The effect of some sulphur compounds on the pitting corrosion of type 304 stainless steel

The potentiodynamic anodic polarization behaviour of AISI 304 stainless ste el (SS) in 0.1 M NaCl containing various concentrations of SO42-, SO32-, SC N- or S2O32- anions has been studied. In pure chloride solutions, the alloy exhibited pitting corrosion. The addition of low concentrations of the sul phur containing anions (except SO42-) decreases the pitting potential. The sulphate anion appears to increase the pitting potential. The electric char ges associated with the pitting initiation and propagation processes were c alculated when the steel was immersed under potentiostatic potential of +32 0 mV for 10 min in 0.1 M NaCl without and with 10(-2) M of the cited anions . The results indicated that SO32-, S-2(-), SCN- and S2O32- anions promote the chloride-pitting propagation kinetics. The pitting potential of 304 SS decreases linearly with increasing chloride concentration while it was temp erature independent in chloride and chloride containing sulphur anions in t he range from 40 to 70 degrees C. Scanning electron microscopy (SEM) observ ations indicated that open pit with 250 mu m in diameter produced on the st eel in 0.1 M NaCl under constant potential of +320 mV. While salt film form ation inside open pits produced in the presence of sulphur containing anion s with chloride. The pit morphology and its diameter depend on the type of sulphur containing anion present. (C) 2000 Elsevier Science S.A. All rights reserved.