FAA Aviation Mechanic Airframe (AMA)Airframe InspectionHard
A technician notices thread-like corrosion filaments spreading in random directions under the paint finish on an aluminum fuselage skin, most prevalent in a high-humidity environment. This condition is best described as which type of corrosion?
- AIntergranular corrosion
- BStress corrosion cracking
- CPitting corrosion
- DFiliform corrosion
Show answer & explanationAnswer & explanation
Correct answer: D. Filiform corrosion
Filiform corrosion appears as thread-like filaments that spread in random directions beneath a coating, typically initiated by moisture penetrating a scratch or defect in the paint film, and is aggravated by high humidity. It differs from pitting (localized cavities) and intergranular corrosion (grain boundary attack).
Why the other options are wrong
- A. Intergranular corrosion follows internal grain boundaries and isn't visible as surface filaments under paint.
- B. Stress corrosion cracking results from sustained tensile stress plus a corrosive environment, producing cracks rather than filaments.
- C. Pitting corrosion creates localized cavities, not thread-like filament patterns.
Filiform Corrosion
A corrosion pattern that appears as random, thread-like filaments spreading under a paint or organic coating, usually caused by moisture infiltration.
- Occurs beneath painted or coated surfaces
- Worsened by high humidity environments
- Initiated by breaks/defects in the coating
Memory trick: Filiform = 'fine filament' worms crawling under the paint