Tin and Tin Alloys

Tin was one of the first metals known to man. Throughout ancient history, various cultures recognized the virtues of tin in coatings, alloys and compounds, and use of the metal increased with advancing technology. Today, tin is an important metal in industry even though the annual tonnage used is much smaller than those of many other metals. One reason for the small tonnage is that, in most applications, only very small amounts of tin are used at a time.

Tin was one of the first metals known to man. Throughout ancient history, various cultures recognized the virtues of tin in coatings, alloys and compounds, and use of the metal increased with advancing technology. Today, tin is an important metal in industry even though the annual tonnage used is much smaller than those of many other metals. One reason for the small tonnage is that, in most applications, only very small amounts of tin are used at a time.

Tinplate. The largest single application of tin is in manufacture of tin-plate (steel sheet coated with tin), which accounts for about 40% of total world tin consumption.

Since 1940, the traditional hot dip method of making tinplate has been largely replaced by electrodeposition of tin on continuous strips of rolled steel. Electrolytic tin-plate can be produced with either equal or unequal amounts of tin on the two surfaces of the steel base metal. Nominal coating thickness for equally coated tinplate range from 0.38 to 1.54 µm on each surface. The thicker coating on tinplate with unequal coatings (differential tinplate) rarely exceed 2.0 µm. Tinplate is produced in thickness from 0.15 to 0.60 mm.

Over 90% of world production of tinplate is used for containers (tin cans). Tinplate cans find their most important use in packaging of food products, beer and soft drinks, but also are used for holding paint, motor oil, disinfectants, detergents and polishes. Other applications of tinplate include fabrication of signs, niters, batteries, toys, gaskets, and containers for pharmaceuticals, cosmetics, fuels, tobacco and numerous other commodities.

Electroplating accounts for one of the major uses of tin and tin chemicals. Tin is used in anodes, and tin chemicals are used in formulating various electrolytes, for coating a variety of products. Tin electroplating can be performed in either acid or alkaline solutions. Sodium or potassium stan-nates form the bases of alkaline tin plating electrolytes that are very efficient and capable of producing high-quality deposits.

Hot Dip Coatings. Coating of steel with lead-tin alloys produces a material called tern plate. It is easily formed and easily soldered and is used as a roofing and weather sealing material and in construction of automotive gasoline tanks, signs, radiator header tanks, brackets, chassis and covers for electronic equipment and sheathing for cable and pipe.
Hot dip tin coatings are used on wire for component leads as well as food handling and processing equipment.

Unalloyed Tin. There are only a few applications where tin is used unalloyed with other metals. Unalloyed tin is well recognized as the most practical lining material for handling high-purity water in distillation plants because it is chemically inert to pure water and will not contaminate the water in any way.

Tin in Alloys. Solders account for the second largest use of tin (after tinplate). Tin is an important constituent in solders because it wets and adheres to many common base metals at temperatures considerably below their melting points.Tin is alloyed with lead to produce solders with melting points lower than those of either tin or lead. Small amounts of various metals, notably antimony and silver, are added to tin-lead solders to increase their strength. These solders can be used for joints subjected to high or even subzero service temperatures.

Both solder compositions and applications of joining by soldering are many and varied. Commercially pure tin is used for soldering side seams of cans for special food products and aerosol sprays. The electronics and electrical industries employ solders containing 40 to 70% tin, which provide strong and reliable joints under a variety of environmental conditions. General-purpose solders (50Sn-50Pb and 40Sn-60Pb) are used for light engineering applications, plumbing and sheet metal work. Lower-tin solders (20 to 35% Sn, remainder Pb) are used in joining cable and in production of automobile radiators and heat exchangers. Low-tin solders are used in large amounts to fill crevices at seams and welds in automotive bodies, thereby providing smooth joints and contours. Solders containing about 2% tin (remainder lead) are used for can side seams to provide hermetic seals. Tin-zinc solders are used to join aluminum, while tin-antimony and tin-silver solders are employed in applications requiring joints with high creep resistance.

Alloys for Organ Pipes. Tin-lead alloys are used in the manufacturing of organ pipes. These materials commonly are named "spotted metal" because they develop large nucleated crystals or "spots" when solidified as strip on casting tables. The pipes that produce the diapason tones of organs generally are made of alloys with tin contents varying from
20 to 90% according to the tone required.

Pewter is a tin-base white metal containing antimony and copper. Originally, pewter was defined as an alloy of tin and lead, but to avoid toxicity and dullness of finish, lead is excluded from modern pewter. These modem compositions contain 1 to 8% antimony and 0.25 to 3.0% copper.

Bearing Materials. Tin has a low coefficient of friction, which is the first consideration in its use as a bearing material. Tin is structurally a weak metal, and when used in bearing applications it is alloyed with copper and antimony for increased hardness, tensile strength and fatigue resistance. Normally, the quantity of lead in these alloys, called tin-base babbits, is limited to 0.35 to 0.5% to avoid formation of the tin-lead eutectic, which would significantly reduce strength properties at operating temperatures.

Lead-base bearing alloys, called lead-base babbits, contain up to 10% tin and 12 to 18% antimony. In general, these alloys are inferior in strength to tin-base babbits, and this must be equated with their lower cost.

Bearing alloys must maintain a balance between softness and strength. Aluminum-tin bearing alloys represent an excellent compromise between the requirements for high fatigue strength and the need for good surface properties such as softness, seizure resistance and embed ability. Aluminum-tin bearing alloys are usually employed in conjunction with hardened steel or ductile iron crankshafts and allow significantly higher loading than tin- or lead-base bearing alloys.

Low-tin aluminum-base alloys (5 to 7% Sn) containing small amounts of strengthening elements, such as copper and nickel, are often used for connecting rods and thrust bearings in high-duty engines. Strict dimensional tolerances must be adhered to and oil contamination should be avoided. Alloys containing 20 to 40% tin, remainder aluminum, show excellent resistance to corrosion by products of oil breakdown and good embeddability, particularly in dusty environments. The higher-tin alloys have adequate strength and better surface properties, which make them useful for crosshead bearings in high-power marine diesel engines.

Battery-grid Alloys. Lead-calcium-tin alloys have been developed for storage-battery grids largely as replacements for antimonies lead alloys. Use of ternary lead-base alloys containing up to 1.3% tin has substantially reduced gassing, and therefore batteries whose grids are made of these alloys do not require periodic water additions during their working life. Two chief methods of grid manufacture are casting and fabrication of wrought alloys including punching, roll forging and expanded metal processes.

Copper Alloys. Copper-tin bronzes were some of the first alloys used by man, and these alloys continue to be used for structural and decorative purposes. True bronzes contain tin in amounts up to 10% as well as very small amounts of phosphorus. Quaternary bronzes containing 5% Sn, 5% Zn, 5% Pb, and remainder Cu are used for general-purpose castings for applications requiring reasonable strength and soundness, such as gears, pumps, and automotive fittings.

Dental alloys for making amalgams contain silver, tin, mercury, and some copper and zinc. The copper increases hardness and strength and the zinc acts as a scavenger during alloy manufacture, protecting major constituents from oxidation. Most dental alloys presently available contain 25 to 27% tin and consist mainly of the inter-metallic compound Ag and Sn. When porcelain veneers are added to gold alloys for high-grade dental restoration, 1% tin is added to the gold alloy to ensure bonding with the porcelain.

Titanium Alloys. Tin strengthens titanium alloys by forming solid solutions. Titanium can exist in the low-temperature alpha phase or the higher-temperature beta phase, which remains stable up to the melting point. In titanium alloys, relative amounts of alpha and beta phases present at the service temperature have profound effects on properties. Aluminum additions raise the transformation temperature and stabilize the alpha phase, but may cause embrittlement in amounts greater than 7%. However, with tin additions, increased strength without embrittlement can be obtained in aluminum-stabilized alpha titanium alloys. Optimum strength and workability can be obtained with 5% aluminum and 2.5% tin; in addition, this alloy has the advantage of being weldable.

September, 2001
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