Steel Corrosion in Highly Carbonated Solutions Mitigated Using Biphosphate Ions as Inhibiting Agents

Abstract

The effect of biphosphate ions as potential inhibiting agents for constructed steel is analyzed in heavily carbonated solution (HCS) simulating concrete ([HCO3-] +[CO3=]) = 0.315 mol L-1). Three conditions are analyzed, always keeping pH=9: 1) a highly carbonated pore simulating solution (HCS), 2) HCS contaminated with NaCl 0.1 mol/L, and 3) HCS contaminated with NaCl 0.1 mol/L and incorporating biphosphate ions as inhibiting agent. In this last condition, three inhibitor dosages are tested, where the ratio [HPO42-] /[Cl-] is 0.2, 0.6, and 1. The results are compared to previous studies in solutions containing ten times less carbonate content. Several electrochemical and surface techniques are used to correlate inhibiting efficiency of biphosphate ions to dosage. The incorporation of biphosphate ions as inhibitors improves the resistance against localized corrosion. The passivating film is composed of a mixture of phosphates and oxides, mainly Fe(III) species. The inhibition efficiency is sensible to the content of biphosphate and carbonate ions in solution. The optimal dosage of Na2HPO4 is 0.06 mol L-1, where the inhibition efficiency evaluated over 6 weeks is higher than 99%. The benefit of using Na2HPO4 in proper concentrations as an efficient corrosion inhibitor is demonstrated for the experimental conditions under study.