Zr₂N₂O Coating-Improved Corrosion Resistance for the Anodic Dissolution Induced by Cathodic Transient Potential

Developing a corrosion-resistant and electrically conductive coating on metallic bipolar plates is essential to mitigate the performance degradation induced by the high cathodic transient potentials (CTPs) in the start-up/shut-down (SU/SD) processes of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a zirconium oxynitride (Zr₂N₂O) coating prepared by atomic layer deposition was used to improve the corrosion resistance of 304 stainless steel (304 SS) toward anodic dissolution at various CTPs. Triangular potential pulses were applied to the specimens to simulate potential variations at the cathode side of the PEMFCs at SU/SD stages. Results show that the Zr₂N₂O coating can provide effective protection at a CTP as positive as 1.1 V versus Ag/AgCl. At all CTPs examined, the peak current density ( i_peak) extracted from the pulse test of the coated specimen (Zr₂N₂O/SS) is 2 orders of magnitude lower than that of uncoated 304 SS, indicating that the presence of the Zr₂N₂O coating remarkably increases the corrosion resistance for the anodic dissolution induced by CTPs. More importantly, upon increasing the CTPs, 304 SS experiences severe intergranular corrosion after 4050 pulses, whereas Zr₂N₂O/SS shows slight pitting corrosion. The quite low i_peak and the mitigated corrosion morphologies of Zr₂N₂O/SS confirm that incorporating oxygen into the protective coating for achieving a high oxidation resistance is a feasible way to restrain the anodic dissolution caused by high CTPs. Analysis of the electron energy level diagrams of the passive film suggests a protective coating with a wider valence band contributing to the improved corrosion resistance toward the transpassive dissolution.