Tin and Tin Alloys

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Introduction: Historical Significance and Modern Applications of Tin

Tin was one of the first metals known to humanity. Throughout ancient history, various cultures recognized the virtues of tin in coatings, alloys, and compounds, with its use increasing as technology advanced. Today, tin remains an important industrial metal even though the annual tonnage used is much smaller than that of many other metals. This limited consumption is primarily because most applications require only very small amounts of tin at a time, highlighting its efficiency and specialized nature in modern manufacturing.

Tinplate Production and Applications

The largest single application of tin, accounting for approximately 40% of total world tin consumption, is in the manufacture of tinplate—steel sheet coated with tin. Since 1940, the traditional hot dip method of making tinplate has been largely replaced by electrodeposition of tin on continuous strips of rolled steel. This modern process allows for precise control of coating thickness.

Electrolytic tinplate 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 ranges from 0.38 to 1.54 µm on each surface, while the thicker coating on differential tinplate rarely exceeds 2.0 µm. Tinplate is produced in thicknesses from 0.15 to 0.60 mm.

Over 90% of world tinplate production is used for containers, commonly known as tin cans. These containers find their most important use in packaging food products, beverages including beer and soft drinks, as well as in holding various household and industrial products such as paint, motor oil, disinfectants, detergents, and polishes. Additional applications of tinplate include fabrication of signs, filters, batteries, toys, gaskets, and containers for pharmaceuticals, cosmetics, fuels, tobacco, and numerous other commodities.

Electroplating and Coating Technologies

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

Hot Dip Coatings

Coating steel with lead-tin alloys produces a material called terne plate. This material is valued for its formability and solderability, making it useful as a roofing and weather-sealing material. It is also used in automotive gasoline tanks, signs, radiator header tanks, brackets, chassis, covers for electronic equipment, and sheathing for cable and pipe.

Hot dip tin coatings are also applied to wire for component leads and to food handling and processing equipment, where tin's non-toxic properties are particularly valuable.

Unalloyed Tin Applications

There are relatively few applications where tin is used unalloyed with other metals. Pure tin is 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 it in any way.

Tin in Alloys: Composition and Applications

Solders

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. When alloyed with lead, tin produces solders with melting points lower than those of either metal alone. Small amounts of various metals, notably antimony and silver, are added to tin-lead solders to increase their strength, making these solders suitable for joints subjected to high or even subzero service temperatures.

Both solder compositions and applications of joining by soldering are diverse. 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 various 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. These lower-tin solders are also used in large amounts to fill crevices at seams and welds in automotive bodies, providing smooth joints and contours. Solders containing about 2% tin (remainder lead) are used for can side seams to provide hermetic seals. For specialized applications, tin-zinc solders join aluminum, while tin-antimony and tin-silver solders are employed where joints require high creep resistance.

Organ Pipe Alloys

Tin-lead alloys are used in manufacturing organ pipes. These materials are commonly called "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

Pewter is a tin-based 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. Contemporary compositions contain 1 to 8% antimony and 0.25 to 3.0% copper.

Bearing Materials

Tin's low coefficient of friction makes it valuable as a bearing material. However, being structurally weak, tin in bearing applications is alloyed with copper and antimony for increased hardness, tensile strength, and fatigue resistance. In tin-base babbits, the quantity of lead is typically 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. These alloys are generally inferior in strength to tin-base babbits, a trade-off for their lower cost.

Aluminum-tin bearing alloys represent an excellent compromise between high fatigue strength and good surface properties such as softness, seizure resistance, and embeddability. These alloys are usually employed 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. Alloys containing 20 to 40% tin, with aluminum making up the remainder, show excellent resistance to corrosion by products of oil breakdown and good embeddability, particularly in dusty environments, making 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 replacing antimonial lead alloys. The use of ternary lead-base alloys containing up to 1.3% tin has substantially reduced gassing, meaning batteries with grids made from these alloys do not require periodic water additions during their working life. The two primary 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 among the first alloys used by humans and 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 the remainder Cu are used for general-purpose castings requiring reasonable strength and soundness, such as gears, pumps, and automotive fittings.

Dental Alloys

Dental alloys for making amalgams contain silver, tin, mercury, and some copper and zinc. The copper increases hardness and strength, while zinc acts as a scavenger during alloy manufacture, protecting major constituents from oxidation. Most dental alloys currently available contain 25 to 27% tin and consist mainly of the intermetallic compound Ag₃Sn. 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, the 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%. With tin additions, increased strength without embrittlement can be obtained in aluminum-stabilized alpha titanium alloys. Optimum strength and workability can be achieved with 5% aluminum and 2.5% tin; additionally, this alloy has the advantage of being weldable.