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.