Spatially-Resolved Detection of Hydrogen Absorbed into Pure Iron Using Electrochromic Tungsten Oxide Thin Film

Abstract

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 HxWO3 was formed at the dark blue area in the WO3 film after the hydrogen detection test. Therefore, the formation of HxWO3 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).