Galvanic corrosion is an electrochemical process whereby one metal corrodes preferentially to another when both metals are in electrical contact with each other in the presence of an electrolyte, such as moisture and salts. This is because the dissimilar metals have different electrode potentials. The potential difference between the dissimilar metals is the driving force for the accelerated attack on the metal with the lower galvanic number (i.e., the anode). Over time, the anode metal dissolves into the electrolyte.
Metals are ranked according to galvanic numbers as a qualitative measure of their nobility. These numbers qualify the resistance to corrosion of any metal when in contact with other metals. A larger difference in the galvanic number between two metals in contact with each other indicates a greater potential for corrosion.
The galvanic numbers of the most common metals used in construction are ranked as follows: 1. aluminium; 2. zinc; 3. steel; 4. iron; 5. stainless steel - active; 6. tin; 7. lead; 8. copper; 9. stainless steel - passive.
Galvanic corrosion is a primary concern with metal roof maintenance. Marine environments present an additional concern due to the higher concentration of salts in the air and water.
Copper is one of the most noble metals. It will not be harmed by contact with other metals but it will cause corrosion to some other metals if contacted directly. The principal metals of concern regarding direct contact with copper are aluminium, light-gauge steel, and zinc. Aluminium and steel flashings and galvanized steel fasteners should not be used with copper. Runoff from a copper roof corrodes aluminium and steel guttering. It is not necessary to isolate copper from lead, tin or many stainless steels under most circumstances.
When it is not possible to avoid contact, an effective method of material separation is required. If paints or coatings are used for isolation, they should be compatible with both metals. Bituminous or zinc chromate primers can be used between copper and aluminium. Bituminous, zinc chromate, or a red lead primer can be effective in separating copper from iron and other ferrous metals. Taping or gasketing with non-absorptive materials or sealants are effective in separating copper from all other metals. In areas with severe exposure, lead or similar gasketing materials should be used, except between copper and aluminium. Water draining from copper surfaces should be prevented from exposure to aluminium and galvanized steel as traces of copper salts may accelerate corrosion. In some cases, anodizing may protect thicker aluminium, such as aluminium window system mullions.