Electron-Transfer Reactions on Passive Chromium

The kinetics of electron transfer between electrochemically passivated chromium and several outer-sphere r dox couples have been investigated as a function of passive-film thickness, d = 5 to 25/~, in 1M H2SO4. The apparent rate of electron transfer decreased exponentially with increasing thickness of the passivating overlayer. Similarly, during poten-tiostatic film growth, the electron-transfer rate decreased logarithmically with time, t, as a consequence of'the film-growth kinetics, since d varies with log(t). These relationships indicate an electron-tunneling controlled process. However, varia-tions in the film thickness and defects result in electron transfer occurring at distributed sites of enhanced tunneling probability, making a meaningful quantitative analysis difficult. Chromium plays an essential role in the corrosion resis-tance of many engineering alloys. Protection results from the formation of a thin,-5-25 /~, passivating oxide film. The electrochemical characteristics of this electrode are determined by reactions involving both ions and electrons. The passive film blocks ion-transfer processes associated with metal dissolution. The kinetics of film formation indi