Pitting of AISI 304 stainless steel in bicarbonate and chloride solutions

Metastable and stable pitting on 304 stainless steel (SS304) in 0.1-0.5 M NaCl aqueous solution (pH similar to 8) has been investigated using a potentiodynamic, a potentiostatic and a weak anodic current galvanostatic techniques. The addition of bicarbonate (0.025-0.5 M) to the solution had an inhibiting effect manifested by a shift of the pitting potentials to more positive values, a longer induction time for pitting and reduced pit nucleation. The pit nucleation frequency and growth, in the metastable and stable states, decrease as the NaHCO3/NaCl molar concentration increases and pitting is no longer observed for NaHCO3/NaCl molar ratios higher than 3 (four times larger % wt of NaHCO3). Pitting events never occurred at a potential below -0.1 V, fewer metastable events were observed at higher temperature (50 degrees C) and none were observed in chloride free bicarbonate solutions. The inhibiting behaviour of oxyanions is discussed.