Corrosion Investigation of Ni-Free Nitrided Stainless Steel Material As Bipolar Plate for Polymer Electrolyte Fuel Cell

Abstract

Bipolar plates are particularly required mechanical strength and corrosion resistance for automobile application of polymer electrolyte fuel cell (PEFC). Thin metallic bipolar plates enable to realize small-size and light-weight PEFC stacks. However, there is some problem for cost, durability, and so on. In this study, we report an improvement of the corrosion resistance of Ni-free stainless steel (SUS445) by a nitriding heat treatment. Nitriding heat treatment of Ni-free stainless steel (SUS445) was performed using a vacuum furnace under nitrogen atmosphere at 1473 K. Electrochemical measurements were carried out using a three-electrode electrochemical cell having a Pt counter electrode, an Ag/Ag2SO4 reference electrode, the SUS445 and the nitrided stainless steel (SUS445N) as a working electrodes in Ar-saturated 0.5 mol dm-3 H2SO4 solution at room temperature. Linear sweep voltammogram was obtained from a rest potential to 1.1 V vs. SHE at a sweep rate of 0.33 mV s-1. Linear sweep voltammograms of SUS445, SUS445N, and SUS316 are shown in Figure 1. The voltammogram of SUS445 has current peak corresponds to a formation of passivation layer at -0.2 V vs. SHE. The onset potential of the SUS445N voltammogram shifts toward the positive direction and there is no current peak of the passivation layer formation. The current density of SUS445N is one order of magnitude lower than that of SUS445 and SUS316. It is found that the corrosion resistance of SUS445 improved by the nitrogen heat treatment. Moreover, PEFC operation test was performed using SUS445N bipolar plates and graphite one for a comparison. The two cell voltage at 0.25 A cm-2 were almost the same up to 2500 h power generation. Therefore, the Ni-free nitrided SUS445 exhibited a high durability as the bipolar plate of PEFC. Figure 1