Kinetic and Thermodynamic Properties of Locally Sourced Calcium in the Zinc Phosphating of Mild Steel

Abstract

The present study is on the establishment of the kinetic and thermodynamic performance of the locally sourced calcium used in the zinc phosphating of mild steel substrates. Calcium oxide was obtained from the calcination of periwinkle shells (at 800 °C for 4 h) and then applied as an additive in the zinc phosphating bath formulation. Metal samples were coated in the bath at different temperature (50 °C and 70 °C), different coating time durations (35, 50 and 65 min) and different CaO concentrations (0, 0.7, 1.4 and 2.0 g/L). The coated samples were subjected to 3.5wt.% NaCl solution, for corrosion test. The surface morphology of phosphate coatings was examined using SEM coupled with EDX to establish the quality and uniformity of the coating on the mild steel. The addition of calcium in the coating process significantly enhanced the corrosion resistance of mild steel relative to uncoated specimens, as justified by the lowest corrosion rate results when calcium was utilized at 70 °C, 65 min and 2.0 g/L CaO concentration. Higher positive value of the equilibrium adsorption constant, Kads, obtained (2.857) at 70 °C compared to the value (0.139) at 50 °C indicated a higher and stronger affinity of the calcium molecules for the adsorbent surface, with Langmuir isotherm fitness of R2 of 0.9319. The feasibility of the electrochemical process and the spontaneity of the adsorption process were affirmed by the negative values of the change in Gibbs free energy obtained (−592.69 kJ/mol of Δ G at 50 °C and −335.51 kJ/mol of Δ G at 70 °C). −3.271 kJ/mol of Δs (negative change in entropy) implied that the particles actually move from a disordered state (liquid) to an ordered state (solid) and forming a complex compound on the metal surface. 16.431 kJ/mol of the Enthalpy change ( Δ H) revealed the amount of heat gained for the coating (adsorption) to take place. The significance of this study, as justified by the results obtained, is that the mass transfer coefficient, kf, is directly proportional to the calcium concentration and temperature, as observed with a good driving force of 1.9267 × 10− 7 kg/m2.s at CaO concentration of 2.0 g/L and coating temperature of 70 °C.