Formation and protectiveness of Fe/Ca carbonate layer on X80 steel in high pressure CO2 corrosion environments

The effect of calcium ions (Ca²⁺) on corrosion of API 5L X80 carbon steel in carbon dioxide (CO₂)-saturated brines was studied. Tests were carried out in brines containing 0, 1000 or 5000 ppm of calcium ions with a constant chloride ion concentration, at temperatures of 35 and 60 ^oC, and pressure of 80 bar. The corrosion rates were determined by mass loss, and the protective properties of the film was evaluated by carrying out 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 in the absence of 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 ^oC the mixed iron-calcium carbonate scale offered better protection when compared to the pure FeCO₃ scale counterpart. However, at 60 ^oC, where a thicker carbonate scale was formed, the increase of Ca²⁺ content had a minimal effect on the corrosion rate.