Reference: Abbott, Richard. Analysis and Design of Composite and Metallic Flight Vehicle Structures 3 Edition, 2019
From (AC43-4A, 1991):
“Corrosion is the electrochemical deterioration of a metal because of its chemical reaction with the surrounding environment. While new and better materials are continually being developed, this progress is offset, in part, by a more aggressive operational environment. This problem is compounded by the fact that corrosion is a complex phenomenon. It can take many different forma and the resistance of aircraft materials to corrosion can drastically change with only a small environmental change.”
For corrosion to occur the following 4 conditions must exist
- Presence of a metal that will corrode (anode)
- Presence of a dissimilar conductive material (cathode) which has less tendency to corrode
- Presence of a conductive liquid
- Electrical contact between the anode and cathode (usually metal to metal contact or a fastener)
Elimination of any of these conditions will stop corrosion.
Factors that influence Corrosion:
- Type of metal
- Heat treatment and grain direction
- Presence of a similar, less corrodible metal
- Barrier protection
- Operating Environment
Note that the only electrically inert product used in aircraft structural applications is glass fiber laminate.
This means that any other material can be used in combination with glass fiber laminates without the risk of corrosion. Glass fiber layers are used as a corrosion barrier between aluminum fittings and carbon fiber laminate back-up structure.
Electrical isolation is a good way to prevent corrosion. However, electrical isolation, especially in composite airframes can cause problems with grounding and lightning protection.