FILIFORM CORROSION MORPHOLOGIES ON PAINTED ALUMINUM-ALLOY-3105 COIL MATERIAL

Filiform corrosion (FFC) morphologies were examined on a rolled Al 310 5 (UNS A93105) aluminium substrate coated by wet polyurethane paint af ter various pretreatment processes. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of FFC filament cr oss-section morphologies, obtained by ultramicrotomy, revealed the rel ative importance of chemical and mechanical factors con tributing to t he propagation mechanism the latter originating from the lift of corro sion products. Filaments were shown ro propagate by formation of succe ssive pits, in which the exposed intermetallic particles acted as cath odic sites. Oxygen reducing at these sites diffused to the sites from the tail end of the filaments. Chromating the surface as a pretreatmen t 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 pretr eated surfaces, although slow, required extensive attack of the substr ate to reach a given length Propagation on as-degreased surfaces, with an intact mill-finish metal surface and without the oxide sublayer, w as enhanced by delamination of the coating along preferred paths, with less pitting of the substrate. The lifting effect combined with the q uality of coating adhesion to determine the general shape of the filam ents formed.