Influence of chloride and sulphate anions on the electronic and electrochemical properties of passive films formed on steel reinforcing bars

Reinforced concrete structures are suffering from the application of de-icing salts, particularly due to premature reinforcement corrosion. To understand and, thereby, identify solutions to this problem, a study has been conducted of the chloride- and sulphate-induced instability of the semiconductor passive films formed on carbon and 2304 stainless steel alloys in a simulated concrete pore solution environment. Steels' vulnerability to corrosion when depassivation occurs, typically in the presence of chloride and/or low pH, makes it essential to understand the nature of their passive films in the effort to mitigate the effect of corrosion. The influence of major alloying elements in five different reinforcing bars and testing on different surface roughness was investigated in the first part of the study. In this second part, linear polarization resistance and potentiodynamic cyclic polarization techniques and Mott-Schottky (M-S) analysis were used to investigate the influence of varying ionic concentrations and, thus, pH of simulated concrete pore solutions on passive films formed on duplex 2304 stainless steel and carbon steel reinforcing bars. It was found that chloride ions increased the defect densities of passive films, most significantly in the lower pH solution, while sulphates decreased the densities. This is attributed to the formation of stable solid sulphides of Fe and Ni (as FeS, FeS2, NiS, Ni3S2) in the outer Fe-rich layer of the passive films as predicted by ThermoCalc(C) calculations.