Electrochemical corrosion behavior of pulse and DC electrodeposited Co-P coatings

Cobalt phosphorous (Co-P) coatings were electrodeposited from cobalt sulfamate bath using direct current (DC) and pulse plating methods. Four different baths were prepared using 0, 2, 5 and 10 gpl sodium hypophosphite (NaH2PO2) concentrations. Energy Dispersive Analysis of X-ray (EDX) was carried out to find the composition of the as-deposited Co-P coatings. It was found that coatings deposited using 2, 5 and 10 gpl NaH2PO2 concentration by pulse plating had 7, 9 and 10 wt.% P respectively, whereas, DC deposited samples showed 2, 9 and 10 wt.% P respectively. Up to 5 gpl of NaH2PO2 concentration the phosphorous content increases and thereafter has no significant influence on it. The electrochemical corrosion behavior of Co-P coatings was characterized in non-deaerated 3.5% NaCl solution at room temperature condition using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The potentiodynamic polarization data obtained for Co with 9 wt.% P exhibited lower corrosion current density and higher polarization resistance for both pulse and DC deposited samples. Nyquist and Bode plots obtained from the EIS measurements for DC deposited coatings showed single time constant whereas, pulsed deposited coatings exhibited double time constant behavior. Higher charge transfer resistance (Rct) and lower double layer capacitance (Qdl) values were obtained for pulsed deposited coatings. The overall corrosion rate was lower for the pulse deposited coatings than the DC deposited coatings. Surface morphology of the coatings before and after electrochemical corrosion tests was investigated using Field Emission-Scanning Electron Microscopy (FE-SEM) to understand the mechanism of corrosion.