Corrosion Inhibition on Mild Steel using Zinc Phosphating Modified with Magnesium and Calcium Additives

Abstract

MS is a very versatile and useful alloy in metal industry, since it is available and affordable, but it is prone to corrosion. This study investigated MS corrosion inhibition by modifying Zn₃(PO₄)2 bath with MgO and, then, with combined MgO and CaO additives. Different phosphating times (40, 60 and 80 min) and T (65 and 80 °C) were considered. Ct from 0 to 1.8 g/L MgO and combined MgO/CaO additives were added to Zn₃(PO₄)2 bath. SEM analysis of the coated samples was carried out. The specimens were also immersed in 3.5 wt% NaCl, for WL testing. Obtained results showed that the highest IE(%) of 56% was obtained for the MS sample at a phosphating T of 65 °C and Ct of 1.8 g/L MgO additive. For combined MgO and CaO additive, an IE(%) of 32% was obtained, at 65 °C, with a Ct of 0.9 g/L. MgO and CaO adsorption process (separately and combined) onto MS followed Freundlich’s adsorption model. ΔG indicated a physical adsorption process, and ΔH indicated an exothermic adsorption process, while ΔS during coating revealed a decrease in the process randomness degree.