Effect of Surface Modification and Mechanical Deformation on Electrochemical Property of Ferritic Stainless Steel for PEMFC Bipolar Plates

Abstract

The proton exchange membrane fuel cell (PEMFC) is an ideal candidate for automotive application due to its high efficiency and cleanliness. The bipolar plate is a major part of PEMFC stack, which accounts for most of the total weight and cost of stacks. Stainless steel is a possible candidate of bipolar plate material due to its reasonably cheap cost and good mechanical property. In this study, high Cr ferritic stainless steel (FSS) is used for feasibility evaluation for the bipolar plate material due to its reasonably good corrosion resistance.To improve the corrosion resistance and interfacial contact resistance, the effects of acidic solution treatment is investigated. The HCl solution is selected to modify the surface of FSS specimen. The potentiodynamic and potentiostatic polarization tests were performed to investigate the electrochemical properties of FSS. The interfacial contact resistance (ICR) test between the carbon paper and specimen was performed to measure the electrical conductivity of the specimen. After immersion test, surface of specimen was analyzed by SEM, EDS and AES. Profilometer was used to measure the surface roughness.The immersion test did not significantly affect the corrosion resistance of specimen, but it decreased ICR value largely. It seems to be due to the roughness increase, precipitation of conductive particle on the surface of specimen and change of passive film composition. After a long term potentiostatic polarization test, the conductive particles were disappeared, Cr/Fe ratio was increased and ICR value was increased while the roughness was maintained. Though the ICR value increased after a long term polarization test, it was acceptable value for using it as bipolar plate of PEMFC. In the manufacturing process, the metallic bipolar plates are made by stamping process. Because the thin steel plate should be locally deformed, there can be electrochemical potential difference between the deformed area and non-deformed area, and the galvanic current between the two areas may induce the galvanic corrosion. To investigate the effect of mechanical deformation on the corrosion behavior of FSS, galvanic corrosion test on the specimen deformed with INSTRON was performed. The corrosion behaviors were evaluated by the corrosion test, potentiodynamic and potentiostatic polarization test, galvanic test and SVET. After galvanic test, there was little galvanic current between the deformed area and non-deformed area. Furthermore, the deformed specimen had higher corrosion current density than non-deformed one. However, through the SVET test and long term immersion test, it was confirmed that the galvanic corrosion between the deformed area and non-deformed area may not affect significantly the electrochemical properties of PEMFC bipolar plate material.