FAA Aviation Mechanic Airframe (AMA)Airframe InspectionMedium
During a corrosion inspection of a heat-treated aluminum wing spar, a technician notices a form of corrosion that progresses along the grain boundaries of the metal, often invisible from the surface until it causes exfoliation. What type of corrosion is this?
- AIntergranular corrosion
- BFiliform corrosion
- CFretting corrosion
- DGalvanic corrosion
Show answer & explanationAnswer & explanation
Correct answer: A. Intergranular corrosion
Intergranular corrosion attacks the grain boundaries of heat-treated aluminum alloys, often starting internally and progressing until it surfaces as exfoliation. It is difficult to detect visually in early stages and often requires ultrasonic or eddy current inspection.
Why the other options are wrong
- B. Filiform corrosion forms thread-like filaments under paint or coatings, unrelated to grain boundaries.
- C. Fretting corrosion results from repeated rubbing motion between mating surfaces.
- D. Galvanic corrosion occurs between dissimilar metals in contact with an electrolyte.
Intergranular Corrosion
A form of corrosion that progresses along the grain boundaries of a metal, common in heat-treated aluminum alloys, and can lead to exfoliation.
- Occurs at grain boundaries, not across grains
- Common in 2000 and 7000 series aluminum alloys
- Precursor to exfoliation corrosion
- Often requires eddy current or ultrasonic inspection to detect early
Memory trick: Intergranular = corrosion 'between the bricks' of the metal's grain structure