Mild steel corrosion control through locally sourced calcium oxide on zinc phosphating process

Abstract

Mild steel is vulnerable to corrosion, so its prevention is essential in order to sustain its wide applications in petroleum, food, electrical, chemical, and construction industries. This study investigated the corrosion control action of the locally sourced calcium oxide on the zinc phosphating process of mild steel. Varied concentrations of calcium oxide (0 – 2 g/L) and coating (phosphating) time (40–70 min) were considered during the phosphating of the pretreated mild steel samples at coating temperatures of 60 °C and 80 °C. The phosphate samples were subjected to weight loss tests utilizing a solution of NaCl (3.5 wt.%) at 25 °C, in order to determine both the sample corrosion rates and corrosion inhibition efficiencies. As both the calcium oxide concentration and phosphating temperature increased, the results trend (in general) showed a decrease in corrosion rate and increase in inhibition efficiency. Sample with the operating conditions of 70minutes coating time, 2.0 g/L calcium oxide in the bath at 80 °C coating temperature exhibited the best inhibition efficiency of 81% and lowest corrosion rate of 0.029 mm/year. SEM analysis revealed that the crystals of the coated surface became finer and had a more surface coverage owing to the increase in the concentration of calcium oxide (from 0.7 to 2.0 g/L). ΔGads values of close to -20 kJmol−1 obtained (from both Langmuir and Freundlich adsorption isotherms) revealed a spontaneous and physical adsorption process of calcium oxide on the sample surfaces. The calculated positive values of ΔHads and ΔSads established the phosphating (coating) process to be an endothermic process with increased randomness at the solid/liquid interface of the sample surface and bath solution.