FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesMedium
During a borescope inspection of an engine component, a technician finds corrosion that has attacked along the metal's grain boundaries with little surface pitting visible, weakening the material internally. Which type of corrosion is this?
- AFretting corrosion
- BIntergranular corrosion
- CGalvanic corrosion
- DSurface (uniform) corrosion
Show answer & explanationAnswer & explanation
Correct answer: B. Intergranular corrosion
Intergranular corrosion progresses along the grain boundaries of a metal, often due to improper heat treatment, and is difficult to detect visually since surface pitting may be minimal even as internal strength is severely compromised.
Why the other options are wrong
- A. Fretting corrosion results from small relative motion between mated surfaces under load.
- C. Galvanic corrosion occurs between dissimilar metals in contact with an electrolyte.
- D. Surface corrosion produces visible, generalized pitting or discoloration on the exterior.
Intergranular Corrosion
A form of corrosion that progresses along the grain boundaries of a metal, often caused by improper heat treatment, and is difficult to detect visually despite significant internal weakening.
- Often results from improper quenching/heat treatment
- Can lead to exfoliation corrosion if unchecked
- Hard to detect with visual inspection alone; needs NDT
Memory trick: 'Grain boundaries crumble quietly inside'