Filiform corrosion is appearing mostly on aluminum
Filiform corrosion is special kind of corrosion appearing mostly on aluminum. The phenomenon resembles a worm creeping under the coating, always starting from a cut edge or a damage in the layer.
Filiform corrosion develops easily when the coated object is exposed to salt in combination with temperatures 30/40°C and relative humidity 60-90%. This problem is therefore limited to coastal areas and linked with unfortunate combination of aluminum alloys and pre-treatment.
To minimize filiform corrosions it is advised to ensure a proper alkaline etching followed by an acidic wash prior to the chrome conversion coating. An aluminum surface removal of 2g/m2 (minimum 1.5g/m2) is recommended.
Anodizing as pre-treatment for aluminum is a technology specially developed to prevent filiform corrosion. A special anodization process is required when the thickness and porosity of the anodization layer is of vital importance.