Corrosion resistance of Au/Ni thin film-coated stainless steel used as a polymer electrolyte fuel-cell separator

Abstract

Recently, as a new clean and renewable energy, much attention has been paid to the polymer electrolyte fuel cell (PEFC) because of energy and environmental problems, such as depletion of petroleum and global warming. PEFC has the characteristics of a simple structure, is easy to maintain and has a low cost. One of the parts of the PEFC is called the 'separator'; metal separators have a major issue regarding compatibility with corrosion resistance and electrical conductivity, in addition to low cost. Using current technology, a stainless steel AISI316L with an Au/Ni skin film has here been selected. Therefore, there is a need to make a thinner skin film, especially an Au top film for suppressing the PEFC total cost. When thinking about skin film, defects in the Au top film increase gradually as the thickness of Au film decreases. As a result, extensive corrosion is generated due to defects in the Au/Ni skin film. There are few reports evaluating the correlation between the corrosion characteristics of stainless steel with an Au/Ni skin film under a PEFC environment and existing defects in the Au/Ni skin film. Therefore, in this study, the pinhole defect ratio especially in Au film was evaluated using the critical passivation current density (CPCD) method. Au/Ni double layered coating was formed on AISI316L stainless steel employing the sputtering method because of its high adhesion strength. As the quasi-PEFC environment, 1 M H2SO4 aqueous solution was selected and then the pinhole defect ratio was evaluated by the CPCD method. As a result, the pinhole defect area ratio of 80 nm thick Au film was obtained to be about 1.1% in 1 M H 2 SO 4 aqueous solution.