FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician inspecting an aluminum wing skin near a fuel vent finds numerous small, isolated cavities on the surface with a white or gray powdery deposit at the bottom of each cavity. There are no fine intergranular cracks or thread-like filaments present. Which type of corrosion is most consistent with this finding?
- APitting corrosion
- BFiliform corrosion
- CIntergranular corrosion
- DExfoliation corrosion
Show answer & explanationAnswer & explanation
Correct answer: A. Pitting corrosion
Pitting corrosion appears as small, localized cavities or holes on the metal surface, often with a white or gray powdery oxide deposit, and results from breakdown of the protective oxide film in a localized area. It lacks the subsurface cracking of intergranular corrosion or thread-like pattern of filiform corrosion.
Why the other options are wrong
- B. Filiform corrosion produces thin, thread-like filaments under paint or finish.
- C. Intergranular corrosion progresses along grain boundaries below the surface, often invisible until advanced.
- D. Exfoliation corrosion causes surface layers to lift and separate like pages of a book.
Pitting Corrosion
Localized corrosion that forms small cavities or pits on a metal surface due to a breakdown of the protective oxide layer, often leaving powdery deposits.
- Appears as isolated small holes/cavities
- Common starting point for stress corrosion cracking
- Caused by moisture, salt, or chemical exposure
- Detected visually or with a fine probe/depth gauge
Memory trick: Pitting = pockmarks, like tiny craters on the moon.