15/08/2025
What is Filiform Corrosion?
Appearance: Thin, thread-like “worm tracks” under the coating, usually starting at edges, scratches, or defects in the coating.
Cause: It’s a form of underfilm corrosion. Once moisture and oxygen get underneath the paint, corrosion creeps between the metal and coating in branching, filament-like patterns.
Typical Causes:
1. Surface Contamination -- Residual grease, oil, or salts not properly cleaned before painting. Hand sweat or fingerprints left before coating.
2. Poor Pretreatment -- Inadequate chemical conversion coating (e.g., chromate, phosphate, or zirconium pretreatment). Incomplete rinsing after pretreatment.
3. Humidity Exposure -- High humidity (>60%) during coating application or curing. Storage of coated items in damp conditions before full cure.
4. Coating Defects -- Pinholes, micro-cracks, or damage in paint film. Too thin a film build.
