Development of High Corrosion Resistance Surface Treatment for Stainless Steels
Authors: Ishii Yuya , Watanabe Hikari , Hoshi Yoshinao , Shitanda Isao , Itagaki Masayuki , Hirano Masaki , Kogure Tomonari , Yanagimoto Hiroshi 
Published: December 2020

Polymer electrolyte fuel cells (PEFC) is energy converter with high efficiency and operates at low temperatures of 80oC, thus it gains attention as an automotive power plant. Stainless steels, which could be used as a PEFC bipolar plate material, can be made very thin1), and has the advantage that the power density of a single cell can be improved. However, it has been reported that the proton conductivity of the electrolyte membrane decreases due to the dissolved iron and chromium ions from the polarized stainless steels due to the current fluctuation during PEFC operation2-3). In this study, the surface treatment method of stainless steels for improving the corrosion resistance was proposed and the electrochemical properties were evaluated.
The anodic polarization curve of stainless steels was measured by a three-electrode system using an electrochemical measurement system (HZ-7000, Hokuto). The working electrode was SUS316L with various surface treatments. A platinum wire and a silver-silver chloride electrode were used for counter and reference electrodes. The electrolyte was 1.0 M H2SO4 (pH 3) containing 1 ppm NaCl and 3ppm NaF. and this electrolyte is defined as a PEFC simulation environment. The measurement temperature was 80oC, the potential scan range was from the immersion potential to 1.5 V vs. SSE, and the potential scan rate was 100 mV min-1.
Three types of stainless steels were used for the working electrode: I. SUS316L, II. SUS316L treated by chemical conversion (CC SUS316L), and III. SUS316L anodized in electrolyte for chemical conversion (anodized SUS316L). The electrolyte for chemical conversion was a 1:1 mixture of 0.1 M Na2WO4 aqueous solution and 0.1 M Na2MoO4 aqueous solution, and the temperature of electrolyte for chemical conversion was 25oC. In the treatment II, SUS316L was immersed in the electrolyte for chemical conversion for 5 min. In the treatment III, SUS316L was polarized in a electrolyte for chemical conversion at an applied potential of 1.0 V vs. SSE for 5 min.
Figure 1 displays the anodic polarization curves of SUS316L with various surface treatments in the simulated PEFC environment. The anodic polarization curve of SUS316L showed passive range from the immersion potential -0.3 V vs. SSE to 0.7 V vs. SSE and small current peak due to the formation of the passive film around 0 V vs. SSE. In the transpassive region at the potential nobler than 0.7 V vs. SSE, the current density region increased sharply. At potentials nobler than 1.3 V vs. SSE, the current density increased sharply due to electrolysis of water. The shape of the anodic polarization curve of the CC SUS316L was almost the same as that of the SUS316L, however, the polarization curve showed a small current density in the passive and transpassive regions. No passive region was observed in polarization curve of the anodized SUS316L. And compared to SUS316L and CC coating SUS316L, the immersion potential shifted to noble and the current density in the transpassive region decreased.
References
1) F, Barbir. “PEM Fuel Cells Theory and Practice”, Academic Press, 4 (2005), 100, 101.
2) A. Pozio, R. F. Silva, M. De Francesco, L. Giorgi, Electrochim. Acta, 48 (2003), 1543-1549.
3) H. Wang, J. A. Turner, J Power Sources, 183 (2008), 576-580.

Figure 1