Filiform corrosion typically appears under the film of which finish system components?

• A. Polyurethane or other dense finish system components
• B. Primer only
• C. Bare metal surfaces
• D. Undercoat adhesives

Answer: (A)

Filiform corrosion is an under-film corrosion that grows beneath a coating, forming threadlike lines along the metal/coating interface. It occurs when a continuous, dense finish coat traps moisture and diffuses it to tiny defects or holidays in the film. Moisture then reacts with the metal at the interface and the corrosion products spread along the interface as filaments. This behavior is most typical with polyurethane or other dense finish systems because the film is sufficiently continuous and impermeable to drive and sustain this under-film reaction. Primer-only films, bare metal, or undercoat adhesives don’t provide the same persistent barrier, so the characteristic filiform pattern is not produced under those conditions.