Filiform corrosion occurs as which feature?

• A. Threadlike filaments under an organic coating
• B. Cracking of coating edges
• C. Pitting at coating surface
• D. Fretting at interface

Answer: (A)

Filiform corrosion is a form of corrosion that propagates under an intact organic coating, appearing as threadlike, worm‑like filaments beneath the coating. It starts where a small defect or pinhole in the coating lets moisture and ions get to the metal–coating interface. The corrosion then advances forward under the coating, producing narrow filamentous trails of corrosion products that grow beneath the coating and can emerge as fine lines on the coating surface. This behavior is distinct from surface pits, edge cracking of the coating, or fretting wear at the interface, which is why the threadlike under‑coating filaments best describe filiform corrosion.