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The origin of the art of coating metal with tin is buried in antiquity. History records that in the days of Pliny (23 AD) articles of iron were dipped in molten tin to give them a protective coating. Writing long before Pliny, the Greek philosopher Aristotle was surprised at the light weight of melted tin. The practice of coating articles with tin was also carried on extensively by the Romans and commercial trade in tin was carried on extensively by the Phoenicians and then the Greeks as early as 500 BC. Just who or what nation first gave iron a protective coating of tin is not known, but the process has grown into an essential industry of world-wide importance.
The tin coating on present-day steel is a protective coating, as it was for the articles made of iron by the ancients. When exposed to air, steel oxidizes or corrodes; rust forms on the surface, while tin is practically unaffected. Steel reacts in much the same way to acids such as those found in foods. Tin itself is too soft a metal for practical use, but a coating of tin over steel combines the strength of steel with the protective or corrosion-resistant qualities of tin.
Long ago, tin-coated articles (usually made of iron) were so valuable and so efficient that down through the ages countries have been trying to protect the tin plating industry within their borders. For example, during the reign of King Henry VIII (1509 to 1547) an act of Parliament was passed forbidding the importing of tinned articles. However, the success of the tin plate industry in England must be attributed to the adoption of the rolling mill for reducing plates, instead of the old practice of pounding them first, was introduced and the fame of the English tin plating industry spread. As the industry grew in England, the United States became the principle user of tin plate and was estimated to have been consuming about seventy per cent of Britain's total production by 1890. In that same year, the McKinley Tariff emulated the proclamation of King Henry VIII. The McKinley Tariff put a stiff price on imported tin plate and encouraged the rapid expansion of the American tin plate industry.
J&L Steel (forerunner of LTV Steel) entered the tin plate industry in 1910 when the Tin Plate Department was established in the newly-build Aliquippa Works. J&L, at that time, was the first independent producer to break into the "Tin Plate Trust" and the first fully integrated independent steel manufacturer to complement semi-finished production with tin plate as a finished product. The ensuing years have seen many changes and improvements leading up to today's operations at the works and LTV's position as the third largest producer of tin mill products in the nation. Over half of LTV's tin plate output is produced at Aliquippa Pennsylvania.
The Aliquippa Tin Plate started with 12 single stand two-high rolling mills and auxiliary equipment. Additional units were added in subsequent years until in 1914 there was a total of 32 hot mills. These mills produced tin plate by what was known as the hot rolling method. This method consisted of cross rolling sheet bar, eight or ten inches wide, approximately five-eight's of an inch thick, and of a prescribed length into sheets of the specified finished gauge. To reduce this bar to the finished gauge, it was necessary to heat and roll the bar or pack four separate times. During the process the pack was doubled twice until the original two bars became a pack of eight sheets. These sheets in turn were cut from their ultimate length of from seventy to ninety inches into multiple cuts approximately the finished size desired by the consumer. These multiple cuts were separated by hand (termed opening), pickled in a relatively strong solution of sulfuric acid and piled in annealing boxes for "Black Annealing". This anneal eliminated strains set up by hot rolling, leaving the sheet almost dead soft. Following the Black Annealing, these sheets were again opened and cold rolled on two-high cold mills to flatted the sheets and develop a certain stiffness required in its ultimate use. After cold rolling, the sheets were White Annealed to relieve a part of the strain of this rolling. Both of the aforementioned annealings were accomplished in boxes in which the sheets had been placed in piles approximately 40 inches high. After the second annealing, called White Annealing, the sheets were then ready for the Tinning operation.
The principle method, at this time by which tin plate was made commercially was by hot dipping. In the hot-dip method tin coating was applied by immersing the sheets in a bath of molten tin, and then passing them through a set of rolls which distributed the tin evenly over the surface. The last six of the Tin Plate Department's hot-dip tinning machines which once numbered 32, were shut down November 5, 1965.
When the 32 hot mills were operating each had a crew of nine men. Production averaged about six tons per crew for each eight-hour shift. In order to accomplish the many multiple single sheet operations in the entire mill a daily force of approximately 1,800 men was required to produce 14,000 tons of tin plate per month. Today the site of these old mills is where the continuous pickle line was and where the five stand tandem cold reducing mill stands. World War II was the biggest single stimulant to go to the more modern electrolytic tinning process. Normal supplies of tin were shut off and tin conservation was a matter of national concern. After World War II when electrolytic tinning lines had been installed in the United States, twice the tonnage of tin plate was produced using the same amount of pure tin. The first tinning lines installed at Aliquippa were lines number one and two installed in 1943 and have since been shut down.
Present Tin Mill operations, besides three and four electrolytic tinning lines, include the related equipment for preparing steel for tinning: the five stand tandem mill manufactured by Mesta Machine Company and installed in October 1947, Four cleaner manufactured by Wean Engineering Company and installed August 1951, hydrogen annealing furnaces manufactured by LOI and installed 1992, continuous annealing line manufactured by Wean Engineering and installed March 1960, three stand tandem cold reducing mill for producing tin plate in thinner gauges manufactured by E.W.Bliss Company and installed in October 1962, two stand 48" temper mill manufactured by Bliss and installed November 1951, numbers three and four coil preparation lines installed January and February 1952 and number six coil preparation line installed April 1965. Three and four tin lines were build by Wean Engineering and installed in October 1951 and December 1960.
When the five stand tandem mill was installed in 1947 it established a record for its time, with the fasted speed ever attained by any steel rolling mill in the world. At top speed it could go as fast as 6250 feet per minute. Coils today can weigh as much as 37,000 pounds and when cold reduced into steel band can be over seven miles long, coiled at the exit end. The mill was also unique in being the first tandem mill installed with a separate generator for each stand. This mill, installed 51 years ago, is still competitive and has most of its original controls with only some minor changes.
The three stand cold double reducing mill, designed to reduce strip as thin as .0050" will roll gauges to almost foil like thickness. A new x-ray with computer controlled automatic gauge control was installed in 1987. With the plate of this mill, six tin coated steel cans can be made with the steel used in five cans made from conventional plate. This mill consumes enough electrical power during operation to supply the city of Niagara Falls.
The basic principle of the electrolytic tin plating process on three and four tin line depends upon the fact that an electrical current flows from the positive electrode called the "anode" to the negative electrode known as the "cathode". In the electrolytic unit, the current travels through a plating bath or "electrolyte," a solution which conducts a current of electricity from the anode of pure tin to the steel which serves as the cathode. The electro chemical reaction is such that tin is corroded from the anode and deposited in a thin, even coat on the steel, the cathode, as it passes through the plating bath.
Aliquippa's electrolytic tinning operation is a continuous process. As each new coil of steel enters the line it is welded to the back end of the coil being processed in the line. To overcome the necessity of stopping the operation, the tinning line utilizes a looping tower, or looping pit which stores enough of the steel coil to permit the plating unit to operate at constant speed while the stop is made for welding. The steel is then cleaned, pickled (removal of inorganic contaminants by using a slightly acidic solution) and rinsed thoroughly as preparation for plating the strip surface. The plating section is made up of a series of containers or "cells" into which are placed pure tin bars and a conductive solution or electrolyte. The strip passes through the cells horizontally, coming in contact with both the electrolyte which carries electrically charged tin ions and a steel roll witch allows completion of the plating circuit . The amount of coating deposited on the steel is regulated by the amount of current supplied from the anode to the cathode by the solution concentration, by temperature, and by speed of the steel through the unit. After plating the steel is rinsed and dried. At this point it has the appearance of dull silver. To obtain a bright surface the coated steel is passed through high frequency coils where the tin coating is fused, or re-flowed to a mirror like finish. Following a rinsing in a chromic acid solution, the product is thoroughly washed and dried by passing through a hot air drier. It then goes through an oilier which applies a thin oil coating. Finally the tin coated steel is rewound into a coil.
Today, the manufacture of tin plate at Aliquippa is quite a contrast to the old hot rolled, hot dipped plate. There is not a single procedure in the present day manufacture of tin plate duplicating the old production methods.
The Aliquippa Tin Mill beginning with J&L Steel, now LTV Steel, has played a major role in the tin plate industry for over 88 years. LTV Steel, backed by its modern steel-making facilities and the quality workmanship of its employees, will keep the Aliquippa Tin Mill, an ISO 9002 certified plant, a major "player" for a long tome to come.
