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Page 19
Zinc surfaces, after a brief exposure to the air, become coated with a
thin film of oxide--insoluble in water--which adheres tenaciously,
forming a protective coating to the underlying zinc. So long as the
zinc surface remains intact, the underlying metal is protected from
corrosive action, but a mechanical or other injury to the zinc coating
that exposes the metal beneath, in the presence of moisture causes a
very rapid corrosion to be started, the galvanic action being changed
from the zinc positive to zinc negative, and the iron, as the positive
element in the circuit, is corroded instead of the zinc. 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 | Weight of Zinc | Percentage |
| Article | 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 |
+--------------------------+--------------------------+-------------+
Tin is often added to the hot bath for the purpose of obtaining a
smoother surface and larger facets, but it is found to shorten the
life of the protective coating very considerably.
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
4-1/2 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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