Dissimilar-metal contact, galvanic corrosion, electrolysis and isolation detailing
Dissimilar-metal contact becomes a galvanic cell when two metals with different potentials touch in the presence of an electrolyte such as rain, condensation or salt spray. The exam tests which metal corrodes, how area ratio changes the risk and how to isolate details.
Core rules
- The more active metal becomes the anode and corrodes; the more noble metal becomes the cathode and is protected. Copper is cathodic to aluminum, zinc and galvanized steel, so copper runoff or direct contact attacks them.
- A small anode connected to a large cathode is dangerous because the anode area is small; a large anode to small cathode is less severe but still not acceptable for critical flashings.
- Isolate with bituminous coating, compatible tape, plastic washers, sleeves or separate fasteners. Coating must be continuous, including cut edges and fastener threads.
- Avoid copper or stainless fasteners on aluminum in wet chloride service without isolation. Use compatible fasteners and keep runoff from noble metals away.
- Galvanized steel is not a safe separator between copper and aluminum; the zinc coating is sacrificial and can be consumed quickly.
Worked example
An aluminum gutter is supported by copper hangers and brackets on a coastal building. Rain wets the joint, copper is cathodic, and aluminum is anodic. The aluminum around each hanger pits and perforates, while the copper remains sound. The small aluminum area at each contact accelerates attack. The fix is not to paint the gutter after installation; it is to use aluminum or isolated compatible hangers, keep copper runoff away, and specify 316 stainless only with isolation where chloride exposure is severe. Verify fastener compatibility with the gutter manufacturer.
Traps
Assuming stainless is always safe; using galvanized fasteners with copper; forgetting cut edges; allowing copper roof runoff onto zinc or aluminum; relying on a thin paint film at a sliding joint.