FAA Aviation Mechanic Airframe (AMA)Airframe StructuresHard
A technician inspecting an aluminum wing spar attach fitting notices fine cracks that appear to follow the direction of the original grain flow of the metal, occurring after exposure to a corrosive environment and sustained tensile stress. This is most consistent with which type of corrosion damage?
- AIntergranular corrosion
- BFretting corrosion
- CUniform surface corrosion
- DGalvanic corrosion
Show answer & explanationAnswer & explanation
Correct answer: A. Intergranular corrosion
Intergranular corrosion occurs along the grain boundaries of a metal, often due to improper heat treatment, and produces cracks that follow the grain flow, especially under sustained tensile stress (stress corrosion cracking is a related form). Uniform corrosion affects the surface broadly, galvanic corrosion occurs between dissimilar metals, and fretting corrosion results from vibration between mating surfaces.
Why the other options are wrong
- B. Fretting corrosion results from small oscillatory movement between mating parts, not grain-boundary cracking.
- C. Uniform corrosion produces even surface pitting/etching, not grain-following cracks.
- D. Galvanic corrosion requires two dissimilar metals and an electrolyte, not described here.
Intergranular Corrosion
A form of corrosion that progresses along the grain boundaries of a metal, often caused by improper heat treatment, weakening the material without much visible surface change.
- Follows grain boundaries rather than the surface broadly
- Often caused by improper heat treatment of aluminum alloys
- Can progress into exfoliation corrosion if severe
Memory trick: 'Inter-granular travels between the grains, like a rat through walls.'