Formation and Protectiveness of Fe/Ca Carbonate Layer on X80 Steel in High-Pressure CO₂ Corrosion Environments

The effect of calcium ions (Ca²⁺) on the corrosion of API 5L X80 carbon steel in carbon dioxide (CO₂)-saturated brines was studied. Tests were performed in brines containing 0 ppm, 1,000 ppm, or 5,000 ppm of calcium ions with a constant chloride ion concentration at temperatures of 35°C and 60°C, and pressure of 80 bar. The corrosion rates were determined by mass loss, and the protective properties of the film were evaluated by performing electrochemical measurements in a separate vessel containing a standard brine. The results showed that adding Ca²⁺ to the brine slightly reduced the average corrosion rate, even without a crystalline corrosion product scale. For longer exposure times, it promoted the growth of a mixed iron-calcium carbonate (Fe_xCa_yCO₃) scale with increasing calcium molar mass, shifting the scale morphology from prismatic crystals (pure FeCO₃) to globular (mixed carbonate). At 35°C the mixed iron-calcium carbonate scale offered better protection when compared to the pure FeCO₃ scale counterpart. However, at 60°C, where a thicker carbonate scale was formed, the increased Ca²⁺ content had a minimal effect on the corrosion rate.