alloys are almost unchanged during prolonged immersion. By contrast, Ni-10Ta-5P and Ni-10Ta-10P alloys with insufficient phosphorus additions are not passivated spontaneously and continue to dissolve actively. Thus, a tantalum-enriched film cannot be formed for initial short periods of immersion. A large amount of dissolution of Ni-10Ta-5P and Ni-10Ta-10P alloys results in accumulation of tantalum ions on the alloy surface, and tantalum species precipitate and are concentrated in corrosion products on the alloy surface. Accordingly, the concentration of tantalum ions in the corrosion product film is increased slowly to more than 90% of cations, which is significantly higher than that in the stable passive film formed on the alloys with high phosphorus additions. © 1995.

Tailoring new corrosion-resistant alloys has recently been performed mostly by the sputter deposition technique. This technique is suitable for forming a single-phase solid solution even when the boiling point of one component is lower than the melting points of the other components and/or when one component is immiscible with another component in the liquid state. Aluminium-refractory metal, chromium-valve metal and molybdenum-chromium-nickel alloys have been successfully prepared in a single amorphous phase. Amorphous aluminium-refractory metal alloys are corrosion resistant in 1 M HCl and chromium-valve metal alloys are spontaneously passive in 12 M HCl, showing a better corrosion resistance in comparison with the alloy components. The amorphous aluminium-refractory metal alloys also have an extraordinarily high hot corrosion resistance. Their sulphidation resistance at higher temperatures is far higher than any other known metallic materials and their oxidation resistance is comparable to chromia- or alumina-forming alloys. © 1995.

The corrosion resistance of amorphous Fe-8Cr-P alloys depends very strongly on the amount of phosphorus addition. The corrosion rates of crystalline Fe-8Cr-7P and Fe-Cr-13P alloys in 9 M H2SO4 solution at 30°C are extremely high in comparison with those of amorphous Fe-8Cr-P alloys with 16.5 at% or more phosphorus. An increase in the phosphorus content improves the corrosion resistance. The amorphous Fe-8Cr-16.5P and Fe-8Cr-18P alloys which are composed of very fine bcc phase dispersed in the amorphous matrix turn black after immersion for about 10 min, while the amorphous Fe-8Cr-20P alloy maintains the metallic luster after immersion for 168 h. Intense spectra of elements in the metallic state are observed in XPS analysis even though the specimen surface turns black. This metallic species found by XPS corresponds to the amorphous Fe-Cr-P phase which is not corroded. The thick corroded layer formed on Fe-8Cr-16.5P and Fe-8Cr-18P alloys seems to be the amorphous alloy phase after dissolution of the microcrystalline bcc Fe, the amorphous Fe-Cr-P phase being covered with a passive film. The amorphous Fe-8Cr-20P alloy does not show initial active dissolution, and the corrosion loss of this alloy is very small. Therefore, at least 20 at% addition of phosphorus to the Fe-8Cr alloy is necessary to form the completely amorphous structure and to provide the high corrosion resistance. © 1995.

Amorphous Ni-40Nb alloys containing one atomic per cent of platinum group metals were prepared by rapid quenching and were employed for electro-oxidation of propane in 1 MH(2)SO(4) saturated with propane gas at 30 degrees C. Two types of electrodes were used; one is ribbon-shaped electrode activated by immersion in HF solution and subsequent cyclic polarization in the sulfuric acid solution, and another one is gas-diffusion electrode by using alloy powder catalysts obtained by prolonged immersion of the amorphous alloys in HF solution. The electrode prepared from amorphous alloys containing rhodium and ruthenium is the most active for propane oxidation and shows considerably higher than that of platinized platinum. The optimum content appeared to be Ni-40Nb-0.5 at%Ru-0.5 at%Rh. The activity for the gas-diffusion electrode is remarkably higher than that of the ribbon-shaped electrode.

X-ray photo-electron spectroscopy (XPS) has been used to investigate the oxidation behavior of amorphous Fe8CrxMo13P7C(x = 0, 2, 4, 6) alloys at room temperature. Thicknesses of surface films on these alloys increase continuously throughout air exposure for more than one week. XPS analysis reveals that air oxidation of specimens at ambient temperature results in an increase in concentration of oxidized iron species in surface films due to preferential oxidation of iron. Contrary to iron, concentrations of chromium and molybdenum species in the surface films decrease as oxidation proceeds. The addition of molybdenum to FeCrPC alloys hinders thickening of surface films and retards the decrease in concentrations of chromium and molybdenum in the films. Very little oxidized phosphorus species have been detected in the surface films at all oxidation times. © 1995.

Amorphous Mo-Zr alloys were prepared in a wide composition range of 40-80 at% zirconium by a magnetron sputtering method. The sputter-deposited Mo-Zr alloys exhibit lower corrosion rates than those of molybdenum and zirconium in 12 M HCl solution open to air at 30 °C. The corrosion resistance of Mo-Zr alloys increases with increasing alloy zirconium content, but molybdenum and zirconium improve the corrosion resistance synergistically. Mo-Zr alloys containing 60 at% or more zirconium exhibit a passive region in a wide potential range without transpassive dissolution of molybdenum. The presence of zirconium suppresses both the cathodic reaction for hydrogen evolution and the anodic reduction. The air-formed film is composed of a double oxyhydroxide of zirconium and molybdenum. Spontaneously passivated films on the alloys containing less than and more than 40 at% of zirconium are composed of molybdenum-enriched and zirconium-enriched oxyhydroxides, respectively. They seem to consist of a bilayer structure; the outer zirconium and inner molybdenum oxyhydroxides. Weak points in each layer in the bilayered passive film seem to be protected mutually by another layer. This appears to be responsible for the high corrosion resistance of the Mo-Zr alloys in comparison with alloy constituents. © 1995.

The addition of 3 at% phosphorus to amorphous Ni-30Ta alloy results in two orders of magnitude decrease in the corrosion rate in 12 M HCl at 30 °C. The amorphous Ni-30Ta-P alloys with 2 at% or more phosphorus were spontaneously passive. The open circuit potential of the phosphorus-free amorphous Ni-30Ta alloy decreases with time and becomes constant in the active region of nickel. By contrast, the open circuit potentials of the phosphorus-containing alloys increase with time of immersion. XPS analysis reveals that surface films on the Ni-30Ta alloys are composed exclusively of Ta5+ ions as cations regardless of the presence of phosphorus. The film formed on the amorphous Ni-30Ta alloy is significantly thicker than that on the amorphous Ni-30Ta-5P alloy. The thick surface film on the Ni-30Ta alloy consists of a high concentration of OH- ions, while the passive film on the Ni-30Ta-5P alloy contains a high concentration of O2- ions. © 1995.

Corrosion rate of the Al-Cr-Ti alloys with relatively high chromium content in 1 M HCl solution are almost one order of magnitude lower than those of Al-Cr alloys. The ternary alloys are passivated spontaneously, while the open circuit potentials of Al-Cr alloys are in the active region even if a large amount of chromium is added. The higher corrosion resistance of the Al-Cr-Ti alloys in comparison with Al-Cr and Al-Ti alloys can be explained in terms of the beneficial effects of titanium addition preventing the enrichment of aluminum in the surface film and chromium addition enhancing the passivity and cathodic reactions even in the chloride containing strong acid.

A series of amorphous Zr-Cu alloys prepared by means of rapid quenching were exposed to a laboratory air (RH = 60±10%, i.e. wet air) for 15 months and in a dessicator (RH ≤ 30%, i. e. dry air) for 23 months. The surface of alloys after exposure was examined by SEM/EDX, XRD and AES. The original surface film on the alloys consisted mainly of ZrO2, and was protective. By exposure to the wet air, metallic copper and ZrO2 were formed as corrosion products. The ZrO2 formed on the Zr-Cu alloys was not in form of film and was not protective. The ZrO2 formed was crystalline and mixture of monoclinic and tetragonal phases, the content of the latter phase increased with the increase in the alloy copper concentration. The surface film formed on the region in contact with the cooling wheel during quenching was rather unstable as compared with that formed on the freely solidified surface. Contaminants, such as C ℓ and S were often observed on the sites where corrosion started. The higher the copper content or the relative humidity, the larger was the corrosion rate.

A variety of alloys including Cr-valve metal, Mo-valve metal and Cr-Mo-Ni alloys have been successfully prepared in a single amorphous phase. All these alloys have extremely high corrosion resistance. Besides, because of the formation of a single phase solid solution, these alloys are suitable for study of the role of alloying element in the alloy matrix in enhancing the corrosion resistance. Cr-valve metal alloys form spontaneously passivated double oxyhydroxide films containing both chromic ion and valve metal cation. Cr-Mo-Ni alloys form a passive hydrated chromium oxyhydroxide film. Zr-Mo alloys form a bi-layered passive film of zirconium and molybdenum hydroxide. The difference in the stability of the passive film affects the corrosion resistance and passive film composition, particularly when the stability of the film is high and when the corrosion potential is high.

The corrosion behavior of the amorphous Fe-8Cr-metalloid alloys containing phosphorus as a metalloid element is largely affected by the amount of phosphorus. Immersion tests in concentrated 9 M H2SO4 solution open to air at 30°C show that the corrosion resistance of amorphous Fe-8Cr-20P alloy is very much higher than that of amorphous Fe-8Cr-13P-7C alloy. The corrosion loss of the amorphous Fe-8Cr-20P alloy is hardly detected by a microbalance even though the open circuit potential is initially in the active region of chromium. By contrast, the amorphous Fe-8Cr-13P-7C alloy shows active dissolution initially and the corrosion loss of this alloy increases gradually with immersion time. Only 30 mV difference in the initial open circuit potentials is responsible for the difference in the passivation behavior, with spontaneous passivation of the Fe-8Cr-20P alloy without initial active dissolution and delayed spontaneous passivation of the Fe-8Cr-13P-7C alloy with initial active dissolution. The presence of phosphorus ennobles the open circuit potentials of Fe-8Cr-13P-7C and Fe-8Cr-20P alloys sharply from the active region to the passive region of chromium by suppressing the anodic dissolution and accelerating the cathodic reactions of hydrogen evolution and oxygen reduction. XPS analysis indicates that the chromium content in the passive film increases with time of immersion following the ennoblement of the open circuit potential. The passivity of the Fe-8Cr-20P alloy which has a higher corrosion resistance than that of the Fe-8Cr-13P-7C alloy is maintained by the passive film in which chromium is less enriched in comparison with the passive film on the Fe-8Cr-13P-7C alloy. Phosphorus and chromium enhance synergistically the corrosion resistance. Phosphorus stimulates passivation and chromium forms the passive oxyhydroxide film. © 1994.

The synergistic effect of chromium and molybdenum in enhancing the corrosion resistance in the very aggressive acid has been studied by electrochemical measurements and XPS analysis. Amorphous alloys are formed in a wide composition range by the addition of nickel to chromium-molybdenum alloys. The corrosion weight loss decreases with increasing molybdenum content, and Cr-Ni-Mo alloys containing 27 at% or more molybdenum exhibit almost the same corrosion rate as that of molybdenum. Cr-Ni alloys dissolve actively, similarly to chromium, while the molybdenum addition to the Cr-Ni alloys results in spontaneous passivation. The molybdenum addition suppresses the anodic dissolution current because of the formation of tetravalent molybdenum oxyhydroxide having a relatively high activity for both cathodic oxygen and hydrogen reduction with a consequent ennoblement of the open circuit potential. The ennoblement of the open circuit potential leads to the formation of the passive chromium oxyhydroxide film. This film is protective to avoid transpassive dissolution of molybdenum under anodic polarization conditions. The increase in the molybdenum content leads to an increase in the passive current density and to an increase in the unstable Mo6+ content in the film under anodic polarization conditions. © 1994.

Amorphous NiTi alloys were prepared by d.c. magnetron sputtering. The corrosion behaviour of amorphous NiTi alloys in 1 M HCl was investigated by electrochemical measurements. Particular attention was paid to the compositions of the surface film and underlying alloy surface just below the surface film. The amorphous NiTi alloys were spontaneously passive. An X-ray photoelectron spectroscopy analysis revealed that the thickness of the surface film was increased linearly with an increase in the polarization potential. The growth rates for amorphous Ni32Ti and Ni62Ti alloys were the same as each other and slightly lower than the growth rate of sputter-deposited titanium. The passive film formed on the sputter-deposited amorphous NiTi alloys was enriched in titanium cation. In the underlying alloy surface just below the surface film after polarization for 1 h, the titanium content increased linearly with the increase in the potential. At relatively low potentials, nickel was concentrated in the underlying alloy surface, while at relatively high potentials the content of titanium was higher than that in the bulk alloy. © 1994.

Catalytic decomposition by hydrolysis of CCl2F2 (CFC-12) to HF, HCl and CO2 was studied for the amorphous binary NiZr and ternary NiZrCr alloy catalysts. The activity and durability for each catalyst were measured and compared. The amorphous Ni40at.%Zr alloy maintained its high activity for 70 h at 723 K, after which it decreased drastically. The amorphous NiZrCr alloy, activated at 823 K for several hours, showed a high activity and gave much longer durability at 723 K. The increase in the alloy chromium content enhanced the activity and elongated the life time. Among the NiZr15at.%Cr alloys, the specimen containing 40 at.% Zr showed the highest activity for the decomposition of CFC-12 but the specimen with 70 at.% Zr could not even be activated. The characterization of the reacted specimens was carried out using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. Their structures were significantly changed by the reaction, because they were exposed to corrosive HCl and HF, formed by the decomposition reaction of CFC-12. © 1994.

Amorphous Ni-valve metal alloys containing a few atomic per cent of platinum group elements were prepared by rapid quenching and were employed for electrochemical oxidation of sulfite in 0.075 M Na2B4O7 + 0.3 M H3BO3 + 1 M Na2SO3 at 30°C. Of the amorphous alloy electrodes containing ruthenium, rhodium, iridium or platinum, the platinum-containing electrodes showed the highest activity, which was almost the same as or even higher than that of the platinized platinum black electrode. Furthermore, the combined addition of platinum and ruthenium enhanced the catalytic activity. When ruthenium was substituted for part of the platinum, the maximum activity for amorphous Ni40NbxPt(1 - x)Ru alloys was observed at about 0.8 at.% Pt. © 1994.

The beneficial effect of phosphorus on the corrosion and passivation behavior of amorphous Fe8Cr13P7C alloy was studied in 9M H2SO4 solution at 30 °C. The corrosion loss of the amorphous Fe8Cr13P7C alloy in an initial period of immersion was relatively high but was scarcely changed afterwards. The corrosion potential was initially in the active region of this alloy but was shifted positively up to the passive region within 30 min. The air-formed film was enriched in chromium but was not protective and dissolved quickly in the initial stage of corrosion. The concentration of chromic ions in the surface film increased with the immersion time. Elemental phosphorus was accumulated on the alloy surface during the initial dissolution. The formation of the elemental phosphorus layer made the open circuit potential more positive as a result of the acceleration of the cathodic hydrogen evolution and by the suppression of the anodic dissolution current. This led to spontaneous passivation as a result of the formation of the chromium-enriched passive film. © 1994.

Amorphous Fe-valve metal alloys containing a small amount of Pt group elements were activated by immersion in HF acid for surface roughening and for surface enrichment of Pt group elements, and were used for methanation of CO2. The FeZrRu alloy showed an activity about one order of magnitude higher than those with the alloys containing other Pt group elements and produced only CH4. The less active Pd- and Ir-containing alloys produced CO as well as CH4. The addition of Nb to the highly active Fe70Zr2Ru alloy led to a significant decrease in the activity for CH4 production, but only CH4 was produced, even if Nb was added. X-ray photoelectron spectroscopy analysis revealed that the Ru and Fe contents in the surface were not markedly changed by the addition of Nb. However, the peak binding energy of Ru 3d 5 2 electrons of metallic Ru changes with the Nb content of the surface. The change in the electronic state of Ru by the addition of Nb seems to be one of the reasons for the decrease in the activity for the methanation of CO2. © 1994.

Surface-activated amorphous Ni-valve metal (Ta, Nb and Zr)-Pd alloys have been used for the decomposition of nitrogen monoxide. The alloys, which are first activated by immersion in hydrofluoric acid and subsequently oxidized by exposure to oxygen at 500 °C, show a high activity for the decomposition of nitrogen monoxide and have superior deterioration resistance. The catalytic activity of the alloy catalysts is affected by the valve metal elements. The tantalum- and niobium-containing alloy catalysts show higher activities and nitrogen formation selectivities than those of the zirconium-containing alloy. Double oxides of NiTa2O6 and NiNb2O6 are formed during the reaction at temperatures higher than 750 °C and, at the same time, the catalytic activity increases sharply. The synergistic effect of palladium and the double oxides seems to contribute to the high catalytic activity of the Ni40Ta1Pd and Ni40Nb1Pd alloys. © 1994.

To prepare highly corrosion-resistant amorphous CrNiP alloys, the amorphizable composition range was determined and the corrosion resistance was assessed from the weight loss in concentrated HCl and HF solutions. All the amorphous CrNiP alloys showed extremely high corrosion resistance in both concentrated HCl and HF solutions. The effects of crystallinity and the chromium content on the corrosion resistance were clearly observed in the concentrated HCl solution. In the concentrated HF solution, all the CrNiP alloys were extremely corrosion resistant, while the binary CrNi alloys had low corrosion resistance. The amorphous CrNi19P alloys also exhibited the low corrosion rate of about 0.004 mm per year in the mixture of aggressive 12 M HCl and 47% HF solution. The effects of crystallinity and alloying with phosphorus and chromium on the corrosion resistance in HCl and HF solutions were explained in terms of the homogeneity of alloys, the formation of an elemental phosphorus layer during the initial dissolution, and the formation of the highly protective, passive chromium oxyhydroxide. © 1994.

Sputter-deposited amorphous Cr-Nb and Cr-Ta alloys possessing higher corrosion resistance than that of alloy constituents were spontaneously passivated in 12 M HCl solution. Their open circuit potentials were higher than those of alloy constituents in 12 M HCl solution, and the passive current densities of amorphous Cr-Nb and Cr-Ta alloys were lower than those of niobium and tantalum metals, respectively, and decreased with increasing chromium concentration. XPS analysis revealed that air-formed films and passive films on the amorphous Cr-Nb and Cr-Ta alloys were composed of double oxyhydroxides of chromium and niobium or tantalum. The higher corrosion resistance of the amorphous Cr-Nb and Cr-Ta alloys in comparison with their constituent elements is attributed not only to the uniformity and homogeneity of the amorphous alloys but also to the formation of the double oxyhydroxide film of chromium and niobium or tantalum, both of which act synergistically in improving the corrosion resistance, partly because the double oxyhydroxide films possess high activity for cathodic oxygen reduction with a consequent ennoblement of the open circuit potential. © 1994.

A method of forming continuous silica and alumina coating films using a sol-gel dip coating and a protectiveness of the coating films are presented. The protectiveness which is related to the defects in the coatings is evaluated by electrochemical methods including polarization curve and open circuit potential measurements in deaerated 0.5M H2SO4 at 303K. The transparent, continuous and adhesive sol-gel silica coating films are obtained when the coatings are applied to single layer alumina-coated Type 316 stainless steel substrate, while the films directly coated onto the mechanically polished substrate tends to produce some defects. The appearance and protectiveness depend on the withdrawal speed and the baking temperature. For instance, the most suitable withdrawal speed and baking temperature are 1.07 and 1.80×10-3m/s, and 673K, respectively. The high protective coating can be obtained by two-layer silica-coatings due to reduction of the defect densities in underlying coatings. © 1994, Japan Society of Corrosion Engineering. All rights reserved.

Binary CrNb and CrTa alloys containing 29-63 at% niobium and 26-69 at% tantalum, respectively, with a single phase amorphous structure, were formed by a sputtering method. Corrosion rates of amorphous CrNb alloys were about four orders of magnitude lower than that of chromium metal and about one fifth of that of niobium metal in 12 M HCl solution. Corrosion rates of amorphous CrTa alloys were lower than that of tantalum metal and were not detected even after about 1 week immersion in 12 M HCl solution. The CrNb and CrTa alloys were spontaneously passive in 12 M HCl solution, in spite of the fact that chromium dissolved actively in 12 M HCl solution. © 1993.

Amorphous binary CrZr alloys containing 11-66 at% Zr were successfully prepared by a magnetron sputtering method. Amorphous CrZr alloys thus prepared were spontaneously passive in 6 M HCl solution. Their corrosion rates were four orders of magnitude lower than chromium metal and one-third of zirconium metal and decreased with alloy chromium content. XPS analysis revealed that airformed films and passive films on the amorphous CrZr alloys were composed of double oxyhydroxides of chromium and zirconium. This appears to be responsible for the high corrosion resistance of the amorphous CrZr alloys in comparison with their constituent elements; the formation of chromium-containing passive film is more protective than the film on zirconium metal, despite the fact that chromium metal dissolves actively in 6 M HCl solution. © 1993.

The corrosion behavior of amorphous FeCr13P7C, FeCr13P7B and FeCr20B alloys in 9 M H2SO4 solution at 30°C was investigated. An increase in the alloy chromium content improved the corrosion resistance of the amorphous FeCr13P7C and FeCr13P7B alloys. When only B is contained as a metalloid, alloying with chromium up to 15 at% is ineffective in increasing the corrosion resistance. The addition of phosphorus accelerates the cathodic reaction of hydrogen evolution on the amorphous FeCr alloys. This leads to sharp ennoblement in the corrosion potential of amorphous Fe8Cr13P7M (M C,B) alloys from the active state to the passive state with immersion time, while the corrosion potential of Fe8Cr20B alloy is unchanged. Chromium is concentrated in the passive film on the amorphous Fe8Cr13P7M (M C,B) alloys formed by immersion in 9 M H2SO4 solution, and is further concentrated by prolonged immersion. In particular, the concentration of chromic ions in the passive film on the Fe8Cr13P7C alloy is higher than that in the film on the Fe8Cr13P7B alloy. © 1993.

The preparation of binary Ti-Cr alloys was achieved by magnetron sputtering using a titanium target with small chromium disks on it. The alloys containing 37-73 at% Cr were amorphous. Amorphous Ti-Cr alloys were spontaneously passive showing significantly lower corrosion rates in comparison with those of titanium and chromium metals in 1 M and 6 M HCl solutions. XPS analysis revealed that air-formed films and passive films on the Ti-Cr alloys were composed of a chromium-titanium oxyhydroxide rather than a mixture of chromium and titanium oxyhydroxides. The passive chromium-titanium oxyhydroxide film was more protective, stable and resistant against depassivation in comparison with passive films on chromium and titanium. © 1993.

Surface-activated amorphous Co&ndash;Zr alloys containing platinum group elements were used for decomposition of nitrogen monoxide. The alloys were immersed in hydrofluoric acid for activation. The HF-treated amorphous alloys were initially inactive for decomposition of nitrogen monoxide at 773 K, but became active after oxidation by exposure to nitrogen monoxide at 773 K. XRD and XPS analyses revealed that the active catalysts were composed of platinum group elements, Co<SUB>3</SUB>O<SUB>4</SUB> and ZrO<SUB>2</SUB>. Platinum group elements in both metallic and oxidized states were supported on the oxides. The catalytic activity depended on platinum group elements. The palladium-containing alloy showed the highest activity and even higher activity than that of a palladium-black. The high activity of the palladium-containing catalyst was attributed to the synergistic effect of palladium and oxides of cobalt and zirconium.

Magnesium additions have been made to enhance the passivity of AlTi alloys prepared by sputter deposition. The corrosion behavior of amorphous AlMgTi alloys in a neutral borate-boric acid solution with chloride ions has been investigated by electrochemical measurements. Particular attention is paid to the effect of magnesium on the corrosion behavior and surface film composition. Alloys containing relatively low concentrations of titanium and low concentrations of magnesium possess a better resistance against pitting corrosion in comparison with the AlTi binary alloys with the same titanium contents, because an increase in alloy reactivity by the magnesium addition leads to preferential dissolution of not only magnesium but also aluminum with a consequent enrichment of titanium cations in the surface film. The addition of excessive amounts of magnesium lowers the pitting potential and increases the passive current density due to a decrease in the stability of the passive film and due to an excessive increase in alloy reactivity. © 1992.

This is a review of laser and electron beam processing and sputter deposition for the preparation of corrosion-resistant amorphous surface alloys and of the characteristics of thus prepared surface alloys. Amorphous surface alloys with a large surface area were prepared by repetition of instantaneous melting of a very restricted volume of the surface by irradiation with a CO2 laser or electron beam and subsequent self quenching by the cold bulk substrates. The materials consisting of the amorphous surface alloys and bulk crystalline metals are quite suitable for corrosion-resistant materials with other specific properties. Sputter deposition was used for preparation of various amorphous alloys such as Al-Ti, Al-Zr, Al-Nb, Al-Ta, Al-Cr, Al-Mo, Al-W, Cr-Ti, Cr-Zr, Cr-Nb, Cr-Ta, Cu-Nb and Cu-Ta. The corrosion resistance of amorphous aluminum alloys was very high and could be changed from that comparable to 304 stainless steels to that far exceeding corrosion-resistant nickel-base alloys by changing the alloying element and its concentration. The corrosion rates of amorphous chomium-valve metal alloys are several orders of magnitude lower than those of alloy constituting elements. The amorphous Al-Mo alloys show high resistance to both sulfidation and oxidation at high temperatures. Sputter deposition is a potential method to produce new materials with specific properties. © 1993.