Filiform corrosion imaged beneath protection layers on Al alloys

Abstract

Aluminium alloys used extensively in aircraft, ships and land transport vehicles are typically protected by a thin conversion coating based on chromium compounds followed by a surface protection layer of polymer paint. Breeching of the protection layer and exposure to a salt spray induces the growth of filiforms from the breech across the aluminium surface under the protective layers. The growth of the filiform is promoted by the formation of a galvanic cell based on chlorine chemistry. In this paper we study the elemental composition of the filiforms using a nuclear microprobe with 3 MeV proton beams. The deep penetration of this beam allows the composition of the intact filiform to be probed in situ through the surface layers. We present elemental maps of the intact filiforms that clearly highlight the presence of Cl in the growing end of the filiform, where the Cl concentration exceeds 20 wt.%, and the peculiar role of potassium in the trail of oxide left behind the growing filiform head.