THE ANODIC BEHAVIOUR OF IRON AND STEEL IN AQUEOUS SOLUTIONS WITH CO_2, HCO_3~-, CO_3~(2-) AND Cl~-

摘要

Laboratory experiments were carried out in deoxygenated brine with one bar CO_2 and different concentrations of hydrogen carbonate. The experiments were of short duration, usually less than two hours. The electrochemical techniques used were cyclic voltametry and electrochemical impedance spectroscopy (EIS). The cyclic voltammetry was performed with an AC modulated signal, so also impedance at a given frequency was obtained besides the DC results. The anodic dissolution rate below a pH about 6 took place with two different mechanisms. The change of mechanism presented itself in cyclic voltammetry by a maximum in the dissolution rate and a minimum in impedance. The anodic reactions did not follow Butler-Volmer kinetics. Impedance studies suggest that adsorption phenomena controlled the kinetics. For HCO_3~- concentration above 0.1 M carbon steel exhibited real passivity above a critical potential in chloride free solutions. When the potential was lowered in the back scan of cyclic voltammetry, significant changes of the film occurred, including reduction of iron in the passive film from the three-to two-valent state. Passivity was maintained despite large changes of the properties of the passive film. Chloride induced passivity breakdown and localized attack occurred above a pitting potential. However, the pitting potentials where this occurred were much higher than the corrosion potential of steels exposed to the formation water from oil-and gas wells.