Dissimilar metals contact between clamp and standing seam roof sheet
If two different metals are placed in electrical contact and bridged by an electrolyte, a current flows through the solution from the anodic to the cathodic or more noble metal. As a result, the cathodic metal tends to be protected but the anodic materials may suffer corrosion. A guide to the compatibility of various metals and alloys in contact is show in figure.
One exception to the necessity for the two metals or alloys to be in direct electrical contact for metallic corrosion to occur, is when the noble metal corrodes slightly and dissolves in water which subsequently flows over a less noble material.
The corrosion product from the more noble metal may then deposit on the less noble metal forming a true bi-metallic contact. An example of this reaction is when copper can dissolve very slightly in some natural waters and re=deposit on zinc or zinc/aluminum coated steel. Hence bi-metallic corrosion may be experienced when two dissimilar metals or alloys, not in direct contact are nevertheless connected electrically.
Obviously bi-metallic corrosion is more severe under immersed conditions than when exposed to normal atmospheric environments. In the case of atmospheric environments, corrosion will only occur when the bi-metallic couple is wet.
In the case of aluminum clamps on the standing seam roof by BlueScope Steel, as in the table it is ok to use without isolation.
Dissimilar metals contact between clamp and standing seam roof sheet
Source: Corrosion technical bulletin 12 by BlueScope Steel
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