Effect of chromium content and sweep rate on passive film growth on iron-chromium alloys studied by EQCM and XPS

Growth of passive films on Cr and Fe-Cr alloys with Cr concentrations ranging from 15% to 54% was studied in situ With the electrochemical quartz crystal microbalance (EQCM). Mass change and current transients were measured as the response to a potential change in the passive region from 0 to 800 mV (SHE) in a pH 1.5 sulfate electrolyte. From these measurements, the thickness change as well as integral and differential growth fractions were calculated. The growth fraction expresses the fraction of oxidized metal that remains in the film. It was found that higher Cr contents gave thicker films and an increase in the growth fraction. The thickness change curves were compared to film growth models assuming rate control at either a film interface or in the film itself through high field conductivity. The integral growth fraction for the potential change experiment was found to vary approximately linearly with the Cr bulk composition. The fraction of dissolved Cr calculated from EQCM data matched well with previous solution analysis results. (C) 2002 Elsevier Science Ltd. All rights reserved.