FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician is installing a steel bracket directly against an aluminum wing skin. To prevent galvanic corrosion at the interface, what should the technician do before assembly?
- AApply a coat of engine oil between the parts
- BIncrease torque on the fasteners to eliminate any air gap
- CUse stainless steel washers only at the fastener heads
- DInsert a nonconductive barrier such as a faying-surface sealant or primer between the metals
Show answer & explanationAnswer & explanation
Correct answer: D. Insert a nonconductive barrier such as a faying-surface sealant or primer between the metals
Dissimilar metals in direct contact, especially with moisture present, create a galvanic cell that accelerates corrosion of the more anodic metal (aluminum). Isolating the metals with a nonconductive sealant, primer, or gasket material at the faying surface interrupts the electrical path and prevents galvanic corrosion.
Why the other options are wrong
- A. Engine oil is not an approved corrosion-prevention barrier and washes away.
- B. Increasing torque does not address the electrochemical cause of corrosion.
- C. Washers at fastener heads do not isolate the large faying surface area in contact.
Galvanic (Dissimilar Metal) Corrosion
Corrosion that occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, causing the more anodic metal to corrode preferentially.
- Farther apart on the galvanic series = greater corrosion risk
- Prevention: isolate metals with sealant, primer, or nonmetallic barrier
- Moisture acts as the electrolyte completing the circuit
Memory trick: 'Keep Metals Apart, Keep Corrosion at Bay' — isolate with a barrier.