In-situ microgravimetric studies of passive alloys: potential sweep and potential step experiments with Fe-25Cr and Fe-17Cr-33Mo in acid and alkaline solution

The quartz crystal microbalance (QCM) has been applied to the measurement of weight changes taking place during potential sweep and potential step experiments in the passive potential region in an acidic and a basic electrolyte. The alloys Fe-25Cr and Fe-17Cr-33Mo were studied as well as pure chromium and iron. The metal and alloy films used were prepared by magnetron sputter deposition on an AT cut quartz crystal. The composition of passive films after anodic polarization was determined by X-ray photoelectron spectroscopy (XPS) depth profiling. In 0.1 M H2SO4 + 0.4 M Na2SO4 solution, both alloys exhibited a mass decrease when the potential was increased in the passive region. The mass decrease resulted from preferential dissolution of iron and led to an enrichment of the passive him in chromium oxide. The Fe-17Cr-33Mo alloy exhibited a higher dissolution rate in the passive region that the Fe-25Cr alloy. Below the potential for transpassive dissolution passive chromium, contrary to the alloys, showed a mass increase with increasing potential.. In 0.1 M NaOH passive Fe-25Cr exhibited a mass gain when the potential was increased. XPS showed that iron cations accumulated in the outer part of the passive film because iron oxide does not dissolve in alkaline solutions. (C) 1999 Elsevier Science Ltd. All rights reserved.