In order to shed more light on the anodic reaction mechanism in CO2 corrosion of mild steel, two different kinds of electrochemical
measurements were used : potentiodynamic sweep and galvanostatic
measurements. Experiments were conducted in a glass cell at room temperature (T=22 +- 1 C), different CO2 partial pressures (0 -1 bar), over a broad pH range (2-7) using a rotating cylinder at 4000 rpm.. Distinct and different anodic mechanisms were observed for pH<4 and for pH>5. In the intermediate area there seems to be a transition from one mechanism to another. New orders of reaction and Tafel slopes were extracted, very different from what was previously assumed. A coherent ensemble of mechanisms was proposed for the anodic reaction which is consistent with the experimental results. Consequences of the present findings for the CO2 corrosion area are discussed. For example, in the case of undissolved iron carbide layers, it is no longer possible to discriminate the roles of galvanic coupling and internal acidification.
Keywords : CO2 corrosion, anodic reaction, mechanism, carbon steel.