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STAINLESS STEEL
ELECTROPOLISHING:
Stainless steels and irons are electro polished on a
considerable scale. The process is particularly attractive in
view of the high costs of mechanically finishing these alloys.
Components treated range in size from kitchen sink units to
small surgical needles; typical examples are windscreen wipers,
wirework, automobile and cooker trim and surgical instruments.
The Process
Principle :
Electropolishing is, in principle a
reversal of the electroplating process. Metal is removed by
applying a dc current to the anodically connected work piece
submerged in a special acid electrolyte. Removal takes place
without inducing stress and preferentially affects the surface
micro-roughness. The surface becomes smooth and bright, with
burrs, edges and corners being removed preferentially.
The results can be repeated with a high degree of precision so
that components with tight tolerances can also be treated in
safety.
Electropolished surfaces are
distinguished by:
- Improved corrosion resistance and durability
- Reduced friction and wear
- Easier cleaning, reduced build-up of products on the
surface, reduced bacterial growth
- Reduced radioactive contamination and improved
decontamination behaviour
- Prevention of catalytic wall reaction
- Reduced pumping times to achieve high vacuum
- Improved high frequency conductivity
- Improved adhesion of plating deposits
- Optimum reflectivity
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The Process Results :
Electropolished and mechanically produced metal surfaces have
fundamental differences.
Mechanical operations such as machining, drilling, grinding and
polishing will all adversely affect the structure and properties
of the material surface. Contamination resulting from tool
abrasion and contact with coolants, can lead to corrosion and
wear. Dependent upon the degree of the manufacturing forces
applied, the surface structure can be affected to a depth of 50
microns.
In contrast to this, Electro-polishing will remove metal from
the component without mechanical, thermal or chemical
transformation. Electro-polishing will eliminate damaged surface
layers and allow the original properties of the material to be
fully utilized.
The areas of Application:
This processes and the electro-polished surfaces produced
by these processes have been employed in a wide range of varying
applications in practically all fields of engineering and
industry:
- Chemical, biochemical, pharmaceutical industries
- Food and beverage industries
- Electrical and electronic engineering
- Plant and equipment manufacture
- Machine and tool manufacturing
- Precision engineering and associated equipment
- Textile and paper industry
- Automotive and vehicle manufacturing
- Aviation and aerospace engineering
- Nuclear research and nuclear industry
- Vacuum and cryogenic engineering
- Ornaments and fancy metal goods
- Architecture and interior decoration
- Household and kitchen equipment like wet grinder parts, mixe parts etc,.
STAINLESS STEEL
ELECTROPOLISHING SOLUTION
Solution Composition
The electro polishing solution is supplied in a prepared form
ready for use.
Equipment:
For this solution a tank
lined with chemical lead is employed.. The provision of mild air
agitation is necessary in order to provide solution movement
during processing and to prevent layering.. For solution heating
silica cased immersion heaters may be used, alternatively a hot
water jacket may be provided.
For
cathodes, lead or copper strips 150 to 200 mm (6 to 8 in) wide
are used. The cathodes should be of sufficient area to give a
cathode current density of 5 to 8 amp / dm2 (50 to
75 amp /ft2). The cathodes will require periodic
brushing to remove the scale formed upon the surface.
JIGS :
The use of titanium jigs is
advised. Alternatively copper or brass jigs with bronze contacts
may be used but these will only have a limited life. The jigs
should be of adequate size in order to carry the high currents
employed.
Operating Conditions :
Anodic Current Density.: 16
to 45amp / dm2 (150 to 400 amp / ft2)
dependent on alloy
used and shape of article.
Voltage. :
10 to 20volts dependent upon the size of the load, the current
density applied, the
anode / cathode distance and ratio of the
surface areas of the
anode and cathode.
Temperature. : 60
to 90` C (140 to 195`F).
Time.
: 4 to 10 minutes. To ensure maximum solution life the life
the
Processing times should be kept as
short as possible.
Process Sequence :
The following sequence is advised:
1.
Cleaning. Solvent degrease in a solvent degreaser or,
alternatively, hot clean in anodic cleaner, depending upon the
surface condition. Where solvent degreasing is employed it is
advisable to follow with hot alkaline cleaning..
2.
Swill
3.
Hot swill and dry.
4.
Electropolish for 4 to 10 minutes.
5.
Swill.
6.
Swill.
7.
Passivate in a 5 per cent volume sulphuric acid – 1 per
cent sodium dichromate solution at room temperature, or in a 20
per cent volume nitric acid at 50` C (120`F) for a few minutes.
8.
Swill.
9.
Swill.
10.
Hot water and dry.
Solution
Maintenance :
The solution is
controlled by adjustment of the density and the iron content.
Density
As supplied the Stainless
Steel Electropolishing Solution will having a density of
approximately sp. gr. 1.75 at 15 0 C (62 0
Be). In use the density of the solution should be maintained
within the following limits:
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Solution temperature |
60 0 C |
90 0 C |
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Density
– Minimum value
Density
– Maximum value |
Sp. gr.
1.70 60 0Be
Sp. gr.
1.81 65 0Be |
Sp. gr.
1.64 57 0 Be
Sp. gr.
1.75 62 0 Be |
In use the density of
solution will normally rise. If, however, the density of the
solution tends to fall, this will be due to excess carry –over
of water on the work or absorption of moisture from the
atmosphere. Articles should be dried before being immersed in
the electropolishing solution and the tank should be covered
when not in use.Excess water may be driven off the solution by
heating the solution to 95to105 0 C (200 to 220
0F) for a period.
Iron Content:
The
degree of brightness will tend to fall when the iron content
reaches 60 g /l (10 oz / gal) and the iron content should
normally be kept below 50 g / l (8 oz / gal) by drag-out or the
removal of a small proportion of the solution. Periodical
elimination of 10 - 20 % of the total solution and replenishment
with fresh solution.
As a guide, the use of the solution to the extent
of four-ampere hours per litre will increase the iron content by
about 1 g/l. (114 ampere hours per gallon use will raise iron
content by 1 oz/gal.)
Most of the nickel and
some of the iron dissolved from the processed components will
precipitate as sludge. It will, therefore, be necessary for the
tank to be emptied occasionally and the sludge removed.
If you want to
start an electro polishing unit
contact us
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