The electrolyte renewal effect on the corrosion mechanisms of API X65 carbon steel under sweet and sour environments

Abstract

Abstract Sweet and sour services are the most common media found inside the pipelines of petroleum industry. They are one of the main causes of degradation by corrosion that lead to several financial and environmental losses worldwide. They also are responsible for the formation of corrosion products that might play a protective feature on the metal surface diminishing the corrosive processes. However, conditions in the field are not necessarily favourable for the precipitation of protective structures given the physicochemical dynamism inside the pipes. Then, the need to address this issue that occurs inside the tubes becomes crucial for the correct assessment of the development of protective corrosion products hence to give a more realistic approach to the lab conditions. Therefore, this paper presents a methodology to investigate the electrolyte renewal effects on API X65 carbon steel corrosion under CO2 and H2S (thiosulfate) environments at 4,5\xa0bar of pressure, temperatures of 90\xa0°C and 120\xa0°C over 150\xa0h. Electrochemical techniques (LPR and EIS), weight loss and characterizations were conducted in two different average pH, 3,3 and 4,5. The proposed methodology was capable to delay the development of protective scales in renewed media. The corrosion rates were higher for conditions with electrolyte renewal and higher temperature. FeCO3 formation was favoured at high pH and 120\xa0°C whilst FeS structures were dominant at 90\xa0°C. Sulphides were more cracked and uneven in renewed media whereas carbonate crystals presented higher anchorage to the substrate and were able to develop in sizeable diameters. Unrenewed media presented the best electrochemical responses.