Galvanic Corrosion and Metal Compatibility

When two different metals are in electrical contact and wetted by an electrolyte such as rain, dew or salty air, they form a small battery. The more active metal (the anode) corrodes faster than it would alone, while the more noble metal (the cathode) is protected. On roofs this explains why some flashings, fasteners and gutters fail early when mixed with the wrong metal.

The galvanic series

Engineers rank metals by an "anodic index". From more noble to more active, approximate values are:

• Copper about -0.35 V
• Stainless steel (18% chromium) about -0.50 V
• Tin and tin-lead solder about -0.65 V
• Lead about -0.70 V
• Carbon steel about -0.85 V
• Wrought aluminium about -0.90 V
• Galvanized steel about -1.20 V
• Zinc about -1.25 V

Outdoors, a difference of more than about 0.15 V is considered risky. Copper with aluminium, zinc or galvanized steel is a classic bad pairing.

Area and flow

A small anode attached to a large cathode corrodes fastest, so a steel screw in copper sheet fails quickly, while a copper rivet in steel is less severe. Water flowing from a noble metal onto an active one carries dissolved ions that also cause attack: copper run-off onto Galvalume, zinc or aluminium gutters is a common problem.

Practical rules

• Use fasteners of the same metal or a compatible, more noble one: copper, bronze or 300-series stainless steel with copper; stainless or aluminium fasteners with aluminium, never bare or ordinary steel.
• Separate dissimilar metals with bituminous coatings, membranes or non-conductive spacers.
• Keep copper and its run-off away from aluminium, zinc, galvanized steel and aluminium-zinc coated steel.

Famous example

The Statue of Liberty suffered galvanic attack where its copper skin met its wrought iron framework after the original shellac insulation failed; restoration replaced the insulation with PTFE.