Corrosion, Passivity, and Pitting of Carbon Steel in Aqueous Solutions of HCO3–, CO2, and Cl–

Abstract

A new type of passivity in carbon (C) steels that occurs in the correct range of carbon dioxide (CO2) bicarbonate (HCO3–), sodium chloride (NaCl), and potential was discovered. C steels normally become passive at about -0.65 VSCE in sodium bicarbonate (NaHCO3) solutions. In sodium chloride (NaCl) solutions, pitting occurs above about -0.5 VSCE. These processes do not occur normally for C steels exposed to the water phase of sweet oil and gas wells. This type of passivity is disregarded for sweet wells with respect to carbonate concentration (CO32– region) and potential. The margin between the free corrosion potential and that of the chloride (Cl–)-induced pitting normally will be sufficient to prevent this type of attack. A breakdown of this new type of passivity that occurs in the correct range might explain localized corrosion of C steels in sweet wells. Corrosion mechanisms under various environmental conditions in aqueous systems with CO2, HCO3–, CO32–, and NaCl were reviewed.