Hydrogen-assisted degradation of high-strength stainless steel with a newly developed aluminum-based coating in high-pressure hydrogen gas environment

The paper presents the hydrogen-entry, tensile, and fatigue properties of a precipitation-hardened martensitic stainless steel, JIS-SUS630, with a newly developed coating, whose thickness ranges from 10 to 20 μm. The newly developed coating consists of alumina, aluminum, and ferroaluminum, and has an excellent resistance to hydrogen entry in 100-MPa hydrogen gas at 270°C. The hydrogen entry in the coated specimen occurred under a diffusion-controlled process and the effective hydrogen diffusivity was approximately one thousandth of that of the base steel. Although the hydrogen diffusivity of JIS-SUS630 was two orders of magnitude larger than that of JIS-SUS304, the effective hydrogen diffusivity of the coated JIS-SUS630 was nearly equal to that of the coated SUS304. In our previous study with secondary-mass ion spectroscopy (SIMS), the coating's excellent resistance to hydrogen entry was attributed to interfacial hydrogen trapping between the aluminum and ferroaluminum layers. The experimental result obtained in this study suggested that the excellent resistance to hydrogen entry demonstrated by the developed coating can be attributed to the reduction in the permeation area induced by the interfacial trapping of hydrogen. The tensile tests of a smooth, round-bar specimen and fatigue tests of a circumferentially notched specimen with exposure to 100-MPa hydrogen gas at 270°C were performed in air at room temperature (RT). The test results showed that the tensile and fatigue properties of the coated specimens were not degraded by hydrogen exposure, whereas those for the non-coated specimens were significantly degraded. Hydrogen-pressure cycle tests of the coated, tubular specimens with an inner notch were also carried out with 95-MPa hydrogen gas at 85°C, demonstrating that the fatigue life of the tubular specimen was improved by the developed coating.