Method of protection for iron and steel against electrochemical corrosion. Two types of cathodic protection are: (1) Active, in which a low voltage (usually 40 to 50 volt) direct current is imposed between the exposed parts of a structure and the ground. (2) Passive, in which an anode (positive electrode) made of a more reactive metal (such as a magnesium alloy) is sacrificed to protect a structure acting as a cathode (negative electrode). Also called sacrificial protection. Cathodic protection is the most widely applied anticorrosion control technique in electrolytically conducting environments such as seawater and soils containing water. Invented in 1824 by the UK scientist Sir Humphrey Davy (17781829) to protect the coppercladded wooden ships from seawater corrosion. See also anodic protection.