Electrochemical and XPS analytical investigation of the accelerative effect of bicarbonate/carbonate ions on AISI 304 in alkaline environment

Passivation behavior and surface chemistry of 304 stainless steel in alkaline solution with different HCO3(-)/ CO32- concentrations are studied with a combined electrochemical and XPS analytical investigation. Cathodic and anodic reactions are inhibited and accelerated, respectively, attributed to the depletion of acceptable oxygen and variation of passive film properties. Increasing HCO3(-)/ CO32- concentration does not alter the bilayer structure but leads to an intensification in electric field strength and composition changes including enrichment of oxidized Cr, increase of Fe(II)/Fe(III) ratio, and decrease of OH-/O2- ratio of the film. Meanwhile, the film thickness is reduced and the point defect density is increased due to the enhanced film dissolution. High HCO3-/CO32- concentration increases the critical chloride concentration of 304 SS in alkaline solution, attributed to the inhibition of the localized acidification within the pits and suppression of the stable pit development.