NFPA 780 – Installation of Lightning Protection Systems, Paragraph 4.2.1 states, “Combinations of materials that form electrolytic couples of such a nature that, in the presence of moisture, corrosion is accelerated shall not be used.” Such combinations are commonly called, “dissimilar metals”, and the photo shows what happens when, for example, copper components are in contact with steel or even painted steel.
Each type or alloy of metal has an intrinsic electrochemical or galvanic potential. For example,
- copper alloys -0.35 to -0.40 V
- stainless steel -0.60 to -0.50 V
- carbon steel -0.85 V
- aluminum -0.90 to -0.95 V
- galvanized steel -1.20 V
In the presence of moisture or another electrolyte, a current is created between metals with dissimilar potential; the greater the difference in potentials, the greater the probability of corrosion. For example, the difference between the copper and steel materials in the photo have a whopping differential of about 0.50 V, and the resulting corrosion is visible. If aluminum lightning protection products had been used in proximity of the steel, the difference in potential would have been about 0.05 V, not enough to cause significant corrosion in ordinary environments.
UL and other lightning protection standards have specific requirements for connecting dissimilar metals, and East Coast Lightning Equipment manufactures a full line of UL-approved bi-metallic connectors. These fittings incorporate lead or stainless steel barriers to prevent contact between copper and aluminum components to moderate galvanic potential differences.
Photo: A big thank you to Andrew Mulholland of Boston Lightning Rod Company, Inc. who spotted this on a roof he was inspecting.