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Galvanic Effects in Electrical Contacts

It is well known that in cells or batteries two dissimilar substances immersed in an appropriate electrolyte will provide a source of electrical energy. The generation of the latter in, for example, the Daniel cell involves the dissolution of the zinc electrode whilst copper is deposited from the electrolyte on to the copper electrode. If the two electrodes are not electrically coupled then the rate of dissolution of the zinc or the deposition of the copper will be very markedly reduced whilst immersed in the same electrolyte. Any two metals, providing they are sufficiently dissimilar in the electrochemical sense, in electrical contact will undergo essentially similar responses in that the dissolution, or corrosion, of one of the metals will be accelerated by contact with the other metal. The extent of such galvanic or bimetallic corrosion depends upon a number of factors but, as the following Table shows, it depends critically upon the metals coupled, reflecting their different electrochemical characteristics. (reference)

Weight losses for iron and coupled metals of the same relative surface areas immersed in a 1 per cent chloride solution

Metals coupled to iron Wt loss of iron (mg) Wt loss of coupled metal (mg)
Copper 183.1 0.0
Nickel 181.1 0.2
Tin 171.1 2.5
Lead 183.2 3.6
Aluminium 9.8 105.9
Cadnium 0.4 307.9
Zinc 0.4 688
Magnesium 0.0 310.4