How does filiform corrosion typically appear on an aircraft structure?

• A. Pits on the metal surface
• B. Thread like lines of puffiness under the film of polyurethane or other dense finish system components
• C. Uniform rust staining
• D. Cracks in the coating

Answer: (B)

Filiform corrosion is corrosion that grows under a coating along the coating–substrate interface. Moisture gets behind defects in the finish and travels under the film, forming slender filaments that advance ahead of the corrosion front. This produces thread-like lines of puffiness or wormlike trails beneath a polyurethane or other dense finish. That distinctive under-coating pattern is what you’d look for on aircraft structures, rather than pits on the metal surface, uniform rust staining, or cracks in the coating.