Filiform corrosion (Topic 11029)

Filiform corrosion occurs under surface layers such as paint. It depends on the relative moisture of the air and the quality of the surface treatment preparation prior to coating. Filiform corrosion has the appearance of thin threadlike attacks progressing along the surface beneath a surface layer.

The mode of attack is similar to pitting corrosion in that the front of the attack is supported by moisture which penetrates the surface layer and becomes depleted of oxygen making the area anodic. Filiform corrosion mainly has an aesthetic effect, but the corrosion products formed may cause deformation in narrow crevices or delamination of surface treatment.

According to one recent investigation, a reduction of the Cu content in the aluminium alloy below 0,3% reduces filiform corrosion substantially[56]. The use of an inhibiting primer or a conversion coating can also be effective. However anodising used as a pre-treatment seems to be the best solution.