Studies on the physical and electrochemical properties of Ni-P coating on commercial aluminum as bipolar plate in PEMFC

Abstract

AA6061 samples were coated with electroless Ni-P after a pretreatment which consisted of either single, double or triple zincating. It was observed that increasing the number of zincating reduces the amount of zinc on the surface, and this has a direct effect on the initial rate of deposition of the Ni-P coating, which was confirmed through pH measurements. The corrosion performance of uncoated and coated AA6061 was evaluated at open circuit conditions with conventional electrochemical techniques in 0.5 M H2SO4 solution bubbled with H2 or O2 to simulate the anodic and cathodic environments respectively of a Proton Exchange Membrane Fuel Cell (PEMFC). In both environments the corrosion rate was lower in the presence of Ni-P coating compared to uncoated AA6061 and it also decreased while increasing the zincate pretreatment steps. This latter effect was attributed to higher P content and amorphous nature of the coating. Furthermore, it was observed that for all coated samples the corrosion rate was higher in the simulated cathodic environment. Potentiostatic tests were also carried out to further discuss the stability of the coatings and the results showed that the measured current density followed the same trend as the corrosion current density. It decreased with increasing number of zincate steps in both environments. Also, measured currents were lower in the simulated anodic environment, confirming a better stability in the anodic side.