Recent efforts and studies made it possible for the hydrogen energy to be utilized in the fields of reusable energy and mobility such as fuel cell electric vehicles (FCEVs). In a high-pressure hydrogen environment, hydrogen penetrates into the metals which causes hydrogen embrittlement (HE). The materials have generally different surface roughness depending on the fabricating method used, and the surface condition especially affects the corrosion resistance of metals. Therefore, the HE behaviors of metals according to the surface roughness should be evaluated to ensure the safety when metals are used for various devices under high-pressure hydrogen environments.
In this study, for austenitic steels of STS316L steel and 18wt%Mn steel and ferritic steels for API-X70 steel and 2.25Cr-1Mo steel, specimens were fabricated by both wire-cutting and polishing. They were tested under 10 MPa hydrogen environment using the in-situ small punch (SP) test method which is a simple test method established by ANU group. The tests were performed at temperatures of room temperature (RT) and -40°C for ferritic steels, and at temperatures of RT, -40°C and -60°C for austenite steels. Through the tests, the effect of surface roughness on the HE behaviors in each steel specimen was examined and analyzed by observing the gas exposed surface using a scanning electron microscope (SEM). As a result, it was found that the surface roughness did not influence the HE characteristic in both steels regardless of the crystal structures such as fcc or bcc.
Furthermore, the ferrite content of austenite STS316L steel was measured before and after the in-situ SP test using a FERITSCOPE in order to understand the HE mechanism occurred. At lower temperatures, stability of the austenite phase decreased and the martensite transformation occurred. when the ferrite content exceeded a certain level, for example, 4.5% the HE became significant. The martensite phase transformation induced resulted in the hydrogen diffusion within the specimen and caused the HE damage of the specimen even at a fast punch velocity like 1.0 mm/min.