Galvanic Corrosion is an electrochemical process which occurs when dissimilar metals are in contact with each other in the presence of an electrolyte, most commonly being moisture and oxygen. For example rain water and salt water make especially good electrolytes. An electrolyte could be any non-metal matter that will conduct an electric current and are predominantly liquids or moisture. Every metal has been rated for nobility and then placed on galvanic scales according to nobility. Basically nobility is a measurement of the resistance to corrosion, especially of one metal contacting another metal. Metals that are the least noble means they are very anodic, electropositive or high potential and will corrode most easily. Whereas metals that are the most noble, means they are highly cathodic, electronegative or low potential and will be the most resistant to corrosion. The most corrosive effects will occur between metals from the opposite ends of the galvanic scale or ranking of nobility. This difference in electrical potential between two or more metals causes galvanic corrosion. Dissimilar metals in contact with each other in the presence of an electrolyte causes current to flow through their points of contact at the expense of the metal with the higher potential or less nobility. The much less noble metal is gradually consumed in the Electro-chemical reaction and will deteriorate or wear away as the metal ions migrate away from the very anodic metal to the more noble cathodic one. The more noble metal's corrosion resistance actually increases from this transfer of ions to it from the less noble metal, while the other metal is gradually getting consumed. Below is an abbreviated galvanic scale showing ranking of the most common construction metals from the least noble up to the most noble in a normal construction setting where both moisture and oxygen are present.
Galvanic Scale (Nobility of Common Metals)*
Stainless Steel – Active
Stainless Steel – Passive
Electropotential of Various Metals
Since dissimilar metals in contact with each other in the presence of oxygen and moisture will result in the less noble metal being corroded or eaten away the more noble one, contact between dissimilar metals should be avoided. For example, copper as one of the most noble of metals should be kept separated from less noble metal such as aluminum and galvanized steel. If contact between dissimilar metals cannot be avoided, their surfaces should be insulated as much as possible at the least with a non-metallic, non-conductive coating, such as bituminous or zinc chromate primers or paint. Separating the dissimilar metals with tape, gaskets, waterproof membrane, sealants or other non-conductive material that does not absorb and hold moisture can be used effectively. Care must also be taken to avoid the liquid wash run off or discharge from the drains from noble metals to less noble ones. For example rain water run off from a copper roof going into aluminum gutters and downspouts will result in traces of copper salts in the run off wash accelerating corrosion of the aluminum. Additionally, the use of galvanized nails, screws, flashing or drip edge on copper roofing or copper gutters should likewise be avoided as such less noble fasteners and metals will corrode rapidly. While rain water is a good electrolyte so too is high humidity or moisture laden air especially when it contains a high concentration of corrosive acids as is typically found in larger cities or near industry. In industrial or heavily populated areas, harmful electrolytes are formed by the absorption of gases by rain and fog to form acids and salts, while in coastal areas a saline electrolyte is formed by the combination of salt with the moisture laden air.
As copper has one of the highest galvanic numbers or nobility of construction metals, copper will not be harmed by contact with any of the common metals. However, copper will promote corrosion of other less noble metals if placed in direct contact with them. Copper can be used most appropriately with lead, tin, lead-tin solder or stainless steel under almost any circumstance. The primary metals of concern in terms of galvanic corrosion contact with copper are aluminum and zinc. Large differences in the thermal expansion rate between copper and these metals may cause additional problems. Also avoid using copper and some brass fasteners with aluminum, zinc and most ferrous metal. Paints or coatings that are used for isolation must be compatible with both of the dissimilar metals. The bituminous or zinc chromate primers and paints can be used between copper and aluminum surfaces. These same coatings or a red lead primer can also be effective in separating copper from iron and the other ferrous metals such as galvanized steel or mild steels.
Another type of corrosion called erosion corrosion is caused by the flow of acidic water concentrated into a very small area. While especially true for the least noble and most corrosive of metals, but capable of affecting all metals, acidic water allowed to concentrate on a small area, for example rain water from a large area like a roof diverted towards a flashing, valley or gutter can cause erosion corrosion. Also acidic water or liquids pooling on metal surfaces or trapped without air circulation between roofing or sheathing can quickly corrode the metal. Solution is to eliminate such pooling, maintain air circulation underneath metal roofs or between inert roofing material and drip edge, valleys and flashing. Another solution is to use replaceable reinforcing insulative strips between the inert materials and metal. Raw zinc and aluminum especially are susceptible to prolonged contact with acidic moisture, as for example the underside of a zinc roof. But care should be taken even with copper and stainless steel. Copper and zinc possess a superior property in that both will gradually develop a long term protective patina that helps guard against such corrosive effects for a very long period of time providing initial safeguards are exercised such as drying out provisions or temporary coatings.
Example of the protective patina that formed on these copper dome roofs
*Note: Galvanic chart rankings involving sea water immersion and low oxygen conditions will vary somewhat - the more typical nobility given here is for land based construction.