Appropriate material selection for high temperature and high hydrogen sulfide/carbon dioxide (H2S/CO2) partial pressure environments within oil and gas wells is essential for a robust corrosion management strategy. Corrosion resistant alloys (CRA) are routinely considered as an alternative to carbon steel for oil and gas applications under such conditions. However, published research relating to CRA passive layer composition and their protective properties in combined H2S/CO2-containing environments is limited, particularly in the presence of chloride ions. This study focuses on understanding the pitting corrosion mechanisms of solution annealed and precipitation hardened UNS N07718. This paper provides an in-depth analysis of the microstructure of both specimens using XRD, SEM/EDX and TEM. Corrosion experiments are conducted in simulated environments analogous to the PuGuang field at 80°C and the testing brine containing chloride and sulfate ions. A combination of elevated temperature/sour gas conditions with electrochemical methods are employed to evaluate the corrosion performance and particularly its localized corrosion performance under the influence of H2S and CO2 partial pressures. Results from potentiodynamic polarization experiments showed an anodic branch that exhibited a complete passivation for both UNS N07718 samples.
Extremely corrosive environments of today's oil and gas exploration requires more expensive Corrosion Resistant Alloys (CRAs) to be used for equipment such as tubulars. Material selection for oil and gas wells, especially those containing high hydrogen sulfide (H2S) and carbon dioxide (CO2) partial pressure, is very crucial. Hydrogen sulfide and carbon dioxide are quite aggressive to the materials used in oil field environments. The material of choice for these oil wells has to be reliable and cost-effective. This makes the material selection process a very complex and difficult task involving both financial and safety risks analysis.
Although adding corrosion inhibitors along with carbon steel tubulars was employed as a method to control corrosion in some of the fields containing high concentration of sulfate and chloride ions, it was not found reliable especially at elevated temperatures. Another concern is related to the high cost of the effective inhibitors and the high amount required to be continuously dosed. Increasing wall thickness of the carbon steel manufactured tubes was another alternative which has its own disadvantages such as increased weight and reduced interior dimensions. Therefore, employing CRAs has been preferred as an alternative for manufacturing oil and gas well components. This will be useful considering the life-cycle economy and safety regulations of the industry.