With the increasing global demand for petroleum products, the oil and gas industry has started targeting production from deeper wells under more challenging conditions. High temperature corrosion is an increasing problem with the shift towards more unconventional reservoirs. The corrosion behaviour and performance of materials can change substantially with variations in temperature and other harsh environmental conditions. Corrosion Resistant Alloys (CRAs) are one method to combat the environmental stresses associated with these aggressive environments, including high H2S content. An understanding of the operating performance for these materials under demanding conditions has to be determined in order to reduce failures, improve corrosion performance and maintain structural integrity. In sour environments, when the H2S gas is dominant, the pitting behaviour of CRAs changes rapidly. The breakdown and re-passivation processes of the passive film are strongly influenced by the existence of H2S gas in the environment. In this study, four CRAs have been tested, which are Super 13Cr SS, 316L SS, 25Cr DSS and Inconel 825 under different temperatures including 25°C, 80°C and 150°C in CO2 and 90% CO2/10% H2S environments with 3.3 wt.% NaCl. These four CRAs have different Pitting Resistance Equivalent Number (PREN) and are common for sweet and sour services. The effects of CO2, H2S and temperature on the passive film have been determined and analysed using the cyclic polarisation technique in short-term experiments. A combination of Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Surface Profilometry (NPFLEX), Microscopy (Leica Microscope), and Transmission Electron Microscopy (TEM) techniques have been used to further analyse the surface, and passive film characteristics and chemical compositions under different surface conditions (wet-ground surface with passive film formed in air and pre-pitted surface which was polarised in the testing environment prior to the main polarisation test to study the behaviour of the passive film formed in the environment with pits existed on the surface). Under CO2 environments, S 13Cr SS showed low resistance to pitting which decreased marginally as the temperature was increased to 150°C. 316L SS had good resistance to pitting under 80°C, however, the susceptibility of the material to pitting decreased substantially and more pitting was observed on the surface. The re-passivated layer on 316L has better resistance to pitting compared to the passive film formed in air when polarised after 48 hours of exposure to the 3.3 wt.% NaCl, CO2 saturated under 25°C and 80°C according to the surface profilometry data. The inner layer of the passive film formed in air on 316L SS is mainly in form of Cr2O3, and the outer layer is in form of Fe2O3. Both 25Cr DSS and Inconel 825 specimens showed high resistance to pitting under both temperatures. Although 25Cr DSS has higher PREN (42.5) compared to Inconel 825 (33.5) . When the temperature increased to 150°C, 25Cr DSS showed a marginal drop in breakdown potential and Inconel 825 had higher breakdown potential. The increase in temperature has a negative impact on the resistance of CRAs to pitting and the breakdown of the passive film. The addition of H2S to the environment has increased the susceptibility of 25Cr DSS to pitting corrosion, where Inconel 825 had better resistance to pitting. The re-passivated layer in the H2S environment has better resistance compared to the passive film formed in air after the environment is switched to CO2 saturated.
