CHEMISTRY, ELECTROCHEMISTRY, AND ELECTROCHEMICAL APPLICATIONS Contents Aluminum Carbon Iron Hydrogen Lead Lithium Manganese Nickel Oxygen Platinum Group Elements Silver Zinc Aluminum Q Li, JO Jensen, and NJ Bjerrum, Technical University of Denmark, Lyngby, Denmark & 2009 Elsevier B.V. All rights reserved. Introduction Aluminum is the most abundant metallic element, making up about 8% by weight of the Earth’s crust. It is a silvery- white metal and belongs to group III of the periodic table. Its atomic number is 13 and atomic weight 26.981 54. Pure aluminum is soft and ductile. However, it can be alloyed with small amounts of copper, magnesium, and silicon to increase its strength and impart a number of useful prop- erties such as high strength, good ductility, and low density. The most common aluminum alloy, A6061, for example, contains copper (0.15–0.6%), magnesium (0.8–1.2%), silicon (0.4–0.8%), zinc (o0.25%), and iron (o0.7%), and is widely used as a vital structural component in aerospace, automotive, railroad, and other industrial applications. Chemically, aluminum is very reactive. Whenever a freshly created aluminum surface is exposed to air or water at room temperature, an oxide film forms immediately and grows to a thickness of about 5 nm in air and to a somewhat greater thickness in water. This oxide layer is impervious and adherent to the metal surface, protecting aluminum from further corrosion. It is this oxide layer that makes aluminum remarkable for its corrosion and wear resistance. Aluminum surface coatings onto other metallic substrates are a well-known technology for corrosion protection. Anodizing, an electrolytic passivation process to increase thethicknessofthenaturaloxidelayeronthesurfaceof metal parts, forms an important part of aluminum elec- trochemistry and offers better use of the oxide layer. The electrochemically prepared oxide layers can be either porous or barrier type, the former providing corrosion re- sistance and allowing for coloring the surface with organic dyes, pigment impregnation, or electrolytic deposition of other metals and the latter being dielectric and character- izing the capacity of aluminum electrolyte capacitors. From an energy storage and conversion point of view, aluminum is a very attractive anode material. The for- mation of the oxide surface layer on an aluminum anode, however, causes a significant decrease in the reversible electrode potential as well as a time lag in reaching the operating potential of a battery. Activation of the alu- minum electrode and depression of the parasitic cor- rosion are the main focus for developing aluminum alloys as anodes for batteries as well as for cathodic protection. Electrochemically, aluminum belongs to the group of metals with very negative electrode potentials. Electro- chemical deposition of aluminum from any aqueous media is therefore impossible owing to the hydrogen evolution at the cathode. Consequently, electrolytic production, refining, and plating of aluminum as well as development of re- chargeable aluminum batteries require nonaqueous elec- trolytes, that is, either molten salts or organic electrolytes. This article starts with a brief summary of chemistry of aluminum with emphasis on its behavior in different 695 . CHEMISTRY, ELECTROCHEMISTRY, AND ELECTROCHEMICAL APPLICATIONS Contents Aluminum Carbon Iron Hydrogen Lead Lithium Manganese Nickel Oxygen Platinum. silicon (0.4–0.8%), zinc (o0.25%), and iron (o0.7%), and is widely used as a vital structural component in aerospace, automotive, railroad, and other industrial applications. Chemically, aluminum. small amounts of copper, magnesium, and silicon to increase its strength and impart a number of useful prop- erties such as high strength, good ductility, and low density. The most common aluminum