Study On Properties Of Nitriding Layer And Cr_xN_y Coating On Stainless Steel Bipolar Plate For PEM Fuel Cell

Proton exchange membrane fuel cell（PEMFC）is widely used because of its high power density,low operating temperature,high efficiency and zero pollution.Although PEMFC bipolar plate,stainless steel has many advantages in strength,cost and processability.But stainless steel is easy to be corroded in PEMFC working environment.In order to promote the commercialization of stainless steel bipolar plates,surface modification is necessary.In this paper,the surface modification of316L stainless steel by plasma enhanced balanced magnetron sputtering technology was studied.nitriding layer,Cr/CrN_x coating and Cr/Cr₂N coating were prepared on316L surface.The effects of deposition parameters on the morphology,phase structures,contact resistance and corrosion resistance of the coating was studied.The results show that:The effect of substrate bias voltage on the morphology,phase structure,contact resistance and corrosion resistance of plasma nitriding layer on 316L austenitic stainless steel was studied.The surface of nitriding layer has sharp grain boundaries and extensive slip steps.Diffraction peak of nitriding layer broadenings,asymmetry,high index diffraction peak decreases,and diffraction peak shifts to small angle.No nitride precipitation is found at grain boundary.Nitrided at-500V,the surface morphology of the sample is relatively smooth,the lattice distortion is small,and the ICR value is 170mΩ/cm².The corrosion resistance of the sample in PEMFC working environment is good and the operation is stable.The passivation film was deposited on the nitriding layer surface by potentiostatic method for 1 hour at 0.6V film forming potential.The passivation film exhibited the characteristics of p-type semiconductor.Nitrided at-500V,the chemical reaction speed at the interface was slower and the corrosion resistance was better.The effect of filament current on the morphology,phase structure,contact resistance and corrosion resistance of Cr/CrN_x coating by Nitrogen and chromium codeposition method was studied.When filament current reachs 4A,the grain size is uniform and the lattice distortion is small.At this time,the ICR value of Cr/CrN_x coating is about 7.643mΩ/cm² and the coating composes of Cr and CrN.The corrosion resistance of the sample in PEMFC working environment is good and the Nyquist plot of the coating including charge transfer step and diffusion step at 4A.The coating passivation film transformes from p-type to n-type semiconductor.The effects of nitriding time on the morphology,phase structure,contact resistance and corrosion resistance of Cr/Cr₂N coating deposited by nitrided after Cr plated was studied.With the increase of nitriding deposition time,the grains changed from long cellular Cr to circular Cr₂N.There was no obvious pore at the junction between the cells,and the cells were smaller and without longitudinal voids.The peak of Cr（200）transformed to that of Cr₂N（111）and the peak of Cr（200）gradually weakened.When nitriding deposition time at 0 h,the Cr coating has good corrosion resistance,but the contact resistance is 567mΩ/cm²,which is about seven times as much as that of the Cr/Cr₂N coating.On the whole,when nitriding deposition time is4h,the coating has cylindrical structure,a strong Cr₂N peak,good corrosion resistance and stable operation.The ICR value is about 80mΩ/cm² at 4A.The passive film on Cr coating shows n-type semiconductor characteristics,while the passive film on Cr/Cr₂N shows p-type semiconductor characteristics.