Effects of oxidation treatment on electrochemical characteristics and interfacial contact resistance of CrAlN-coated metallic bipolar plates for PEMFCs

The effects of oxidation treatment on the electrochemical charactersitics and the interfacial contact resistance of CrAlN coatings were investigated for application to metallic bipolar plates of the polymer electrolyte membrane fuel cells. Three types of CrAlN-coated specimens were prepared: unoxidized, chemically oxidized and electrochemically oxidized specimens. On the surface of oxidized CrAlN coatings, defects such as micro-/nano-particles and pores were decreased, but the oxygen content increased. Compared with the unoxidized specimens, the chemically oxidized specimens showed lower corrosion current density, a wider passive region, higher activation overpotential and faster stabilization. The electrochemically oxidized specimens exhibited the highest electrochemical stability because of the formation of an oxide film in defect regions, which prevented the penetration of corrosive ions. In the unoxidized specimens, corrosion initiated around micro-particle regions. This is attributed to the fact that the growth defects in the CrAlN coating layer caused crack propagation, thus creating a pathway for penetration and attack by corrosive ions. However, the CrAlN-coated specimens with defects sealed by the oxide film exhibited relatively superior corrosion resistance. The chemically and electrochemically oxidized specimens had an interfacial contact resistance of less than 10 m ohm center dot cm2 under a compaction force of 138 N/cm2 after potentiostatic polarization experiments.