FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
During a walk-around inspection, a technician notices thin, thread-like filaments of corrosion product forming random worm-like trails beneath the paint film on an aluminum wing skin, without any visible pitting on the bare metal surface. What type of corrosion is this most likely to be?
- AExfoliation corrosion
- BGalvanic corrosion
- CStress corrosion cracking
- DFiliform corrosion
Show answer & explanationAnswer & explanation
Correct answer: D. Filiform corrosion
Filiform corrosion appears as thin, thread-like or worm-like trails under a paint or coating film, typically starting at a coating defect or edge and spreading randomly in the presence of moisture and humidity trapped under the finish.
Why the other options are wrong
- A. Exfoliation corrosion causes layered lifting/flaking of extruded metal along grain lines, not thin threads.
- B. Galvanic corrosion occurs at dissimilar metal contact points and produces localized pitting, not thread-like trails.
- C. Stress corrosion cracking produces intergranular cracks under sustained tensile stress, not surface filaments.
Filiform Corrosion
A form of corrosion appearing as random, thread-like filaments beneath a paint or organic coating, caused by moisture entering through coating defects.
- Occurs under paint/coating films
- Thin, worm-like random trails
- Promoted by high humidity and coating breaks
Memory trick: Filiform = Filament worms crawling under paint.