FAA Aviation Mechanic Airframe (AMA)Airframe InspectionMedium
A technician inspecting a control surface hinge bracket finds fine, reddish-brown powder (aluminum oxide) accumulating between two closely fitted parts that experience small-amplitude vibratory movement against each other. What condition does this describe?
- AExfoliation corrosion
- BGalvanic corrosion
- CFretting corrosion
- DFiliform corrosion
Show answer & explanationAnswer & explanation
Correct answer: C. Fretting corrosion
Fretting corrosion occurs when two tightly fitted surfaces experience small, repetitive rubbing motion, generating fine oxide powder (reddish-brown on aluminum, black on steel) and often leading to surface wear and eventual fatigue cracking. It is common at hinge points, bolted joints, and press-fit bearings subjected to vibration.
Why the other options are wrong
- A. Exfoliation corrosion causes layered lifting of the metal, not powdery debris from motion.
- B. Galvanic corrosion results from dissimilar metals with an electrolyte, not mechanical rubbing.
- D. Filiform corrosion produces thread-like trails under a coating, not powder from rubbing.
Fretting Corrosion
A form of wear corrosion caused by small-amplitude repetitive motion between two tightly fitted surfaces, producing fine oxide debris and surface damage.
- Occurs at hinge points, bolted joints, bearing fits
- Produces reddish-brown powder on aluminum, black on steel
- Can lead to fatigue crack initiation if undetected
Memory trick: Fretting = friction + vibration = fine dust