The corrosion behavior and mechanical properties of CrN/NiP multilayer coated mild steel as bipolar plates for proton exchange membrane fuel cells

This research mainly studies on the interfacial conductivity and corrosion resistance of mild steel (MS)/electroless plating nickel (EN)/CrN composite coating. The MS/EN/CrN composite coating is deposited on a mild steel substrate using a combination of electroless nickel plating process and closed field unbalanced magnetron sputter ion plating system. The simulative working environment of proton exchange membrane fuel cell (PEMFC) is implemented to test the electrical and corrosion properties of uncoated mild steel and coated samples. Results show that the performance of the MS/EN/CrN coated samples was greatly improved. The SEM results indicate that the surface of MS/EN/CrN is dense and smooth. CrN (111) and Ni (111) are observed in the MS/EN/CrN coating by XRD. The potentiodynamic polarization and EIS tests show that the incorporation of electroless nickel deposits into the CrN coating can provide better corrosion resistance. Potentiostatic polarization tests reveal that MS/EN/CrN coating has the lowest current density both in anodic and cathodic environment. The Interfacial contact resistance (ICR) results show that the ICR of MS/EN/CrN coated sample reduces to 3.3 mΩ cm2 at a compaction force of 140 N/cm2. Moreover, after potentiostatic polarization, the ICR values of the MS/EN/CrN coating are still less than DOE's 2020 target. In addition, compared with other samples, the hardness and adhesion property of the MS/EN/CrN multilayer coating were greatly improved, which is beneficial for resisting the shock and wear during the assembly process of fuel cell stacks.