An Analysis Of Zinc Coating Applied By The Hot Galvanizing Process
When galvanized iron is immersed in a corrosive liquid, the zinc is attacked in preference to the iron, provided both the exposed parts of the iron and the protected parts are immersed in the liquid. The zinc has not the same protective quality when the liquid is sprinkled over the surface and remains in isolated drops. Sea air, being charged with saline matters, is very destructive to galvanized surfaces, forming a soluble chloride by its action.
As zinc is one of the metals most readily attacked by acids, ordinary galvanized iron is not suitable for positions where it is to be much exposed to an atmosphere charged with acids sent into the air by some manufactories, or to the sulphuric acid fumes found in the products of combustion of rolling mills, iron, glass, and gas works, etc., and yet we see engineers of note covering-in important buildings with corrugated and other sheets of iron, and using galvanized iron tie rods, angles, and other constructive shapes in blind confidence of the protective power of the zinc coating; also in supreme indifference as to the future consequences and catastrophes that arise from their unexpected failure.
The comparative inertia of lead to the chemical action of many acids has led to the contention that it should form as good, if not a better, protection of iron than zinc, but in practice it is found to be deficient as a protective coating against corrosion.
A piece of lead-coated iron placed in water will show decided evidences of corrosion in twenty-four hours. This is to be attributed to the porous nature of the coating, whether it is applied by the hot or wet (acid) process.
The lead does not bond to the plate as well as either of the other metals—zinc, tin, copper, or any alloys of them.
The following table gives the increase in weight of different articles due to hot galvanizing:
Description of Article Weight of Zinc per Square foot % of Increase of
Weight
Thin sheet-iron 1.196 oz. 18.2
5/16-in. plates 1.76 " 2.0
4-in. cut nails 2.19 " 6.72
7/8-in. die bolt and nut approximately 1.206 oz. 1.00
A portion of a zinc coating applied by the hot process was found to be very brittle, breaking when attempts were made to bend it; the average thickness of the coating was .015 inch.
An analysis gave the following result:
- Tin, 2.20;
- Iron, 3.78;
- Arsenic, a trace;
- Zinc (by difference), 94.02.
A small quantity of iron is dissolved from all the articles placed in the molten zinc bath, and a dross is formed amounting in many cases to 25 per cent of the whole amount of zinc used.
The zinc-iron alloy is very brittle, and contains by analysis 6 per cent of iron, and is used to cast small art ornaments from.
A hot galvanizing plant, having a bath capacity of 10 feet by 4 feet by 41/2 feet outside dimensions, and about 1 inch in thickness, will hold 28 tons of zinc.
With equal amounts of zinc per unit of area, the zinc coating put on by the cold process is more resistant to the corroding action of a saturated solution of copper sulphate than is the case with steel coated by the ordinary hot galvanizing process; or, to put it in another form, articles coated by the cold process should have an equally long life under the same conditions of exposure that hot galvanized articles are exposed to, and with less zinc than would be necessary in the ordinary hot process.
The hardness of a zinc surface is a matter of some importance. With this object in view aluminium has been added from a separate crucible to the molten zinc at the moment of dipping the article to be zinced, so as to form a compound surface of zinco-aluminium, and to reduce the ashes formed from the protective coverings of sal-ammoniac, fat, glycerine, etc.
The addition of the aluminium also reduces the thickness of the coating applied.
Cold and hot galvanized plates appear to stand abrasion equally well. Both pickling and hot galvanizing reduce the strength, distort and render brittle iron and steel wires of small sections.
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