MODERN METHODS of METAL FINISHING' RAYMOND R. ROGERS Columbia University, New York City
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N THIS age of metals it is not surprising that the chemical engineer is developing numerous methods of finishing which greatly increase the usefulness of the metals. At the present time the finishing of metals is carried on chiefly to increase their resistance to corrosion or abrasion and to improve their appearance. Finishing to Improve Corrosion-Resistance and Apfiearance.-Much has been written about the corrosion of metals, and rightly so. Iron, which is by far the most important metal a t the present time, is particularly subject to corrosive attacks, not only by many strong chemicals, but also by comparatively mild materials, such as those which are present in the ordinary atmosphere. Thus, when exposed to the atmosphere, iron soon becomes unattractive and its usefulness is frequently decreased to a great extent. The most obvious way to avoid this diEculty is to cover the iron with a film of some material which is less susceptible to the action of corrosive substances. This film may be of
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A course of lectures in Metal Finishing is given by the author in the Department of Chemical Engineering at Columbia University.
(1) another metal such as copper, tin, zinc, nickel, or chromium, (2) a compound of iron such as the oxide or phosphate, (3) non-ferrous inorganic material such as vitreous enamel, (4) a substance which is entirely organic or else a combination of organic and inorganic materials such as paint, varnish, lacquer, or rubber.
Another method of decreasing the corrosion of iron is to add other metals, such as nickel, chromium, and silicon to i t to form an alloy. Strangely enough, anumber of these alloys are less noble than iron itself and yet, under many conditions, they corrode less. One important reason for the corrosion resistance of such alloys is the fact that extremely thin "natural finishes," probably oxidic in nature, are formed upon their surface. Then, in many cases, the properties of iron and its aUoys make i t necessary to abandon them entirely and to choose some other metal or alloy which does possess the desired properties. The use of such metals as nickel, aluminum, and magnesium, and of such alloys as brass and monel metal is increasing annually.
Some of these metals (as copper, nickel, and aluminum) are protected by "natural finishes" more or less similar in nature to those on the surface of the iron alloys already mentioned. Some of them (as aluminum, zinc, magnesium, and their alloys) arefrequently coated with another metal or with some material of the paint or lacquer type. An oxide film exhibiting many important properties may be produced electrolytically on the surface of aluminum and some of its alloys. This film is very hard, is resistant to certain types of corrosion, will withstand rather high voltages and is capable of being colored. Finbhing to Imprme Resistance to Abrasion.-The resistance of iron to abrasion may be increased by processes similar to those used to increase its resistance to corrosion and to improve its appearance. I t is well known that electroplating iron with chromium will greatly increase the resistance of the iron to abrasion. Also, if a film consisting largely of iron compounds sucb as the carbide or nitride is formed on the surface of iron, the hardness will be increased. The addition to iron of certain metals such as tungsten will improve its resistance to abrasion. Highspeed steel containing as much as eighteen per cent. of tungsten will retain its hardness even a t quite high temperatures. Sometimes i t is advisable to use some non-ferrous material. For example, certain alloys composed of cobalt, chromium, and tungsten (called stellite) are becoming increasingly well known for their hardness, even a t red heat. Coating with Another Metal.-One metal may be coated with another metal in a variety of ways. If the coating metal has a comparatively low melting point, i t may be applied in a molten condition by the process of hot dipping. The nobler metals are frequently plated by electrodeposition from aqueous solutions of their compounds. Hot metal powder may be applied by cementation processes. Finally, almost any metal may be sprayed not only upon metal surfaces but also upon materials as easily destroyed as paper. Coatings of Iron Compounds.-The pleasing appearance of the surface colors produced by heating iron in the air is well known. Other iron oxide coatings which are valuable because of their protective properties and their appearance may be produced by processes which are somewhat more complicated. The importance of phosphate coatings on iron, either alone or beneath a film of paint, enamel, or lacquer, is rapidly increasing. Iron carbide, iron nitride, or a combination of the two can be produced on the surface of iron, depending upon the process used. Vitreous Enamels.-Although vitreous enamels were used as long ago as three or four centuries before Christ, the production of enameled metals is one of the most complex procedures in the field of metal finishing,
and much remains to be learned about it. Perhaps the most important requirement in order to obtain good results in this field is a knowled~eof hieh-temoerature chemistry. Paints, Varnishes, Lacquers, Rubber.-At the present time the amount of research which is being carried on in this field is enormous. There is the meatest rivalrv between enamels and lacquers and between other kinds of finishes in this same general field. Also, there is a keen competition between these finishes and the metallic coatings. The production of new alloys, many of which must be protected by some kind of finish, has still further added to the feverish activity of the paint, varnish, and lacquer chemists. Rubber-coated metal is being used extensively, especially in the production of containers for highly corrosive materials. Importance of Clean Metal Surfaces.-One important requirement is common to most of the different metal finishing processes. The surface of the iron or other basis metal must be clean. It must not only be free from metallic oxides but also from traces of grease if the best results are to be obtained. The oxides may be removed by chemical or electrochemical pickling and the grease and other d i i , whicb is frequently associated with it, by chemical or electrochemical cleaning. There are many possible cleaning and pickling procedures, and it is necessary to choose the one which best fits the properties of the basis metal which is to be finished. Cladding.-It has been mentioned already that either non-ferrous metals or alloys of iron may be used advantageously for many purposes. Unfortunately, these materials are frequently very expensive, and for that reason their use is much more limited than it otherwise would be. For a number of years attempts have been made to "clad ordimary, inexpensive iron with a fairly thin coating of a more expensive metal. In this way the desired properties of the expensive material could be obtained at a more reasonable price. Until recently it was difficult, if not impossible, to obtain a sufficiently strong weld between many of the cladding metals and the backing of ordinary iron due to the impossibility of eliminating the "natural finishes" from the surface of the cladding metals. Now, however, a method has been developed which does eliminate these "natural finishes," with the result that excellent welds may be produced quite readily. It is now possible to obtain ordinary iron clad with sucb metals and alloys as nickel, stainless irons, 18-8 chromium-nickel steel, high carbon steel, high speed steel, and stellite. Also, some of the cheaper metals and alloys such as copper and brass may now be clad with noble metals such as gold, silver, and platinum. It is hoped that enough has been written in this brief outline to show the extent and importance of the rapidly expanding field of metal finishing.
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