FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesMedium
A technician inspecting an aluminum wing panel finds white powdery deposits forming at a joint where a steel bolt contacts the aluminum skin in the presence of moisture. This is most likely an example of which type of corrosion?
- AStress corrosion cracking
- BFretting corrosion
- CGalvanic (dissimilar metal) corrosion
- DFiliform corrosion
Show answer & explanationAnswer & explanation
Correct answer: C. Galvanic (dissimilar metal) corrosion
Galvanic corrosion occurs when two dissimilar metals (steel and aluminum) are in contact in the presence of an electrolyte such as moisture, creating a small electric current that corrodes the more anodic metal, in this case the aluminum. Stress corrosion requires sustained tensile stress, fretting requires relative motion between surfaces, and filiform is a thread-like corrosion pattern under coatings.
Why the other options are wrong
- A. Stress corrosion cracking requires sustained tensile stress plus a corrosive environment, not just dissimilar metal contact.
- B. Fretting corrosion results from small oscillatory movement between mating surfaces, not simply dissimilar metals.
- D. Filiform corrosion appears as thread-like trails under paint/coatings, unrelated to dissimilar metal contact.
Galvanic Corrosion
Corrosion that occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, causing the more anodic (active) metal to corrode.
- Requires dissimilar metals, electrical contact, and an electrolyte
- The more active metal in the galvanic series corrodes preferentially
- Greater difference in galvanic series = more severe corrosion
Memory trick: 'Different metals, wet weather, worse together' - galvanic corrosion needs a couple and moisture