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 stainless steel (SS, UNS S31603) was investigated in aqueous 0.5 M sodium chloride (NaCl) solution using potentiodynamic polarization, the abrading electrode technique, alternating current (AC) impedance spectroscopy combined with x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Addition of HCO₃⁻ ions to NaCl solutions extended the passive potential region in width and, at the same time, raised the pitting potential in value on the potentiodynamic polarization curve. Potentiostatic current transients obtained from the moment just after interrupting the abrading action 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 presence of HCO₃⁻ ions. The pit number density decreased with increasing HCO₃⁻ ion concentration. Moreover, addition of HCO₃⁻ ions to NaCl solutions retarded lateral pit growth, while promoting downward pit growth from the surface. The bare surface of the specimen repassivated preferentially along the pit 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 carbonate ([Fe,Cr]CO₃) that formed from preferential adsorption of HCO₃⁻ ions.