Information Sheet No - British Stainless Steel Association

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SSAS Information Sheet No.5.01
British Stainless Steel Association Issue 03 12 th March 2001
Page 1 of 5
Introduction
Surface Finishes of Stainless Steel
Surface finishing treatments applied to stainless steels can take many forms. The main
objective of these treatments is to restore optimum corrosion resistance following thermal
and mechanical processing, and to enhance cosmetic appeal.
The treatments in frequent use are:-
• Acid treatment – of which descaling is the most common
• Mechanical polishing, abrasive blasting and electropolishing
• More specialist surface treatments such as colouring may also be applied
Acid Treatments
1. Pickling and Descaling
Oxide scale resulting from heat treatment or welding can be removed by immersion in an
acid solution containing diluted hydrofluoric acid. Hydrochloric acid based pickles, such as
those employed for pickling carbon steels, should be avoided due to the risk of pitting
corrosion. For items too large for immersion treatment, spray pickling processes can be
used. For use on vertical or overhead surfaces or where the treatment needs to be
localised, as on welds, pastes or gels, which can be applied with a brush, are beneficial.
Exposure time will vary depending on thickness and type of scale, strength and
temperature of the pickling product employed. Complete removal of oxide scale and other
surface contaminants is essential for good corrosion performance. Thorough washing with
water should follow all acid treatments.
2. Passivation
Stainless steel owes its corrosion resistance to a very thin oxide film on its surface. This
passive film will form when a clean stainless steel surface comes into contact with oxygen
in the air. This action can be accelerated by immersion in a dilute nitric or citric acid
solution and this is referred to as passivating. Spray-on solutions and paste type products
can also be used for passivating.
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SSAS Information Sheet No.5.01
Issue 03 12 th March 2001
Page 2 of 5
Mechanical Polishing
This method of surface treatment is generally carried out with the objective of creating an
aesthetically pleasing cosmetic finish or a controlled surface roughness/brightness
condition of specific applications. Where measured finishes are specified these will
normally be as ‘Roughness’ (CLA) in microns Ra or Reflectivity in Gloss Units.
Mechanically polished finishes can be applied to virtually all products before fabrication i.e.
Plate, Sheet, Bar, Tube, etc and can subsequently be blended by conventional hand
polishing after assembly or fabrication.
Grinding/polishing operations are carried out using iron free abrasive belts or wheels of
either Aluminium Oxide or Silicon Carbide construction. Buffing operations may be carried
out using nylon web rolls/flapwheels or sisal mops whilst Bright/Mirror finishes would be
produced on cotton mops using an appropriate compound.
Consideration should always be given to operating conditions in service when specifying
polished finishes as contamination may occur in certain circumstances should an inferior
finish be used.
Abrasive Blasting
This process which can be carried out either wet or dry is an alternative to acid cleaning
and descaling. It is important to use the correct iron free abrasive in order to avoid surface
contamination. Any abrasives, which are re-circulated, should be cleaned to ensure they
are free from contaminants. After treatment components should be thoroughly washed in
clean water and dried.
Barrel and Vibratory Finishing
Both these treatments can be carried out on stainless steel parts. The precautions listed
for abrasive blasting apply equally for these types of finishing operations.
Electropolishing
This well proven process electrochemically removes a fine surface layer, leaving a micro
smooth, highly corrosion resistant, reflective surface with an excellent oxide film. The
treatment can be employed for decorative and functional applications or for a combination
of both. The ultimate finish will largely depend upon the surface condition prior to
electropolishing. The process is also suitable for removing fine burrs from machined
components and pressings.
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SSAS Information Sheet No.5.01
Issue 03 12 th March 2001
Page 3 of 5
Special Finishes
In addition to the methods described overleaf it is possible to achieve special effects on
Stainless Steel by using any of the following techniques.
Surface finishes as supplied on Flat Rolled Products.
Finishes are specified in BSEN 10088:2 for flat products. This standard now replaces the
obsolete BS1449:2 1983, which was withdrawn in 1999. The BSEN 10088:2 finishes follow
the following convention and are shown in more detail on the table on page 4.
Hot Rolled material will have the first digit ‘1’
Cold Rolled material will have the first digit ‘2’
Special finishes will take the first digit of the base material and should be specified as to
whether required on one side or both sides.
1. Electroplating
Many different electro-deposits can be applied; to increase surface hardness, to improve
lubrication, to avoid seizing, to enhance solderability and to minimise bi-metallic (contact)
corrosion. Chromium, autocatalytic (electroless) nickel, silver, cadmium, nickel and tin
plating can all be applied.
2. Colouring
Achieved by chemical surface treatment, which modifies the normal protective passive
oxide film. An attractive range of colours can be produced which enhance the decorative
appeal of Stainless Steel.
3. Specialised Surface Treatments
Bonded dry-film lubricants (molybdenum di-sulphide), ion implantation and PVD (physical
vapour deposition) for wear resistance. Organic coatings – PTFE type resins for low
friction, special electrophoretic lacquers for tinting purposes and metal spraying can also
be supplied.
4. Cosmetic Patterned Finishes
There are a number of patterned finishes available, which are cold rolled onto the surface.
These include linen, leather, letters and mosaic effects.
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SSAS Information Sheet No.5.01
Issue 03 12 th March 2001
Page 4 of 5
European Standard
Number EN 10088-2
Surface Finishes Specified in BSEN 10088:2
Redundant
BS 1449-2
Description
Notes
HOT ROLLED
1U - Hot rolled No further treatment
1C 0 Hot rolled Softened
1E 1 Hot rolled Softened, Mechanically descaled
1D 1 Hot rolled Softened, pickled
COLD ROLLED
2D 2D Cold rolled Heat treated, Pickled
2B 2B Cold rolled Heat treated, Pickled, Skin passes
on bright rolls
2A 2A Cold rolled Bright Annealed
2H - Cold rolled Work hardened
SPECIAL FINISHES
1G/2G 3A Ground Uniform finish usually obtained with
80-100 grit size. Ra 2.5µm approx
2J 3B Dull Buffed Uniform straight grain finish usually
achieved on 180-220 grit. Ra
1.25µm approx
1J/2J 4 Dull Polish Unidirectional texture – not very
reflective. Final polishing on 180-
240 grit. Ra 0.6µm approx
1K/2K 5 Satin Polish Clean cut surface to achieve
adequate corrosion resistance for
marine and external architectural
applications. Ra

SSAS Information Sheet No.5.01
Issue 03 12 th March 2001
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may meet the general description in the table. Specifiers need to be aware that the finish
supplies meets their individual requirements.
Mill Finishes
Sheet and strip supplied with mill finishes (1U through 2H) may, unless otherwise agreed,
be supplied with only one surface inspected to the required finish.
In such instances the manufacturer should indicate the prime surface either by marking the
material or the packaging or by some other agreed convention.
Polished Finishes
Other polished finishes (1G through 2P) may also be available.
Before commencing any task ensure that you have received the appropriate health and
safety literature from the supplier and fully understand it. If in doubt seek advice.
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SSAS Information Sheet No.5.01
Issue 03 12 th March 2001
Page 6 of 5
This Information Sheet is an update of BSSA Information Sheet No.2
Technical Advice: Advice and assistance provided without charge are given in good faith but without responsibility