The cause of filiform corrosion appears to be associated with________.

• A. Hydrogen embrittlement
• B. Stress corrosion cracking
• C. Mild surface contamination of solid particles deposited from the atmosphere or residue on the metal surface after processing
• D. Thermal oxidation

Answer: (C)

Filiform corrosion under a coating is driven by contaminants on the metal surface that create a localized electrochemical environment. Mild surface contamination—residues or small particles left on the metal after processing or picked up from the atmosphere—traps moisture and forms a conductive path at the coating–metal interface. This allows anodic dissolution to begin at those contaminated spots and propagate under the coating as threadlike fronts, producing the characteristic worm-like filaments. The other options describe different failure modes: hydrogen embrittlement involves hydrogen weakening the metal itself, stress corrosion cracking requires tensile stress and a corrosive environment causing cracks, and thermal oxidation is oxide growth from high temperatures—none of which describe the under-coating, contamination-driven growth seen in filiform corrosion.