Filiform corrosion commonly affects which metals?

• A. Titanium
• B. Steel, aluminum, and magnesium
• C. Copper and brass
• D. Plastic

Answer: (B)

Filiform corrosion is a form of underpaint corrosion where slender, worm-like trails of metal loss propagate beneath a coating. It starts at defects or porosity in the coating, where moisture and an electrolyte reach the metal surface, creating a local anodic area that drives corrosion under the coating. The corrosion front advances in a filament-like pattern, which is why you see those characteristic thread-like channels.

This type of attack is most commonly seen on steels, aluminum, and magnesium because these metals are widely used with organic coatings and are readily etched or passivated by moisture once a defect exists. The coating-sealed environment on these materials provides the right conditions for the electrolyte to form under the coating and for the anodic process to push forward as filaments. Copper and brass can corrode under coatings as well, but the distinct filamentary pattern under typical service conditions is far more characteristic for steel, aluminum, and magnesium. Plastic isn’t metallic, so filiform corrosion doesn’t apply.