A model for the corrosion of steel subjected to synthetic produced water containing sulfate, chloride and hydrogen sulfide

A model was developed for the prediction of corrosion rates associated with steel subjected to synthetic produced water. The corrosive species included in the model, identified through water analysis conducted in the field, are sulfate, chloride and hydrogen sulfide. The effect on corrosion of these species was examined through polarization experimentation using a three electrode glass corrosion cell and potentiostat. Samples of carbon steel, used in sub-sea pipeline systems, were used at the working electrode and the experiments were carried out at similar physicochemical conditions observed in pipeline systems in the field. The model was based on heterogeneous reactions at the metal surface, with electrochemical parameters determined through experimentation employed in the model to describe the anodic and cathodic processes involved in the corrosion of steel. The model consists of a system of equations with Butler–Volmer kinetics describing the charge transfer and the Nernst diffusion model the mass transfer processes occurring in the corrosion system. The solution is based on a charge balance between the reduction and the oxidation processes which occur at the steel surface. Current density convergence criteria were used in the model to solve the system of equations for corrosion potential, surface species concentration and component current densities. The corrosion rate is determined as the rate of oxidation of iron at the surface and model results have been validated using experimental data. The model demonstrates a reasonable qualitative match with corrosion data collected in the potential region close to the corrosion potential in general, with good qualitative match in the anodic region near the corrosion potential. Some deviation occurs between model and experimental values where overpotentials become large but the model is shown to respond well to changes in input parameter values and predicts the corrosion potential and corrosion rate for each system within experimental variability and the accepted standards of accuracy.