Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/373

IRON 357 gas at the solidifying temperature as at the higher temperature, and consequently of preventing the separation of gas during solidifi cation ; or else to its chemical reaction upon the dissolved carbon oxide forming particles of silica disseminated through the mass and setting free carbon, so that the metal containing less dissolved carbon oxide can thus retain more hydrogen ; experimental evi dence is wanting finally to decide the question, but the latter view seems to be highly probable. 45. Protection of Iron from Oxidation by Surface Appli ances. One of the greatest inconveniences in connexion with the use of iron and steel for constructive and general purposes is the tendency of the metal to oxidize and rust in the air under ordinary atmospheric conditions, i.e., in presence of free oxygen together with moisture and small quantities of carbon dioxide (and in the case of the air of seaside places of saline spray, and in that of coal-consuming districts of sulphur acids, &c,). Highly polished iron can be kept in perfectly dry air without rusting at all ; and in contact with a solution of an alkali, such as caustic soda or ordinary carbonate of soda, the tendency to oxidize is far less than in simple spring water, sea-water, or moist air ; in certain cases this property may be utilized for the pre servation of bright steel objects kept in stock, a coating of ordinary whitewash (chalk and water, or preferably lime and water) being brushed over them and allowed to dry on ; a similar coating of lime or whiting and oil is even more efficacious, especially if the oil be a non-drying one, i.e., one that does not spontaneously take up oxygen and become more or less acidified. Protective coatings of paints of various kinds tar, melted pitch, &c. are generally applied to the exterior of large iron constructions, such as bridges, pillars, girders, rainwater spouts and conduits, railings, and the like ; the function of these is more mechanical than chemical, the coating simply preventing the metal from coming in contact with the oxidizing medium ; but in some of the paints used the basic character of certain of the materials probably also diminishes the tendency to oxidation. In certain cases the corrosion of iron can be diminished by placing a more active metal in contact with it (e.g., zinc), so that by a galvanic action the oxidation is largely limited to the zinc ; by causing the surface of the iron to be closely adherent to the protect ing zinc coating (by dipping the brightened metal in fused zinc), a sort of permanent metallic paint coating is obtained, which acts as a preservative in the threefold manner of mechanically preventing contact with air, of galvanically confining the oxidation to the zinc, and of chemically causing the iron to be coated with a basic film of zinc oxide (when the zinc has become slightly oxidized). Many other metallic protective coatings can be similarly applied. Of these the most frequently employed is tin, forming ordinary " tin plate," the manufacture of which dates back a considerable length of time. Other coatings can also be applied by means of electricity, at least to small articles ; electro-coppered iron goods and nickelized steel articles thus prepared are now frequently manu factured. It has long been noticed that iron bars, plates, &c., from the forge or rolling mill are coated externally with a film of magnetic oxide formed by the action of the air on the heated metal ; during rolling this film becomes somewhat thick and peels off, forming " mill-scale " ; when the film is but thin and is closely adherent, it has long been known that the metal does not rust so readily at the parts thus covered up with a protective coating as at points where this coating is removed. Similarly it has been long known that certain natural forms of magnetic oxide, of iron, and in parti cular the "iron sands" of New Zealand, Canada, and elsewhere possess the power of resisting the rusting action of sea-water and atmospheric air conjoined. The idea or purposely coating iron articles with a film of magnetic oxide prepared in such a way as to resist to the greatest possible extent natural pcroxidation, and so closely adherent to the underlying metal as not to be readily de tached by ordinary usage, has accordingly been successfully carried out in practice by more than one person. Barjfs process for pro ducing such a coating is based on the reaction occurring between iron and steam at moderately elevated temperatures, viz., 3Fe + 4H 2 = Fe 3 O 4 + 4 H 2 , hydrogen and magnetic oxide being produced ; the articles to be protected, having first been worked into shape, are placed inside a large muffle or chamber of brick heated to a suitable temperature (400 to 600 C. ), and subjected to the action of superheated steam for a period of from five to ten hours according to the thickness of the coating required. Hoiver s process consists in the exposure of the iron articles to be protected to the action of air and carbon dioxide in a chamber or retort heated externally and capable of being closed air-tight ; when hot, air is blown in from time to time so as to produce a thin coating of oxide on the surface. A modification of the process consists in alternately oxidizing and reducing the sur face, the fuel being burnt inside the chamber ; heated air, in excess of that requisite for the combustion, is introduced, and the surface oxidized, so that the outer film is peroxide, magnetic oxide under lying ; by shutting off the air supply for a short time the peroxide is reduced and a uniform film of adherent magnetic oxide produced. Both processes can be so worked as to give a highly protective coating to the iron, so that the articles treated will resist ordinary atmospheric influences for long periods of time. The chief weak point in each case (excluding the question of cost) is that it is impossible to apply any amount of force to the treated articles with out cracking or stripping off chips of the coating, so that protected iron articles cannot be hammered, rivetted together, bent, brother- wise subjected to mechanical strains, but must be worked to the dimensions and shape (however large) that are ultimately required before treatment ; hence the processes become too costly for large girders, &c. Tin and Tcrnc Plates. In order to protect thin iron plates from oxidation and to enable them to be readily soldered together so as to manufacture the numerous articles in ordinary use made by the " whitesmith " or "tin-man," they are coated over with a closely adherent layer of metallic tin, forming tinplate ; when instead of pure tin an alloy of tin and lead is used as a protective coating, the product is known as terncplatc. The charcoal plates ( 23) prepared from a good quality of iron used for the tinning process and pro perly annealed are thoroughly scoured with sand and water and "pickled" in dilute sulphuric acid alternately until perfectly clean and bright ; they are then washed and immersed in a pan full of melted grease until all adherent water has boiled away, and dipped into a bath of melted tin (covered with grease to prevent oxidation) which adheres to the surface more or less completely ; the first bath is of less pure tin than that contained in a second into which the plate is further dipped so as to complete the coating ; the plate is then taken out, and wiped with a hempen rubber to remove super fluous tin and make sure that the surface is completely covered, and dipped for a third time into another bath, after which it is placed in a grease bath of tallow and palm oil at a moderately high tem perature, so that the surplus tin may run off, and then into a cooler grease bath to avoid too sudden chilling, which would impair the lace of the coating, and finally taken out and cooled in the air. As the tin in the third bath becomes alloyed with iron from the opera tion, it is removed into the second, fresh pure tin being used instead ; similarly the metal from No. 2 bath is by and by removed to supply the first one, so that the plate is covered with less and less ferruginous tin successively as it passes through the three baths. For terne plates the process is much the same, only an alloy of about half tin and half lead is used instead of pure tin ; in con sequence, the surface produced is not so brilliant. When tin plates are partially etched by dilute aqua fortis mixed with common salt or sal ammoniac, they acquire a peculiar spangled appearance, owing to the dissolving away of adherent tin, leaving behind a less readily attacked crystalline tin-iron alloy ; the " moiree metallique" thus prepared, after varnishing to prevent oxidation, is frequently used for ornamental purposes. Galvanized Iron. When perfectly cleansed iron is immersed in melted zinc instead of tin, the zinc adheres to the surface just as tin does in the tinplate manufacture, forming "galvanized iron," the name being derived from the circumstance that the coating is analo gous to that producible by electrical means. Norwood and Rogers apply a thin coating of metallic tin to the iron before dipping it in the zinc bath, by putting in a wooden tank alternately granulated zinc and the cleaned iron plates, the tank containing a dilute solution of chloride of tin, so that the tin is deposited by a kind of galvanic action. By the ordinary processes of electroplating manufactured iron and steel goods can be covered over with protective coatings of other metals, notably copper and nickel. Similarly by various processes silver and gold coatings can be laid on, especially by mciinsof the application of mercurial solutions of the precious metals (water gild ing), the mercury being volatilized by heat and the residual gold or silver made more adherent by burnishing, the process being repeated several times if necessary. " Tyrosilver " goods are prepared by heating the silvered steel whereby the silver sinks, as it were, into the metal; successive coatings are then applied and burnt in," until the sinkage ceases ; in this way a coating is obtained not readily detachable by wear and tear. Protective coatings of various forms of enamel are often employed for various classes of iron goods, more especially advertisement boards, i.e., sheets of enamelled iron