Pitting corrosion is a significant concern for the broader application of stainless steel in modern industries in which metal and metal alloy are detached preferentially from susceptible parts on the surface, resulting in the creation of holes in passivated alloys that are exposed to an aqueous, neutral electrolyte containing corrosive species. Exposure of SS to brines leads to the localized loss of surface passivity and the onset of isolated pitting, which render the equipment or piping unfit for service. In the present study, the passive layer behavior and the pitting corrosion of the modified martensitic stainless steel (MMSS) were evaluated in a saturated CO2 environment (pH~5) with different NaCl concentrations and temperatures, using various electrochemical techniques. It was found that by increasing the temperature up to 60 °C, the corrosion resistance of the MMSS increased; however, the corrosion rate dramatically increased at 80 °C, indicating the destruction of the oxide layer. According to the point defect model (PDM) results, the calculated values of polarizability (α), metal cation diffusivity (D), and the rate of annihilation of cation vacancies (jm), reveal a strong dependence on the solution temperature.

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The limits and nature of the passive behavior exhibited by modified martensitic stainless steels (MMSS) is studied in brine solutions at different temperatures using electrochemical methods and the point defect model (PDM), in an environment at pH is 3 and temperatures up to 150°C, using autoclaves with CO2 gas up to 2.8 MPa. The results show that passivity on the MMSS is characterized by the metal cation vacancy diffusivity (D) which indicates that the passivity up to temperatures of 75°C is more stable than at higher temperatures up to 150°C. The value of D is used to describe the transition between the passive layers at the threshold temperature of 75°C; at which exhibits the largest stability. MMSS specimens passivated at 75°C exhibit higher passivity stability in experiments performed at 25°C, when compared the pre passivation, suggesting that the change experienced at 75°C is at least partially irreversible.