The function of hydrogen in the embrittlement of steels has been examined with respect to plasticity involved in the fracture process. Fractographic features and effects of stress state on the hydrogen embrittlement indicate promoted crack formation by hydrogen associated with strain localization. Hydrogen also enhances the creation of vacancy-type defects during plastic deformation, and correlations are shown between the density of strain-induced vacancies and the susceptibility to the hydrogen embrittlement.
Analyses of the role of hydrogen in the fracture process have shown that hydrogen promotes the initiation of micro-cracks and reduces the resistance to the successive crack growth, being originated in vacancy formation. Interrelations between hydrogen effects in fatigue and delayed fracture have been shown, supporting the common effects of hydrogen through enhanced creation of vacancies.
Various models of the mechanism of hydrogen embrittlement have been briefly and critically reviewed. Characteristic features of hydrogen embrittlement are the best explained with the hydrogen-enhanced strain-induced vacancies model that claims the primary role of vacancies rather than hydrogen itself in the embrittlement.

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Fuel-cell vehicles, one of the applications of hydrogen energy system, are getting into wide use in near future. Structural materials with high strength and less hydrogen embrittlement are required for extending the cruising range and reducing the costs of the vehicle. Hydrogen environment embrittlement has been evaluated usually using a chamber of high pressure hydrogen gas containing a specimen inside and loading from outside. However, this kind of method requires facilities of high pressure hydrogen for mechanical properties testing.
A simple testing method for mechanical properties evaluation under high pressure hydrogen gas was developed without using a high pressure gas chamber and, in this report, this method was applied easily to low temperatures.
Tensile properties of stainless steels, SUS 304, 304L and 316L, obtained by this simple method are in good agreement with previous data obtained in a high pressure chamber, which proves the effectiveness of this simple method.

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Hydrogen Environment Embrittlement (HEE) susceptibility in high pressure gaseous hydrogen was investigated on austenitic stainless steels and A6061-T6 aluminum alloy. Tensile properties of these materials were evaluated by Slow Strain Rate Test (SSRT) in gaseous hydrogen pressurized up to 90MPa in the temperature range from-40 to 85°C. HEE susceptibilities of austenitic stainless steels strongly depended upon the chemical compositions and testing temperatures. A6061-T6 aluminum alloy showed no degradation by hydrogen. Fatigue properties in high pressure gaseous hydrogen were evaluated by the external cyclic pressurization test using tubular specimens. The tubular specimen was filled with high pressure hydrogen gas. The outside of the specimen was cyclically pressurized with water. Type 304 showed a decrease in the fatigue life in hydrogen gas, while as for type 316L and A6061-T6 the difference of the fatigue life between hydrogen and argon environments was small. HEE susceptibilities of investigated materials were discussed based on the stability of an austenitic structure.

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Fretting fatigue is one of the major failure modes at the joint or contact part between components in machines and structures. To clarify the effect of hydrogen gas environment, fretting fatigue tests were performed. Fretting fatigue strengths at 3×10⁷ cycles obtained in air and hydrogen gas were compared. The test materials were low alloy steel SCM435H, heat resisting alloy SUH660 and two kinds of austenitic stainless steels SUS304 and SUS316L. Stainless steels were hardened by giving 30% strain under monotonic tension. Nitrided material was also used in the experiment to investigate prevention of fretting in hydrogen gas environment. The pressure of hydrogen gas was 0. 12MPa in absolute pressure. Fretting fatigue life in hydrogen gas rather increased in the short-life region. It was found that the extension of fatigue life was caused by the delay of start of stable crack propagation. In air, there was no decrease of fretting fatigue strengths between 1×10⁷ and 3×10⁷ cycles. In hydrogen gas environment, fretting fatigue strengths continued to decrease exceeding 10⁷ cycles. As the results, fretting fatigue strengths at 3×10⁷ cycles were lower in hydrogen gas environment than in air. The reduction was 12% for SCM435H, 18% for SUH660, 7% for SUS316L and 24% for SUS304. Nitriding was effective for the improvement of fretting fatigue strength not only in air but also in hydrogen gas environment. The tangential force coefficient in hydrogen gas environment increased. Detail observations of fretted surface and initiation and early propagation of fretting fatigue crack in hydrogen gas environment showed that adhesion between contact surfaces might have an effect of crack initiation at the stress level which is lower than fretting fatigue limit in air. The effect of absorbed hydrogen and high-pressurized hydrogen gas environment are the problems that remain to be solved.

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Cold cracking sometimes occurred in long-term operated petroleum pressure vessels due to hydrogen embrittlement by thermal stress and diffusible hydrogen after multi pass repair welding. The cracking was caused by the hydrogen concentration at the base metal of 2. 25Cr-1Mo steel⁄overlay weld metal of austenitic stainless steels during the service with high temperature and hydrogen partial pressure. The hydrogen content at the interface was calculated by the theoretical analysis using the diffusion equation based on activity. The multi pass weld cracking susceptibility was raised with increase in hydrogen content at the interface. In addition, the theoretical analysis results indicate that the cracking could be prevented by reduction of hydrogen content at the interface to control the repair welding thermal cycles and de-hydrogen heat treatment.

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It is difficult to clarify the detailed propagation mechanisms of back-side metal loss of bottom floors because this task is complicated by several factors, including the soil and the environment. Therefore, authors assumed the propagation of back-side metal loss of the bottom floors to be a stochastic process and applied the directed percolation model to the phenomena. The back-side metal loss generated on the bottom floors was simulated using the directed percolation model with various percolation probabilities. It was then verified that similar risk curves for the dataset measured with the continuous thickness measurement device were created with a simulated dataset. Using the simulated dataset, the dependency of number of data for calculating the Corrosion Risk Parameter (CRP) obtained from the statistical analysis using the measured dataset was discussed. Compared with the CRP calculated from the simulated dataset and the maximum depth of the clusters, the more than 1, 000 data were needed for the calculation of the appropriate CRP.

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