The early stages of FeCO₃ scale formation kinetics in corrosion

In a carbon dioxide (CO₂) corrosive environment, when the product of the local concentrations of iron (Fe²⁺) and carbonate (CO₃²⁻) ions exceed the solubility limit, the precipitation of iron carbonate (FeCO₃) on the internal walls of carbon steel pipelines can significantly reduce the corrosion rate. In the following work, static glass cell corrosion experiments were conducted to understand the precipitation behavior in the early stages of FeCO₃ development and the factors favorable to protective film formation. The corrosion and precipitation rates were followed using a combination of mass gain, mass loss, electrochemistry and surface analysis techniques. At the conditions studied, the protectiveness and the rate of film formation was observed to vary significantly. The results indicated that the early stages of FeCO₃ precipitation consists of a complex simultaneous nucleation and growth process. FeCO₃ precipitation is shown to be dependent on surface species concentrations which can be significantly different to that of the bulk solution. Additionally, the role of crystal surface coverage and the blocking of actively corroding sites on the steel surface is examined and is shown to play a critical role in reducing precipitation kinetics at low levels of bulk super-saturation.