MOLYBDENUM
Metal Coatings Two main types of metal coatings making use of molybdenum are described in the literature. In the first, generally known as moly-blacks, molybdenum is the principal constituent of the coating. Such coatings are often applied electrolytically. In the second type, molybdenum acts as an accelerator or promoter for coatings consisting mainly of other ingredients. Several of the so-called iron phosphate-type prepaint treatments belong to this class. A few patents describe processes belonging to neither of the foregoing classes.
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DONALD PRICE 123 East 92nd S t . , New York 28, N . Y .
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H E R E are two ways in which molybdenum is used in the formation of protective or decorative coatings on metal. I n the first, molybdenum is a major constituent of the coating itself, and in the second it acts as an accelerator or promoter in the formation of a coating made up primarily of other ingredients. The so-called moly-blacks belong to the first class, and the complex iron phosphate type coatings, where molybdenum is used as an accelerator, belong to the second. COATINGS I N WHICH MOLYBDENUM I S MAJOR CONSTITUENT
Although they have only recently come into extensive use, the moly-blacks are by no means new. Kollock and Smith (6) described the formation of these coatings in 1901, and they are further described in Smith’s well-known textbook, “Electroanalysis,” which went through some six editions (15). Both in the Kollock and Smith paper and in the Smith book the purpose in view was the determination of molybdenum. Smith described the method as follows: A solution of sodium molybdate acidified with sulfuric acid is electrolyzed for several hours, whereupon the dark blue color of the solution gradually disappears and the cathode becomes coated with a black deposit described as the hydrated sesquioxide of molybdenum. The deposit was brilliant black and so adherent that it could be washed without detaching any particles. T o complete the analysis, the deposit was dissolved in dilute nitric acid while still moist, evaporated to dryness on a hot plate, and the residue of white molybdic acid weighed. 4 German patent issued to Pacz (11)in 1930 describes a process for coloring zinc and cadmium by cathodic treatment in a dilute solution of ammonium molybdate using platinum, carbon, or other nondissolving electrodes. By the addition of a definite amount of ammonia a black color is obtained. Other metals may also be colored by using a neutral or acid solution and a soluble anode. The claims cover the addition of ammonium salts and the use of a zinc anode and specifically exclude sulfomolybdate. The application of a thick, strongly adherent coating of metallic molybdenum is claimed in a German patent issued to the Gesellschaft fur electrische Gluhlampen (18)in 1932. According to the process claimed in this invention, colloidal molybdic anhydride is first prepared from ammonium molybdate by heating it in a vacuum a t 300’ C. The parts to be treated are then painted with the resulting colloidal paste and heated in a reducing atmosphere of hydrogen or a hydrogen-nitrogen mixture a t 900’ C. It is claimed that the process is suitable for coating medical instruments. Price and Brown (13) obtained smooth, adherent, steel-gray deposits of molybdenum on iron or copper cathodes a t 50’ C.‘ and 36 amps per square decimeter from an aqueous solution prepared by the electrolytic reduction of molybdic acid anhydride August 1955
dissolved in sulfuric acid using platinum electrodes. They studied the effect of molybdic acid concentration, acidity, current density, and temperature. The rate of deposition decreased as soon as the cathode was covered with the gray deposit, and no addition agent could be found that would give bright deposits of molybdenum. Blum and Hogaboom ( 1 ) describe the Price and Brown solution as viscous and of a golden bmwn color and state that it probably contains salts of the lower oxides of molybdenum. There was no conclusive evidence that pure molybdenum have ever been electrodeposited consistently from aqueous solution. Hoffman and Hull described the electrodeposition of molybdenum black finishes ( 4 ) from baths consisting essentially of ammonium molybdate, nickel sulfate, and boric acid in the proportions of 4 to 5 ounces per gallon of the molybdate to 3.5 ounces per gallon of nickel sulfate. Molybdenum had not been used previously with nickel to give black deposits and the bath possessed two unusual features, high rate of deposition and high throwing power. Whether or not the metals deposit directly in colloidal form as basic compounds under the pH a t the cathode was not known. The rate of deposition of molybdenum-nickel coatings described by Hoffman and Hull is surprisingly high and amounted t o 0,001 inch in 10 minutes a t 2 to 5 amps per square foot. Under the same conditions nickel gives a coating only about 1 / 2 0 as thick. The coating was true black and showed no colors under an intense beam of light in a dttrkroom. Over iron the coating showed rust quickly, but on zinc i t increased the salt spray life by 150 hours over that for plain zinc deposits. It retained its black color and gave good outdoor exposure. Hoffman and Hull concluded that black molybdenum plating offers attractive possibilities as a process markedly superior in many respects t o existing black nickel plating and similar black coating processes. They predicted that the unusually lustrous deep black deposit produced, coupled with simplicity in application and cpntrol of solutions, would give the black molybdenum process an important place in metal finiehing. A process for the electrodeposition of a molybdenum black from a bath quite similar in composition to that used by Hoffman and Hull is described in a patent granted to Schweikler ( 1 4 ) . This patent, however, discloses the use of a polyhydric alcohol as an addition agent to give more adherent coatings. The inventor also claims that his coatings may be plated on steel. copper, and brass, but that best results are obtained on zinc. cadmium, tin, or aluminum. If a black finish is desired even traces of copper must be excluded from the bath and for this purpose a small amount of sodium thiocyanate is added. The coatings may be used either for decorative purposes or as electrical insulators where a thin nonconducting coating is required.
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ENGINEERING, DESIGN, AND EQUIPMENT Coatings of the type described are used commercially particularly for blackening zinc, electrodeposited zinc, and zinc-base alloys. They may be deposited not only electrolytically but in certain cases also by immersion. On zinc surfaces they give surprisingly good outdoor weathering and form excellent bases for air-dry painting, lacquering, or enameling. COATINGS IN WHICH MOLYBDENUM ACTS AS AN ACCELERATOR
Westbrook in a patent assigned to the Du Pont Co. ( 1 7 ) describes an improved process for zinc plating in which molybdenum is used; prior to this invention zinc plate had an objectionable dark color that limited it largely to use as a protective coating. By means of this invention a mirrorlike finish is obtained using a zinc cyanide bath containing a substantial proportion of molybdenum in the form of a soluble compound. The deposits are not pure zinc but alloys of zinc with small amounts of the alloying metal.
Steel panels with painted finish over phosphate type coating with molybdenum (right) and without molybdenum (left) after 600 hours in salt spray
The so-called phosphate coatings are applied to iron and steel articles prior to the application of paint or other finishes for the purpose of increasing rust protection and improving the adherence of the finish. If the finish is damaged the nonconducting phosphate coating retards the spread of corrosion under it and markedly increases its life. The foundation for the present day phosphate coating industry was laid by Coslett who filed a patent in 1906 covering the treatment of iron and steel objects with hot, dilute phosphoric acid in the presence of iron phosphates. The purpose of the iron salts was to moderate the action of the raw acid on the iron or steel surface. The Coslett coatings probably consisted of a very thin layer of iron phosphate and even though they afforded only meager rust protection they were soon applied industrially. However, they suffered from one serious drawback; it required several hours to apply them. Accordingly, both Coslett and others sought means to reduce this time. A major improvement was made by Coslett himself who substituted zinc phosphate for the iron phosphate in the original bath. This not only gave more uniform coatings with better corrosion protection, but reduced the time to about half an hour to an hour. Richards later introduced the use of primary manganese phosphate. These two discoveries form the basis for most phosphate coating baths in use today. Even with the improvements introduced by Coslett and Richards, the time required for the application of phosphate coatings was still too long. If phosphating was to be used on a large scale it was necessary to find some means of speeding up the process. Little by little the problem was solved by the discovery of various accelerators, among the most important of which are nitrates, nitrites, and chlorates. The present day baths containing accelerators and other addition agents t o improve the 1512
quality of the coatings may be applied in a few minutes or even less than a minute when pressure spray machines are used. The improvements in the phosphate coating process disclosed in the hundreds of patents that have issued in the field consist for the most part in the use of various additives among which ia molybdenum. The iron phosphate-type bath has also been revived in improved form and for many purposes offers distinct advantages over the conventional baths based on zinc and manganese. The best of the iron phosphate-type processes m&e use of molybdenum and also contain a small amount of molybdenum in the coating. A patent issued in 1918 (9) and assigned to the Parker Rust Proof Co. claims the rust proofing of iron and steel surfaces by treatment with a boiling dilute solution of a metaphosphate of molybdenum or tungsten in combination with a metaphosphate of iron. The preparation of the iron metaphosphate is described; it is used to reduce the cost of the process. A large amount of irop metaphosphate seems t o have no objectionable effect. Notwithstanding the inventor's claims such a process would almost certainly be too expensive for practical use. Boulanger ( 2 ) describes a process for protecting aluminum, magnesium, or their alloys against corrosion which consists in treating articles made of these metals in a bath containing sodium carbonate, one part of alkali sulfomolybdate and five parts of alkali phosphochromate. Pacz describes a phosphate coating process using molybdenum in a French patent issued in 1932 ( I O ) . In addition to 2 to 5% of a soluble molybdate per liter, his bath contains sodium silicofluoride, a mixture of iron and manganese phosphates, and free phosphoric acid. The coating consists of iron and manganese phosphates together with oxides of molybdenum, and it has good corrosion resistance. However, this process has the disadvantage of requiring 15 minutes to apply. A phosphate coating bath for coating aluminum, magnesium, and zinc is described in a U. S. patent issued to Prier in 1933 ( I S ) . The bath contains phosphochromate and sulfomolybdate, trisodium phosphate, and a salt of a hydroxy acid such as tartaric acid. The purpose of the latter is to prevent the precipitation of alumina. Thompson in a patent assigned to the Parker Rust Proof Co. (16) claimed a process for coating iron and steel using a bath containing phosphoric or sulfuric acid and sodium sulfite with a small amount of molybdenum oxide as an accelerator. Since the bath is said to be used a t 100' F. it is hard t o understand what purpose is served by sodium sulfite in acid solution. A patent covering a phosphate coating process for iron and steel in which molybdenum is used was issued to Dodd and Ayres in 1950 (5). This patent describes a phosphating composition consisting of an acid salt of phosphoric acid and a water soluble compound of molybdic or tungstic acids together with a phenol. The preferred ingredients are a compound of molybdenum and a polyphenolic compound such as a tannin. The latter apparently acts as a complexing agent for the molybdenum. The coatings obtained with the solutions described by Dodd and Ayres belong to the so-called iron phosphate type and have met with wide commercial success. For many uses they are preferable to the well-known zinc phosphate coatings. The coating weights of the iron phosphate-type coatings are considerably lower than those of the zinc type. The better grades of the former run 60 to 75 mg. per square foot against 250 mg. per square foot or higher for the zinc-type coatings. The iron phosphate coatings are dense and amorphous in contrast with the zinc phosphate coatings in which crystals are plainly visible. This is an advantage where a metal article has to be partly fabricated after it is painted. The crystalline coatings tend to crack and flake off whereas the amorphous iron-type coatings stand considerable working without damage. Kronstein and his coworkers (6) a t New York University recently made studies of four different commercial phosphate pretreatments, two of the zinc type and two of the iron type. They determined the weight in milligrams per square foot of each coating and compared its surface with that of the bare steel by means of a Brush Analyzer. They also determined the salt spray life of each coating under a standard olive drab (OD) enamel and under a gilsonite test coating by the conventional method as well as by the electrographic printing method developed by Kronstein, Ward, and Roper ( 7 ) . Their studies revealed that one of the iron phosphate treatments in which molybdenum is used stood up just as well as either of the zinc phosphate types, although the latter coatings were more than twice as heavy as the iron-type coating. It is,
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MOLYBDENUM therefore, apparent that the paint bonding characteristics of a phosphate coating do not depend solely on coating weight but on the character of the coating itself. Molybdenum definitely exerts a beneficial effect on the coating in this case.
A U. S. patent recently issued t o Miller and assigned t o Kelite Products, Inc. (8) describes a process for the rust protection of ferrous metals by treatment with a n aqueous solution, the solute of which consists of 5 t o lo’% of molybdic anhydride, 2 to 10% of sodium acid sulfate, and 80 t o %yoof sodium sulfate. The sodium acid sulfate serves to form molybdic acid in solution and to maintain the pH between 2 and 4. The process has the advantage that i t can b,e done at room temperature. This treatr ment does not belong to the phosphate class since it contains no phosphoric acid or phosphates. According to the inventor the coating formed in the process is ferric molybdate. The literature covering the use of molybdenum in protective and decorative coatings is not extensive. But molybdenum shows definite promise in several directions. What is needed is further research.
Dodd, S. R., and Ayres, E. F. (to Oakite Products, Inc.), E. 8. Patent 2.502.441 (Ami1 4. 1950). Hoffman, R. A,‘, andHull, R. O., Proc. Am. Electroplaters’ Soc., 27, 45-50 (1939).
Kollock, L. G., and Smith, E. F., J. Am. Chem. Soc., 23, 669-71 (1901).
Kronstein, M., DeLong, L. F., and Norman, A. W., Paint V a r n i s h Production. 42. 2-12 (December 1952). Kronstein, M., Ward,’M.’M., and Roper, R., IND. ENG.CHEM., 42, 1568-72 (1950).
Miller, D. E. (to Kelite Products, Inc.), U. S. Patent 2,557,509 (June 19, 1951). Oeschger, W. I. (to Parker Rust Proof Co.), Ibid., 1,254,263 (Jan. 22, 1918). Pacz, A., French Patent 728,364 (July 5, 1932). Pacz, A., German Patent 510,380 (Oct. 18, 1930). Price, W., and Brown, 0. W., Trans. Electrochem. SOC.,70,423-9 (1936).
Prier. Pierre. U. 9. Patent 1.923.502 (Aua. 22, 1933). Schweikler, E. W. (to E. I. du Pont de Nemours & Co.), Ibid., 2,351,639 (June 20, 1944). Smith, E. F., “Electro-Analysis,” 6th ed., Blakiston, Philadelphia, 1918. Thompson, J. S. (to Parker Rust Proof Co.), U. S. Patent
2,302,643 (Nov. 17, 1942).
LITERATURE CITED
(1) Blum, W., and Hogaboom, G. B., “Principles of Electroplating and Electroforming,” pp. 348-9, McGraw-Hill, New York, 1949. (2) Boulanger, Charles (to Soc. Cont. Parker), U. S. Patent 1,811,298 (June 23, 1931).
Westbrook, L. R. (to E. I. du Pont de Nemours & Co.), Ibid., 2,080,520 (May 18, 1937). Wolff, H. (to Gesellschaft fur elektrische Gluhlampen m.b.H.), German Patent 580,748 (July 15, 1933). RECEIVEDfor review January 19, 1955.
ACCEPTED June 6 , 1965.
Alloys for the Chemical J
rocess Industries The action of molybdenum in improving the corrosion resistance of alloys has led to the use of many molybdenum-containing alloys in the chemical industry. The four main types of alloys that have long proved their value in this field are (1) austenitic chromium-nickel steels containing molybdenum ; (2) austenitic chromium-nickel steels containing molybdenum and copper; (3) high silicon irons containing molybdenum; and (4) nickel-base alloys containing molybdenum. Properties and applications of these alloys are discussed and mention is made of the potentialities presented by the unusual corrosion resistance of metallic molybdenum.
J . Z . BRIGGS Climax Molybdenum Co., New York 36,
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HE chemical industry is mainly interested in alloys con-
taining molybdenum because of their corrosion resistance. The importance of corrosion resistance is not merely a question of the direct cost of corrosion-in 1949 this was estimated (8) to be $5.5 billion a year for the United States alone-but also the indirect cost through overdesign, loss of product, shutdowns, reduced efficiency, explosion, and contamination. Historically, the development of corrosion resistant alloys has more or less coincided with the greatest period of chemical development. Since the beneficial effect of molybdenum was recognized in the early work on corrosion resistant alloys, the chemical industry has had many years of experience with some of the molybdenum-containing alloys. As a matter of fact, they have become standard for many types of corrosion resistance. Emphasis is placed on four groups of alloys to indicate the wide variation in corrosion resistance that can be obtained with commercially available materials. August 1955
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ALLOYS CONTAINING MOLYBDENUM
Austenitic Chromium-Nickel Steels Containing Molybdenum. Molybdenum was one of the earliest additions to be used generally in the austenitic chromium-nickel steels of the so-called 18-8 type (about l8y0 chromium and 8% nickel). Almost 20 years ago, John A. Mathews, one of the pioneers in this field, wrote (6) Molybdenum in amounts of from 2 to 4% is probably the moat generally useful of all the additions to 18-8 , , i t enhances general corrosion resistance against many chemicals and so far aa the writer is aware never lowers it.
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Later experience has confirmed these remarks and demonstrated the usefulness of 18-8 with molybdenum for construction of chemical process equipment (6) in 1. Organic syntheses and processing, particularly those involving organic acids of relatively low molecular weight t 2. Certain inorganic processes requiring more r e ~ i s t a nmaterial than the molybdenum-free grades-for example, the molyb-
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