The influence of temperature on the passivation behaviour of a super duplex stainless steel in a boric-borate buffer

The passivation behavior of a super duplex stainless steel has been investigated in a boric-borate buffer solution with and without chloride additions, at different temperatures, by means of potentiodynamic and potentiostatic techniques. X-ray photoelectron spectroscopy was used to detect surface compositional changes of anodic passive films formed as a function of electrode potential and temperature. The resulting passive films were all chromium enriched with respect to the bulk composition, and their thickness and Cr sub ox /Fe exp ox +Cr exp ox ratio were found to vary significantly. In particular, the films formed in the passive domain were more enriched with Cr compared with those grown in the secondary passive region. At a constant potential the temperature increase produced a reduction in the Cr content of the film. These findings can be explained by the different ionic mobilities and dissolution rates at the film/solution and film/alloy interfaces. Pitting corrosion was observed only in the transpassive zone. A temperature increase from 23-60 deg C did not produce a decrease in the pitting potential. This behavior can be explained by the parallel action of two different phenomena, the change in the composition and structure of the passive film and the velocity increases in the process, which yield localized rupture of the passivity by the nucleation and growth of pits in connection with Cr depletion at high potentials.