Understanding the False Positive of Modified Martensitic Steels in Sweet & Sour Service Conditions Using a Point Defect Model Perspective

Abstract

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.