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Page 18
The articles to be tinned must be first thoroughly cleansed, and then
kept in the hot solution until properly whitened. A better result will
be obtained by using the following bath, and placing the pieces in
contact with a strip of clean zinc, also immersed:--
Bitartrate of potassium 14 oz.
Soft water 24 "
Protochloride of tin 1 "
It should be boiled for a few minutes before using.
The following is one of the best solutions for plating with tin by the
battery process:--
Potassium pyrophosphate 12 oz.
Protochloride of tin 4-1/2 "
Water 20 "
The anode or feeding-plate used in this bath consists of pure Banca
tin. This plate is joined to the positive (copper or carbon) pole of
the battery, while the work is suspended from a wire connected with
the negative (zinc) pole. A moderately strong battery is required, and
the work is finished by scratch-brushing.
In Weigler's process a bath is prepared by passing washed chlorine gas
into a concentrated aqueous solution of stannous chloride to
saturation, and expelling excess of gas by warming the solution, which
is then diluted with about ten volumes of water, and filtered, if
necessary. The articles to be plated are pickled in dilute sulphuric
acid, and polished with fine sand and a scratch-brush, rinsed in
water, loosely wound round with zinc wire or tape, and immersed in the
bath for ten or fifteen minutes at ordinary temperatures. The coating
is finished with the scratch-brush and whiting. By this process
cast-or wrought-iron, steel, copper, brass, and lead can be tinned
without a separate battery. The only disadvantage of the process is
that the bath soon becomes clogged up with zinc chloride, and the tin
salt must be frequently removed. In Hern's process a bath composed
of--
Tartaric acid 2 oz.
Water 100 "
Soda 3 "
Protochloride of tin 3 "
is employed instead of the preceding. It requires a somewhat longer
exposure to properly tin articles in this than in Weigler's bath.
Either of these baths may be used with a separate battery.
SECTION VII.
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 amp�res 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.
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