Effect of carbonate in alkaline tritiated water on the oxide layer of S318.03 stainless steel

The electrochemical behavior of S318.03 steel in presence of tritiated water containing carbonate, was studied using cyclic voltammetry and electrochemical impedance spectroscopy to provide an indication of mechanisms and oxide layer modifications. Carbonate addition produces several effects. The corrosion potential changes and the potential of the active peak shifts slightly towards more negative values on increasing carbonate concentration. In addition, the active peak and the passive current are higher with carbonate. Depending on passive potentials and carbonate concentration, the passive oxide layer and its characteristics change showing the importance of the carbonate concentration in passivity. Impedance spectroscopy shows a Warburg impedance especially for the prepassive potentials, near the corrosion potential, indicating that there is ionic diffusion during the oxide formation. The diffusion coefficient inside the complex oxide layer was determined. The prepassivity shows two regions depending on potential. This indicates that there are definitely successive reactions with the oxide layer and that there are several processes occurring: formation of {Me (O [3]H) 2-oxide phase} then adsorption of CO 2− 3 with formation of two passive oxide layers in the prepassive region, and passivation with Me 2O 3 formation at the higher passive potentials. The oxide capacitance value is low showing that this is insulating. The vacancy diffusibility in the oxide layer is shown to play a major role. It is shown that S318.03 steel is partially protected in presence of carbonated oxide.