Tinning

Tinning is the process of making tinplate, which consists of sheets of iron or steel that have been thinly coated with tin by being dipped in a molten bath of that metal. Hence the process is more precisely described as hot-dipped tin plating. This is done in order to prevent the iron from rusting. Another method is galvanizing.

Introduction

Terne-plate is a similar product, but the bath is not of tin, but of tin and lead mixed, the latter metal constituting from 7.59% of the whole; it has not the bright luster of tinplate, whence its name, from terne, dull, tarnished. The sheets employed in the manufacture are known as black plates, and are now of steel, either Bessemer steel or open-hearth. Formerly iron was used, and was of two grades, coke iron and charcoal iron; the latter, being the better, received a heavier coating of tin, and this circumstance is the origin of the terms coke plates and charcoal plates by which the quality of tinplate is still designated, although iron is no longer used. Tinplate is consumed in enormous quantities for the manufacture of the tin cans in which preserved meat, fish, fruit, biscuits, cigarettes and numerous other products are packed, and also for the household utensils of various kinds made by the tinsmith or silversmith; terne-plates, which began to be produced in England about the middle of the 19th century are widely employed in the United States for roofing purposes.

History

The manufacture of tinplate was long a monopoly of Bohemia, but about 1620 the industry spread to Saxony. In 1665, Andrew Yarranton (1616-1684), an English engineer and agriculturist, was commissioned to go to Saxony and if possible discover the methods employed. According to his own account, he was very civilly treated and was allowed to see the whole process. On his return to England his friends undertook the manufacture on an experimental scale, but though they were successful they had to abandon it, because their method became known and a patent for it was trumped up by a rival, who, however, from lack of technical skill was unable to work it. Half a century later the manufacture was revived by Major John Hanbury (1664-1734) at Pontypool; the method of rolling iron plates by means of cylinders, said to have been devised by him, enabled more uniform black plates to be produced than was possible with the old plan of hammering, and in consequence the English tinplate became recognized as superior to the German. During the next hundred years or so the industry spread steadily in England and Wales, and after 1834 its expansion was rapid, especially in Wales, Great Britain becoming the chief source of the world's supply. In that year her total production was 180,000 boxes of 108 lb each (around 50 kg, in America a box is 100 lb), in 1848 it was 420,000 boxes, in 1860 it reached 1,700,000 boxes. But subsequently the advance was rapid, and the production, which was about 2,236,000 lb in 1891, had by 1900 increased to more than 849,000,000 lb, of which Over 141,000,000 lb were terne-plates. The total imports in that year were only 135,264,881 lb. In later years, again, there was a decline in the American production, and in 1907 only 20% of the American tinplate mills were at work, while the British production reached 14 million boxes.

Processes

There are two processes for the tinning of the black plates: hot-dipping and electroplating.

Electroplating

In electroplating, the item to be coated is placed into a container containing a solution of one or more tin salts. The item is connected to an electrical circuit, forming the cathode (negative) of the circuit while an electrode typically of the same metal to be plated forms the anode (positive). When an electrical current is passed through the circuit, metal ions in the solution are attracted to the item.

Hot-Dipping

In the hot dipping process, which is the older, the plates, after being properly annealed, are scoured with sand and water and pickled in dilute sulfuric acid alternately until they are perfectly clean and bright. They are then washed in water, and after being boiled in palm oil to remove all traces of acid and water are dipped into a bath of molten tin, covered with oil to prevent oxidation. They are then taken to a second bath containing purer tin than the first. After this they are scoured with a hempen rubber and dipped in a third bath containing the purest tin of all; then they are passed through rolls to finish the surface and regulate the thickness of the coating. As the tin in the third bath becomes alloyed with iron from the operation, it is removed into the second, pure fresh tin being substituted; and similarly the metal of the second, as the amount of iron in it increases, is removed to the first. In the acid process only a single bath of tin is required. The molten metal is covered with a layer of muriate of zinc, which acts as the flux, and by means of rolls the plates are passed through this down into the tin, to be brought out at another point in the bath where there is a layer of oil on the surface.