Metals are rated according to their “nobility”. When dissimilar metals are in contact with each other in the presence of oxygen and moisture, the more noble metal will corrode the less noble. Contact between dissimilar metals should always be avoided. If contact can not be avoided, the adjacent surfaces should be painted with bituminous paint or zinc chromate primers or paints. Taping or gasketing with non-absorptive materials is also effective.
Care should be taken to prevent the wash from copper alloy surfaces onto adjacent, exposed, less noble metal surfaces since the traces of copper salts carried in the wash can accelerate the corrosion of less noble metals.