Anodic films on electrodes

One of the earliest areas of electrochemical application of ellipsometry was in studies of films on metals in the passive state. The passive films that can be ellipsometrically studied in a relatively 

Passive films on metals such as iron, chromium, cobalt, nickel, gold, and platinum present a more subtle problem because the film thicknesses are only a few to several nanometers at the most and the refractive indices are complex (k O). Accurate determination of variations in A and ij/ with the film formation, rather than their absolute values, is necessary. Pioneers in this area include Kruger s and Bockris groups.Their work, as well as research since the 1970s by Sato, Ord, De Smet, and Szklarska-Smialowski, among others, opened a new way of investigating thin passive films in situ. 

Anodic films on platinum were analyzed with ellipsometry and related techniquesand by three-parameter ellipsometry. The film formed in sulfuric acid solution at potentials higher than about 1.0 V versus NHE was found to be extremely thin (2 5 A, depending on potential) and light-absorbing ( 2.8, 1.7). 

One of the difficulties encountered in studying anodic films is that in many cases the films are not completely removed by cathodic reduction. Once the iron, nickel, or cobalt electrode surface is subject to anodic potential to form passive layers, the ellipsometric readings on the surface usually do not return completely to the original values of the film-free surface on cathodic treatment, which indicates that a residual film remains. This makes it difficult to repeat the experiment on the same specimen of metal because there is no way of having a reference state assured. Szklarska-Smialowska et al reported that if EDTA is added to the electrolyte solution, complete removal of anodic film on iron is achieved on cathodic treatment. This was confirmed to work also 

Thickness (x) and the Optical Constants (it, k) of Anodic Films on Metab Determined by Three-Parameter Ellipsometry 

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