Similar Metals

4. Because of contaminants that are always present, both rainwater and ground water are electrolytes. When rainwater or ground water comes in contact with a building assembly that includes two different metals, a galvanic reaction can occur, generating an electrical current that will corrode one of the metals with astonishing rapidity. The safest approach in detailing is to use the same metal in all of the components of an exterior detail: aluminum nails with aluminum roofing sheet, aluminum screws and bolts in aluminum cladding components, copper nails with copper roofing, and steel bolts in steel structural shapes.

5. It is often necessary, however, to confront such problems as attaching aluminum cladding sheets to a steel frame or an aluminum mast to a copper roof. To solve these problems, we must look more deeply into the nature of the galvanic reaction between dissimilar metals. Metals varv in their chemical activity. When two different m metals are brought together and bathed with an electrolyte such as rainwater, an exchange of electrons takes place that protects the less active metal while corroding the more active one. The greater the difference is between the activities of the two metals, the greater the potential for corrosion.

6. In the accompanying table, the common architectural metals are ranked with respect to their relative activities: the most active metals are at the top and the least active arc at the bottom. Metals with similar levels of activity are grouped together. In general, it is safe to combine metals that arc in the same group.

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