Influence of oxide structure on the elevated temperature pitting behaviour of Fe-Cr-Ni alloys

The influence of temperature, potential, alloy and electrolyte composition on the growth of hot water oxide layers on stainless steel type AISI 321 and Fe-Cr-model alloys is presented. The correlation between the oxide layer properties and the pitting corrosion behaviour is discussed. In situ electrochemical investigations have been carried out in aqueous electrolytes (pH 8) on hydrothermal conditions varying from 150 to 250 C. After hot water exposition the steel surfaces were examined microscopically. The porosity of these layers increases with rising temperature due to a change in crystallinity from a fine-crystalline to a coarse-crystalline structure. It is shown that the pitting initiation mechanism is strongly dependent on the morphology of the oxide layers. At 150 C pitting corrosion occurs, whereas at 250 C the formation of extended hollow shaped corrosion has been observed. The cause of the detected inhibiting effect of sulphate ions on the chloride induced pitting is discussed. Furthermore, evident dependencies of the pitting susceptibility on the chromium content of the alloy and on the oxide layer preformation potential have been observed. (orig.)