THE ROLE OF THIOSULFATES IN THE PITTING CORROSION OF FE-17CR ALLOYS IN NEUTRAL CHLORIDE SOLUTION - ELECTROCHEMICAL AND XPS STUDY

The role of thiosulfates in pitting induced by chlorides on Fe-17Cr al loys in neutral medium was investigated by electrochemical and XPS tec hniques. The concentration was 30 ppm S2O32- in 0.02 M NaCl. The studi ed alloys contained different contents and kinds of sulfide inclusions (Ti or Mn sulfides). Electrochemical experiments (i-E curves and pitt ing probability measurements) show the deleterious effect of thiosulfa te addition. The detrimental effect of the thiosulfates on the pitting resistance increases with increasing sulfur content in the alloy. The Ti-stabilized alloy exhibits a better pitting resistance than the Nb- stabilized alloys. For the XPS study, an ultra-pure Fe-17Cr alloy was used. The XPS study was performed after (a) immersion of the alloy (cl eaned by argon sputtering) in the solution 0.02 M NaCl +30 ppm S2O32- and (b) addition of the thiosulfates after passivation in 0.02 M NaCl. The XPS results give the evidence that the thiosulfates are reduced o n the metallic surface whereas they do not interact strongly with the passive film surface. Sulfide islands are formed at the bare alloy sur face and prevent repassivation. (C) 1997 Elsevier Science Ltd.