Sodium Silicate and Phosphate as Corrosion Inhibitors for Mild Steel in Simulated Cooling Water System

Abstract

Two inorganic inhibitors were used to protect mild steel in cooling water system (single and binary sodium silicate and phosphate). Four different concentrations of silicate and phosphate ions were used (1 × 10−3, 5 × 10−3, 1 × 10−2 and 0.2M) and (1 × 10−3,5 × 10−3, 1 × 10−2 and 0.1M), respectively, in addition to two mixtures of \({{\rm SiO}_{3}^{2-}}\) and \({{\rm PO}_{4}^{3-}}\). Corrosion parameters were measured by electrochemical method using potentiostat such as corrosion potentials, corrosion current densities and Tafel slopes to calculate polarization resistance. Protection efficiencies P% were calculated to know the best concentration of inhibitors; these results indicate that the best concentration of \({{\rm SiO}_{3}^{2-}}\) and \({{\rm PO}_{4}^{3-}}\) is 1 × 10−2M that gave P% 74.088 and 95.494, respectively, while \({{\rm PO}_{4}^{3-}}\) gave better protection than \({{\rm SiO}_{3}^{2-}}\) and its mixtures. The adsorption of \({{\rm SiO}_{3}^{2-}}\) and \({{\rm PO}_{4}^{3-}}\) on the steel surface in simulated cooling water obeys the Langmuir adsorption isotherm model.