Failure analysis of a wet gas pipeline due to localised CO2 corrosion

CO2 corrosion is one of the main causes of failures of pipelines transporting wet gas. It can happen in general and localised form. In this study, the impact of the presence of iron carbonate scale which formed during process on CO2 corrosion behaviour of API 5L X65 carbon steel has been investigated by electrochemical measurements. The chemical composition and morphology of the scale has been studied via X-ray diffraction and scanning electron microscope. The scales were found to be iron carbonate scale with trace amount of iron sulphide. The result of electrochemical analysis revealed that the uniform corrosion rate of carbon steel decreases in the presence of iron carbonate scale. Furthermore, the bare and continuously covered sample with iron carbonated did not show susceptibility to localised CO2 corrosion. However, the presence of uneven iron carbonate scale or formation of thin, porous or defected scale can result in a potential difference between this area and adjacent area covered with compact scale followed by localised corrosion.