Corrosion inhibitive effects of calcium-modified zinc phosphate coating on A36 mild steel

Abstract

In this work, the corrosion inhibitive performance of calcium modified zinc phosphate coating on A36 mild steel was studied. Treated A36 mild steel samples were subjected to coating through immersion in phosphating bath containing varied concentrations of calcium oxide (0–2.5 g/L) at different operating temperatures (60–80 °C) and immersion time (30–60 min). The coated samples were then subjected to corrosion tests (weight loss and inhibition efficiency determination) in 3.5% NaCl solution, to examine the anticorrosion behaviour of the coated mild steel in NaCl solution. The calcium modified phosphating process which was carried out at 80 °C with calcium concentration of 2.5 g/L gave a better and more uniform coating on A36 mild steel surface, as confirmed by visual inspection and SEM/EDS analysis. Also, Freundlich adsorption isotherm was found suitable for the prediction of the adsorption of both calcium and zinc particles on A36 mild steel, as indicated by the coefficient of determination (R2) values of approximately one. The adsorption of both the zinc and calcium particles on A36 mild steel was found to be spontaneous and physical process, as revealed by the values of Kads and ΔGads.