Corrosion and Stress Corrosion Cracking Behavior of Super 13Cr Martensitic Stainless Steel in CO2-Saturated High Temperature and High Pressure (HTHP) Downhole Environment

Abstract The service environments in deep/ultra-deep oilfields are particularly corrosive due to the high temperatures, chloride contents and CO2 partial pressures, which pose significant challenges to the integrity of tubing materials. In this research, we investigate the corrosion and stress corrosion cracking behavior of Super 13Cr martensitic stainless steel (S13Cr MSS) exposed to formation brine at 200°C with a 5.2 MPa CO2 partial pressure through immersion tests and slow strain rate (SSR) tests. The corrosion products composed of amorphous Cr(OH)3 and Cr2O3 were identified, which acted as a barrier to slow down the corrosion process at the material interface. The general corrosion rate of S13Cr MSS in a simulated downhole environment was 1.70 mm/y in the first 5 hours then decreased to 1.37 mm/y after 20 hours immersion and the general corrosion rate of 1.04 mm/year was recorded over 7 days exposure. Meanwhile, the pitting rate was high (2.27 mm/year) and the corresponding pit depth was 1.30 μm after 5 hours of exposure. After 7 days, the pitting rate was reduced to 0.37 mm/y. The results of SSR tests indicate that the fractures of S13Cr MSS initiated from the pits and were characteristic of ductile behavior at 200°C. Moreover, the stress corrosion cracking susceptibility of S13Cr MSS increased for the pre-corroded sample, suggesting that the increase in the level of stress concentration is due to the decrease in radius to depth ratio of pits during the corrosion progress.