How does filiform corrosion usually appear on an aircraft structure?

• A. As thread-like lines of puffiness under a film of polyurethane or other dense finish system topcoats.
• B. As rust spots on exposed metal.
• C. As blistering under the paint.
• D. As delamination at fasteners.

Answer: (A)

Filiform corrosion is a subsurface process that develops under a coating. It starts at tiny defects or pinholes in the topcoat and moisture gets beneath the film. The corrosion then advances along the metal–coating interface in slender, threadlike filaments, which push the coating up into fine, wormlike lines. This is why it’s seen as thread-like lines of puffiness under a dense finish such as polyurethane on aluminum structures. The damage remains under the coating, so you don’t typically see rust spots on exposed metal, blistering of the paint, or delamination at fasteners; those are different failure modes. The distinctive threadlike appearance under the paint is the hallmark of filiform corrosion.