A morphological study of filiform corrosive attack on cerated AA2024-T351 aluminium alloy

SEM and EDS studies were carried out to characterise filiform attack on a cerated AA2024-T351 aluminium alloy with a polyurethane topcoat. The filiforms developed on AA2024-T351 were sectioned, stripped of corrosion product and etched to reveal the grain structure. Examination of sections through the filaments and the filaments themselves, revealed severe local attack in the form of pitting resulting in grain etch out, grain boundary attack and subsurface etch out. Chloride ions were detected deep within pits and the subsurface etch out. The observations were similar to those found with filiform corrosion on chromated and coated surfaces. The observations led to development of a filiform corrosion model naming the volume expansion of the corrosion product as the principal cause for delamination.