How it works
Filiform corrosion is a special case of under-film corrosion driven by differential aeration. It starts where moisture and contaminants such as chlorides reach the metal through a defect in the coating. A small electrolyte cell forms under the film. The front of the filament, the head, is starved of oxygen, acidic and anodic, so metal dissolves there. Behind it, oxygen diffuses through the porous tail of corrosion products, which acts as the cathode. The head keeps advancing, lifting the coating as it goes and leaving a trail of oxide behind, much like a moving crevice. The filaments wander, branch and rarely cross one another.
Filiform attack needs a humid but not wet environment: under full immersion or condensation the same conditions tend to produce blistering instead. On steel, filament heads often look blue-green and tails reddish-brown; on aluminum, the products are white or grey.
Where it shows up
It is best known on painted or powder-coated aluminum — architectural profiles, enclosures, transport panels and aircraft skins — and on coated steel such as packaging, appliances and lightly painted equipment. Coastal and industrial atmospheres, where soluble salts deposit on surfaces, make it more frequent. It is mostly a cosmetic and adhesion problem at first, but uncontrolled filaments undermine the coating and open the way to wider corrosion.
How it is tested and prevented
ISO 4623 specifies laboratory tests of resistance to filiform corrosion, with Part 1 for steel substrates and Part 2 for aluminum substrates: a scribed panel is contaminated, then held in a humid atmosphere and the filaments are assessed. Test conditions and acceptance criteria come from the standard and the product specification.
Prevention relies on the layers below the topcoat:
- clean, salt-free substrates before coating;
- a proper pretreatment, such as conversion coating or anodizing on aluminum;
- adequate film thickness, especially on edges and corners;
- low-permeability coatings and prompt repair of mechanical damage.
When filaments are already present, the affected coating is removed beyond the visible tips, the surface is cleaned and pretreated again, and the area is recoated.