ficiently high voltage, sodium metal will be produced at one electrodeand chlorine gas at the other. A similar electrolytic process is used to obtain purealuminum from solutions of aluminum oxide.Electrolysis is important in silverplating. In this process an electric current ispassed through an object that is immersed in an appropriate solution of a silvercompound. If the voltage is sufficient, silver ions (Ag+) will accept electrons fromthe object being plated. The ions thereby change to silver atoms (Ag), whichplate the surface. A similar technique is used in electroplating copper,chromium, and gold.CORROSION The chemical deterioration of a material, usually a metal or metal alloy, iscalled corrosion. The most common causes of corrosion are contact with waterand oxygen, though other substances in the earth and in the atmosphere canalso cause corrosion. The material with the greatest economic importance thatis most affected by corrosion is iron. The corrosion of iron is called rusting.The corrosion of metals such as aluminum, tin, copper, and zinc generallystops after a thin layer of metal oxide forms on the exposed surface of themetal. This layer serves as a barrier to further contact with oxygen. Even wheniron combines with oxygen, a thin, almost invisible coating of iron oxide formsthat prevents further rusting when no water molecules are present. When wateris present, however, the oxide that forms is bulky and porous, allowing oxygencontinued access to the iron below.Other metals also corrode slightly under normal atmospheric conditions.Copper and its alloys brass and bronze are protected from continuous andpenetrating corrosion by the formation of a green patina, or film, calledverdigris, which is composed of copper carbonate. In many instances buildingswith copper-clad roofs and trim are deliberately allowed to develop patinasbecause the color is considered attractive.Corrosion takes place at a much faster rate in heavily indust...
