Corrosion evaluation of Super 13Cr martensitic stainless steel under high temperature and high pressure downhole environments

The service environments of tubing materials in deep/ultra-deep oilfields and geothermal wells are particularly corrosive due to the extremely high chloride content, temperature and CO2 partial pressure, which pose significant challenges to the integrity of tubing materials. Super 13Cr martensitic stainless steel (S13Cr MSS) is widely employed in high temperature and high pressure (HTHP) downhole environments owing to its excellent mechanical properties and corrosion resistance. Nevertheless, its corrosion behaviour under extreme downhole conditions remains to be further investigation investigated to facilitate its application and modification.
The current study aimed to identify the failure modes of S13Cr MSS in HTHP CO2-saturated formation brine by HTHP immersion test, slow strain rate tensile test (SSRT), in-situ electrochemical measurements, and surface analysis technologies. The study also investigated the influence of environmental factors (temperature and CO2 partial pressure) as well as austenite content on the corrosion behaviours of S13Cr MSS and the evolution of corrosion product films.
This thesis demonstrates that S13Cr MSS faces a severe risk of uniform corrosion under HTHP condition (200°C/5.2 MPa CO2). The research suggests that the corrosion process occurs in three distinct stages: 1) Early stage: native passive film degradation; 2) Middle stage: localized preferential corrosion; 3) Final stage: uniform corrosion. Variations in CO2 partial pressure (0.27/5.2 MPa CO2 at 200°C) affected the precipitation behaviour of calcium carbonate, and altered the corrosion morphology during the middle stage as well as the composition of the corrosion product film. Increasing temperature did n’ot change the thermodynamic mechanism of corrosion, but kinetically accelerated each corrosion stage. The relationship between the native passive film degradation time and temperature was found to obey the Arrhenius law. Austenite, acting as a cathode, forms a galvanic couple with the martensite matrix, thereby accelerating the corrosion of S13Cr MSS. The austenite-free S13Cr MSS demonstrated the best corrosion resistance.