Mechanism

Due to the electrical potential difference that develops when two dissimilar metals or alloys are connected together in an aqueous solution the base metal will become anodic and the more noble metal will act as a cathode. The noble metal is in effect, cathodically protected by the more reactive metal which is corroded.

Galvanic Series

A galvanic series of metals and alloys can be listed for given corrosive environments, for example seawater, to show which material is liable to corrode in a galvanic couple, table 1.

Table 1. Simplified galvanic series for metals and alloys. The relative position in the series will depend on the corrosive environment and on the passivity of the surface of the metal or alloy.

Noble (cathodic) Base (Anodic)

Platinum Gold Graphite Titanium Silver Stainless steels Nickel Monel Cupronickel Tin bronze Copper Cast iron Steel Aluminium Zinc Magnesium

Attack on the base metal will usually be more severe at the junction with the noble metal, but the extent of the damage will depend on the electrochemical differences between them, i.e. the wider their separation in the galvanic series, the greater is the attack on the base partner. The relative surface areas of the two metals exposed to the corrosive media and the nature of that media will also have an affect. When small surface areas of base metal are connected to much larger areas of noble material the attack on the base metal will be rapid.

This is illustrated by the first recorded example of the galvanic effect with the detachment of copper sheets from the hull of HMS Alarm in 1761. This was as a result from attack on the iron nails which had been use to attach the copper to the timbers.

Minimising the Effect of Galvanic Attack

Galvanic attack can be minimised, as can other forms of corrosion, by correct design. The use of galvanically compatible materials and the use of electrical insulation between dissimilar materials will help. Not coating the anodic surface in case of pinhole damage to it is also useful as this could give rapid local attack.

The galvanic effect is the reason why different phases and segregated regions in alloy microstructures will have varying resistance to corrosion. This effect is made good use of when polished specimens are selectively attacked by etching in order to reveal and study microstructural features under the microscope. In stainless steels Cr-depleted zones around Cr-rich second phases will be less noble and as such will be subject to highly localised attack leading to interdendritic and/or intergrannular forms of corrosion.