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 FexCayCO3 (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 FexCayCO3 (x + y = 1) were indeed observed, along with non-uniform corrosion. Determining relative molar fractions of Ca2+ and Fe2+ in FexCayCO3 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 FexCayCO3. 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.