Influence of Chloride and Sulfate Ions on the Electrochemical Behavior of Carbon Steel in Highly Alkaline Media

This work explores the electrochemical behavior of carbon steel in highly alkaline solutions representative of concrete pore environments. The alkaline medium was prepared using NaOH, KOH, and Ca(OH)₂ to ensure conditions favorable to steel passivation. Chloride and sulfate ions were then added at different concentrations in order to study their individual and combined effects on corrosion behavior.

Electrochemical tests were carried out using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization measurements at several immersion times. In solutions with no chloride or low chloride content, the steel showed stable passive behavior. This was reflected by relatively steady OCP values, high impedance levels, large phase angles, and low corrosion current densities, all indicating the presence of a protective passive film in the alkaline environment. When the chloride concentration was increased, this passive state became less stable. A clear decrease in impedance and polarization resistance was observed, together with higher corrosion current densities, suggesting degradation of the passive film and the onset of localized corrosion.

Compared to chlorides, sulfate ions had a milder effect on corrosion but still influenced the electrochemical response by modifying the solution chemistry and the characteristics of the passive layer. In media containing both chloride and sulfate ions, the corrosion behavior was found to depend on their relative concentrations and the exposure time. In some cases, competitive effects were observed, while in others, the combined influence of the two ions affected the stability of the passive film and the overall corrosion kinetics.

Overall, the results highlight the key role played by aggressive anions in controlling the corrosion behavior of carbon steel in highly alkaline media. This study provides useful insight into corrosion processes in concrete-related environments and underlines the importance of ion composition when assessing the durability of steel in alkaline pore solutions.