GALVANIZED IRON. This name is given to iron which has been coated with zinc, to prevent it from rusting. The iron is simply dipped in zinc, and is not coated by any galvanic process, as its name would imply. Zinc, after a short exposure to air, becomes coated with a film of oxide, which does not increase, and this preserves both the zinc itself as well as the iron beneath it from further decay, provided there is no gal vanic action going on between the two metals. Probably, however, this action does take place, and the iron is in that case protected by its acting as an eleetro-negative element to the zinc. But whatever the true explanation of the matter may be, practi cally it is found that iron so coated resists the oxidizing effects of air and water extremely well.
The process of galvanizing iron is now practiced on a most extensive scale, and like some other so-called inventions of modern times. has been claimed by patentees who were plainly not the original inventors. The French chemist Dumas states that so long ago as 1742, Malouin knew of a plan for coating iron with zinc. There is no doubt of the fact, at all events, for it is stated in bishop 1Vatson's Chemical Essays, issued in 17S6, 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 sonic details of the operation are given. The first English patent for galvanizing iron was granted to Mr. H. W: Craufurd in 1837, and another for the zincing of iron which had been previously tinned•was taken out by Mr. E. Morewood in 1821. Like most patents for processes or machines which are commercially successful, these were subjected to much dispute and litigation, and their validity deserved to be attacked. We prefer now adays to coat our iron cooking-vessels with tin instead of zinc; but in a recently pub lished volume of great interest on the Industries of Birmingham, it is stated that " gal vanized iron buckets, hardly known 25 years ago, arc now sold by tens of thousands weekly." The process as employed by Mr. Craufurd—and it is still much the same—was first to remove the rust and scale from the iron, by is, immersing it in dilute sulphuric or hydrochloric acid, either hot or cold; but the former state was preferred; and for this purpose 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 have been acted upon by the acid for a few minutes more or less, according to their requirements, they are 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, previous to its being melted, is coated with a thick layer of dry sal-ammoniac (chloride of ammonium); 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., arc dipped in, in bundles, or in the case 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.
The important article of telegraph wire, of which the single firm of Messrs. John ston Co., annually turns out some thousands of miles, is managed entirely by machinery. The iron wire is brought from the drawing--mill, and passed through a pipe kept at white heat by passing through a furnace; this furnace having a row of such pipes set like the tubes of a boiler, but all at the same level, and open at each end, so that the wire has a clear passage through. Next the wire passes at a dull red heat through dilute hydrochloric acid, and immediately after that through a bath of zinc, the process being performed by a self-acting apparatus, having a series of drums. The use of galvanized iron is daily increasing. It is largely consumed in the form of sheets, both plain and corrugated, for roofs, sheds, cisterns, and arches of fire-proof floors; in the state of wire, besides that used in telegraphs, a large quantity is employed 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 last ing and less troublesofne than paint 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 cannot be recommended; and in these circumstances other plans should be resorted to for the protection of time iron.
The plan adopted by Morcwood and Rogers for making the variety of galvanized iron called galvanized tinned iron, referred to above, is as follows: The sheets or other articles, after being pickled, and scoured, and washed, as in the usual process, are trans ferred to a large wooden bath. On the bottom of the bath is first placed a layer of finely granulated zinc, then a sheet of the 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, prepared 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 prop erties are much the same.