Modified 13Cr martensitic stainless steels (UNS S41426) are a class of materials used for operations involving natural gas production in sweet and moderately sour service conditions. Discrepancies between experimental results and field services have posed problems in identifying the window of service with experiments often overestimating these results. This false-positive is likely attributed to changes in passive film composition and stability with respect to temperature and H2S activity. Electrochemical tests were performed using a 1L autoclave holding 35 bar of CO2 gas with and without H2S at temperatures varied from 25 °C to 150 °C in salt brine solutions with a pH of 3.5. The point defect model approach is used in conjunction with potentiodynamic polarization and chronoamperometry to observe changes in cation vacancy diffusivity that contribute to strengthening and instability. These results indicate a critical point in temperature in the tests with only CO2, where resistance to pitting susceptibility is maximized due to a decrease in the cation vacancy diffusivity.
Modified 13Cr (UNS S41426) (M13Cr) are advantageous as components for wellbores in oil and gas upstream units due to their high strength capabilities and tremendous corrosion resistance in sweet environments with minimal H2S levels. However, previous studies speculate disparities between an overestimation in the application limits for the 110 ksi grade material. Previous experimental results associate this to microstructural differences from varying heat treatments. The proprietary procedures used to manufacture, emphasize a lack of quality control among suppliers.1
Most of the current studies on M13Cr have relied on mechanical testing methods in verifying windows of service under a range of Cl− activities, H2S partial pressures, and pH. Limited studies have been performed in investigating the electrochemical responses that contribute to passive layer breakdown, pit nucleation, and propagation while varying the aforementioned parameters across a broad range of temperatures to identify changes in stability of the passive layer.
