Comparison of corrosion behaviors between SS304 and Ti substrate coated with (Ti,Zr)N thin films as Metal bipolar plate for unitized regenerative fuel cell

• A (Ti,Zr)N coating is deposited on SS304 and titanium substrates.
• This coating contains a nano-composite structure of TiN and ZrN phases.
• Effective improvement of corrosion resistance on substrates has been obtained.
• Coating on SS304 has better enhancement of corrosion resistance than that on Ti.
• It can be used for bipolar-plate material of the unitized regenerative fuel cell.

In this study, a (Ti,Zr)N coating consisting of nano-composite structure of the TiN and ZrN phases is deposited on SS304 and titanium substrates as the metal bipolar plate through the cathodic arc evaporation techniques. This improves the corrosion resistance of SS304 and Ti metal when used in a unitized regenerative fuel cell (URFC). The corrosion behaviors of these two substrates are compared. Scanning electron microscope, energy dispersive spectrometer, and X-ray diffractometer are used to characterize the microstructure, thickness, chemical composition, as well as crystalline and phase states of the deposited thin films. The potentiodynamic polarization of the (Ti,Zr)N coating is tested in a simulated URFC H2 environment, i.e., 0.50 M H2SO4 with 3 ppm NaF solution purging with H2 gas at 60 °C. Experimental results demonstrate that the (Ti,Zr)N coating improves the corrosion resistance of SS304 (about 215 times that of uncoated SS304) better than that of the Ti substrate (about 200 times that of uncoated Ti). The enhanced corrosion resistance is attributed to the nano-composite structure of TiN and ZrN, which has a dense and columnar microstructure that is impermeable to a corrosive medium. The coating of the (Ti,Zr)N layer has better corrosion resistance than the increased sheet resistance for both substrates. Therefore, the (Ti,Zr)N–SS304 and (Ti,Zr)N–Ti samples, specifically the (Ti,Zr)N–SS304 sample, are good candidates as the material for URFC H2 side metal bipolar plate.