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Home » Surface Finishing For Stainless Steel

Upload time：2011.03.07 Sources:Special Steel - Supplies Special Steel, Supplies Tool Steel, Stainless steel, Steel Stockists Suppliers Special Steel Stockists Browse:

Background

To a very large extent Stainless steels are used because of the corrosion resistance of their surfaces. This excellent corrosion resistance can only be achieved if proper cleaning and finishing operations are carried out after any fabrication process which has impaired the surface condition.

Passivation

This process is recommended where the surface must be free of iron. The presence of any iron, cast iron, mild steel, carbon steel or low alloy steel particles on the surface of Stainless Steel will promote pitting corrosion at the cells set up between the "free" iron and Stainless steel. This potentially very serious (and certainly unsightly) problem most often occurs due to contamination by scraping with carbon steel tools or fixtures, or from grinding swarf. Passivation also aids in the rapid development of the passive surface layer on the steel.

The removal of the iron can be readily carried out by the procedures in Table 1.

Table 1. Passivation Procedures. Refer ASTM A380

Grade	Treatment	Temperature	Time
Grades with at least 16% Chromium (except free machining grades e.g. 303)	20-50% nitric acid	Room temp. to 40°C	30-60 minutes
Grades with less than 16% Chromium (except free machining grades e.g. 416)	20-50% nitric acid	Room temp. to 40°C	60 minutes
Free machining grades such as 303, 416 and 430F	20-50% nitric acid + 2-6% sodium dichromate	Room temp. to 50°C	25-40 minutes

Notes

1. If no dulling of the metal surface can be tolerated a trial treatment should first be carried out.

2. All passivation treatments must be followed by thorough rinsing.

3. Observe all precautions for handling acids - nitric acid is highly corrosive and dangerous to exposed skin.

Pickling

Pickling is an acid treatment to remove high temperature scale produced in welding, heat treatment or hot working. It also removes red rust from corrosion of the steel or from corrosion of contaminant iron or steel particles. Note that passivation is not sufficiently aggressive to remove this corrosion Product after the free iron has begun to rust. High temperature dark scale is not only undesirable for aesthetic reasons - it also results in a reduced corrosion resistance of the underlying steel surface layer.

The type of scale and hence the methods to remove it will depend upon the steel grade and the heating conditions involved. The straight-chromium grades such as Grades 410, 416 and 430 scale more readily and unfortunately the resulting scale is also more tenacious.

All pickling operations result in metal removal, and the outcome is therefore to some degree a dulling of the visual brightness and perhaps also a significant reduction in dimensions.

The best solution to the scale problem is not to create it in the first place! Heat treatment in a vacuum or a good controlled atmosphere, such as bright annealing, eliminates the need for pickling, and generally results in a better final surface finish.

If pickling does need to be carried out the treatments given in Table 2 can be used. An initial pickle in sulphuric acid is often beneficial as this softens the scale so that it can more readily be removed by subsequent pickling in hydrofluoric and nitric acids.

Pickling Paste

A very convenient method for pickling is use of "Pickling Paste". This is a prepared mix of strong acids in a stiff paste which enables it to be applied to small areas and to vertical or even overhanging surfaces. It is especially useful for pickling to remove heat tint following welding. Again precautions for handling acids must be followed and the residue flushed thoroughly to a suitable waste stream after completion. Most commercial pickling paste is formulated for the austenitic grades, so if these are used to clean lower alloyed grades such as 3CR12 the process must be closely monitored to ensure the paste is quickly removed and very thoroughly rinsed off afterwards.

Table 2. Pickling procedures. Refer ASTM A380

Grade	Treatment	Temperature	Time
All Stainless steels except free machining grades. (Useful to loosen heavy scale prior to other treatments)	8-11% sulphuric acid	65-80°C	5-45 minutes
Grades with less than 16% Chromium (except free machining grades e.g. 416)	15-25% nitric acid + 1-8% hydrofluoric acid	20-60°C	5-30 minutes
Free machining grades and grades with less than 16% Chromium	10-15% nitric acid + 0.5-1.5% hydrofluoric acid	20-60°C	5-30 minutes

Notes:

1. Trial treatments should be carried out first to confirm that dulling is acceptable.

2. Pickling should preferably be carried out on fully annealed Stainless steels due to risk of grain boundary attack. This problem is especially relevant to steels sensitised in welding.

3. All pickling treatments must be followed by thorough rinsing.

4. Observe all precautions for handling acids - sulphuric, nitric and especially hydrofluoric acid are highly corrosive and dangerous to exposed skin.

Degreasing

Grease, oil, cutting fluids, drawing compounds and other lubricants must be removed from the surface of Stainless steel components before heat treatment (to prevent carbon pick-up) or final passivating treatments (to enable full access by the treatment). Parts must also be degreased prior to further assembly by welding, again to prevent pick-up of carbon at high temperature.

Both liquid and vapour degreasers are used. Liquid cleaning is often by hot alkaline detergents; proprietary mixes may also contain various additives. The parts should be thoroughly rinsed afterwards.

Organic solvents can be applied by spraying, swabbing or vapour degreasing. These treatments should again be followed by thorough hot water rinsing.

As with cleaning operations on other metals, the rate of cleaning can be increased by the use of brushing, jetting or stirring etc. during the operation.

Electropolishing

Electropolishing is an electrochemical process which brightens the steel surface by selective dissolution of the high points - it is the opposite of electroplating, and is carried out with similar equipment.

The process is able to produce a very attractive and hygienic finish, but trials should first be conducted to determine the optimum prior surface condition and polishing parameters. Electropolishing of some surfaces results in a frosted rather than smooth finish.

Grinding and Polishing

Stainless steels can be readily ground, polished and buffed, but certain characteristics of these materials require some modification of standard techniques for best results. Most notably, the high strength, tendency to "load up" abrasive media, and low thermal conductivity of Stainless Steels all lead to build-up of surface heat. This in turn can produce heat tinting (surface oxidation) or surface smearing, and in extreme cases even sensitisation of austenitic Stainless steels or "burning" (re-hardening) of heat treated martensitic grades. Techniques that help prevent build-up of surface heat include (a) use of lower speeds and feeds, and (b) careful selection of lubricants, and of proper grit size and type, so as to minimise loading of the abrasive.

Corrosion resistance of Stainless steels may be adversely affected by polishing with coarse abrasives. Corrosion resistance is often adequate following polishing to a No.4 (approx 180-grit) finish. Polishing with fine alumina or chromium oxide to obtain still higher finishes - such as buffed finishes No.7 and No.8 - removes fine pits and surface imperfections and generally improves corrosion resistance. Buffing can also be carried out by using a "Scotch-brite" buffing wheel.

Iron contamination must be avoided or removed if polished Stainless Steel surfaces are to have good corrosion resistance. Abrasives and polishing compounds must be essentially iron-free (less that 0.01% for best results), and equipment used for processing Stainless steels must not be used for other metals. If these conditions cannot be met, a cleaning/passivation treatment (after pre-cleaning to remove polishing compounds and lubricants) will be required to restore good corrosion resistance.

Mechanical Cleaning

Problems associated with chemical cleaning processes can be avoided by using mechanical cleaning. With all mechanical cleaning processes great care must be taken to prevent the Stainless steel surface from becoming contaminated by iron, steel or iron oxide particles.

Sand Blasting

Sand blasting is often used - this must be with clean silica or garnet sand. Shot, grit and cut wire blasting must be done with Stainless steel media, equal in corrosion resistance to the metal being cleaned, and scale particles must be continually separated from the shot.

Wire Brushing

Wire brushing is useful to remove light heat tint, but again brushes must be of stainless steel, and these must never be used on materials other than Stainless Steels.

Barrel Finishing and Vibratory Finishing

Barrel finishing and vibratory finishing both use abrasive media to mechanically polish small parts and are widely used on fasteners such as screws and bolts and on pipe fittings.

Mechanically cleaned parts are not quite as corrosion resistant as acid pickled material because mechanical cleaning leaves some scale residues and often some residue from cleaning. It can be used as a preparatory step before acid pickling.

Blackening

Blackening to produce a non-reflective black oxide surface can be accomplished by several methods. Immersion in sulphuric acid/potassium dichromate solutions at 80 to 99°C or immersion in a molten salt bath of sodium dichromate at 400°C are two common methods. Other proprietary treatments are also available. Most treatments are performed by specialist firms, who should be consulted regarding a specific application.

The surface of Stainless Steel is actually and extremely thin but stable and passive Chromium rich oxide film, on which Stainless Steel relies for its excellent corrosion resistance. The surface finish on Stainless Steel should therefore be developed and maintained to ensure this vital property, and also for the secondary reason of the pleasing aesthetic appearance of Stainless Steel.

STANDARD MILL FINISHED - FLAT ROLLED PRODUCTS

The Standard Mill Surface Finished are laid down in Specifications (BS 1449, Part 4, and the Committee of Stainless Steel Producers, American Iron & Steel Institute).

The finished are designated by a system of numbers, and these are broadly described hereunder relative to the finishing operations employed. It should be remembered that different grades of Stainless Steel can result in a variation of visual appearance for the same finishing operation. The thickness can also have an effect, generally the thinner the material the smoother the surface finish.

The thicker gauge sizes of Stainless Steel are hot rolled. This is done at high temperatures and will always result in a scaled surface.

Stainless Steel Flat Product is supplied in the annealed ie fully softened condition.

This is also a high temperature operation and unless carried out in a very closely controlled inert atmosphere, will result in oxidation (scaling) of the surface.

The scale is usually removed by a pickling process, that is the removal of the scale by use of suitable acids, and the passivated by the use of Nitric Acid.

No. 0 Finish

Also referred to as Hot Rolled Annealed (HRA). The plate is hot rolled to required thickness, and then annealed. No pickling or passivation operations are effected, resulting in a scaled black finish.

This does not develop the fully corrosion resistant film on the Stainless Steel, and except for certain high temperature heat resisting applications, this finish is unsuitable for general end uses.

No 1 Finish
Plate is hot rolled, annealed, pickled and passivated. This results in a dull, slightly rough surface; quite suitable for industrial applications which generally involve the range of plate thicknesses. Grinding marks may be visible in isolated areas.

Some of the thinner thicknesses within the plate range are Cold Rolled; but Sheet, Coil and Strip gauges are produced by Cold Rolling, ie rolled without and heating of the material. Cold Rolling hardens the material, and the thinner sizes may have to be subjected to an intermediate anneal and pickle, or bright annealed, during the reduction of thickness to final gauge.

The starting material for Cold Rolling always has a No. 1 finish. Cold Rolled material is supplied with the following standard mill finishes.

No 2D Finish
A no. 1 Finish after being Cold Rolled, Annealed, Pickled and Passivated. This results in a uniform dull matt finish, superior to a No. 1 Finish.

Suitable for industrial application, and eminently suitable for severe deep drawing as the dull surface, (which may be polished after fabrication) retains the lubricant during the drawing operation.

No 2B Finish
A 2D Finish is given a subsequent light skin pass cold rolling operation between polished rolls.

This is the most common finish produced and called for on sheet material. It is brighter than 2D and is semi-reflective. It is commonly used for most deep drawing operations, and is more easily polished to the final finished required than is a 2D finish.

No 2BA Finish
This is more commonly referred to as a BRIGHT ANNEALED (BA) FINISH. Material with a No. 1 finish is Cold Rolled using highly polished rolls in contact with the steel surface. This smooths and brightens the surface.

The smoothness and reflectiveness of the surface improves as the material is rolled to thinner and thinner sizes. Any annealing which needs to be done in order to effect the required reduction in gauge, and the final anneal, is effected in a very closely controlled inert atmosphere. No oxidation or scaling of the surface therefore occurs and there is no need for additional pickling and passivating.

The final surface developed can have "MIRROR" type finish similar in appearance to the highly polished No. 7 and No. 8 Finishes.

Note
Much of the 2B Finish sheet imported is not a true 2B Finish. Mills which operate bright annealing facilities will often carry out all the annealing operations of Cold Rolled material in such facilities. This leads to a superior "2B" finish as no oxidation or scaling takes place during the annealing operation, even though the actual rolling may be effected on polished rolls as for normal 2B Finish, but not highly polished as would be needed to produce a BA finish.

The following finishes are all mechanically produced polished finishes. As well as being standard mill finishes, they are also applied to stainless steel articles and components to meet the required aesthetic criteria. It should be appreciated that factors such as hand polishing vs. mechanical polishing; polishing a flat product as against a component of complex shape; thickness and composition of material can affect the visual appearance of the final surface.

No. 3 Finish
This is a ground unidirectional uniform finish obtained with 80 - 100 grit abrasive.

It is a good intermediate or starting surface finish for use in such instances where the surface will require further polishing operations to a finer finish after subsequent fabrication or forming.

No. 4 Finish
This is a ground unidirectional finish obtained with 150 grit abrasive. It is not highly reflective, but is a good general purpose finish on components which will suffer from fairly rough handling in service (eg restaurant equipment).

No. 6 Finish
These finishes are produced using rotating cloth mops (Tampico fibre, muslin or linen) which are loaded with abrasive paste.

The finish depends on how fine and abrasive is used, the uniformity and finish of the original surface.

The finish has a non-directional texture of varying reflectiveness. "Satin Blend" is an example of such a finish.

No. 7 Finish
This is a buffed finish having a high degree of reflectiveness.

It is produced by progressively using finer and finer abrasives and finishing with Buffing compounds. Some fine scratches (grit lines) may remain from the original starting surface.

No. 8 Finish
This is produced in an equivalent manner to a No. 7 Finish, the final operations being done with extremely fine buffing compounds.

The final surface is blemish free with a high degree of image clarity, and is the true mirror finish.

Note
The finer polished finishes (No. 4, No. 6, No. 7 and No. 8) are generally only produced one side of the sheet, the reverse side being either a 2B or No. 3 Finish.

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