AN ELLIPSOMETRIC STUDY OF SURFACE-FILMS GROWN ON IRON AND IRON CARBON ALLOYS IN 0.05 M KOH

Ellipsometric and electrochemical studies were performed at room temperature in 0.05 M KOH on Fe and Fe-C alloys with up to 0.87% C. A rotating split-ring disc electrode was used to detect soluble Fe2+ and Fe3+ species formed on Fe. The results obtained suggest that at potentials below peak II of anodic polarization curves, the surface films consisted mainly of Fe(OH)2. At potentials comprising peaks II and III, films could be composed of Fe(OH)2, colloidal Fe(OH)3, and/or FeOOH. At higher potentials, films could be composed of FeOOH, Fe3O4, and hydrated Fe2O3. Medium carbon contents (about 0.4% C) increased the anodic dissolution rate of iron and lowered the complex refractive indices of surface films, whereas high carbon contents (about 0.7% C) hindered dissolution and increased the refractive indices. It is suggested that the beneficial effect of high carbon contents on the protective properties of the film is associated with an easier formation of magnetite in the presence of a carbon residue on the metal surface. © 1990.