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To
generate a continuous copper layer so that the subsequent acid galvanisation
solutions do not attack the castings.
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Unclosed
crevices or seams are attacked by solutions used later.
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Gloss,
half-gloss or silk gloss nickeling
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For
corrosion protection and as decorative finish.
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Burr
formation if too thick a layer.
Insufficient galvanisation in corners if too thin
a layer.
Peeling (flaking) caused by effects of heavy stresses and loads.
Formation of small nodes is usually the result
of solution impurity.
Penetration of previously applied layers leads to
corrosion.
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As
a coloured finish to prevent tarnishing or tempering coloration
effects, and to improve corrosion resistance.
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Encasement
of solution components with subsequent corrosion.
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Silver-plating,
brassing, gold-plating, etc.
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For
corresponding colour and decorative effects.
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Layers
that are too thin provide insufficient cover.
Impurities in the pools lead to colour deviations
(particularly brass).
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A
baking temperature that is wrongly set, or
defective mixing, can lead to the formation of
bubbles or the copper layer becoming brittle and peeling.
Distortions can result from too great a stress
and load during clamping.
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Metallic
coatings |
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Gold coating improves the quality of cheaper base materials;
it provides corrosion protection, resistance against acids
and gases, good solderability, and such materials then have
constant physical properties at high temperatures (spec. resistance,
contact resistance).
Gold-plating
is particularly suitable for contacts (plugs, pins, blades)
in electronics and electrical engineering. In order to prevent
diffusion of the base material into the gold layer it is customary
to provide electrical components with a diffusion block. These
parts are nickel-plated before gold-plating. Decorative gold-plating
is used for costume jewellery, uniform accessories, furniture
fittings, and so on. |
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The velour nickel process allows the depositing of silk-matt,
gloss-free nickel coatings of excellent evenness and traction.
A fine surface roughness of about 0.8 my is characteristic
of the velour nickel surface, also largely compensating for
any unevenness of the base material.
Velour
nickel coatings can be chromated or given other metal coatings,
e.g. brass, silver, or gold, without difficulty. |
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During chemical or flowing nickeling, nickel-phosphorous layers
(P = approx. 8%) are deposited evenly on edges, in bored holes,
hollow spaces and on flat areas, thus allowing accurate coatings
in the micro range. In terms of structure, this coating differs
markedly from galvanic nickeling.
The
effective and high-quality protection against corrosion and
wear makes chemical nickel plating a process that is of particular
use in the area of functional electroplating.
Chemical
nickel achieves a hardness of 500 - 700 HV, and with heat
treatment as high as 900-1000 HV, thus almost achieving the
values of hard chrome. The precipitation is non-magnetic,
further, chemically nickel-plated products also remain easy
to solder even after longer periods of storage.
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Due
to chrome’s high level of corrosion resistance, this
metal is widely used on products directly exposed to the atmosphere.
Chrome may be a highly reflective or matt deposit, which makes
a suitable pre-treatment (preferably a gloss or matt nickeling)
of decisive importance. |
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Copper precipitation is often used as an intermediate layer
before nickeling, but for certain purposes also a desirable
process within electrical engineering due to the good conductivity.
Finally, copper coatings, in conjunction with an appropriate
metal coloration, give parts made from different base materials
an “antique” appearance.
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Brass layers are mainly used for purely decorative purposes.
Hence, mostly deposited as gloss brass coatings with complete
sub-nickeling and then subsequently treated to improve protection
against temper colour.
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Zinc coatings are widely used on steel, because of their cost
advantage and good wide-ranging anti-corrosion properties.
The galvanic coating of steel products with zinc provides
optimum corrosion protection and a decorative effect using
low amounts of the raw material, because the depositing of
layers can be tenacious, ductile, and dense and have a high
gloss. Galvanic zinc layers are deposited as standardised
in DIN 50 961.
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Non-metallic
coatings |
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Applied to steel, iron or cast iron, browning involves a decorative
layer of black oxide, formed by a chemical conversion of the
metal surface, retaining the accurate dimensions, hardness
and low friction of the object.
Browning layers on steel are standardised in DIN 50 938.
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Chromating (often also called passivation) is carried out
using a treatment containing chrome ions (chrome-VI bonds),
and generates a supplementary corrosion-resistant layer on
galvanic coatings. This considerably delays corrosion of the
metal coating.
Chromating is principally used with zinc and cadmium coatings
as a subsequent treatment. One can obtain transparent, blue,
yellow, olive-coloured or black protective layers depending
on the process used. When transparent or bluish chromating
is used, a chrome-like appearance of a highly glossy zinc
or cadmium coating is obtained.
The chromating of zinc and cadmium coats are standardised
in DIN 50 941.
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Sealing
of chromating layers
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Treatment of the chromed product in watery solutions of organic
compounds seals the chromed goods. The introduction of these
materials into the fresh, gel-like chromating layer followed
by hardening leads to a substantial improvement in corrosion
resistance compared to unsealed goods.
Whilst a variety of applications are available one of the
most commonly used processes is better known as “Aquares”. |
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