The effects of coating thickness and substrate roughness on corrosion behavior of electroless Ni-P deposits on Al5083 alloy

In the present research, electroless Ni-P deposits were obtained on Al5083 substrate and the effects of the coating time on morphology, phosphorus content and corrosion behavior of the deposits were investigated. The scanning electron microscopy (SEM) of specimens obtained in various coating times, were studied and compared in their surface morphology, thickness and uniformity of the coatings. Surface morphology of the coatings exhibited a nodular feature with a typical cauliflower-like structure. Corrosion resistance of the coatings was evaluated in 3.5 wt.% NaCl solution by potentiodynamic polarization and electrochemical impedance (EIS) methods. The results indicated that by increasing thickness of coatings, the number of porosities decreases and the corrosion resistance and coating thickness increases. The corrosion resistance of electroless deposits on a smooth substrate in comparison to a rough substrate is higher, probably due to the fact that by increasing substrate surface roughness, the number of defects and porosities of the coatings increas.