The initial stage of hydrogen embrittlement is hydrogen absorption. In atmospheric environments, hydrogen absorption into steel occurs as a result of the cathodic reaction during corrosion process. Because the hydrogen absorption behavior is sensitive to surface heterogeneity, it is an important subject to clarify the distribution of hydrogen absorption on steel surface for reducing the risk of hydrogen embrittlement.

Electrochemical hydrogen permeation cell developed by Devanathan et al.¹⁾ is effective in evaluating the hydrogen permeation rate and the diffusion coefficient of hydrogen, but it cannot measure the distribution of hydrogen absorption. A new and easy detection system to obtain the distribution of hydrogen absorbed into steel has been required.

We therefore fabricated a detection system for hydrogen absorbed into steel using WO₃. After WO₃ thin film was formed in the back side of an iron sheet (hydrogen detection side), hydrogen was absorbed into the iron sheet by the cathodic polarization in the front side (hydrogen entry side). Then, the change in the optical property of the WO₃ thin film by hydrogen permeating through the iron sheet was examined.

Pure iron sheets 1 mm in thickness were used for the hydrogen detection test. Prior to the test, the surface of the hydrogen detection side was electrochemically plated with Pd. After the Pd-plating, the WO₃ thin film was formed by reactive rf magnetron sputtering. The hydrogen evolution and absorption were performed by the potentiostatic polarization at -0.720 V in a 0.1 M H₂SO₄ solution. The surface of the WO₃ film in the hydrogen detection side was observed by a video camera during the polarization.

The hydrogen detection test with the WO₃ thin film shows that the color in the WO₃ film gradually changed from a light blue to a dark blue. In addition, the color of the WO₃ film around the area corresponding to the electrode region in the hydrogen entry side mainly altered. This result suggests that the optical property of the WO₃ thin film in the hydrogen detection side changed by hydrogen absorbed into the iron sheet in the hydrogen entry side. XPS result shows that H_xWO₃ was formed at the dark blue area in the WO₃ film after the hydrogen detection test. Therefore, the formation of H_xWO₃ by the absorption of hydrogen seems to result in the color change of WO₃.

1) M. A. V. Devanathan and Z. Stachurski, Proc. Roy. Soc., A270, 90-102 (1962).