GALVANIZING.
Galvanizing, as a protecting surface for large articles, such as enter
into the construction of bridges, roofs, and shipwork, has not quite
reached the point of appreciation that possibly the near future may
award to it. Certain fallacies existed for a long time as to the
relative merits of the dry or molten and the wet or electrolytical
methods of galvanizing. The latter was found to be costly and slow,
and the results obtained were erratic and not satisfactory, and soon
gave place to the dry or molten bath process, as in practice at the
present day; but the difficulty of management in connexion with large
baths of molten material, and the deterioration of the bath, and other
mechanical causes, limit the process to articles of comparatively
small size and weight. The electro deposition of zinc has been subject
to many patents, and the efforts to introduce it have been lamentable
in both a mechanical and financial sense. Most authorities recommend a
current density of 18 or 20 ampères per square foot of cathode
surface, and aqueous solutions of zinc sulphate, acetate or chloride,
ammonia, chloride or tartrate, as being the most suitable for
deposition. Electrolytes made by adding caustic potash or soda to a
suitable zinc salt have been found to be unworkable in practice on
account of the formation of an insoluble zinc oxide on the surface of
the anode and the resultant increased electrical resistance; the
electrolytes are also constantly getting out of order, as more metal
is taken out of the solution than could possibly be dissolved from the
anodes by the chemicals set free on account of this insoluble scale or
furring up of the anodes, which sometimes reaches one-eighth of an
inch in thickness. To all intents and purposes the deposits obtained
from acid solutions under favourable circumstances are fairly adhesive
when great care has been exercised to thoroughly scale and clean the
surface to be coated, which is found to be the principal difficulty in
the application of any electro-chemical process for copper, lead, or
tin, as well as for zinc, and that renders even the application of
paint or other brush compounds to futile unless honestly complied
with. Unfortunately these acid zinc coatings are of a transitory
nature, Their durability being incomparable with hot galvanizing, as
the deposit is porous and retains some of the acid salts, which cause
a wasting of the zinc, and consequently the rusting of the iron or
steel. Castings coated with acid zinc rust comparatively quickly, even
when the porosity has been reduced by oxidation, aggravated no doubt
by some of the corroding agents--sal-ammoniac, for instance--being
forced into the pores of the metal. Other matters of serious moment in
the electro-zincing process, apart from the slowness of the operation,
were the uncertain nature, thickness, and extent of the coating on
articles of irregular shape, and the formation of loose, dark-coloured
patches on the work; the unhealthy and non-metallic look and want of
brilliancy and the lustre prevented engineers and the trade from
accepting the process or its results, except for the commoner articles
of use. To obviate any tendency of the paint to peel off from the zinc
surface, as it generally manifests a disposition to do, it is
recommended to coat all the zinc surfaces, previous to painting them,
with the following compound: 1 part chloride of copper, 1 part nitrate
of copper, 1 part sal-ammoniac, dissolved in 61 parts of water, and
then add 1 part commercial hydrochloric acid. When the zinc is brushed
over with this mixture it oxidizes the surface, turns black, and dries
in from twelve to twenty-four hours, and may then be painted over
without any danger of peeling. Another and more quickly applied
coating consists of, bi-chloride of platinum, 1 part dissolved in 10
parts of distilled water, and applied either by a brush or sponge. It
oxidizes at once, turns black, and resists the weak acids, rain, and
the elements generally.
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.
THE END.
INDEX.
A
Amalgam process in tin-plating, 59.
Appliances and apparatus used in japanning and enamelling, 29.
B
Battery process in tin-plating, 59.
Black grounds, 11.
---- japan grounds on metal, common, 12.
---- paints, 52.
---- pigment, 46.
---- stain for iron, 53.
---- varnish for sewing machines, 56.
Blue japan grounds, 9.
---- pigment, 46.
Brass, polished, colours for, 49-57.
Brick ovens, 33.
Bright pale yellow grounds, 10.
Bronzing composition, 49.
Brown japan, 57.
Bunsen burner, 33.
C
Carriage varnish, 51.
Colours for polished brass, 49.
Common black japan grounds on metal, 12.
Composition for bronzing, 49.
Cream enamel, 8.
E
Enamelling and japanning stoves, 29-46.
---- ---- ---- ---- heated by direct fire, 34.
---- ---- ---- ---- heated by hot-water pipes, 36.
---- or japanning metals, 20-28.
---- old work, 27.
F
First stage in the japanning of wood, 5.
---- ---- in the japanning of leather, without a priming, 5.
G
Galvanized iron, painting on, 49.
Galvanizing, 61-66.
Golden varnish for metal, 51.
Green japan grounds, 10.
---- pigment, 46.
Ground, red japan, 10.
---- scarlet japan, 9.
---- tortoise-shell, 12.
Grounds, black, 11.
---- black japan, 12.
---- blue japan, 9.
---- bright pale yellow, 10.
---- green japan, 10.
---- japan, 6-19.
---- orange-coloured, 11.
---- purple, 11.
---- white japan, 7
H
Heating stoves by direct fire, 34.
---- ---- by hot-water pipes, 36.
Hern's process in tin-plating, 60.
I
Immersion process in tin-plating, 59.
Iron, black stain for, 53.
---- galvanized, painting on, 49.
Ironwork, varnishes for, 55.
J
Japan, brown, 57.
---- ground, red, 10.
---- ---- scarlet, 9.
---- ---- grounds, 6-19.
---- ---- black, 12.
---- ---- blue, 9.
---- ---- green, 10.
---- ---- white, 7.
---- work, painting, 13.
---- ---- varnishing, 17.
Japanese gold size, 14.
Japanese lacquer, 47.
Japanning and enamelling stoves, 34.
---- ---- ---- ---- heated by direct fire, 34.
---- ---- ---- ---- heated by hot-water pipes, 36.
---- leather without a priming, first stage, 5.
---- or enamelling metals, 20-28.
---- tin, 25.
---- wood, first stage, 5.
L
Lacquer, Japanese, 47.
M
Metal, golden varnish for, 51.
---- polishes, 51.
Metals, japanning or enamelling, 20-28.
Modern japanning and enamelling stoves, 34.
N
Natural Japanese lacquer, 47.
---- lacquer, 45.
O
Oil vehicle, 14.
Old work, enamelling, 27.
Orange-coloured grounds, 11.
P
Painting japan work, 13.
---- on galvanized iron, 49.
---- ---- zinc, 49.
Paints, black, 52.
Pigments suitable for japanning with natural lacquer, 45.
---- black, 46.
---- blue, 46.
---- green, 46.
---- red, 46.
---- white, 45.
---- yellow, 46.
Polished brass, colours for, 49.
Preparing the surface to be japanned, 4.
Priming the surface to be japanned, 4.
Processes for tin-plating, 58.
Purple grounds, 11.
R
Red japan ground, 10.
---- pigments, 46.
S
Scarlet japan ground, 9.
Sewing machines, black varnish for, 56.
Shellac varnish, 6.
Stoves, modern japanning and enamelling, 34.
Stove, the enamelling and japanning, 29-45.
Surface to be japanned, priming or preparing the, 4.
T
Tin, japanning, 25.
Tin-plating, colours for, 58.
Tin-plating, amalgam process, 59.
---- battery process, 59.
---- Hern's process, 60.
---- immersion process, 59.
---- Weigler's process, 60.
Tortoise-shell ground, 12.
U
Urushiol, 47.
V
Varnish, carriage, 51,
---- for iron and steel, 57.
---- for metal, golden, 51.
---- shellac, 6.
Varnishes for iron work, 55.
Varnishing japan work, 17.
W
Weigler's process of tin-plating, 60.
White japan grounds, 7.
---- pigments, 45.
Wood, first stage in the japanning of, 5.
Y
Yellow grounds, bright pale, 10.
---- pigments, 46.
Z
Zinc, painting on, 49.
ABERDEEN: THE UNIVERSITY PRESS
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ENAMELS AND ENAMELLING
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