Galvanic Action - Dissimilar metals in contact with each other will undergo a reaction in the presence of water. This is called galvanic action, which results in the corrosion of one of the metals. The following is the galvanic scale. When any two of the metals noted are in direct contact, the metal lower on the list will corrode. The farther apart the metals are on the list, the faster corrosion occurs.
GALVANIC SCALE Lowest <<<< Resistance to Galvanic Corrosion >>>> Highest Aluminum Zinc Steel Tin Lead Brass Copper Bronze
For example, if you attach copper with steel nails, the nails will corrode. To prevent dissimilar metals from contacting each other, they must be isolated with underlayment, bituminous paint or another nonconducting material.