Influences of Chloride and Fluoride Ions on Localized Corrosion of Stainless Steels at Anodic Polarization

Tuesday, 3 October 2017
Prince George's Exhibit Hall D/E (Gaylord National Resort and Convention Center)
C. Tsukioka, M. Ueyama, Y. Hoshi, I. Shitanda, M. Itagaki (Tokyo University of Science), S. Takata, and T. Kogure (Toyota Motor Corporation)
Polymer electrolyte fuel cells (PEFCs) are environmentally friendly generators and operate at relatively low temperatures, so they have been put into practical use as power sources for automobiles. A carbon plate is generally used for the separator of PEFC. However, there are problems in using carbon separator such as low mechanical strength and cost, thus metal separators are used for automotive PEFC. Stainless steels are considered to be suitable metal among several candidates for material of metal separator because it is highly corrosion resistant, high workable and relatively reasonable. Since the inside of the PEFC becomes particular corrosive environments, dissolution of metallic ions due to localized corrosion of stainless steel is feared. Typical conditions of the particular corrosive environments assumed in this research are as follows, chloride ion concentration: 10 ppm, fluoride ion concentration: 30 ppm, pH 3, electrode potential: 0.8 V vs. SSE, temperature: 80 degree centigrade. A schematic diagram of the anodic polarization curve of stainless steels is shown in Fig. 1. The anodic polarization curve is divided into three potential regions and they are defined as a passive region, a first transpassive region, and a second transpassive region. Anodic polarization curves of SUS 447 in three kinds of solutions are shown in Fig. 2. The following three solutions were used for measurements. A: pH 3 1 M H2SO4 (80℃) without halide ions, B: 1 M H2SO4 containing 10 ppm NaCl, C: 1 M H2SO4 containing 30 ppm NaF. The anodic polarization curve of SUS 447 in solution A without halide ion shows the passive region and the second transpassive region at nobler potential than 1.2 V vs. SSE. The anodic polarization curve of SUS 447 in the solution B containing chloride ions shows the first transpassive region and the current increase above 0.8 V vs. SSE is observed. If the potential of the PEFC cathode is assumed to be 0.7 ~ 0.8 V vs. SSE, it is near the boundary between the passive region and the first transpassive region, and metallic ions dissolution may occur. Furthermore, in the anodic polarization curve in SUS447 in a solution C containing fluoride ions, the current value in the first second transpassive regions is much larger than that in the solution B, indicating the acceleration of the dissolution rate. It is considered that corrosion protections such as surface treatment and inhibitor addition are necessary when stainless steels are used in the particular corrosive environment in PEFC. In this study, we investigated the influences of chloride and fluoride ions on localized corrosion of stainless steel by electrochemical measurements, and discuss the corrosion protection method of stainless steel as a metal separator of PEFC.