On the kinetics of passive film formation of iron and iron - chromium alloys

Ellipsometric, surface analytical and electrochemical studies were conducted on the kinetics of passive film formation on iron and iron - chromium alloys in various electrolytes at room temperature. It will be shown that with increasing pH of the solution and increasing Cr content of the alloy the thickness of the passive film grows by a logarithmic time law in agreement with the Cabrera - Mott model. However, at low pH and low Cr contents the passive film is formed in a very short time and its thickness remains constant with time or even decreases in some cases. This experimental finding and the time dependence of galvanostatic transients are indicative of film formation by precipitation from the solution. The experimental results are compared with a simple model on solid film formation. The concept of precipitation provides an explanation for the abrupt change of the critical current density around a Cr content of 10 wt.%.