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

Abstract

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, mixed metal carbonates with the formula Fe_xCa_yCO₃ (x+y=1) can form. This inhomogeneity may 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 inter-planar spacings to unit cell parameters, Xray diffraction (XRD) data yielded values for the molar fraction of Ca²⁺ in Fe_xCa_yCO₃. Procedures in the current experimental study were designed to develop a range of specific corrosion product layers on mild steel samples. Experiments were conducted at constant Cl- concentration with and without 10,000 ppm Ca²⁺ in stagnant conditions, 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.

Introduction

The effect of calcium cations (Ca²⁺) on the formation and protectiveness of iron carbonate (FeCO₃) layers in aqueous carbon dioxide (CO₂) corrosion of mild steel was discussed in a previous study.¹ It showed that the isostructurality of calcium carbonate (CaCO₃) and FeCO₃ allowed the incorporation of Ca²⁺ into the FeCO₃ structure; thus, the morphology and chemical properties of FeCO₃ were altered.

The importance of FeCO₃ formation on corrosion protection of mild steel has been well documented.^2-7 In a stagnant aqueous CO₂ solution, the water chemistry at the corroding steel surface is not the same as the bulk water chemistry. As a consequence of the corrosion process that consumes hydrogen (H⁺) and releases ferrous iron (Fe²⁺) to the solution, the pH and Fe²⁺ concentration increase adjacent to the steel surface. This leads to a higher degree of FeCO₃ saturation near the steel surface and a higher probability of protective FeCO₃ layer formation. However, in a turbulent well-mixed solution a corroding bare steel surface has almost the same water chemistry as the bulk solution, making protective FeCO₃ layer formation less probable.^2,6-9 In addition, at very high flow rates, there is a possibility of removal of protective FeCO₃ layers, leading to localized corrosion. ^8,9