Which form of corrosion develops on metal surfaces in humid atmospheres?

• A. Pitting corrosion
• B. Filiform corrosion
• C. Galvanic corrosion
• D. Uniform corrosion

Answer: (B)

Filiform corrosion shows up as thread-like filaments that grow under a protective coating when moisture is present. In a humid atmosphere, water diffuses through coating defects or pinholes and creates a thin electrolyte layer at the coating–metal interface. The corrosion cells then form and advance along the surface beneath the coating, producing those characteristic filament patterns. This is why it’s the best fit for a situation described as developing on metal surfaces in humid environments—the humidity interacts specifically with coatings to drive this distinctive under-coating progression.

Uniform corrosion would affect the whole exposed surface more evenly, not just under a coating and not in the same filamentary pattern. Pitting is localized to individual pits, not long filaments, and often relates to aggressive species like chlorides. Galvanic corrosion requires two different metals in electrical contact with an electrolyte, not just humidity acting on a single metal surface.