Corrosion performance of zinc phosphate coatings deposited on AA6061-HDG steel

Zinc phosphate conversion coating prior to spraying is essential in the automotive industry to enhance corrosion resistance as well as promote the adhesion of the subsequent top coatings. In this work, the corrosion performance of zinc phosphate coatings deposited on aluminium alloy (AA6061) and hot-dip galvanized (HDG) steel has been investigated in 3.5 wt% NaCl solution using potentiodynamic polarization method. The effects of bath parameters (i.e. concentration of Zn2+, and F− ions, temperature, and time) on the morphology of coatings are presented. Scanning electron microscope (SEM) attached to Energy Dispersive spectroscopy (EDS), were used to characterize the coatings. Surface roughness measurements of coated samples were examined with Atomic Force Microscope (AFM). The results show that Zn2+ did not have any significant effect on the coating and corrosion performance of the product mix of AA6061 alloy and HDG steel. Surface analysis of the coatings showed that nucleation and growth of phosphate crystals were faster on HDG steel than on AA6061 for bath conditions considered. The results indicated that phosphate coatings deposited at 45 °C in 118 mg/L fluoride for 90 s were more uniform and continuous on HDG steel and thus had the better corrosion performance in the investigated electrolyte than AA6061 alloy.