638 CREATIVE CHEMISTRY electrons and liberate hydrogen gas. Oxygen gas of the air in the presence of water will also accept electrons to produce hydroxyl ions, (0H)~. Iron and steel will therefore corrode in very weakly acid solutions or even in the presence of water containing an abundance of dissolved oxygen. Cathodic protection is successfully used for buried pipe lines, cables, etc. It consists in placing "anodes" in the soil and maintaining an electromotive force of a few tenths of a volt between these "anodes" and the metal to be protected, which, as long as it is cathodic, resists corrosion because it cannot lose electrons as long as it is positively charged. The anodes are slowly sacrificed by electrolysis, but they can be replaced at low cost. Different points on the surface of a metal may vary in their tendency to lose electrons, due to differences in the composition of the metal at different points on the surface, differences in hardness, strains due to poor annealing and other causes, differences in crystal orientation within the metal, differences in exposure or radiation, differences in the composition of the dust particles at different points on the surface, differences in dust concentration, differences in moisture, and many other factors. Although corrosion is a very complex problem, great progress is being made in methods of combating it. In nearly every case corrosion is prevented by covering the metal with a surface film which will prevent contact with oxygen or water. Iron, properly cleaned and dried, is painted, and as long as the paint film is maintained intact and impervious to moisture, no corrosion will take place. The first coat of paint usually contains red lead as the pigment, although zinc chromate is replacing it, particularly for under-water work. Exposure to sunlight, water, and air quickly oxidizes the paint vehicle and therefore allows the pigment to dust off. Iron may also be protected for some uses by giving it an initial protective coating of iron oxide. This is done in a number of different ways. Black iron oxide is produced by dipping the articles in melted potassium nitrate, KNO3. This produces the familiar blue coat on such articles as watch hands. A film of oil or grease prevents rust. Likewise, the porcelain-like enamel of stoves and kitchenware prevents rust as long as no portions are cracked or chipped off. Once broken, however, the rust spreads under the enamel, and it begins to scale off. In many cases rusting is prevented by coating the iron with a metal which is not so easily corroded. Nickel was once used widely for this purpose, but it is now supplanted by the even less corrodible and more beautiful chromium. Electroplating with chromium is an achievement of the last
METALLURGY 639 twenty years. Copper is sometimes used to coat iron, but copper is so readily tarnished itself that it is not always a desirable coating for iron, although it will prevent rust as long as it covers the whole surface. Metallic coats may be applied by electrolysis; by electrochemical displacement, as in the case of copper, zinc, tin, nickel, chromium, and cadmium; by dipping the iron into the molten metal as in the case of zinc, tin, and aluminum; by spraying with the recently invented metallic-spray guns, which melt the metal and spray it on a surface in one operation; by cladding, i.e., heating thin sheets of such metals as copper, nickel, or aluminum into the surface; by using the finely divided metal as a paint pigment; and by various other methods. As mentioned above, tin and zinc may be applied to iron by dipping in the molten metal. Tin is the favorite material for cans and pans. It affords excellent protection as long as the surface is unbroken because it reacts with the oxygen of the air to form on its surface a thin film of tin oxide, which resists further action. Tin is below iron in the electrochemical series and therefore accepts electrons from the iron, once the surface is scratched, so that the iron then rusts even more rapidly than it would in the absence of the tin. Zinc, on the other hand, is above iron in the electrochemical series, and all of the zinc will go into solution before the iron is attacked. Galvanized iron is therefore preferred to tin for many purposes, but it cannot be used in contact with foods because it may dissolve to produce toxic zinc compounds. Cadmium, a metal similar to zinc, is now being used as a metallic coating, especially of hardware articles. Zinc may be applied to iron by the Sherardizing process, in which the articles are heated with zinc dust in a tight drum to 800° F., thus forming alloys at the surface. Aluminum powder in fine flakelike form is now widely used as a pigment in paints for the protection of bridges, oil tanks, and many other metal structures. Aluminum, like tin, does not easily corrode, because it forms a protecting layer of aluminum oxide on its surface; if it were not for this fact, aluminum would corrode more readily than iron because it is above iron in the electrochemical series and, therefore, has a greater tendency to give up electrons. In 1907 Thomas Coslett, an English chemist, invented the process which now is known as the " Parkerizing Process." It consists of producing a coating of basic iron phosphate by dipping the iron into a hot, dilute solution of iron phosphate. This process gives a pleasing dull-black finish that serves as an excellent base for paint or enamel.