Ellipsometric Study of the Formation of Films on Iron in Chromate Solutions

Film growth on iron was studied by ellipsometry in aerated or deaerated chromate solutions of different pH's at open-circuit potentials and at various anodic potentials. It was found that various films can be formed depending on the experimental conditions. At high anodic potentials iron oxide is chiefly produced with an index of refraction of 2.6 (1-0.129. At open-circuit potential the iron and chromium hydroxides with the index of refraction of 1.6-1.7 or a mixture of iron and chromium hydroxides with iron and chromium oxide (with index of refraction of 1.6-1.9 and various absorption coefficients) is formed. The kinetics of the film growth at open-circuit potential is logarithmic, and the kinetics of iron hydroxide formation at constant potentials follow the cubic law.