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 usin g a potentiodynamic, a potentiostatic and a weak anodic current galvan ostatic techniques. The addition of bicarbonate (0.025-0.5 M) to the s olution had an inhibiting effect. manifested by a shift of the pitting potentials to more positive values, a longer induction time for pitti ng and reduced pit nucleation. Tile pit nucleation frequency and growt h, in the metastable and stable states, decrease as the NaHCO3/NaCl mo lar concentration increases and pitting is no longer observed for NaHC O3/NaCl molar ratios higher than 3 (four times larger % wt of NaHCO3). Pitting events never occurred at a potential below -0.1 V, fewer meta stable events were observed at higher temperature (50 degrees C) and n one were observed in chloride free bicarbonate solutions. The inhibiti ng behaviour of oxyanions is discussed.