Point Defect Equilibria and Diffusion in Siderite (FeCO3) Passive Film Studied Using Density Functional Theory

In a CO₂-rich anoxic aqueous environment as in some oil fields, a layer of siderite (FeCO₃) grows on carbon steel as a byproduct of steel corrosion. This layer acts as a resistive barrier that kinetically slows down further corrosion of steel. Understanding and predicting the rates of  progression of both general and pitting corrosion  requires a fundamental understanding of defect equilibria and diffusion in the passive film [1]. In this work we address this need by modelling point defect equilibria and mobility using a combination of density functional theory (DFT) and thermodynamic analysis. The defects of interest in this study are the native cation and anion vacancies and extrinsic chlorine defects whose concentrations vary depending on temperature and the chemical potential gradient from the steel/siderite interface (iron rich) to the siderite/water interface (carbonate rich). Defect formation energies were calculated using the DFT+U approach and defect mobility’s were computed the nudged elastic band method. Preliminary results indicate that iron vacancies are predominantly fully ionized (2- charge state), whereas oxygen vacancies are mainly neutral and carbon vacancies are never fully ionized (charge state less than 4-) as shown in Figure 1. In addition we identified hole polarons localized on iron cations to be a predominant electronic defect that participate in the equilibria of charged defects in this material.

The predictions of point defects thermodynamics and kinetics in FeCO₃ is major step toward an atomistics-informed corrosion model of carbon steel in oil fields.

[1] D. D. Macdonald, Pure Appl. Chem., 71, 951, (1999).