Prediction of CO2 Corrosion with the Presence of Low Concentrations Acetic Acid in Turbulent Flow Conditions

CO2 corrosion of mild steel with the presence of organic acids typically acetic acid (HAc) is a current concern in the oil and gas industry. However, this problem received relatively little attention in the corrosion analysis of oil and gas systems. Most of CO2 corrosion prediction models do not consider the effect of acetic acid species in corrosivity analysis which strictly limits the prediction to CO2 corrosion only. Recent studies have shown significant effect of acetic acid on CO2 corrosion but contribution of low concentration up to 60 ppm is not addressed. Thus, the objective of this research is to study the kinetics and mechanism of CO2 corrosion with the presence of low concentration of acetic acid in turbulent flow conditions at fixed pH and various temperatures. The electrochemical studies are based on linear polarization resistance (LPR) and potentiodynamic polarization. Rotating Cylinder Electrode (RCE) apparatus was used to simulate turbulent conditions producing shear stress representing pipe-flow condition. The experimental results are compared with the prediction by three openly available models: Norsok, Cassandra and de Waard Milliams models. Based on LPR results, low concentration of HAc below 40 ppm does not contribute much to corrosion rates. An appreciable increase in corrosion rate is observed for HAc concentration more than 40 ppm, whereby a maximum increase of 68 % at pH 5 and 120 % at pH 6 depending on temperatures. HAc increases corrosion rate by extra cathodic reaction which originated from the direct reduction and dissociation reactions. It is also observed that corrosion rate increases with increasing temperature. This is due to acceleration of anodic and cathodic reaction when temperature increases, also this is related with availability of more HAc species at higher temperature. Furthermore, an increase in corrosion rate due to the increase of rotational velocity is recorded in this study until 2000 rpm, beyond this not much effect of rotational velocity is observed. Flow effect is related to the transport of species towards and away from metal surface. Potentiodynamic polarization sweeps showed that the cathodic limiting current slightly increases with the presence of low concentration of HAc. There is no change in the mechanism of anodic reaction. The overall corrosion process is mainly controlled by a charge transfer process. It is indicated by corrosion current (icorr) value lower than limiting current (ilim). The
empirical prediction equation that considers the effect of low concentration of acetic acid is proposed based on LPR tests at pH 5 in turbulent conditions.