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

The potentiodynamic anodic polarization behaviour of AISI 304 stainless steel (SS) in 0.1 M NaCl containing various concentrations of SO42-, SO32-, SCN- or S2O32- anions has been studied. In pure chloride solutions, the alloy exhibited pitting corrosion. The addition of low concentrations of the sulphur containing anions (except SO42-) decreases the pitting potential. The sulphate anion appears to increase the pitting potential. The electric charges associated with the pitting initiation and propagation processes were calculated when the steel was immersed under potentiostatic potential of +320 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 temperature independent in chloride and chloride containing sulphur anions in the range from 40 to 70 degrees C. Scanning electron microscopy (SEM) observations indicated that open pit with 250 mu m in diameter produced on the steel in 0.1 M NaCl under constant potential of +320 mV. While salt film formation inside open pits produced in the presence of sulphur containing anions 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.