Filiform Corrosion Morphologies on Painted Aluminum Alloy 3105 Coil Material

Abstract

Filiform corrosion (FFC) morphologies were examined on a rolled Al 3105 (UNS A93105) aluminium substrate coated by wet polyurethane paint after various pretreatment processes. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of FFC filament cross-section morphologies, obtained by ultramicrotomy, revealed the relative importance of chemical and mechanical factors contributing to the propagation mechanism, the latter originating from the lift of corrosion products. Filaments were shown to propagate by formation of successive pits, in which the exposed intermetallic particles acted as cathodic sites. Oxygen reducing at these sites diffused to the sites from the tail end of the filaments. Chromating the surface as a pretreatment reduced filament growth by inhibiting coating disbonding at the tip as a result of good adhesion. Anodizing suppressed the pitting process by increasing protection to the metal substrate. Propagation on pretreated surfaces, although slow, required extensive attack of the substrate to reach a given length. Propagation on as-degreased surfaces, with an intact mill-finish metal surface and without the oxide sublayer, was enhanced by delamination of the coating along preferred paths, with less pitting of the substrate. The lifting effect combined with the quality of coating adhesion to determine the general shape of the filaments formed.