FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician inspecting an extruded aluminum wing spar cap finds the surface layers separating and lifting like the pages of a book, running parallel to the direction of the original metal grain flow. What type of corrosion produces this characteristic pattern?
- AFretting corrosion
- BFiliform corrosion
- CExfoliation corrosion
- DUniform surface corrosion
Show answer & explanationAnswer & explanation
Correct answer: C. Exfoliation corrosion
Exfoliation corrosion is an advanced, visible form of intergranular corrosion that occurs in extruded or rolled high-strength aluminum alloys, where elongated grain structure causes corrosion products to build up along grain boundaries and lift the surface layers like pages of a book.
Why the other options are wrong
- A. Fretting corrosion results from small relative motion between tight-fitting mating parts, producing reddish-brown debris, not layer separation.
- B. Filiform corrosion produces thin thread-like trails under paint, not layered lifting of base metal.
- D. Uniform surface corrosion is even etching over a broad area, not layered separation.
Exfoliation Corrosion
An advanced form of intergranular corrosion in extruded/rolled high-strength aluminum where surface layers lift and separate along elongated grain boundaries.
- Common in extrusions/forgings with elongated grain flow
- Layers lift like pages of a book
- Advanced form of intergranular corrosion
Memory trick: Exfoliation = EXtruded metal peels like a book's leaves.