Effect of bicarbonate ion additives on pitting corrosion of type 316L stainless steel in aqueous 0.5 M sodium chloride solution

The effect of bicarbonate ions (HCO3-) on pitting corrosion of type 316L st ainless steel(SS, UNS S31603) was investigated in aqueous 0.5 M sodium chlo ride (NaCl) solution using potentiodynamic polarization, the abrading elect rode technique, alternating current (AC) impedance spectroscopy combined wi th x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Addition of HCO3- ions to NaCl solutions extended the passive poten tial region in width and, at the same time, raised the pitting potential in value on the potentiodynamic polarization curve. Potentiostatic current tr ansients obtained from the moment just after interrupting the abrading acti on showed the repassivation rate of propagating pits increased and that the pit growth rate decreased with increasing HCO3- ion concentration Over the whole applied potential, the oxide film resistance was higher in the prese nce of HCO3- ions. The pit number density decreased with increasing HCO3- i on concentration. Moreover, addition of HCO3- ions to NaCl solutions retard ed lateral pit growth, while promoting downward pit growth from the surface . The bare surface of the specimen repassivated preferentially along the pi t mouth and walls, compared to the pit bottom as a result of formation of a surface film with a high content of protective mixed ferrous-chromous carb onate ([Fe,Cr]CO3) that formed from preferential adsorption of HCO3- ions.