Electrochemical quartz crystal microbalance study of the transient response of passive Fe-25Cr alloy

The transient response of a passive iron-chromium alloy is studied by apply ing an anodic potential step and monitoring the resulting current transient and mass change. The Fe-25Cr model alloy used in this study is sputter dep osited on AT cut quartz crystals which are connected to an oscillator circu it. The change in resonance frequency with time resulting from metal dissol ution or film growth is monitored and converted into a mass change using th e Sauerbrey equation. A formalism is developed for the interpretation of:cu rrent and mass transients observed with binary alloys in the passive state. Two selectivity ratios are defined which characterize the reactions at the metal-oxide and oxide-solution interface, respectively. Further quantities of interest are the dimensionless mass/charge ratio determined experimenta lly and the dissolution ratio which characterizes the relative importance o f dissolution and film formation for a given species. It is found that in a cid solution a potential step applied to Fe-25Cr in the passive region lead s to a mass loss due to preferential dissolution of iron. As a consequence chromium becomes enriched in the passive film. In 0.1 M NaOH, on the other hand, iron does not dissolve readily and the electrode exhibits a mass gain upon application of a positive potential step. XPS analysis of the passive films qualitatively confirms the conclusions derived from EQCM experiments . (C) 1999 Elsevier Science Ltd. All rights reserved.