(Invited) Corrosion Behavior and Hydrogen Permeation of Steel Materials Under Wet-Dry Cyclic Environment

Higher strength steels have been required from the viewpoint of safety and energy saving in the fields of transportation equipment and infrastructure. However, it is known that the higher the strength of steels, the higher the risk of hydrogen embrittlement. It has been reported that hydrogen embrittlement occurs even in atmospheric corrosive environment, and solving the problem of hydrogen embrittlement will be the key to the future hydrogen energy society. For that purpose, it is necessary to clarify the hydrogen embrittlement behavior due to atmospheric corrosion. We focused on surface potential measurement as a technique to visualize permeated hydrogen. The purpose of this study is to clarify the hydrogen permeation behavior of steel materials under atmospheric corrosive environment. A 50 x 50 x 0.5t mm of pure iron plate was used as the sample. After grinding both surfaces of samples and degreasing with ethanol, nickel plating was applied to the hydrogen detective side of the sample. Hydrogen penetration was accelerated by a wet-dry cyclic corrosion test using a droplet of 0.5 M NaCl solution. The distilled water was added after second cycles to avoid the accumulation of NaCl. The potential of the hydrogen detective surface was measured with time in the drying process by surface potential device. The surface potential probe is set on the XY stage and the surface potential distribution can be obtained by scanning the probe. In this study, the surface potential of the copper plate in contact with the sample was adjusted at 0V as the reference potential. Although the corrosion occurred on the surface in the drying process after dropping the NaCl solution, no surface potential change was observed on the hydrogen detective side of the sample in that process. The surface potential started to change in a part of the surface of the sample immediately after the droplet was completely dried, and after 1hour, the size and shape of the potential changed area became almost the same as those of the corrosion products. No corrosion occurred on the hydrogen detective side of samples during the surface potential measurement, indicating that the surface potential change is attributed to the atomic hydrogen penetrated through the samples. The surface potential changed area gradually disappeared, and almost no potential change was observed on the surface after 72 hours. The change in surface potential after the second cycle of wet and dry cyclic corrosion test showed the same tendency as in the first cycle, and a potential distribution close to the shape of corrosion products was observed.