Film formation on non-noble metals

The electrochemical fihn formation by oxypolymerization was discussed for inert electrodes Uke Pt or Au. A stable and adherent film can be formed on inert metals. But on metals Uke iron undergoing anodic dissolution during the anodic oxidation process the preparation is more complicated. 

In case of iron or steel, anodic or chemical oxidation is accompanied by dissolution of the metal. The oxidation potential of most monomers is in the region of metal dissolution. Therefore, strong corrosion inhibits film formation. Formation of passivating oxide films, which might be possible at a higher potential, will also be delayed or inhibited. 

In the literature several ways to avoid or minimize the corrosion have been described. 

Beck et al. used oxalic acid to form a layer of iron oxalate to prevent the dissolution and support the formation of the polymer layer. ° Ferreira etched the steel surface with a dilute nitric acid to form an iron nitride layer, which inhibited the iron dissolution. 

For iron and steel parts, a different film preparation was developed that was based on adhesion promoters as described in Section 11.3. After formation of the SAM layer in a solution containing the adhesion promoter, the electrode was transferred into a cell filled with a solution of the monomer molecules and a supporting electrolyte. Then an anodic current pulse was applied for some seconds or minutes. A fihn of poly thiophene formed on the iron or steel parts with a thickness depending on the pulse time. Oxidation of the iron or steel was avoided by this procedure. The film had a compact structure, as was shown by SEM pictures. The adhesion promoter film provided exceUent protection against delaminating. 

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