Galvanic Corrosion & the Area Effect

When two dissimilar metals are in electrical contact and bridged by an electrolyte, such as condensation or salt-laden moisture, they form a cell. The metal that is more active, lower in the galvanic series, becomes the anode and corrodes; the more noble metal becomes the cathode and is protected. The further apart the two metals sit in the series, the greater the driving potential and the more vigorous the attack.

The point that catches candidates out is the area effect. Corrosion current is spread over the anode, so a small anode connected to a large cathode concentrates the whole current into a tiny area and produces very rapid, deep attack. A steel fastener in an aluminium panel is a large anode with a small cathode and corrodes slowly, but an aluminium fastener in a large steel or carbon-fibre panel is a small anode with a huge cathode and can fail quickly. This is why fastener material selection matters more than it first appears, and why carbon fibre, which is strongly cathodic, must never sit directly against aluminium.

Prevention removes one of the four requirements: separate the metals with an insulating layer such as glass-fibre or a sealant to break the electrical path, exclude the electrolyte with paint, primer or sealant, choose metals close together in the series, or fit a sacrificial anode of a more active metal so that it corrodes instead.

• Try this: Set the plate to aluminium alloy and the fastener to stainless steel, then reduce the fastener area. Why does the attack rate change so little, and where is the damage?
• Try this: Now reverse them, aluminium fastener in a stainless plate, and reduce the fastener area to a few per cent. Explain the very high attack rate using the area effect.
• Try this: Select carbon fibre against aluminium alloy. Which corrodes, and what does the manual require between them?