THE INFLUENCE OF TEMPERATURE ON THE PASSIVATION BEHAVIOR OF A SUPER DUPLEX STAINLESS-STEEL IN A BORIC-BORATE BUFFER SOLUTION

The passivation behaviour of a super duplex stainless steel has been i nvestigated in a boric-borate buffer solution with and without chlorid e additions, at different temperatures, by means of potentiodynamic an d potentiostatic techniques. X-ray photoelectron spectroscopy was used to detect surface compositional changes of anodic passive films forme d asa function of electrode potential and temperature. The resulting p assive films were all chromium enriched with respect to the bulk compo sition, and their thickness and Cr-ox/Fe-ox + Cr-ox ratio were found t o vary significantly. In particular, the films formed in the passive d omain were more enriched with chromium compared with those grown in th e secondary passive region. At a constant potential the temperature in crease produced a reduction in the chromium content of the film. These findings can be explained by the different ionic mobilities and disso lution rates at the film/solution and film/alloy interfaces. Fitting c orrosion was observed only in the transpassive zone. A temperature inc rease from 23 to 60 degrees C did not produce a decrease in the pittin g 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 yi eld localized rupture of the passivity by the nucleation and growth of pits in connection with chromium depletion at high potentials. (C) 19 97 Elsevier Science Ltd.