In-situ Mößbauerspectroscopic Investigations of Phase Transformations in Oxide Layers below Organic Coatings

Phase transformations in oxide layers below organic coatings have been investigated by in-situ Mossbauer spectroscopy. During wetting of preoxidized iron surfaces iron can further corrode without diffusion of O 2 through the polymer film. The anodic iron dissolution is compensated by cathodic rust reduction. The reactive rust component γ-FeOOH can be reduced to the spinel-phase Fe 3− δO-4 changing the rust morphology and causing the loss of adhesion of the organic coating. This reaction is controlled by the ionic conductivity below the organic coating which is caused e.g. by impurities adsorbed from the atmosphere before the application of the organic coating. ― In order to retard the redox reactions in oxide scales below organic coatings the oxide surface has been chemically modified e.g. by silanes, phosphates and phosphor-organic compounds. This decreases the rate of rust reduction significantly