Inhibition of pitting on carbon steel

Carbon dioxide and hydrogen sulphide pitting related failures are a prime concern in oil and gas internal pipelines. Often, these types of failure have a great impact on safety and environmental matters and can cause severe financial consequences to the operator. Chemical inhibitors have been used to provide effective protection for the asset whether it is due to localised corrosion or general corrosion. More often than not, the operating conditions determine the type of corrosion product film produced on the carbon steel and in tum influence the type of corrosion inhibitor to provide effective protection for the asset. In order to develop supenor localised corrosIOn inhibitors, a better understanding of the mechanisms of the action and interaction of pitting and the corrosion products are required. An extensive study on the corrosion film, pitting growth and inhibition of pitting corrosion of carbon steel X65 in sweet and sour conditions has been carried out using electrochemical methods and various post test surface analysis techniques. The results revealed that both phosphate ester and imidazoline inhibitors showed excellent performance to stop pit propagation in sweet conditions if the optimum concentration is applied. An in-situ monitoring technique for assessing real and artificial pit growth on X65 carbon steel surfaces in sweet and sour conditions has been developed to investigate the inhibitor action on the pits and surrounding corrosion films. Assessments of the surface chemistry around the pits by FTIR analysis revealed that there are two different mechanisms focused on inhibiting pitting corrosion. The first mechanism revealed that the inhibitor adsorbs at the pit site and is absent on the surrounding film and the second mechanism revealed that the inhibitor adsorbs both on the pit site and on the surrounding films.