Measurements of anodic polarization curves of SUS304, SUS316 and SUS447 were carried out. The working electrode (WE) was polished with diamond paste as a pretreatment and its minimum particle size was 1 μm. After that, it was ultrasonically cleaned in methanol and distilled water. The surface area of the WE was 0.28 cm². The counter electrode was platinum and the reference electrode was a saturated KCl/Ag/AgCl (SSE) electrode. Polarization curves were measured by scanning the working electrode potential at 100 mV min^-1 from open circuit potential to 1.5 V vs. SSE. The electrolyte solution was 1 mol/dm³ H₂SO₄aqueous solution (pH 3) which contained 10 ppm NaCl and 30 ppm NaF.

 Figure 1 shows polarization curves of SUS447 in electrolytes which contain (a) no inhibitor, (b) 1 mmol/dm³ Na₂MoO₄ and (c) 1 mmol/dm³ Na₂MoO₄ + 1 mmol/dm³ Na₂WO₄ as inhibitors. Passive and transpassive regions are observed in all polarization curves and the boundary between passive and transpassive region is approximately 0.8 V vs. SSE. The cathode of PEFC in practical use is between 0.7 V and 0.8 V vs. SSE. It is considered that the potential of cathode is normally in the passive region. However, the current density in the transpassive region should be small to avoid contamination of metallic ions dissolved from stainless steel. In Fig 1, all current density is approximately 5 μA cm^-2 in the passive region. The potential of current density in the solution of (c) is ten times smaller than that of (a) in the transpassive region, indicating that the coexistence of Na₂MoO₄ and Na₂WO₄has significant inhibitor effect for transpassive dissolution of stainless steel. In this study, protection mechanisms of inorganic corrosion inhibitors are proposed on the basic of thermodynamic data.

References

[1] J. Imamura, Y. Tarutani, NIPPON STEEL & SUMITOMOMETAL Technical Report, 396 (2013).

[2] K. Hashimoto, K. Asami, M. Naka and T. Masumoto, Corros. Sci., 19, 857 (1979).