Hi Tom: The majority of the corrosion that kills Coupes comes from inside the aluminum. Due to deficiencies in the original heat treatment, intergranular corrosion starts between the copper alloying granules and the aluminum.
This cannot be seen and progresses to exfoliation. At the latter stage it is discovered when a spar cap can be picked to pieces with your fingers. This is discussed in AC 43-13-1B Para 6-17. Paint and protective coatings will prevent surface corrosion, but might hide early exfoliation. This can sometimes be spotted before the whole part comes to pieces as deep pits in the metal filled with white powder. This the corrosion working it's way to the surface. Little blue spots will be seen from the copper granules. Cheers: Paul NC2273H A & P
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