FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesMedium
During inspection of an aluminum wing spar extrusion, a technician finds the surface has lifted and is separating into thin layers parallel to the surface, giving it the appearance of pages in a book. This condition is characteristic of which type of corrosion?
- AFiliform corrosion
- BFretting corrosion
- CGalvanic corrosion
- DExfoliation corrosion
Show answer & explanationAnswer & explanation
Correct answer: D. Exfoliation corrosion
Exfoliation corrosion progresses along grain boundaries parallel to the surface of extruded or rolled aluminum, causing the metal to swell and delaminate into layers resembling pages of a book. It is commonly found in high-strength aluminum extrusions and forgings.
Why the other options are wrong
- A. Filiform corrosion appears as thread-like filaments under paint or coatings, not layered delamination.
- B. Fretting corrosion results from repeated rubbing between mated surfaces, producing a fine reddish-brown powder.
- C. Galvanic corrosion occurs between dissimilar metals in contact with an electrolyte, not a layered delamination pattern.
Exfoliation Corrosion
Subsurface corrosion that follows grain boundaries in extruded or rolled aluminum, causing the surface to lift and delaminate in layers.
- Follows grain flow direction in extrusions/forgings
- Surface swells and separates like pages of a book
- Common in high-strength 2xxx and 7xxx aluminum alloys
Memory trick: Exfoliation peels like the pages of an old book