Galvanic cell
The Galvanic cell, named after Galvani, consists of two metals connected by an electrolyte which forms a salt bridge between the metals. In 1780, Luigi Galvani discovered that when two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, they made the leg contract. He called this “animal electricity.”. This discovery paved the way for all electrical batteries.
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Scheme of a galvanic cell
The Galvanic cell's metals dissolve in the electrolyte at a different rate, leaving some electrons in the rest of the metal, which charges it. The different dissolving rate thus causes an unequal number of electrons in the two metals. This results in an electric potential between the two metals. If an electrical connection, such as a wire or direct contact, is formed between the two, an electric current flows. At the same time, ions of the more active metal, which forms the anode, are transferred through the electrolyte to the less active metal, the cathode, and deposited there as a plating. In this way the anode is consumed or corroded. A similar process is used in electroplating.
Unwanted galvanic cells are formed whenever two metals are in contact in the presence of an electrolyte, such as salt water, resulting in the galvanic corrosion of the more active metal. There are two common ways of preventing this. One is to insulate the two metals from each other, for example plastic or fibre washers are used to separate steel water pipes from copper-based fittings. The other is the use of sacrificial anodes.
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