Prevention of Rusting by Cathodic Protection

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When two different metals are immersed in an electrolyteAn electrolyte is a substance that dissociates in water into charged particles called ions. Positively charged ions are called cations. Negatively charged ions are called anions. Simply, an electrolyte is a... More and connected by an external wire. The metal which is higher in the electrochemical series becomes the negative electrodeElectrodes are conductors, in the form of wires, rods or plates, through which current enters and leaves the electrolyte. When the current leaves the electrodes it is known as the cathode... More (anodeAn anode is an electrode of a polarized electrical device through which conventional current enters the device. It is the positive part of electrolytes where oxidation takes place. More) and the metal lower in the series becomes the positive electrode (cathodeA cathode is the electrode from which a conventional current leaves the electrolyte. It is the negative part of the cell where reduction takes place. More).

Based on these principles a metal that corrodes easily can be protected by coating it with metals higher up in the series e.g. iron can be protected by coating it with zinc, aluminium, magnesium, calcium etc.

Galvanised iron and tin-plated iron can be used as an example.  

Tin-plated iron is a sheet of iron coated with a thin layer of tin while galvanized iron is coated with zinc. Part of the tin or zinc coat may be broken during the course of handling. The hole formed becomes filled with trioxocarbonate(IV) acid, H₂ CO₃ , formed by dissolving carbon(IV) oxide,