http://mysite.du.edu/~jcalvert/phys/zinc.htm The symbol in the title is an early chemical symbol for zinc, in the spirit of alchemical symbols. It had to be invented by Olof Bergman of Uppsala, in the 18th century, because metallic zinc lacked a traditional symbol. Zinc was named by the Swiss alchemist Theophrastus Bombastus von Hohenheim (Paracelsus, 1493-1541), who coined the new Latin word zincum from antecedents that are not clear. A name was necessary for the newly-prepared metal, although its alloy, brass, had been known since ancient times. In English and French, this became zinc, in German and Dutch zink, in Spanish cinc, in Welsh sinc (pronounced "shink"), in Greek pseudargyros ("false silver") or tsigkos, pronounced "tsingos." In Russian, it is tsink. The kitchen sink has nothing to do with zinc, unless it happens to be made from it. Cadmium was only discovered in 1817 as an impurity in zinc by Stromeyer, who studied the deposits in lead and zinc furnaces called cadmia fornacis, and accordingly the metal was called cadmium. After iron, aluminium and copper, zinc is usually the fourth-most used metal, competing with lead. We probably see it every day, use it nearly as often as a source of electrical power, and handle small parts made mostly of it. This would be enough to make the metal interesting, but it also gives us valuable insights in physics and chemistry, and gives us an excuse to discuss them in this article. Some time ago, when new science courses for liberal-arts majors were being discussed at the University of Denver, a course entitled WATER was proposed, probably because of the role of water in our environment. Professor Edgar Everhart, the comet-chaser, considered this less than attractive to the usual liberal-arts student, and suggested another possibility, ZINC. Well, we didn't follow up either suggestion, probably correctly, but preparing this article recalled the incident to me. So here, then, is ZINC. The metal cadmium will also appear, since it is similar to zinc and closely associated with it, and is also useful, though much less used. The largest use of zinc is as a protective coating for iron. The process is called galvanizing with reference to the cathodic protection the zinc offers to the iron. The name was coined by Sorel, who patented the process in 1836. Some sources say that galvanizing began at Swansea, at the early zinc smelter, in 1740. It could not have been called "galvanizing" then, of course. It is usually carried out by dipping carefully cleaned iron or steel in molten zinc, which gives the rather thick, robust coating required. Tin, another familiar protective coating, is now usually applied electrolytically, since it is easier to make a reliable thin coating this way. Tin is much more costly than zinc, which is an incentive to thin coats. Galvanized steel cannot be used in cans for food preservation as tin is, however, because the zinc coating is attacked by food acids to which tin is immune. Zinc, however, gives excellent protection against the weather and moisture, so it is preferred where this is important, since it is cheaper. Zinc protects the iron by cathodic protection, since it is higher on the electrochemical scale than iron and will sacrifice itself to protect the iron, reducing it to the metal and eliminating rust. This phenomenon was noted by Faraday in 1829. If it sacrifices too much, however, the iron is exposed to oxidation, as is sometimes seen with old or damaged galvanized iron. The life of galvanized steel depends on the thickness of the coating and the environment. The life is approximately proportional to the thickness of the coating, however it was applied. A coating of 1 ounce per square foot, giving a film 0.0018" thick, has a life of about 25 years in rural locations, 10-15 years in an urban environment. Note that the coating is on both sides of a sheet, so the total zinc use will be 2 oz./sqft in this case. Zinc will not give cathodic protection if it becomes passivated, or covered by a closely adherent layer of hydroxide, since then the necessary currents cannot flow. However, the layer will protect the zinc from corrosion, also protecting the underlying metal. Zn(OH)2 is insoluble for pH between 6 and 13, and in this range the hydroxide will protect the zinc under water. Aluminium and chromium are protected and passivated by the oxides, lead by lead sulphate. Iron or steel articles can be mixed with zinc dust and tumbled in a steel drum at about 370°C, below the melting point of zinc. The articles must first be thoroughly cleaned, by pickling in 50% HCl, or a similar process. The zinc alloys with the surface to form a thin but very adherent layer that protects the iron underneath, and will not clog fine details such as threads. About 15 mg/cm2 of zinc is used. This process is called Sherardizing, after Sherard Cowper-Coles, who patented it in 1901. Iron can be coated with chromium or aluminium by similar processes. These procedures are known in general as cementation, in which an alloy is formed without melting. The next use in tonnage is as wrought zinc, that is, as plate, tubing, and other forms made by rolling or other shaping processes. Unlike tin or aluminium, zinc is insufficiently malleable to form good foils, so we do not see them. Sheet zinc was once used for roofing, where it has a very long life in this application destructive to most metals. In contact with the atmosphere, which contains H2O and CO2, a closely-adhering layer of basic zinc carbonate, Zn(OH)2·ZnCO3, forms and protects the metal. Sheet zinc is now used mainly for battery anodes, in which it is effectively "burned" to produce electrical energy. Batteries are little fuel cells burning zinc, and are a more practical device in their field of application than fuel cells burning alcohol or hydrogen and using the oxygen of the air, at least so far. Closely following this use, and perhaps exceeding it at times, is the use of zinc to make die castings. The most common process is pressure die casting, in which the molten zinc is forced into steel dies that make the mold. Zinc expands on cooling, so it fills the mold exactly, like type metal, and can make precision castings requiring very little machining. The low melting point of zinc gives long die life. Automobiles are full of die castings, from brake cylinders and fuel pumps to door handles, many of which are plated to give them a shiny finish. Carburetors at one time were made up nearly completely of die castings. In mass production, die castings are much cheaper than machined parts, since the large cost of the dies can be amortized over the numerous products. Die castings required the production of extremely pure zinc, since the usual impurities caused the castings to swell and "crystallize" in a short time. They didn't actually crystallize, of course, but the impurities migrated to the crystal boundaries and caused embrittlement. A typical die-casting alloy is Zn 94.9, Al 4.1, Cu 1.0, called "Zamak." Lead, cadmium and tin impurities must be kept to a very low level in the zinc used for this purpose. Small amounts of zinc are used for other purposes. Zinc has occasionally been used in coins, such as the Albanian 1/2 Leku and the United States cent. Zinc chloride, ZnCl2 is used as a soldering flux for soldering iron with tin-lead solders in a water solution. It hydrolyzes to give an acid reaction, ZnCl2 + 2HOH → Zn(OH)2 + 2H+ + 2Cl-, which is not as corrosive as pure hydrochloric acid would be. Zinc is used in aluminium solders, such as 75 Zn, 20 Cd, 5 Al, which is Bureau of Standards aluminium solder ZN1. Zinc is also used in silver solders, such as 52 Cu, 38 Zn, 10 Ag, which melts at 820°C. This is actually a silver-bearing brass, which gives the name "brazing" to the process. Zinc oxide, ZnO, is a white pigment, and "blue powder," a colloidal dust of small spheres coated with oxide, is used in paints for protecting ships. Zinc oxide, ZnO, with 0.5% ferric oxide, Fe2O3 is the active ingredient in calamine lotion, which soothes irritated skin. Zinc appears in very small amounts in the mineral supplements favored by those who think eating it will improve the health. Zinc is required in the diet in such small amounts that it is always present in a normal diet without the need for supplementation. Zinc appears to be rather non-poisonous, though in large quantities it is carcinogenic. It is used in several lotions applied topically. If zinc oxide is breathed, the strange nervous malady "oxide shakes" seems to result. Cadmium is a by-product of zinc production, much rarer and used only in small amounts. Cadmium is prettier than zinc, since it is whiter, like tin or silver. A few percent hardens and strengthens copper without decreasing its conductivity greatly. Cadmium copper is used for electrical contact wires, where durability and low resistance are both desirable. It also is used as a protective coating for iron, in very thin films. These films alloy with the iron and are hard to damage in use, so cadmium-plated screws and bolts are found. It is used as the anode in rechargable nickel-cadmium cells. Its main use is in alloys, where it can replace the more expensive bismuth in fusible alloys. It can also replace expensive tin in some applications, such as solders. However, cadmium is extremely poisonous, more so even than mercury or lead. It attacks the kidneys, among other things. Cadmium may be essential to the rat metabolism, it is believed. Its use has been strongly discouraged where its vapor or dust may be created, or it is disposed of carelessly. However, cadmium is being persecuted, like mercury and lead, although it is a completely negligible hazard in its normal uses. --

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