Further strengthening of steel materials is required in recent years from the viewpoint of energy saving and resource saving. However, hydrogen embrittlement is well known to be more sensitive to high-strength steel, and especially it is an urgent matter to solve the problem related to hydrogen embrittlement caused by corrosion. A technology for visualization of penetration or permeation hydrogen is required as a method to solve it. Katayama et al. report that distribution of permeated hydrogen can be visualized by surface potential measurement. In addition, they shows that the time during which the surface potential change due to permeated hydrogen is observed varies depending on the environment of the hydrogen detection side. However, it is not clear what the environment dependency of hydrogen observation time is attributed to. In this study, it is focused on the relative humidity as the environmental factor of the hydrogen detection side, and hydrogen permeation behavior was monitored by surface potential measurement under the environment of hydrogen detection side controlled at a variety of relative humidity.

Pure iron of 50 × 50 × 1 t (mm) was used as a sample. The hydrogen penetration surface of the sample was covered with a rubber type masking material except for about 2mm diameter of bare metal area. The sample was set in the apparatus, and the relative humidity in the acrylic container was controlled by the saturated salt method. Hydrogen penetration on the entry side was accelerated by cathodic polarization at - 2 mA/cm² for 600s in 0.1M NaOH solution. After the polarization was stopped, the surface potential distribution of the hydrogen permeation side was measured every 10 minutes to 1 hour in the range of 20 × 20 (mm) with the copper plate as a reference．

Hydrogen evolution was observed only at the bare metal area of the sample during cathodic polarization, indicating that hydrogen penetration occurred only at the area. The humidity in the acrylic container was set to 68% with a saturated potassium chloride solution, 57% by a saturated sodium chloride solution, and 38% with a saturated magnesium chloride solution, respectively. The potential on the back side of the bare pure iron area gradually went down after cathodic polarization and shifted to the noble direction after showing the maximum. It took above 24 hours at 38% RH until a return to the original surface potential distribution, and that time decreased with an increase of relative humidity. In addition, the peak value of surface potential became lower as the relative humidity decreased.