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Tin and Its Alloys ~-
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BRUCE W. GONSER Battelle Memorial Institute, Columbus, Ohio
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IN has a wide diversity of applications in the construction of equipment for the chemical and food industries. Usually i t serves a8 an alloy or a component with other metals, rather than as block tin. Thus, it is used as a lining or for surface protection of steel and copper, in soft soldering, in bearing metals, and in tin bronzes and fusible alloys. These and other applications are outlined in Figure 1. The solid metal is used particularly as tubing and sheet for applications where nontoxicity is a factor.
some extent in Great Britain, it appears possible to producc hotdip tin plate in coils a t a somewhat lower cost than by the conventional method. Stemming directly from the production of electrolytic tin plate is the use of very thin tin coatings on steel as a base for paint to improve corrosion resistance (20). Coatings only 0.000008 to 0.00003 inch thick have shown marked improvement in experimental studies, but the heavier coatings in this range are recommended. This protection is made more effective by a chromate treatment or by using the Protecta-Tin process developed by thr Tin Research Institute (19, 26). This treatment is given essentially by dipping a tinned can or other tinned steel product into a hot sodium hydroxide solution containing trisodium phosphate, sodium dichromate, and a wetting agent. A tinned container so treated, for example, resists external rusting on exposure to weathering, and internal blackening from sulfur-containing foods. Another treatment for tinned cans to prevent, or a t least discourage, rusting is described by Gillies (16). This consists simply of adding 0.1% of sodium dichromate and 0.1% of a detergent, such as sodium hexametaphosphate, to the cooking water in which the cans are being sterilized. Risk of chromium contamination through seams is considered to be insignificant. Although many of the uses of tin on tin cans are prompted by its corrosion resistance ($7) and because the products of corrosion do not poison the contents of the can, tin is also used as a coating even for dry products because tinned steel solders easily. This is important in sealing the side seams of cans a t high speed.
TIN SUPPLY
Restrictions in the use of tin in the United States have favored use of substitutes or of the minimum amount of tin that would serve a given purpose. Starting near the beginning of World War 11, these restrictions sharply curtailed supply during the war years and have been continued to permit the building of a strategic stockpile for future emergencies. Xow that world production is considerably higher than consumption, gradual removal of restrictions is taking place. The trend in production is shown in Table I. This trend toward producing more than is consumed is expected t o be even more pronounced in 1949, and an excess world production of 32,000 long tons is estimated by the International Tin Study Group (37).
Table I. Tin Statistics (22) World World World I-.S..1.
mine production metal production metal consumption tin-plate production
Long Tons 1947 1848 114,500 152,900 124,500 159,600 138,900 136,900 3,313,221 3,528,750
SOLDERS AND FLUXES
TIN PLATE AND TINNING
As a direct wartime measure to conserve tin, high-speed electrolytic tin-plate lines mere installed by all the steel companies producing tin plate. These have continued to be used and others have been added until nearly 50% of the United States tin plate is now produced on 25 electrolytic lines. Speeds of tinning as high as 1400 feet per minute are attained, yet excellent uniformity of coating is secured. Tin plate produced by the normal hot-dip method has a tin coating that exceeds 1 pound per base box (112 gheets, 14 X 20 inches, or equivalent area); the electrolytic lines can produce practically any thickness, but are currently being used largely in the range of 0.25 to 0.75 pound per base box (5, 21, 24, 43). Because the cost of electrolytic tinning mounts rapidly as the speed decreases to permit a heavier coating, there is still a good field for the heavier hot-dip tin plate in making cans for products that cannot be packed safely in the extremely lightly :oated tin plate A device to measure the thickness of very thin electrolytic and hot-dipped tin-plate coatings, described recently ( 6 ) ,seems to give good results on tin coatings of up to 0.0004-inch thickness, or well qver the range commonly encountered. This Francis thickness tester (15) measures the thickness by measuring the time required to dissolve anodically a known area of coating. Angles, Caulfield, and Kerr (2) have summarized results of some investigations done in England during the war on conditions affecting the quality of electrolvtic tin plate. High-speed production of electrolytic tin plate is now well established in British industrv. By the Sherman process (3),which has been applird to
A rather comprc,hensive and practical booklet on soft soldering is now available on request from the Tin Research Institute (29). This discusses practical methods of soldering both common and special metals and alloys, as well as fluxes and fluxing and properties of the joints. A glutamic acid hydrochloride-urea flux, described for the first time, has found considerable usefulness in soldering parts which require a stronger flux than rosin, but without some of the disadvantages of zinc chloride. Although corrosive as the raw flux, the flux can be effectively decomposed on heating. The residue is not corrosive if all parts of the joint that have come in contact with the flux are strongly heated and thc small amount of hydrogen chloride released is free to escape. Even in absence of such effective treatment, residual active flux is easily removed by nTashing with water, and i t is not hygroscopic unless used on zinc, in which case some zinc chloride might be formed. Some developments in making activated rosin-cored solder have been noted also. 'These may contain aniline hydrochloride, lactic acid, glutamic acid hydrochloride, or other activators of still undisclosed composition. Their action in greatly increasing the fluxing power of rosin has been well demonstrated in many practical operations, but such additions cannot be guaranteed to give a completely noncorrosive flux and should be tested thoroughly before adoption for important operations. Properties of soft solders and soldered joints are treated in a comprehensive way in a publication covering recent vork bv the British Nonferrous Metals Research Association (SO). For highest resistance to creep above 150" C., these investigators found a lead-base solder containing only 1% tin and 1.5% silver to be best. Tin is needed even in such a small amount to give necessary wetting properties. Recent Kork in this coiintry on soiders has been particularly concerned with properties ut subzero trmpvra-
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COURTESY TIN RESEARCH INSTITUTE
Figure 1. Industrial Applications of Tin and Its Principal Alloys
tures (23, 2 6 ) . Solders containing over about 90% lead were found to retain their ductility and t o increase in impact strength down to a t least - 180' C. If the tin content is increased to SO%, the tensile strength is greatly increased as the temperature drops, but a t extremely low temperatures or below - 150' C. serious embrittlement may be encountered. Russian investigators ($8) concluded that a 25 tin-75 lead solder showed the best combination of strength and plasticity at -200" to -250" C. of the solders they investigated, but these did not include lead-base solders having only a small percentage of tin. RIethods found most satisfactory in practice for spectrographically analyzing soft solders (35) and for testing soldered joints for shear strength (42) have been described, as well as equipment for measuring the flow of solder on sheet metals (36). BEARING METALS AND BRONZE
The use of tin in babbitt and bearing metals continues to be of strategic importance, although some substitution has been forced by the curtailed supply. Warburton (44)described the method of the Tin Research Institute for securing good adhesion between white-metal bearings and cast-iron shells, Forrester (12, 13) and Greenfield (14) show=d that alloys with a high copper content have a low bond strength because of a layer of CusSnC formed on the bond. The effect may be inhibited by very rapid cooling. -4 thick layer of FeSnl has a similar effect, The ability of tin and lead-base babbitts to retain oil films is much greater than that of the stronger copper-lead alloys. Boas and Honeycombe (4)discussed the deformation of tin-base bearing metals by heating and cooling, showing that deform$tion by cyclic thermal treatment occurs only in alloys based on non-
cubic metals. Excellent discusions of practical value have been covered in a symposium on bearing materials and hearing problcms (1) and in the comprehensive Metals Handbook (11). An important advance in the field of coppcr-tin bronzes has been the development by the hmerican Smelting and Refining Company of a continuous casting method for producing rod up to 4.5 inches in diameter, tubes, and rectangles (34, 4f). The composition range covers copper with up to 13% tin, up to 25% lead, u p to 10% zinc, and up to 5% nickel. This makes available bronze cast in long lengths to a close tolerance and in a form that favors economies in machining time and cost. Such castings are said to be of uniformly high quality and to give an excellent structure t h a t assures optimum physical properties. Corrosion-resistant bronze compositions, conscqucntlp, can now be obtained for structural purposes in a nider range of compositions and shapcs than heretofore. Considerable work has been in progress in England on improving chill castings of high-tin bronze (32, 33, 38). Conditions have been outlined for making castings of 8 to 14% tin bronze having unusually high strength and elongation. Cold-drawn tubes made from 10% tin bronze have shown excellent resistance to aerated sea ivater and jet impingement and appear t o be well suited for use in evaporators, condensers, and the like (8, 40). TIN-ALLOY PROTECTIVE COATINGS AND MISCELLANEOUS
Speculum plating, or surfacing a base metal with a bright white bronze of about 45 tin-55 copper composition, has been rather a e l l developed in England, but so far has been little used in this country because of the restricted supply of tin. It shows considerable promise as a n attractive finish that closely ap-
October 1949
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Forrester, P. G., “Babbitt Alloys for Plain Bearings,” Tin Research Institute, 1947. Forrester, P. G., J . Inst. Metals, 73, 573 (1947). Forrester, P. G., and Greenfield, L. T., Ihid., 72, 91 (1946). Francis, H. T., Trans. Electrochem. Soc., 93, 79 (1948). Gillies, R. A., Food, 16, 81 (1947). Gonser, B. W., Wire and Wire Products, 22, 207 (1947). Gonser, B. W-., and Strader, J. E., “Corrosion Handbook,” by H. H. Uhlig, pp. 323-9, 829-836, Kew York, John Wiley & Sons, 1948. Hedges, E. S., and Hoare, W. E., Metal Treatment, 13, 197 (1946-47). Hedges, E. S.,and Jordan, L. A , , J . Iron Steel Inst. (London), 152, 429 (1945). Hoare, W.E., Iron & Coal Trades Rev., 157, 767 (1948). International Tin Study Group, Statistical Bull. (A4pril1949). Jaffee, R. I., Minarcik, E. J., and Gonser, B. W., Metal Progress, 54, 843 (1948). Johnson, S . S., and Jenison, G. C., Proc. Am. Electroplaters SOC., 33, 102 (1946). Kalish, H. S.,and Dunkerley, F. J., M e t a h Technol., 15; Tech. Pub. 2442 (1948). Kerr, R. J., J . SOC.Chem. Ind., 65, 101 (1946). Kerr, R. J., Angles, R. M., and Caulfield, K. W., Ibid., 66, 5 (1947). Kostenets, V. I., and Ivanchenko, A. M., J . Tech. Phys. ( L ’ S . S.R.), 16, 551 (1946) (in Russian). Lewis, 1%‘. R., “Notes on Soldering,” Tin Research Institute, 1948. McKeown, J., “Properties of Soft Solders and Soldered Joints.” p. 118, British Nonferrous Metals Research Association, 1948. Mantell, C. L., “Tin,” 2nd ed., New York, Keinhold Publishing Corp., 1947. Pell-Walpole, W. T., Foundry Trade J . , 85, 573 (1948). Pell-Walpole, N‘,T., Netal Treatment, 14, 69 (1947). Smart, J. S.,and Smith, A. -4., Iron .4ge, 162, 72 (1948). Smith, D. M., “Spectrographic Anal>-sis of Tin and Tin-Lead Solder,” Tin Research Institute, 1948. Sperotto, J. J., Trans. A m . SOC.Metals. 41, 940 (1949). Tin Producers’ A4ssoc.,Tin, July 1949, p. 2. Tin Research Inst., “How to Make Improved Chill-Cast Tin Bronzes,” 1946. Tin Research Inst., “Speculum Plating,” 1946. Tin Research Inst., Tin and Its Csea, 19, 4 (1948). Ibid., 20, 11 (1948). (42) Trey, F., .lfetall.forschung, 2, 84 (1947). (43) Van Vleet, H. 8.. XPch. E n g . , 70, 315 (1948). (44) Warburton, H., Mech. World Eng. Record. 124 (3227), 579 (1948).
proaches silver in attractiveness without the disadvantage of darkening easily by sulfur staining ( 7 , 39). To obtain a coating that would withstand severe tropical conditions of corrosion, a tin-zinc electroplated alloy has been developed which is being used commercially in Great Britain (9, IO). The composition is preferably 75 to 80% tin and 20 to 25% zinc. S o unusual difficulties in operation have been reported, and the coating has been adopted only after extensive testing for corrosion resistance. Industrial applications have particularly included use of tin-zinc in place of cadmium plating, with considerable saving in cost. Tinning by replacement, or merely immersion of the article to be coated in a medium containing a tin salt, has been used effectively for coating copper and its alloys. Recently a process has been developed for coating both ferrous and copper-bearing metals by immersion in a molten salt bath containing stannous chloridr ( 1 7 ) . The temperature is sufficiently high to produce a tin alloy with the metal being coated; thus, with copper a white bronze coating is produced. Advantages are a vastly greater speed of coating and production of much heavier coatings than are possible by immersion in aqueous solutions of tin salt. Recent discussions of the corrosion resistance of tin and of tin coatings are given in Uhlig’s book (18). A new edition of Mantell‘s comprehensive book on tin (31) Covers nrw material on tin alloys in industry, tin coatings, arid corrosion of tin and its alloys in food products and various media. LITERATURE CITED
Ani. Soc. Metals, Cleveland, Ohio, “Sleeve-Bearing Materials,”
1949. Angles, R. M., Caulfield, K. K., and Kerr, R.. J . SOC.Chem. Ind., 65, 430 (1946); 66, 5 (1947). Anon., T i n Printer, 23, No. 12-B, 266 (1947). Boas, W., and Honeycombe. R. K.K., J . Insf. Metals, 73, 433 (1947). Bogert. F. W., Metal Finishing, 46, 68 (1948). Caulfield. K. IT., and Hoare, W.E., Sheet Metal Ind., 1949, 753. Cuthbertson, J. W.,J . Electrodepositors’ Tech. Soc., 23, 143 (1947-45). Cuthbertson. J. W., J . Inst. .l.leta/s, 76, 317 (1946). Cuthhertson, .J. W . , T i n and Its Cses, 20, 5 (1944). Cut,hbertson,J. W., and Angles, R. XI.,J . Electrochem. SOC.,94, 73 (1948). Ellis, 0. W., Beck. P. A., and Underwood, h.F., “Metals Handbook,” p. 745, C7rrelmcl. Ohio, American Society for Metals, 1948.
R E ~ F I Y EJuly D 2 5 , 1949.
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WOOD ~
ALFRED J. STAhI31, iY. S . Forest Products Laboratory, Madison, T i s .
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HE chemical engineer who picks wood for various special uses usually does so on the basis of its availability, its relatively reasonable cost, its rarely surpassed strength properties per unit of weight, its ease of fabrication, and its excellent resistance t o a great variety of chemicals. Fortunately, wood has again become readily available in pIactically all common species, and in a properly seasoned condition. Chemicals, such as creosote for the preservative treatment of wood and phenol for making resin-treated dimensionally stabilized xood, are again available after an early postwar shortage. These signs should make the chemical engineer more aware of the merits of wood for use in chemical engineering structures. PRESERVATIVE TRE4TMEYT
Wood used in many of the chemical engineering applications should, when possible, be treated with a wood preservative $0 as to increase its service life, especiallv when used out of doors
in contact with the ground or in applications that tend to maintain a rather high moisture content within the wood. Although reaearch is rontinually being conducted on the toxicity of various chcmirals, no striking npw preservatives have recently been developed. Testing of the effectiveness of preservatives is a long-drawn-out procedure. Before a chemical is universally accepted, it must go through such extended tests that it is no longer considered new. Pentachlorophenol, for example, is just non obtaining this time-tested mark of acceptance (14, 15, 52).
Southern vello\q pine test fence posts pressure-treated x i t h 3 and 5% solutions of pentachlorophenol dissolved in spent crankcase oil until average absorptions of approximately 6.5 pound.: per cubic foot were obtained, after 10 years of eupqsurc under severe conditions in a 1Iississippi test line, gave serviceahilitles of 1 0 0 ~ for o the 5oJ, and 99yo for the 3y0 solutions (one post out of 100 was removed because of decay, 14). These serviceability values compare favorablv with values for creosote-treated posts over
