Electrochemical Study on Hydrogen Entry Initiationin SSRT of Low Alloy Steel in High-Pressure Hydrogen Gas

Abstract

The application of concurrent cathodic hydrogen charging SSRT is being investigated as an alternative to SSRT in high-pressure hydrogen gas, which is used to evaluate the hydrogen compatibility of metallic materials. There are some differences in the stress-displacement behavior and the fracture surface between the two tests. These differences can be attributed to the difference in the timing of the onset of hydrogen entry in the two environments. In SSRT of low-alloy steel in high-pressure hydrogen gas, the amount of absorbed hydrogen up to the yield stress is small and increases rapidly after plastic deformation, Whereas, since in concurrent cathodic hydrogen charging, hydrogen entry is initiated as soon as begins, it is necessary to determine an apporopriate start time. The surface changes of SCM435 specimens during SSRT in NaOH solution were analyzed by electrochemical measurements, and the breakdown of the oxide film that inhibits hydrogen entry during SSRT in high-pressure hydrogen gas was discussed. As a result, the impedance at low frequencies decreased immediately after the yield point, which was attributed to the decrease in the reaction resistance of the oxide film. The rest potential decreased immediately after the yield point and immediately after the fracture, which were attributed to the exposure of the fresh surface due to the breakdown of the oxide film. From these results, it was clear that hydrogen charging in the concurrent cathodic hydrogen charging SSRT should be started just after the yield point.