Spelter used per Ton of Galvanised Sheets.
Kind of Sheets
28 gauge ......
504 1b. ...
26 " ,.......
24 " ......
22 " ......
20 " ......
18 " ......
16 " ........
It will be noticed that, generally, the amount of spelter used in the modern process is only about half that which was placed upon the sheets under the old method. In consequence of this much thinner coating of zinc, the "life" of galvanising sheets is not by any means as long as it was formerly In ordering large quantities of galvanised sheets, not only should the gauge of sheet or weight per square foot be specified, but also the thickness of zinc coating or weight of spelter per square foot to be added, if buyers desire to obtain sheets of lasting quality. By the old method it took from 15 lb. to 20 lb. of spelter to cover both sides of 100 square feet of sheet, whereas by the double-roll system only 9 lb. to 12 lb. of spelter are used in coating the same area.
Usually, when sheets are thicker than 16 gauge (that is, 1/16 in. thick) they are not run through the rolls, but are carefully dried and then plunged bodily into the molten spelter, being drawn up on the opposite side of the pot through a thick layer of sand, or sand and ashes, kept moistened by water being sprinkled upon it.
The purer the iron the better the coat, might be taken as a rule in galvanising, and that is the reason why mild steel never takes as firm a coat as the best iron. Any impurities, such as carbon, silicon, etc., offer resistance to the formation of a tenacious surface alloy of iron and zinc. The harder the steel the more tendency there is for the spelter to peel off when the sheet is bent.
The remarkable liking which zinc has for iron, and which is its chief advantage in forming a good coating on galvanised work, is also at the same time its greatest drawback in the ready formation of dross. As the sheet passes through the pot small particles of iron are detached from its surface, which combine with the zinc, forming an iron-zinc alloy which gradually precipitates at the bottom of the pot, and which has to be periodically removed. Also the molten zinc slowly dissolves away the walls of the wrought iron or mild steel pot, forming additional dross, so that its renewal is required every six to twelve months or so. This is one of the constant worries and expenses of the galvaniser, and will never be altered until a different material for the pot, or another system of carrying out the process, is adopted. In the present writer's experience a pot that lasted longer than any other was constructed of wrought iron, which gave the following analysis: -
For those who are interested in galvanising it may be useful to know the compositions of one or two samples of zinc spelter. The analyses, of four specimens are given: -
Sample No. 1, it will be seen, is an exceptionally pure specimen of virgin spelter. The other three are samples of metal of the kind ordinarily in use. Any iron in spelter is most objectionable, as it all helps to form dross in the pot. Most of the lead present in a spelter falls to the bottom of the pot and there accumulates.
Lead and zinc have very little affinity: hence the former metal usually separates out, and on account of its being heavier than either spelter or dross, settles at the bottom of the galvanising bath, as shown in Fig. 338. When a bath has been in use several months, as much as a 6 in. depth of lead will sometimes have accumulated. In case the pot is shallow, and it is necessary to remove the lead, this can be readily done by standing a tube (one about 12 in. diameter, and a little longer than the depth of molten metal, made of 1/8 in. plate will do) upright on the bottom of the bath, and lading out the spelter, when the molten lead will be forced up the tube, and can be laded out as required. The dross, on account of its greater density than spelter, and being lighter than lead, sinks through the former and floats on the latter, as seen by the layers in Fig. 338. It is usually removed by a perforated spoon or ladle. Its composition is generally something like the following: -
It will be observed that the pot is heated on the sides only; any bottom heating having a tendency to make the dross rise and mix with the spelter, thus causing the surface of articles which are being galvanised to become rough. Great care should be taken not to let the spelter become red-hot, as undue heating spoils the surface of the object to be galvanised (giving it a "dry" appearance), and at the same time accelerates the formation of dross.
As much as possible of the top of the molten metal should be covered with sand, or sand and fine ashes, to prevent the atmosphere from coming into contact with the spelter, and thus forming zinc-oxide. Also it will reduce the loss of heat by radiation if the top of the unused part of the bath is covered with a plate of iron or other suitable material. Small articles can be readily galvanised by fixing up a small iron pot on an ordinary fire, or by gas-heating. In this case, when the spelter has been in use some time, it will be found to have become mixed with dross. To remove this, the molten metal should be allowed to settle, being kept in a liquid state by the application of a gentle side heat, the dross being then laded out.
The amount of salammoniac used per ton of work done will usually be about 9 lb. to 12 lb.
One disadvantage in the galvanising of objects having riveted or lap joints, is that these parts hold traces of acid or flux, which soon sets up corrosion, and causes the parts to decay. It is difficult to avoid this, except by galvanising both before and after manufacture. It is an advantage to wash the joints down with weak soda-water, thoroughly dry, and then force into the joints a little boiled oil. Stains or black spots on sheets are caused in the same way as above. When the surface is brocky or porous, acid is absorbed which shows up a day or so after galvanising, by forcing out black spots of chloride of zinc.
In galvanising odd work, if there are any parts like screw-threads, etc., that do not require coating with zinc, these should be covered with white lead before the article is put into the galvanising bath.
For cooking purposes zinc-coated articles are useless, on account of the readiness with which zinc is dissolved by various organic acids: but for temporary uses, such as with buckets, baths, etc., the zinc-coating is all right.