GALVANIZED IRON. Iron which has been coated with zinc, to prevent it from rusting. The iron is simply dipped or immersed in melted zinc, not coated by any galvanic process, as its name would imply. The process of galvanizing iron is now practiced on a most extensive scale. The French chemist Dumas states that so long ago as 1742 Malouin knew of a plan for coating iron with zinc. At all events, it is stated in Bishop Watson's Chemical Essays, issued in 1786, that a method (essentially the same as that now in use for zincing iron) was then practiced at Rouen for coating hammered iron saucepans with zinc, and some details of the operation are given. The first English patent for galvanizing iron was granted to H. W. Craufurd in 1837, and another for the zincing of iron which had been previously tinned was taken out by E. Morewood in 1821. The process as employed by Craufurd, which is still essentially unchanged, was first to remove the rust and scale from the iron by pick ling—that is, immersing it in dilute sulphuric or hydrochloric acid, either hot or cold, although the former state was preferred; and for this pur pose the acid was kept warm in a large leaden bath, sunk in the ground for easier access. After the sheets or other articles of iron had been acted upon by the acid for a few minutes more or less, according to their requirements, they were plunged into cold water, to remove the acid, and afterwards scoured with sand, and again washed clean with water. The iron being now ready to receive its coating of zinc, it is plunged into a bath of that metal, which, pre vious to its being melted, is coated with a thick layer of dry sal ammoniac (chloride of am monium) ; this melts also, and forms a viscid coating over the metal, which prevents that rapid oxidation to which the molten metal is otherwise liable.
For inferior material the scouring with sand is usually dispensed with. The sheets of iron are then made to pass between two iron rollers in the zinc bath, and are thus more easily drawn through and kept perfectly smooth. Ships' bolts, nails, screws, chains, etc., are dipped in, in
bundles, or in the ease of nails, etc., in iron strainers; when removed, the zinc makes them adhere together; and to effect their separation, they have to be placed in a crucible with powdered charcoal, in which they are heated to redness, and repeatedly shaken as they cool; by this means they are easily separated.
Galvanized iron is largely uked in the form of sheets, both plain and corrugated, for roofs, sheds, and cisterns; in the state of wire, besides that used in telegraphs, a large quantity is em ployed for wire ropes, netting, and the like ; and it has innumerable minor applications, such as for water-vessels, ship-fittings, and many other articles formerly made of wood, copper, brass, slate, etc. For most of these purposes the zinc coating is much more lasting and less trouble some than they would be; but still in certain situations, as where it is exposed to the action of sulphurous compounds in smoke, and where its surface is brought directly into contact with other deleterious chemical substances, its use can not be recommended; and in these circumstances other plans should be resorted to for the protec tion of the iron.
The plan adopted for making the variety of galvanized iron called galvanized tinware is as follows: The sheets or other articles, after being pickled, and scoured, and washed, as in the usual process, are transferred to a large wooden bath. On the bottom of the bath is first placed a layer of finely granulated zinc, then a sheet of iron, then another layer of granulated zinc, and so on as far as convenient; and the bath is filled up with a diluted solution of chloride of tin, so that by means of the galvanic action produced the tin becomes deposited thinly over the sheets of iron. The plates are then taken to the zinc bath, pre pared exactly as in the ordinary process, where they are dipped or passed through the rollers. By this process a very even deposit of zinc is produced, and the material so made is preferred for some purposes to ordinary galvanized iron, although its properties are much the same.