328
NEWS
EDITION
Vol. 11, No. 22
New Uses for Thin Films JUNIXJS D. EDWARDS, Aluminum Research Laboratories, Aluminum Co. of America, New Kensington, Pa. OXIDE FILMS on aluminum are as old as the metal itself. the Eloxal process, and the electrolyte is said to contain 3 per Were it not for the ever-present, self-forming oxide film, alumi- cent oxalic acid plus 0.1 per cent chromic acid. The chromic num—high in the electromotive force series—would not be acid is reduced by the oxalic acid, and the chromium, possibly the stable, corrosion-resistant, structural metal which it is today. as chromium oxalate, gives a purple tint to the electrolyte. The films made in For many years oxalic acid elecscientific interest trolyte are yellow in oxide films on to gray. The use a l u m i n u m cenof s u l f u r i c acid tered in their elece l e c t r o l y t e , the tric or electrolytic Alumilite process, properties. Aluhas r e a c h e d its minum, in certain highest developelectrolytes, h a d ment in the United asymmetric propStates. e r t i e s . Current Certain charpassed when the acteristics of the aluminum was electrolytic oxide cathode, but when coatings on alumiit was anode a film num are essential formed which had for special service high r e s i s t a n c e requirements. and e f f e c t i v e l y Oxide c o a t i n g s blocked the pash a v e been used sage of c u r r e n t . extensively for the This property has protection of cafebeen employed in teria trays. Here the e l e c t r o l y t i c a combination of current rectifier. a b r a sion-r esistA more important ance and corrouse of such films sion-resistance is is in the electrodesired to elimil y t i c condenser. nate the smudging This consists of an experienced when aluminum anode, a soft metal tray an oxide film on is dragged along a its s u r f a c e comtray rail or over posing the dielecWESTXNGHOTJSE MURAL SYMBOLIC OP RADIO BROADCASTING a hard porcelain tric, and an elecKDKA and its associated short-wave station W8XK are used as a central theme in this mural's surface. Or again, trolyte f o r m i n g design. The background is a map of the world which has shrunk to the size of a neighborhood by reason a highly adsorpthe second plate of the far-reaching effects of radio. Concentric rings, centered about the map location of Pittsburgh, the home of Westinghouse and KDKA, bind the whole world together in this universal communication. tive coating is deor electrode of the Side panels on the mural portray radio's benefits; how radio brings in a twinkle of an eye c millio sired t o permit condenser. A n family firesides to the ringside of an important boxing match, to the platform of key political speeches, coloririg by dyes other aluminum to a point of vantage in great public gatherings, to orchestra seats at the theater, concert, and opera, to outstanding religious ceremonies, and to the better night clubs and dance orchestras. for d e c o r a t i v e member may be purposes. Sulfuric used as a current conductor to the electrolyte. The electrolyte may be in liquid acid in certain concentrations and under certain coating conor paste form; the so-called dry electrolytic condenser using ditions has proved a very versatile electrolyte constituent. Bengpaste electrolyte is being made by the millions today and its ston (4?) has found that an electrolyte containing about 60 per cent sulfuric acid gives a coating which is highly adsorptive to development is reaching unexpected goals. The useful films in electrolytic condensers and rectifiers are dyes. Furthermore, the coating structure is such that thin aluvery thin—comparable with wave lengths of light—and show minum sheet, coated and colored, can be stamped and formed typical light interference effects. The past ten years have seen into buttons, tags, and other small articles without injury to the much thicker oxide films the subject of intensive investigation coating. Gower (5), using sulfuric acid in a concentration of and commercial exploitation. These thick oxide films are of about 25 per cent, na.3 produced coatings which combine good interest, not only for their electrical properties, but for me- abrasion-resistance with color adsorption capacity. Tosterud (6) chanical, physical, and chemical properties which are finding has made highly abrasion-resistant coatings with an electrolyte containing about 7 per cent sulfuric acid. a host of uses for them. The discovery by Flick (4) that adsorptive anodic oxide The films used in rectifiers and condensers are always formed electrolytically. It is possible, however, by appropriate chemical coatings could be colored by certain types of dyes marked the reaction, to form oxide films on aluminum. They are generally beginning of a new and remarkable development in the oxide lacking, however, in certain properties—-particularly imper- coating of aluminum. The color penetrates the oxide coating viousness and hardness—which make the thick type of anodic throughout its depth and is adsorbed and held so strongly that coating especially useful. Films for electrolytic condensers are it is not readily dissolved, except by solutions which attack the usually made in solutions of boric acid or ammonium borate. oxide itself. It became possible to coat aluminum with a dense, While anodic, these films may withstand voltages as high as 500 hard, adherent oxide coating colored throughout with almost volts without breakdown. Kemoved from the electrolyte and any color of the rainbow and with an endless number of tints and dried, however, they puncture at a few volts. The films are shades. The dyed oxide coatings made with selected dyes are usually thin because they block the passage of oxide-forming relatively permanent indoors and show little or no fading in a current. If a different type of electrolyte is employed, the film 300-hour Fadeometer test with an arc rich in ultra-violet light. formed may have a sufficiently low resistance in the electrolyte Outdoors, however, the dyes show fading in time. For severe to permit continuous passage of current and, within limits, service conditions, the oxide coating can be colored with, incontinuous building of the oxide coat. Oxide films up to a mil organic pigments formed within the pores of the coating and held firmly adsorbed. The range of colors, however, is somewhat or more in thickness can be made in this way. A variety of electrolytes have been proposed for this purpose; limited. A novel and interesting application of dyed and oxide-coated sulfuric acid, chromic acid, and oxalic acid have received the most retention. Bengough and Stuart (1), in England, developed aluminum is the "Factory Made Mural*' which the Westinga process for the anodic oxidation of duralumin in a 3 per cent house Electric and Manufacturing Co. exhibited at A Century solution of chromic acid in order to enhance its corrosion-re- of Progress Exposition. Sheets of aluminum colored by the sistance. The process has been widely used, but because the Alumilite process are cut to form the pattern of the mural and coating is relatively thin, it has been the general practice to are inlaid in Micarta by a Westinghouse process. The luster of protect the coating with top coats of paint. Paint coatings the colored aluminum against the Micarta background makes a adhere to it well, and the combination system of oxide coating striking contrast that adapts the combination for signs, table and paint gives excellent protection. The films made with the tops, and other utilitarian, as well as decorative, purposes. The abrasion-resistance of the oxide coating is of importance chromic acid electrolyte, particularly on duralumin, are gray. The oxalic acid electrolyte has found its principal applications in many applications. This resistance to wear is greater in in Japan and Germany. In Germany, the process is known as (Continued on page 886)
336
NEWS
New Uses for Thin Films (Continued from page 828) general, the thicker the coating, although the oxide itself will v a r y in character, depending upon the conditions of application. A quantitative measure of abrasion-resistance is obtained b y a test which consists in abrading the surface with an Aloxite wheel under light pressure ($). T h e abrasion-resistance is measured b y t h e number of revolutions which the specimen makes against the abrasive wheel before the coating is worn through. A series of values obtained in this way are given i n the following table: T A B L E I.
ABRASION-RESISTANCE OP O X I D E COATINGS A S A FUNCTION O F THICKNESS ABRASION-RESISTANCE AGAINST THICKNESS OP COATING ALOXITE WHEEL Inches Revolutions 0.00011 15 0.00019 95 0.00031 268 0.00037 365 0.00074 2563 0.00148 7061
These values are much higher than those obtained with baked enamel coatings, such a s are applied in finishing metal furniture, even though the enamel coating be very much thicker. The oxide coatings produced electrolytically on aluminum when subjected to x-ray examination show no evidence of crystallinity a n d appear to b e amorphous. Small amounts of water held in the coating are probably held by adsorption, a s there is no evidence of a n y hydrate of aluminum. How ever, by a special after-treatment the oxide can be converted, a t least in part, into crystalline monohydrate, a change which i s accompanied by a marked decrease in the permeability and a n increase in i t s dielectric a n d protective properties. While, for purposes of coloring, an adsorptive film is desired, for many purposes a film is needed which is completely impervious. A variety of sealing treatments have been developed t o accomplish this result without changing the appearance of t h e coating. Still other sealing treatments markedly increase the corrosionresistant properties of the coating. The uses tor oxide-coated and colored aluminum are on the increase. Several million pounds of aluminum articles of one kind or another are so treated every year, wbi^h is a well-merited tribute t o the serviceability of these coatings.
EDITION
Vol. 11, No. 22
United States producing 17 tons in 1929 or 15 per cent of the world supply. The United States dominates the molybdenum situation, producing 94 per cent. Canada is the nickel nation, producing 90 per cent. Chile is the only commercial source of natural nitrates. World production of potash is concentrated in Germany and France. Silver production is restricted largely to North and South America, especially Mexico and the United States. Tin mining is concentrated in the Malay States, Bolivia, and the Netherland East Indies, with British interests dominating. China supplies half and Burma one-fifth of the world's tungsten. Two-thirds of the vanadium production is consumed in the United States, largely for steel alloying, but Peru with 60 per cent, southwest Africa with 2 0 per cent, and the United States with 17 per cent are the producers. The zinc industry is centered largely in the United States, with 40 per cent production. Mexico, Australia, Germany, and Poland each furnish 10 per cent.
Emanations T H E CHEMICAL
LITERATURE
(With apologies to Kipling) T h e literature. The chemical literature,— When in doubt look i t up in the literature. Every question man can raise, every phrase of every phase of that question is o n record In t h e literature; Thrashed o u t threadbare pro and con In t h e literature. Did t h e universe a t large once carry a positive charge? Why aren't the holes i n macaroni square? From Avogadro's number to t h e analysis of cucumber, if you're interested you'll find it, for it's there In t h e literature. In Journal this or Zeitschrift that, Comptes rendus or Zenlralblatt, It's somewhere In t h e literature. P. G. HORTON
LITERATURE C I T E I
(1) (2) (3) (4) (5) (6)
Bengough and Stuart, U. S. Patent 1,77*,910 (July 29, 1930). Bengston, U. S. Patents 1,869,041 ~ . a 1.869,042 (July 26, 1932). Camp, IND. E N O ° .., *v, oox U928). Flick, U. S. Pe ent 1.5*0,127 (Feb. 10,1925). G',wer. U. S. 'atent 1,869,058 (July 26, 1932). Tosteru.l, U S. Patent 1,900,472 (March 7, 1933).
MINERAL DISTRIBUTION THE A
2.R1CAN INSTITUTE O F M I N I N G and Metallurgical E n
gineers " a-s organized T h e Mineral Inquiry for the purpose of maki .'.r ±actual studies of the world's mineral resources in their p o l i t e Λ and international relations. There is a remarkable COD.· itration of these minerals in a few countries. The informf/ on is particularly interesting, for while synthesis has a r**· .arkable record in supplementing natural resources, certain
