Filiform corrosion is characterized by which feature?

• A. Pitting at coating edges
• B. Threadlike filaments under coating
• C. Uniform rust across surface
• D. Delamination of coating

Answer: (B)

Filiform corrosion is an under-film form of corrosion where moisture and electrolytes get under a painted or coated surface and cause the metal to corrode in a threadlike, filamentous pattern beneath the coating. The key feature is slender, wormlike filaments that extend under the film, often starting from a defect or edge of the coating. This distinguishes it from uniform rust on the surface, pitting at coating edges, or coating delamination, which are different failure modes. Detecting these under-film filaments helps explain why a coating may look intact while corrosion progresses beneath it, and highlights the importance of proper surface prep, coating integrity, and edge/defect management to prevent it.