As galvanising is the commonest process adopted for applying a protective coating to sheet-iron work we shall explain the method followed with some fulness. Essentially the process consists in applying a thin film of zinc to the surface of the iron. We will first explain the plan followed for sheets and work on a large scale, and then give some hints of how best to deal with small articles. Before sheets can be galvanised all scale must be removed from their surface, and this is usually done in a pickling solution composed of equal parts of hydrochloric acid (or muriatic acid, as it is often called) and water. Lead-lined tanks are sometimes used for holding the acid; but the better plan is to have stone tanks, jointed with rubber packing, and held together with tie-rods. During the time the sheets are in the pickle they should be moved continuously, so that all parts of the surfaces may be equally exposed to the action of the acid. The length of time for pickling will depend upon the temperature and strength of the acid and on the condition of the sheet surfaces. If the acid is fresh and the sheets have been close-annealed (that is, out of contact with the furnace gases, then the pickling may be done in about fifteen minutes; but if the acid is partly spent, or the sheets covered with heavy scale (as the result of open-annealing), thirty to forty-five minutes will be required. Heating the acid (done in the early days of galvanising by blowing steam into the tanks) will increase the speed of working, but the character of sheet-surface produced will not be so good as when pickled by the cooler acid. Any increase of temperature over and above that of the atmosphere required for the effective working of the pickle is soon obtained by the heat generated through the chemical action. If the pickling solution is too hot the action upon the sheets is not uniform, and the surfaces will be somewhat rougher. Occasionally a sheet will be found that contains a hard patch of scale or a scab, and this will have to be removed by a scraper or pick before attempting to pass through the galvanising bath. Sometimes a blister (a piece of double sheet which has not been properly welded in the manufacture) is found on a sheet, and great care should be taken to cut this away, as it will act as a receptacle for acid, which, when carried into the molten spelter, may cause a serious explosion. To obtain a good-looking surface after galvanising, the operator should be careful not to over-pickle, as this will cause the sheet to look "dead" and "dry." When properly cleaned the sheets are plunged into a water-tank for washing, and are then ready for the galvanising bath.
The quantity of acid used varies from 1 1/2 to 4 carboys per ton of sheets, depending upon whether the sheets are close or open-annealed, or heavy or light. For economical working, the partly-spent acid from the large tank, when it becomes too slow for sheets, should be used as far as possible for small work, for which the time of pickling is not so important.
Where a large amount of work is done it is usual to test the acid with a Twaddel's hydrometer, the degree of reading, according to the density of the acid, varying from 24° to 30°. Without the acid is fairly pure the reading given on the hydrometer is not an exact indication of the strength of the acid from the galvaniser's point of view. A better and more effective test is to compare the relative amounts of zinc dissolved by equal quantities of acid taken from the different sample carboys. Thus, to give the result of one experiment: A certain quantity of 24° acid dissolves 5 oz. of zinc, whilst the same quantity of 30° acid dissolves 6 oz. of zinc. Their relative values, therefore, to the galvaniser are as 5 is to 6. In this way, by taking cost into account, it can be seen which is the most economical to use.
The amount of waste in pickling runs out to about 4 lb. per 100 square ft. of open-annealed sheet iron to 2 3/4 lb. per 100 square ft. of close-annealed sheet. This gives, as near as possible, 33 lb. to the ton of 16 gauge and 82 lb. to the ton of 24 gauge of the former, galvanised, and 57 lb. to the ton of the latter.
Before proceeding to explain how sheets are passed through the galvanising bath, it is as well to call attention here to the fact that the quality of the galvanised sheet surface will very largely depend upon the kind of surface that is put upon the black sheet. If the iron is of an inferior quality with a coarse surface, or is over-pickled, no amount of care in galvanising will produce a good surface. This, indeed, is true of all surface treatment, whether tinning, painting, lacquering, or whatever it may be.
A sectional elevation of a galvanising bath, with the rolls in position, is shown in Fig. 338. A layer of flux, about 6 in. thick, of crude salammoniac (or muriate of ammonia, as it is called) is allowed to boil up in the flux-box, a bit of tallow being thrown in occasionally. The sheets are taken one by one and passed into the pot through the flux-box, down through the feeding-rolls, and up out between the surface, or leaving-rolls, and taken away, either by hand or travelling chains, boshed in a tank of warm water, and dried by passing through a drying stove.
To ensure a clean galvanised sheet the surface-rolls must be kept clear of all waste flux and scum, and the flux in the flux-box must not be allowed to get too dirty, or else some of it will be carried through to the leaving-rolls and mark the sheets.
Up to within a few years back all sheets were either drawn through the clear or through sand, the feeding-rolls alone being used, and these, of course, simply to carry the sheet through the molten metal. One object of the surface-rolls is to give a more uniform coating of zinc and impart a little better surface to the sheet. The primary object, however, in the use of leaving-rolls is to squeeze as much zinc off the sheet as possible, and thus reduce the cost of manufacture. The result is that galvanised sheets of the present day are altogether inferior to what they were under the old system of manufacture. The following table, which has been compiled from experiments carried out by the writer, will give some indication of the altogether thinner coating of zinc which is now put upon sheets to that which was formerly the case: -