Analytical kinetic study of steel corrosion inhibition by Na3PO4 in synthetic concrete solutions

The inhibitory effect of sodium phosphate Na₃PO₄ has been studied previously by gravimetric measurements, and the maximum efficiency was recorded at a concentration of 7.5 × 10⁻³ mol/L, it reached more than 70% in synthetic concrete pore solution. The influence of immersion time on this optimal concentration was followed by the same gravimetric method; the results obtained show that the inhibitory efficiency decreases slightly with time. In addition, two other parameters that condition the kinetics of corrosion inhibition have been treated, which are the pH of the initial time and final time solution as well as the variation of temperature on the evolution of the inhibitory efficiencies. The increase in temperature slightly decreases inhibitory efficiencies. It is found that the efficiency of sodium phosphate at 25°C reached a value of 69.28% for the optimum concentration of 7.5 × 10⁻³. While at 35 and 45°C, it attained 68.08% and 63.31%, respectively, for the same concentration. The thermodynamic study reveals that the adsorption of the inhibitory molecules on the metal surface follows the Langmuir adsorption model and that is a physical adsorption.