CO₂ Corrosion of Mild Steel Exposed to CaCO₃-Saturated Aqueous Solutions

The behavior of CO₂ corrosion of mild steel was studied in a CaCO₃ saturated aqueous solution. The bulk water chemistry ([Fe²⁺], [Ca²⁺], and pH) was controlled and the mass transfer characteristics inherent to the experimental setup were well defined. A combination of weight loss and electrochemical techniques, as well as surface characterization tools, were used to assess the behavior of corrosion products over long-term experiments. Three corrosion periods were identified based on the analysis of the corrosion rates and Nyquist response values versus exposure time. Residual Fe₃C was the early corrosion product; however, Fe_xCa_yCO₃ (x + y = 1) precipitated within its pores over the course of experiments where a favorable local water chemistry was achieved, regardless of the bulk solution. Results show that the presence of Ca²⁺ did not jeopardize the final protectiveness of corrosion products.