stainless steel surface finishes

8/13/2019 Stainless Steel Surface Finishes

1/5

bc

A a l c o a n d O P T I S H E EN a r e r e g i s t e r e d t r a d e m a r k s o f A a l c o M e t a l s L t d C o p y r i g h t : A a l c o M e t a l s L t d , Th e H e r s h a m C en t r e , H e r s h a m G r e e n , H e r s h a m , Su r r e y K T 1 2 4 H P

1

Stainless Steel Surface Finishes

IntroductionSurface finish is an important element in anyspecification of stainless steel regardless of theintended use. For those applications whereappearance is important, finish is a design

element and must be specified. In non-decorative applications the surface finish mayhave implications for friction, wear,maintenance or corrosion resistance and must,therefore, also be carefully chosen and clearlyspecified.The choice of finish should never be left to thesupplier, or the specification loosely worded,such as Type 304 with a 180 grit finish. Thefinish should be properly identified by astandard industry designation or by a tradename, e.g. O PTISHEEN .

Finishes and DesignThere are a wide range of decorative finishesavailable; therefore, it is important to pay closeattention to the selection of the mostappropriate finish for the application required.For highly visible applications the appearance ofstainless steel is a critical design element and amisunderstanding of the wrong finish can alterthe desired effect. In commercial and hygienicapplications, such as restaurants and hospitals,properly finished stainless steel is easier to keepclean. In consumer products, such as cateringequipment, the lustre from a well polished sheetof stainless steel has strong sales appeal.

In addition to the visual appearance of polishedstainless steel there are a number of functionalconsiderations. In sanitary applicationscorrectly polished stainless steel not only looksgood but it helps to reduce the risk of bacteriabeing retained by the material.

In aggressive environments, such as in thenuclear or offshore industries, a correctlypolished stainless steel surface has a better

resistance to corrosion than a surface that isroughly or badly polished. A smooth surface isless susceptible to an accumulation of depositsand stainless, which often become focal pointsfor localised corrosion. All stainless steelfinishes perform better when cleaned andmaintained and details of correct cleaningprocedures may be found in our publication,

The Cleaning and Maintenance of StainlessSteel.

Finishes and FabricationSome fabrication operations, such as grindingprior to painting or gluing, may require a roughsurface finish but, generally speaking, asmooth, well finished sheet requires less

physical effort than a coarse, rough one when itcomes to blending. Certain finishes are moredifficult to recreate by hand than others,causing fabrication difficulties; this is especiallytrue of the special finishes which cannot beeasily replicated in a fabrications workshop. Forthis reason the fine satin finish (such asOPTISHEEN ) is popular and practical forfabrication shops around the world.

Types of FinishStainless steel is available in a wide variety ofstandard and special finishes. The majority of

finishes can be divided into three categories:mill finishes, mechanically polished finishes andspecial finishes. In each case the finishes aredescribed under either their appropriate BritishStandard number (BS 1449 Part 2) or the nameby which they are commonly known. In someinstances a finish may commonly be known byseveral names, leading to somemisunderstandings (see Finish Variations ).

Satin Finish VariationsFinishes described by a particular industrydesignation, i.e. No. 4 finish, may still vary fromsupplier to supplier and even from batch tobatch from the same supplier. In the case ofmill finishes, this may be the result of differingmanufacturing conditions. In the case of satinpolished finishes, it is probably due to wear ofthe abrasive belts used in these processes. Oneway to overcome these problems is to specify aproprietary brand name finish, such asOPTISHEEN , which has a guaranteedconsistency.

Failing this, reference should be made to a

supplied finish of at least 30 mm square toshow up all the characteristics of the finish andagainst which the final product can be judged.The customer should define and agreeacceptable levels of variations from thestandard to minimise the problems caused bybelt wear and consequent changes in finish.


2/5

bc

A a l c o a n d O P TI S H EE N a r e r e g i s t e r e d t r a d e m a r k s o f A a l c o M e t a l s L t d C o p y r i g h t : A a l c o M e t a l s L t d , Th e H e r s h a m C en t r e , H e r s h a m G r e e n , H e r s h a m , Su r r e y K T 1 2 4 H P

2

BS EN 10088-2 / 10028-7 Finishes

BS EN Finish Old BS Finish Description

Hot Rolled

1C 0 Hot rolled, heat treated, not descaled

1E 1 Hot rolled, heat treated, mechanically descaled

1D 1 Hot rolled, heat treated, pickled

1U - Hot rolled, not heat treated, not descaled

Cold Rolled

2C - Cold rolled, heat treated, not descaled

2E - Cold rolled, heat treated, mechanically descaled

2D 2D Cold rolled, heat treated, pickled

2B 2B Cold rolled, heat treated, pickled, skin passed

2R 2A / (BA) Cold rolled, bright annealed

2Q - Cold rolled, hardened and tempered, scale-free

Special Finishes*

BS EN Finish Old BS Finish Description TypicalGrit

Typical R a

1G or 2G - Ground Grit 120 2.5 to 2.0

1J or 2J 3B Brushed - Unidirectional 180 1.2 to 1.0

1J or 2J 4 Dull Polished Unidirectional 240 0.6

1K or 2K 5 Satin polished Unidirectional 320 0.5 Max

1P or 2P 7 Bright polished Non-Directional with ahigh degree of image clarity

600 0.1

1P or 2P 8 Mirror Finish Non-Directional with a veryhigh degree of image clarity

800 0.05

1M or 2M - Patterned

2L - Coloured

2W - Corrugated

1S or 2S - Surface Coated (Metallic coatings such astin, lead or aluminium)

*Note: Special finishes indicate hot rolled (1) and cold rolled (2) sheets, e.g.:Ground polished hot rolled sheets = 1G / Ground polished cold rolled sheets = 2G


3/5

bc

A a l c o a n d O P TI S H EE N a r e r e g i s t e r e d t r a d e m a r k s o f A a l c o M e t a l s L t d C o p y r i g h t : A a l c o M e t a l s L t d , Th e H e r s h a m C en t r e , H e r s h a m G r e e n , H e r s h a m , Su r r e y K T 1 2 4 H P

3

Mill Finishes

The majority of mill finishes have a dull or mattfinish which is not suitable as a decorative finishfor most products or applications. They aresuitable, however, where appearance is notimportant or where further finishing is to takeplace.

Stainless steel is polished by mills in a range offorms, e.g. sheet, plate, bar & tube, but allfinishes may not be available on all forms. It isimportant, therefore, to check that the finish isavailable in the form required.

1D / 1E Hot Rolled PlateHot rolled, softened (annealed) with the surfacede-scaled. The surface remains rough andheavily pitted and subsequent finishing islimited to heavy grinding. It can be used where

uniformity and smoothness of finish are notimportant and where a heavy ground surface isacceptable.Typical uses include gas turbine parts,incinerators, kiln liners and oil burner parts.

2DThis material is cold rolled followed by softeningand de-scaling. The effect of cold rolling thematerial is to produce a smoother, less pittedsurface. The surface is not only smooth but ofmatt grain appearance. Used where a bettersurface is required or subsequent processing/finishing is necessary.Typical uses include car exhaust systems,furnace parts, builders hardware andpetrochemical equipment.

2BThe material is cold rolled, softened and de-scaled as in the case of the 2D finish and thenreceives a final light pass on polished rolls calleda skin pass or pinch pass. The materialremains grey in appearance but the effect ofthis final pass on polished rolls is to produce asmoother, brighter surface than the 2D finish.This is the general purpose, cold rolled finishand it is suitable for a wide range of applicationswhere stainless steel is specified. It is alsosuitable for a wide range of subsequentpolishing processes such as satin finishing.Typical uses for a standard 2B finish includenon-decorative or functional sheet metalproducts, industrial refrigeration equipment,chemical plant and plumbing fixtures.

2R (Bright Annealed)This finish is also produced by cold rolling andde-scaling but in this case the final annealing isdone in a controlled atmosphere furnace. Theeffect of annealing without the presence ofoxygen is to produce a reflective surface on thestainless steel and it is often used in preferenceto a 2B finish where reflectivity is important,such as catering equipment. The very smoothsurface also makes this surface highly suitablefor further finishing processes such as mirrorpolishing and chrome plating.Typical uses include architectural components,food processing equipment, chute liners,general hardware and catering equipment.Problems can occur in use since the verysmooth, highly reflective surface is prone toscratching. Also different batches can bemarkedly different in appearance.

Acid Treatments

De-scalingScale (high temperature oxides) can beremoved by immersion in suitable acidsolutions. Paste style de-scaling solutions arealso available for localised treatment on largestructures or components.

Passivation / Removing FerrousContaminationStainless steel can pick up carbon from machinebeds, tools, racks, etc., which can result inunsightly rust staining. This can be removed byusing dilute nitric acid and this is sometimescalled calcination because of the ability ofoxidation acids to accelerate the formation ofacid film. Etching operations are conducted onstainless steel by dry or wet methods.

Wet EtchingThe wet methods use strong acidic solutions,typically nitric/hydrochloric andnitro/hydrofluoric acid solutions.

The extent and depth of the etching can becontrolled by the type of solution and by limitingthe contact times of the etching solution.Etching is, to a considerable extent, an art anda skill which is acquired through experience aswell as extensive experimentation.


4/5


5/5

bc

A a l c o a n d O P TI S H EE N a r e r e g i s t e r e d t r a d e m a r k s o f A a l c o M e t a l s L t d C o p y r i g h t : A a l c o M e t a l s L t d Th e H e r s h a m C en t r e H e r s h a m G r e e n H e r s h a m Su r r e y K T 1 2 4 H P

5

Special Finishes

There are a wide range of special finishes thatcan be applied to stainless steel which are usedwhen the standard mill or polished finishes arenot suitable. These finishes may be used fordecorative purposes or on fabricatedcomponents that cannot be polished in thenormal way.

Electroplating or ElectrodepositionIn this instance metal is deposited onto thestainless steel. This process is undertaken foraesthetic reasons, to increase the surfacehardness or to enhance the solderability toimprove lubrication and avoid seizing, toeliminate bi-metallic contact. Some typicalapplications of plating on stainless are:Copper - For lubrication in cold heading andspring making to reduce galling and scaling.Nickel - Most usually in combination with copperas a basis for chromium plating.Chromium - For colour matching as a car trimand to preserve finish on ????pess plates.Gold & Silver - For decorative effect on panels.

Textured, PatternedThis finish is the deep patterned effect by rollingthe sheets of stainless steel between two hard,engraved rollers. The most common patternsare leather effect and linen effect, althoughothers are available. Some of the textures areclaimed to increase the strength and rigidity ofthe material. This material is mainly used

where a high resistance to vandalism anddamage is required, for example car-park lifts.

ColouringThe colouring of stainless steel can be achievedby chemical surface treatment which actuallymodifies the surface oxide film on the metal. Arange of colours is available, including bronze,gold and blue. The coloured finish is attractivebut susceptible to finger-marks, which results ina colour change and is not hard-wearing, andthis should be considered when deciding on theuse. The process uses acidic solution which,

due to the variances in composition of thestainless and the process variables, can beguaranteed to be the same every time.

Metal SprayingA thin layer of metal can be applied to stainlesssteel by a means of spraying. The metals usedare usually lead or tin, because coated stainlesssteel is cheaper than the pure lead or tin.

Paint / EnamelA tougher colour surface can be achieved usingpaint, stove enamel or vitreous enamel. Thesefinishes are usually baked onto the surface toprovide a hard coloured surface. The finisheshave to be applied to clean de-scaled stainlesssteel that is also free from solvents and greaseand they can be used for a wide range ofapplications.

The strength of the bond to the surface candepend on the surface texture. For this reasonit is usually advisable to roughen the surfaceprior to painting. Paint also has the advantageof being easy to confine to limited areas by theuse of masking.

Electro PolishingThis process is particularly suited tocomponents or small pieces of stainless steel.The finish is uniform, highly reflective andsmooth. The process involves immersing thestainless steel in an electrolyte bath in whichthe metal acts as an anode, removing a layer ofmetal and smoothing the surface. Good resultscan only be achieved on material that alreadyhas a good surface.

Blast or Dry EtchingThis finish is achieved using fine abrasiveswhich are fired at the stainless steel in a streamof compressed air. This achieves a uniform,matt grey finish on components and fabricatedparts such as castings. It can also be usedselectively for decorative purposes. Only iron-

free abrasives can be used to avoidcontamination and these can be filtered toremove any traces of dirt.Changes in surface texture produce markeddifferences in light reflectivity and, as a result,contrasting effects can be obtained. Suchcontrasts are most pronounced when theetching is done on polished or buffed finishes.

Barrel / Vibratory FinishesThis is a similar process to base etchinginasmuch that also other media fine abrasivesare used to finish the material. In these cases,

however, the abrasives are vibrated against themetal or tumbled with their components in abarrel type container. The same points applyregarding the type of abrasive as for baseetching and its main use is the same, which isfor the finish of components.

stainless steel surface finishes

Documents