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

The passivation behaviour 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 asa 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-ox/Fe-ox + Cr-ox ratio were found to vary significantly. In particular, the films formed in the passive domain were more enriched with chromium compared with those grown in the secondary passive region. At a constant potential the temperature increase produced a reduction in the chromium 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. Fitting corrosion was observed only in the transpassive zone. A temperature increase from 23 to 60 degrees C did not produce a decrease in the pitting potential. This behaviour 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 increase in the process, which yield localized rupture of the passivity by the nucleation and growth of pits in connection with chromium depletion at high potentials. (C) 1997 Elsevier Science Ltd.