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Diverse
methods can be used to meet these requirements, and there is a mutual
relationship between certain contamination types and the cleaning
processes.
Surface contaminations can be put into two major groups with respect to
formation:
- contaminations that form on the surface(e.g. scale, rust, temper colour),
- adhesive contaminations (e.g. oil, grease, dust).
The suitable pre-treatment procedure or procedures can only be chosen if
the starting condition and the target are set, and the task to be carried
out is properly known. Profound knowledge of the contamination to be
removed is of topmost importance for the pre-treatment of the workpiece,
because the success of surface protection or dressing is crucially
dependent on the condition of the surface.
At any rate, experience shows that appropriately selected and correctly
performed surface pre-treatment already ensures half the success.
There are several methods available for pre-treatment.
They include physical, chemical and physicochemical procedures.
Electroplating methods
Copper-plating
In galvanization technology, coppering is widely applied to make
finishing plating with electrocoating, to coat pressing-cylinders, to
prepare plates with printed wiring, to protect steel parts before
selective annealing, as an intermediate layer in corrosion prevention as a
part of the copper-nickel-chrome plating system.
Electrolytically deposited copper is light red in colour.
On air, a thin oxide layer forms on its surface, and it turns
brownish-black.
It dissolves in oxidising acids (nitric acid, chromic acid) and
sodium-cyanide solution.
Its compounds contain copper with one or two active valences. Univalent
compounds are usually colourless, while bivalent compounds are blue. The
properties of copper layers removed by galvanization are determined by the
type of electrolytes applied and by the conditions of removal.
The following galvanic coppering electrolytes have been developed
in electroplating technology:
- cyanide,
- vitriolic (with sulphates),
- diphosphatic,
- boratofluoric,
- sulphamate, etc. electrolytes.
Copper plating deposited from diphosphatic electrolytes have a fine
crystalline structure and are relatively hard, copper plating deposited
from additive-free vitriolic electrolytes have a coarser crystalline
structure and are soft. In electroplating practice, a significant
proportion of copper plating is deposited from cyanide electrolytes.
Main areas of application:
Cyanide copper electrolites
- intermediate layer in steel, zinc castings as a (brass) plating
in copper-nickel-chrome plating system,
- to prevent copper cementation in the course of galvanizing steel parts,
before the acidic coppering,
- as a protecting layer before azotizing and carbonization.
Vitriolic copper electrolytes
- producing galvanization forms,
- production of plates with printed wiring
- for technical purposes in the graphic and printing industry.
- to deposit a shiny equalising layer after cyanide coppering and prior to
glossy nickelplating.
Nickel-plating
Galvanic finishing with nickel-plating is the most often used procedure
to protect metal objects against corrosion, since it resists corrosive
influences in neutral, alkaline and mildly acidic atmosphere well.
In galvanization technology nickel plating is used for
decorative
protective and decorative
technical purposes.
The material of the object to be protected is usually steel, zinc
casting, copper and its alloys. The product has different forms and
characteristics subject to the conditions of use: matte nickel, glossy
nickel, silky nickel, or hard dispersion (ageing) nickel plating are
available.
For decorative purposes, it is usually used as stand-alone coating or
intermediate layer. The most important feature of these coatings is
their appearance.
Nickel plating for protective and decorative purposes can be used in
itself, or in (copper-)nickel-chrome plating systems. Apart from the
appearance, the anticorrosive nature of the deposited coating is also
essential in these applications.
In the case of plating deposited for technical purposes, the most
important features are hardness, reflecting and anticorrosive features
are the most important depending on the intended purpose of use.
Corrosion resistance.
The galvanic nickel-plating on the surface of steel and zinc castings is
called cathodic coating. This means that the corrosion of the base metal
will always start on places of defect (scratches, holes, etc.).
A thin oxide film develops on the nickel coating when exposed to air,
which results in the significant drop in glossiness, the surface
becoming dull.
To prevent this, a thin chrome layer is deposited on the nickel plating,
and this way the shine of the plating can be preserved for a long time.
The chrome coating deposited this way is not as noble as the nickel
plating.
In air, however, a passive film forms quickly on the chrome surface,
which has a disadvantageous influence on the corrosion resistance of the
nickel layer. Namely, the passivated chrome layer will become nobler
than the (mainly glossy) nickel beneath it. Thus, the nickel beneath the
chrome layer will start corroding, especially in the case of plating
deposited from bright nickel electrolytes.
To prevent the rapid corrosion of nickel, a crack-free chrome layer, or
rather microporous or microcracked chrome coating is deposited,
especially recently with a thin nickel deposit in most cases. Advanced
methods: cleaning, mechanical polishing, cleaning, glossy coppering,
glossy nickel-plating, glossy chroming. Advanced glossy technologies
abandoned a significant part of mechanical polishing, therefore, they
are faster and give an opportunity for automation.
It should be noted, however, that working conditions are more
sophisticate, the solutions and electrolytes more delicate, they require
more care and are more expensive, and one has to consider at all times
the way the additives applied modify the structure of the coating and
its anticorrosion value (e.g. the sulphuric content of glossy nickel
coating).
Sophisticated glossy technologies selected with due care and
thoughtfulness can be used to achieve favourable technical and economic
results. A characteristic solution, for example, is the application of a
double nickel-coating system, as well as the introduction of microcrack
and microporous chroming.
Mass nickel-plating :
Equipment designed for bulk electroplating can achieve efficient nickel
plating of small articles and parts without hanging or stringing.
Using immersion bells proved to be the most appropriate for this purpose.
After preparation, the workpieces are transferred into the nickel
plating bell.
We immerse a perforated oblique plastic bell in the plating bath. Anodes
are lined along the bath wall, while the products are in the immersion
bell.
The bell rotates around its axis at low speed, therefore, keeping the
small parts in constant motion and ensuring equal layer thickness.
We use a tilting device to lift the bell from the bath during loading
and unloading.
The parts remain in the bell until the electrolyte flows and drips back
to the electroplating vat through the perforated holes. We unload the
parts by lifting the rotating bell further.
The finished articles go through rinsing and hot air centrifugal drying.
Chroming
Chromium plating deposited by galvanization is hard, wear-resistant, has
a good resistance to corrosion, oxidizing materials effect the formation
of a thin oxide layer on the surface, which reduces corrosion
Main areas of application:
Decorative chroming
A thin chromium layer is deposited - usually on an intermediate nickel
layer - for decorative chroming applications. Plating has a dual role
here: it reduces the matting of the nickel layer and increases its
resistance to corrosion.
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