Anodic Dissolution of Stainless Steel Covered with Oxide Thin Film in PEFC Simulated Environment
Authors: Kino Yuki , Kato Shinya , Hoshi Yoshinao 
Published: December 2024

Stainless steels have been considered for the bipolar plate materials in fuel cell stack components to reduce the cost of the fuel cell vehicles (FCVs). However, the contact resistance between the gas diffusion layer (GDL) and bipolar plates fabricated by the stainless steels may show large values due to the formation of passive film on the stainless steels 1,2). Further, the anodic dissolution of stainless steels may occur in FC operation, leading to the increase of the contact resistance between the GDL and bipolar plates. In the present study, we developed the surface treatment method for bipolar plates fabricated by the stainless steels using oxide thin film. The dissolution behavior of stainless steels with oxide thin film was investigated in the polymer electrolyte fuel cell (PEFC) simulated environment. In addition, the contact resistance between the GDL and stainless steels with oxide thin film was evaluated before and after corrosion test.
In the present study, the stainless steel sheets covered with oxide thin film were fabricated by the vacuum evaporation. The dissolution behavior of these samples was investigated by an electrochemical measurement by a three-electrode system. To simulate PEFC environment, the test solution containing 3 ppm NaF and 10 ppm NaCl was used as the electrolyte solution. The pH of the test solution was adjusted at pH 3. The stainless steel sheet or stainless steel sheet covered with oxide thin film was used as the working electrode. The platinum wire and the KCl-saturated Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The potentiostanic polarization was performed at an arbitrary potential. The electrochemical impedance was measured under potentiostanic polarization. In this measurement, the AC potential amplitude was 10 mV and the frequency range was 100 kHz to 0.01 Hz at 5 frequencies per decade.
The potentiostatic polarization of the stainless steel sheets and stainless steel sheet covered with oxide thin film was carried out at an arbitrary potential to evaluate the corrosion behavior of each sample. In the stainless steel sheet, the increase and the decrease in the current density were observed under potentiostatic polarization, implying the dissolution of Fe ions or the dissolution and repassivation of the passive film. The time variation of the current density for stainless steel sheet covered with oxide thin film under potentiostanic polarization suggested that the dissolution rate of Fe ions was small under potentiostatic polarization. Based on these results, we measured the impedance spectra of the stainless steel sheet covered with oxide thin film under potentiostanic polarization. In this case, the time variation of impedance was investigated by 3D plots of impedance to determine the instantaneous impedance at arbitrary measurement time 3,4). The charge transfer resistance of each sample, which was estimated by curve fitting using equivalent circuit, at arbitrary measurement time was discussed to examine the corrosion mechanisms.

References

(1) A.Miyazawa, T. Himeno, A. Nishikata, J. Power Sources, 220, 199 (2012).
(2) A.Miyazawa, E Tada, A. Nishikata, J. Power Sources, 231, 226 (2013).
(3) Z.B. Stoynov, B.S. Savova-Stoynov, J. Electroanal. Chem. 183, 133 (1985).
(4) Y.Hoshi et al., Corros. Sci., 232, 112018 (2024).

Acknowledgement

This is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).