Characterization and corrosion behavior of Co and Co–P coatings electrodeposited from chloride bath

Low and high phosphorous Co-P coatings are electrodeposited using direct current (DC) and pulse current (PC) methods from cobalt chloride baths. Low phosphorous coatings from DC and PC methods are crystalline and displayed predominantly fcc structure while the high P coatings are amorphous. Electrochemical corrosion studies show an increase in corrosion resistance with increase in phosphorous in both DC and PC plated coatings. PC coatings show higher polarization resistance (Rp) and lower corrosion current density (icorr) suggesting better corrosion resistance of these coatings. Compositional analysis shows a slight or no increase in P content after corrosion in PC coatings with high P exhibiting better corrosion resistance. Comparisons of surface morphologies before and after corrosion illustrate that PC plated coatings are less affected compared to their DC counterparts. Higher amount of oxidized cobalt in PC electrodeposited coatings is deduced from X-ray photoelectron spectroscopy (XPS) while mainly metallic Co species is found in DC plated coatings. This study demonstrates that the overall corrosion resistance for the PC deposited coatings is better than the DC deposited coatings owing to reasons like high P content, amorphous structure, smooth morphology and higher metal oxide content in the deposits