Iron Calcium Carbonate Instability: Structural Modification of Siderite Corrosion Films

Corrosion research related to CO₂-containing environments has focused over the past few decades on siderite formation (FeCO₃) as a main corrosion product on carbon steel, yet the influence of Ca and other ions on its chemical and structural characteristics is not fully understood. Metal-localized corrosion is the biggest industrial challenge because of the unknown and unpredictable character of this phenomenon that frequently leads to failure. We report here the role of Ca and formation of iron-calcium carbonate (FeCaCO₃) through a spiral growth model as in the calcite system and quantify the replacement of Fe²⁺ by Ca²⁺ ions in the structure of FeCO₃ to form FeCaCO₃. The incorporation of Ca²⁺ inhibits the completion of spiral segments on the growth of the rhombohedral crystals of FeCO₃, promoting an enlargement of its structure along the c-axis. This leads to distortions in the chemical structure and morphology affecting the chemical and mechanical properties. Under flow conditions over time in an undersaturated environment, Ca is leached out from the expanded structure of FeCaCO₃ increasing the solubility of the crystals, weakening the mechanical properties of the resulting corrosion films and stimulating localized corrosion.