DECEMBER, 1936
IXDUSTRIAL AND ENGINEERING CHEMISTRY
1.109
Discussion L. H. ALMY H. J. Heinz Company, Pittsburgh, Pa.
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HE food manufacturer is very much interested in metals. This interest is more intense now than it was twenty or thirty years ago. Much of the wooden equipment of the early period has been replaced with units made of metal because of the development of corrosion-resistant metal. The canner must be cautious, however, in changing to metal or from one type of metal to another. Contamination of food products with small amounts of metals may prove to be detrimental to the healt,h of the consumer and to the quality of the product. Foods come into contact with metals through the container and through the various pieces of equipment used in their manufacture. The common metal container is made of tin plate. The idea that the metallurgist may be able to develop a new type of metal for food containers that will be as practical as tin plate is intriguing, but, the possibility of success along this line is somewhat remote. Tin plate is quite satisfactory as it is, but it could still be improved. I t is pliable, withstands the distortion required in fabricating cans, and is comparatively cheap. I t does seem impractical, however, to be required to protect one metal by plating it with another and then to paint the surface to protect the second metal, as is done in the case of lacquered or enameled cans. Canners have been interested in the stainless steels, particularly the 18-8 metals. They are useful for colloid mills, homogenizers, pumps, st,irrers, pipe lines, dippers., etc. They are also excellent
for dressing up machinery-for example, as a shield around a filling machine. Their resistance to corrosion makes it possible for the manufacturer to keep his kitchens bright, clean, and sanitary, prolong the life of equipment, and minimize the danger of metal contamination. In selecting a metal for equipment in a food factory, due consideration must be given to the question of the solubility of the metal in the foods with which it comes in contact. Organic acid and salt solutions are highly corrosive, much more so than most people realize. Some metals are more suited to certain food products than others. An alloy may not be satisfactory on account of the tendency for one of the metals to “bleed,” or dissolve faster than the other metal or metals. Some have’a low coefficient of heat transfer and are thus not satisfactory for cooking kettles. We really know very little about the toxicity of metals. Considerable research has already been performed but we have not advanced far enough to say definitely that such and such an amount of any metal is harmless. Until we know more, we must be careful what metals we inadvertently add to foods. Most foods contain a sufficient supply of the mineral elements. With few exceptions, we are safe in consuming the foods as mother nature has made them. To keep them so it behooves the canner to use only those metals that’ are highly resistant to corrosion. RECEIVED October 9, 1936.
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Discussion L. S. DEITZ, JR. Nassau Smelting & Refining Company, Tottenville, N. Y
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K A DISCUSSIOK of white metals, it seems appropriate to
bring up a point, which is well known by metal manufact’urers but which needs constant reviewing. This point concerns the sampling of alloys containing any or all of the metals lead, tin, antimony, and copper, where lead and tin predominate. Those who purchase or use these metals often do not realize that segregation is present in the bars or the pigs to such an extent that a systematic method of sampling must be followed if a correct average sample is to be obtained. Different portions of a bar of babbitt may vary several per cent in antimony, copper, and tin, depending on the composition. The variations in different portions are usually sufficient to be outside of the specification requirements. The A . S.T. M. covers a,dequ:ttely the method of sampling these alloys by the alternate saw cut scheme. However, there are still many industries whose samplers knock off a corner of a pig or file off the edge of a bar and arrive a t analyses which are far from representative of the shipment and often cause considerable misunderstanding between buyer and seller. It is therefore to be hoped that users of these alloys will recognize the need for the recommended procedure.
Protection against Corrosion The depreciation of manufacturing equipment and building structures by corrosion of metal surfaces is a constant source of expense. I n order 1.0 combat these problems, a continuous search for more adequate materials for resistance to corrosion is being made, but often the cost of these materials or their physical properties preclude their general adoption. One method of combating corroFion which has proved succes-
ful in many cases is the application of metal coatings by the process of “metal spraying.” This process briefly consists in feeding a metal wire of the desired composition into a special oxy-acetylene gun which melts the wire; as the molten portion is produced, a jet of compressed air disintegrates the molten metal and sprays it upon the desired surface, forming an adherent. film. By repeated applications, any reasonable thickness of film can be produced. The surface must be specially prepared by sandblasting with sharp sand or by blasting with sharp steel grit. The latter method is used for hard surfaces where hard metals are being sprayed. Wires of 10 to 20 gage are commonly specified; the larger wires are made of metals or alloys with a lower melting point. Accurate gage of wire and a smooth surface are required for the successful operation of the guns. Practically all materials which can be furnished in wire form may be applied by this process. At present the chief metals together with type applications are as follows: ZIXC. Zinc is the most widely used a t the present time. I t resists exposure to moist air and to brine solutions. Zinc wire of 99.9 per cent grade, which has more corrosion resistance than a galvanized coating, is used for the spray. Coatings 0.01 to 0.03 inch thick are usually applied. Refrigerator plants are using this material for maintaining ice cans and protecting equipment and superstructures. The S. S. Normandie is reported to have its entire ventilating system, refrigerating plant with its superstructure, interior of stacks, and other important units sprayed with this metal. Most of the work was done after the equipment was assembled or in place.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Superstructures in plants subject to moist corrosive conditions are a fertile field for this material. The preparation of the surfaces and spray is usually done on the job and just before erection. It is claimed that a saving is made owing to the fact that lighter structural members may be used since the factor of safety can be lowered. In Europe zinc is used for the coating of the surfaces of rails and fish plates where rails are joined. The coatings allow better conductivity of joints as well as tighter contacts. I t is anticipated that this practice will follow in the United States. LEAD. For surfaces subject to corrosion of sulfuric acid or sulfur dioxide, where abrasion is not a factor, lead coatings are used. The linings of condensers, tanks, and pipes of large sizes have been coated with satisfactory results. Here again superstructures in certain industries are being covered. A wool scouring plant has found this method economical for its building trusses. TIN. For surfaces exposed to food products, such as milk holders, pasteurizers, and parts subject to wear in food industries, tin coatings are being used. In some cases where glass-lined equipment has become cracked through some accident, the entire glass lining has been removed by grit blasting and replaced by sprayed tin, thus salvaging the equipment with minimum expense and delay. ALUMINUMfinds application in two general fields; (1) where corrosion resistance is desired a t normal temperature and (2) where high-temperature corrosion is encountered. In this case, a surface is produced similar to a calorized surface. The oil industry has found that sprayed aluminum coatings have been effective a t both high and low temperatures in their refining equipment. MONELMETALhas many applications in lining pumping equipment subject to corrosion and moderate abrasion. KICKELis used in special cases. The interior of a tank car for caustic soda has been completely covered with this metal. STEEL. Where abrasion and corrosion are severe, such as the shafts of large centrifugal pumps and blowers, stainless steel is applied to the shafts a t the bearing surfaces, or worn shafts are built up, then ground to size, and polished. A hardness up to 450 Brinell may be obtained if desired by the selection of a proper alloy. High-carbon and low-carbon steels are used in a similar manner. By the same procedures, in addition to preparing surfaces for resistance to corrosion and for building up worn parts, metal sprays may be used for decorative purposes. Several articles in recent literature (2) cover the general field of metal spraying and its uses. Metal-sprayed surfaces are about 15 per cent less dense than the solid metal. This is not considered detrimental since the corresponding reduction in strength is usually not an important factor. Also the pores are not continuous and may become filled with protective corrosion products such as zinc oxychloride in cases of zinc, and lead sulfate in the case of lead.
Storage Batteries One of the largest consumers of lead and antimony is the storage battery industry. The majority of batteries are made for pleasure cars and trucks, but power plants and communications systems consume a considerable tonnage. The Faur6 (paste) cell, with sulfuric acid electrolyte, lead cathodes, and lead peroxide anodes, is the chief type produced. The grids which support the active ingredients of the paste type cells have been cast for many years from alloys of lead and antimony, because of the superior casting properties and strength of these alloys over pure lead. From 6 to 9 per cent antimony has been generally used, and the grid content of the plates has been approximately 65 per cent of their weight. But in the past few years, with increasing competition and with additional loads on automobile batteries due to electrical appliances such aa
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heaters and radios, efforts have been made to increase the capacities without increasing the space required, resulting in the use of a larger number of thinner grids and greater paste ratios. The grids have been reduced so that the thinnest ones are now approximately 0.055 to 0.060 inch thick. In order to produce these grids, the antimony content of the alloy has been increased to 12 per cent since this alloy casts better and has a greater tensile strength than the alloys of lower antimony content, Improvements have also been made in mold design, in sprays for mold surfaces, and in casting machines. From the standpoint of the manufacturer of grid metal, it appears from customer’s specifications for grid metal that there are many differences of opinion on the desired antimony content and limits for such elements as tin, arsenic, bismuth, and copper. Undoubtedly many data are available on these points, and it would be interesting and instructive to have them correlated. For many years the use of antimony-lead alloy for grids has been considered undesirable from the electrochemical viewpoint ; since antimony is electropositive toward lead, it promotes excessive sulfation of the negative plate through local action, thus causing static discharge to the cell. For this reason, attempts have been made to substitute other alloys of lead. Lead containing approximately 0.05 to 0.10 per cent calcium has been tested ( 3 ) and found superior to antimony-lead alloy in electrochemical properties, especially in the static discharge rates. Plates of calcium-lead alloy discharge only one-tenth as rapidly as antimony-lead grids. Since calcium-lead is a hardenable alloy, it requires a heat treatment or an aging period to develop the tensile strength comparable to that of the antimony-lead grids. I t is therefore softer than antimonial lead when cast, and a different technic of manufacture with probably different mold drafts will be required. The alloy has a much higher fatigue life under corrosion. The commercial features of thiq alloy are being studied further since it offers the possibility of improving the electrical properties of these types of batteries as well as the physical properties of the grids.
Penetration of Molten Solder into Metal Surfaces Occasional failures of tension members of soldered or sweated joints have indicated that the penetration of solder into certain metals is much greater than was formerly realized. Considerable data are available on the penetration of different metals by brazing solder and also by lead-tin solder, but the effect on the fatigue properties and tensile strength of some metals, especially steels, has not been generally recognized. Within the last two years several investigations (1) have determined the effects of solder and other alloys of low melting point on different metals commonly employed in general engineering practice. The general conclusions to be drawn from these data are as follows: (1) Nearly all of the usual metals and alloys are subject to intercrystalline penetration to some extent. (2) Nickel, Monel metal, and cupronickel are slightly affected. (3) The low-carbon steels and the lower carbon heat-resisting steels are only slightly affected, but similar steels with higher carbon content are more subject to penetration. (4) The nickel-chromium steels are quite susceptible. Although failures were originally observed and recorded in airplane construction the uses of solder are so extensive that similar effects must be present to some extent in other fields.
Literature Cited (1) Ewijk, L. J. G. van, J. Inst. Metals, 56, Advance copy No. 695, 109-18 (1935); Austin, G.W.,Ibid., to be published. (2) Mathey, E. L., Metal Progress, 29, No. 4, 52-6 (1936); Gruber, A. V., paper presented before meeting of Fla. Eng. SOC.,1936. (3) Schumacber, E. E.,and Phipps, G. S., Trans. Electrochem. SOC., 68 (preprjnt), 1935; Haring, H. E., and Thomas, U. B., Ibid., 68 (preprint), 1935. RECEIVED October 9, 1936.
