Galvanic Corrosion

Galvanic corrosion (also called electrolysis or bimetallic corrosion) is a common but often misunderstood roofing issue. It occurs when two metals with different electrochemical potentials are in direct contact and exposed to an electrolyte — typically rainwater. The more "active" or "anodic" metal in the pair corrodes sacrificially to protect the more "noble" or "cathodic" metal. For example, if a zinc-coated roof sheet is in contact with a copper downpipe, the zinc will corrode at an accelerated rate. Common galvanic corrosion risks in roofing include: using stainless-steel screws in Zincalume sheets without appropriate washers; allowing copper or brass fittings to contact Colorbond or galvanised steel; run-off from copper or lead flashings flowing over zinc-coated metal below; and direct contact between Zincalume steel and wet treated timber (the copper in the treatment solution acts as the dissimilar metal). In Gippsland, galvanic corrosion is most frequently seen where old and new roofing materials have been mixed during partial repairs — for example, new Colorbond valley irons installed alongside old galvanised sheets, or new stainless-steel fixings used in old galvanised gutters. Town & Country Roofing ensures material compatibility on every installation, using compatible fasteners, isolation washers, and separation strips where dissimilar metals must be in proximity.