High-strength steels are known to be susceptible to hydrogen embrittlement. Even small amounts of hydrogen which enter the high-strength steels under corrosion conditions could cause the hydrogen embrittlement. In order to elucidate the mechanism of hydrogen entry in corrosive environments, and to prevent the corrosion-induced hydrogen embrittlement, the simultaneous analysis of the corrosion behavior and the hydrogen entry is required. Some techniques, such as scanning Kelvin probe force microscope, have been reported for visualizing the hydrogen distribution in metals with a high spatial resolution. However, it is still challenging to visualize the hydrogen distribution of an observation area of several tens of millimeters with high spatial and temporal resolutions. In this study, we succeeded the visualization of the distribution of hydrogen entering a pure Fe sheet immersed in a NaCl aqueous solution using a conductive polymer.

A pure Fe sheet (99.5 %) was used as a specimen. The Fe sheet was electrically polished in a mixed solution of perchloric acid and acetic acid. The thickness of the Fe sheet was 0.4 mm after the polishing. In order to visualize the hydrogen distribution in the Fe sheet, a polyaniline (PANI) layer was formed on one side of the Fe sheet. It was reported that the PANI layer changes its color owing to the reaction with atomic state hydrogen in Fe sheets.¹ To enhance the sensitivity of the PANI layer to hydrogen, a Ni interfacial layer was formed between the PANI layer and the Fe sheet. The PANI layer was formed on the Ni interfacial layer through electrolytic polymerization at a constant voltage of 1 V in a 0.5 M H₂SO₄-0.5 M aniline aqueous solution. On the other side (bare Fe side) of the Fe sheet, an electrode area of 1 cm × 1 cm was prepared using a resin. Only the bare Fe side was immersed in a 0.1 M NaCl aqueous solution. In addition to the open-circuit potential measurements, the corrosion behavior was observed in situ using a digital camera during the corrosion test. The PANI layer was exposed to air without contacting the NaCl solution. During the corrosion test, the color of the PANI layer was observed in situ using another digital camera. It was confirmed that the small amount of hydrogen entering the Fe sheet due to corrosion can be visualized using the PANI layer. The distribution of hydrogen that entered the Fe sheet was analyzed based on the color distribution of the PANI layer. The in situ simultaneous observation of the corrosion behavior and the hydrogen entry revealed that the hydrogen entry was promoted on the dissolved area of the Fe sheet in the NaCl solution.

Reference

1. H. Kakinuma, S. Ajito, T. Hojo, M. Koyama, E. Akiyama, Adv. Mater. Interfaces, 2101984 (2022).