Stainless steel is considered as a suitable material of PEFC metal separator because it has good durability, formability, chemical stability and reasonable price. Whereas, localized corrosion would occur on stainless steel due to potential and temperature change during PEFC operation because a metal separator contacts with acidic solution containing fluoride and chloride ions. It is generally known that a few percent of molybdenum as an alloy element inhibits localized corrosion. Hashimoto et. al.¹⁾ proposed that dissolved molybdenum from stainless steel affected protection of steel surface. In previous study²⁾, we discussed anti-corrosive effects of molybdate ions as an inhibitor in solution. In present study, we investigate inhibitor mechanisms of sodium molybdate, sodium tungstate and sodium phosphate for localized corrosion of stainless steel by electrochemical impedance spectroscopy.

In this study, measurement of polarization curve and electrochemical impedance were conducted by three electrode methods. The working electrode was S44700 (Surface area : 0.28 cm²). The counter electrode was platinum wire and the reference electrode was a saturated KCl/Ag/AgCl (SSE) electrode. The electrolyte solution simulating PEFC operation environment (PEFC simulating solution) was 1 mol/dm³ H₂SO₄ aqueous solution (pH 3) which contained 10 ppm NaCl and 30 ppm NaF. Furthermore, some inorganic inhibitors were added to PEFC simulating solution as follows: 1 mmol/dm³ sodium molybdate, sodium tungstate or sodium phosphate. 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. Impedance measurements were carried out at potentials in passive and transpassive regions. Imposed AC potential amplitude was 10 mV and frequency ranges were from 10 mHz to 10 kHz.

Figure 1 presents polarization curve and impedance spectra of S44700 in PEFC simulating solution. The polarization curve is composed of passive region from open circuit potential to 0.8 V vs. SSE and transpassive region from 0.8 V to 1.5 V vs. SSE. Impedance spectra show blocking behavior at passive region (0.7 V, 0.8 V vs. SSE) and inductive loops at transpassive regions (0.9 V, 1.0 V vs. SSE). In present study, we investigate inorganic inhibitors reducing transpassive dissolution current. Then, we propose inhibitor mechanisms of sodium molybdate, sodium tungstate and sodium phosphate by EIS.

References

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

2) M. Ueyama, C. Tsukioka, Y. Hoshi, I. Shitanda, M. Itagaki, S. Takata, and T. Kogure, C01 – 697, 232 nd ECS Meeting.