Filiform corrosion aluminium alloys

This appears as a random non-branching white tunnel of corrosion product either on the surface of non-protected metal or beneath thin surface coatings. It is a structurally insensitive form of corrosion which is more often detrimental to appearance than strength, although thin foil may be perforated and attack of thin clad sheet (as used in aircraft construction) may expose the less corrosion resistant aluminium alloy core. Filiform corrosion is not commonly experienced with aluminium, as reflected by the insignificance afforded it in reviews on the phenomena (Section 1.6). 

The lowest potential is measured in the center, where corrosion (i.e. anodic dissolution of iron) attacks most aggressively. At the edges, the potential increases somewhat in this zone oxygen reduction proceeds. The potential changes around the drop imply the presence of an ultrathin electrolyte film because the potential reaches values of the bare iron surface only at a considerable distance from the edges of the macroscopically observed drop [213]. Filiform corrosion of automotive aluminium alloy AA6016 has been studied with SKP [221]. 

Hughes, A.E., Mol, J.M.C., Hinton, B.R.W, and van der Zwaag, S., A morphological study of filiform corrosive attack on ceratedAA2024-T351 aluminium alloy. Corrosion Science,20Q5.Al y. 107-124. 

Coil-coated strip in aluminium alloys is not prone to this type of corrosion. After the introduction of new lacquering techniques by electrostatic deposition of powders on profiles or sheets in aluminium alloys of the 3000,50(X) and 60(X) series, filiform corrosion appeared in the 1980s, mainly in the coastal areas of Northern Europe. 

The nature of the alloy is not an important factor, because filiform corrosion may affect all aluminium alloys. A recent collaborative study conducted by three European transforming companies, Alusuisse, Hydro Aluminium and Pechiney, showed that for the most commonly used alloys in the construction industry, 6060 and 6063, the alloy composition has no influence, except when the copper concentration exceeds 0.1%. 

Standard guide for testing filiform corrosion resistance of organic coatings on metal Standard practice for measurement of corrosion potentials of aluminium alloys... 

X. Zhou, G.E. Thompson, and G.M. Seamans, The influence of surface treatment on filiform corrosion resistance of painted aluminium alloy sheet, Corros. Sci. 45(8), 1767-1777 (2003). 

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