Effect of Concentrations of Fe²⁺ and Fe³⁺ on the Corrosion Behavior of Carbon Steel in Cl⁻ and SO₄²⁻ Aqueous Environments

The concentration of metal ions in aqueous environments significantly affects the formation of corrosion products and further metal corrosion. In this paper, the electrochemical behavior of carbon steel in the presence of Fe²⁺ or Fe³⁺ concentration in Cl⁻ and SO₄²⁻ aqueous environments have been investigated using conventional electrochemical methods such as linear polarization and alternating current impedance spectroscopy. The morphology and composition of carbon steel corrosion products were studied using scanning electron microscopy and laser Raman spectroscopy. The effects of corrosion products and iron ions concentration on the corrosion of carbon steel were discussed. Corrosion products of carbon steel in aqueous environments were γ-FeOOH, γ-Fe₂O₃ and a small amount of α-Fe₂O₃. The addition of Fe²⁺ affected the cathode reaction of the electrode reaction, and promotes the formation of γ-FeOOH and Fe₃O₄. And with the increase of Fe²⁺ concentration in the solution, the anode process of electrode reaction was suppressed. The addition of Fe³⁺ promoted the formation of γ-Fe₂O₃ and Fe₃O₄. Fe³⁺ affected the ionization of water, causing the pH of the solution to drop. Fe³⁺ undergoes a redox reaction with the matrix. Addition of Fe³⁺ ions promoted the formation of FeOOH, an intermediate product, which reacts with the anode product Fe²⁺. These factors all accelerated the corrosion process.