International Journal of Hydrogen Energy, Vol.45, No.40, 20632-20646, 2020
Evaluation of the corrosion resistance of Ni(P)Cr coatings for bipolar plates by electrochemical impedance spectroscopy
In the present research, the corrosion resistance of Ni-P and Ni-P-Cr coatings on AA7075-T6 aluminum plates under simulated anodic and cathodic conditions of polymer electrolyte membrane fuel cells (PEMFC) has been studied by electrochemical impedance spectroscopy (EIS). Three Ni-P coatings 20 mu m, 30 mu m, and 40 mu m thick applied by electroless deposition were tested. Besides, a two-layer Ni-P-Cr coating 30 mu m thick was also analyzed. It was formed by an inner Ni-P layer, and an outer 10 mu m thick chromium one added by electroplating. Corrosion tests were combined with interfacial contact resistance (ICR), roughness, contact angle, and SEM-EDX measurements. The best results were obtained for the 20 mu m Ni -P and the two-layer Ni-P-Cr coatings, although the latter showed a high ICR value due to the high electrical resistivity of the chromium oxide surface formed. It was verified that coating degradation occurs when the electrolyte penetrates the micro-cracks and the nodular surface interfaces, reaching the base metal and causing the coating delamination.This behavior is associated with a sharp decrease in the polarization resistance (R-p) of the equivalent circuit model fitted to the EIS results. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.