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.