Study of the Possible Mechanisms of Steel Passivation in CO₂ Corrosion Available to Purchase

"Superprotective" corrosion layers, frequently observed in CO₂ corrosion tests, can be due to a passivation of the steel surface beneath the porous corrosion layers. By potentiodynamic scans on bare steel in $CO2/HCO3−$ solutions of various pH, an anodic peak and a passive domain are effectively observed. However the passivation pH of carbon steel lies within the pH range of the carbonate/bicarbonate buffer, and it should not normally be attained in the presence of CO₂.

Possible passivation mechanisms are then discussed. Two alternative ways are essentially open:

• the achievement of locally very high pH values, through the local disconnection of CO₂ from bicarbonate (due to the rate limiting hydration of CO₂ into H₂CO₃ and the resulting depletion of H₂CO₃ by cathodic reactions), followed by a further increase in pH due to the local generation of carbonate by the cathodic reduction of bicarbonate.

• at lower pH, the involvement of mill scale and rust layers containing Fe^III compounds. Then the permanent release of Fe³⁺ ions and their cathodic reduction can contribute to pass the critical current for passivation, when it cannot be supplied only by H⁺ reduction.