Effect of Incorporation of Calcium into Iron Carbonate Protective Layers in CO₂ Corrosion of Mild Steel

The two dominant cationic species in reservoir brine are sodium (Na⁺) and calcium (Ca²⁺). Calcite (CaCO₃) and siderite (FeCO₃) are isostructural, thus Ca²⁺ incorporates readily into the hexagonal FeCO₃ lattice, and vice versa. In aqueous carbon dioxide (CO₂) solutions where both Ca²⁺ and ferrous iron (Fe²⁺) are present (such as downhole gas reservoirs or deep saline aquifers after CO₂ injection), inhomogeneous mixed metal carbonates with the formula Fe_xCa_yCO₃ (x + y = 1) can form; their presence on steel has been hypothesized to lead to localized corrosion. During carbon steel corrosion experiments conducted in electrolytes containing high Ca²⁺ concentrations, inhomogeneous corrosion product layers with the composition Fe_xCa_yCO₃ (x + y = 1) were indeed observed, along with non-uniform corrosion. Determining relative molar fractions of Ca²⁺ and Fe²⁺ in Fe_xCa_yCO₃ is paramount to predicting the relative properties and stability of such mixed metal carbonates. Using Bragg’s law and equations to relate interplanar spacings to unit cell parameters, x-ray diffraction patterns yielded values for the mole fraction of Ca²⁺ in Fe_xCa_yCO₃. Procedures in the current study were designed to develop a range of specific corrosion product layers on mild steel samples. The compositional analysis of these surface layers was used to develop a relationship with the observed corrosion mechanisms. Experiments were conducted at constant chloride (Cl⁻) concentration with and without 10,000 ppm Ca²⁺ in stagnant conditions and for two different flow conditions. In stagnant conditions, localized corrosion was associated with the presence of Ca²⁺ and the inhomogeneity of the corrosion product layer. The corrosion attack became uniform when flow was introduced.