Filiform corrosion is commonly observed beneath coatings. Which option best describes the area affected?

• A. Bare metal
• B. Organic coatings
• C. Insulation
• D. Glass

Answer: (B)

Filiform corrosion is a form of underfilm corrosion that propagates beneath an intact organic coating. It starts at small defects or holidays in the coating, where moisture and ions at the metal–coating interface create a thin electrolyte. From there, corrosion propagates as filament-like channels under the coating, so the area affected is beneath the organic coating itself. This is why organic coatings are the best description of the area involved. Exposed bare metal would show corrosion on the surface after coating failure, while insulation or glass aren’t involved in this metal–coating interaction.