Biofilm effect on the cathodic and anodic processes on stainless steel in seawater near the corrosion potential. Part 2 : oxygen reduction on passive metal

The electrochemical behavior of stainless steel in both abiotic and biotic seawater is studied and compared to the behavior of a gold electrode in the same environments. The distinction between stoichiometry and thermodynamics on one hand and mechanism and kinetics on the other hand is highlighted. The biofilm does not seem to directly affect the oxygen reduction process near the oxygen equilibrium potential and far from the oxygen diffusion-limit current. Also, hydrogen peroxide (H2O2) and manganese oxides do not seem to play a direct role at potentials > 300 mV vs saturated calomel electrode (SCE) in the oxygen reduction process. The oxidation of the organic material present in the biofouling with currents able to affect the value of the passive material corrosion potential is highlighted. The oxidation of the organic material in the biofouling at the surface is characterized by anodic currents with the same order of magnitude of the anodic currents corresponding to passive currents or higher. The oxidation of the organic material seems to affect the value of the corrosion potential of the complex electrode system comprising metal/passive film/biofouling/seawater. The oxidation of the organic material lowers the pH at the near-electrode surface, and it contributes to enhancing the kinetics of the oxygen reduction and to increasing the corrosion potential. The anodic oxidation of the same organic material gives an anodic process that must be added to the metal dissolution, and it contributes to lowering the measured corrosion potential. The role of the passive film is underlined, and its indirect effect on the mechanism of oxygen reduction through the dissociative adsorption of water is suggested.