Sodium Sulphate Solution for Pickling Stainless Steel

The attached files may be a very useful for Pickling_and_Passivation_of_Stainless Steel.

Dear Sir:

Thank you for this information.

I work for a company that builds stainless steel products and owns a company that pickles and passivates for the main company as well as other customers. Prior to 10 years ago, the company I work for did this at the plant I worked in, and I was in charge of the mixing and use of the pickling chemicals. All I have ever seen in use for pickling is a commercially available solution of 25% nitric acid and 10% hydrofluoric acid. This is mixed with equal amounts of DI water. This appears to be the most common pickling method. (you do want to pickle, not passivate?)

Never saw the sodium sulphate used. The company has never considered it for use.

This acid bath is quite effective at pickling stainless steel, but there are definite problems with the acid bath I described:

1. It is an acid, and must be treated with care - personal protective equipment is a must. The hydrofluoric acid is a very dangerous acid - very difficult to neutralize once in contact with skin. I had some very painful grey skin under my fingernails once, the effects continued for several weeks, from a chance encounter from some rinse waters I did not get removed quickly enough. It attacks calcium bearing materials. (bone) A calcium product, such as a stomach acid neutralizer, will help to abate the effects of the acid.

2. The company has a waste treatment line and uses it to neutralize the rinse waters from many other acids before disposal to the sewer, with no environmental problems, but the above mix creates tremendous amounts of sludge to dispose of, which is very costly. The company has finally resorted to paying an offsite environmental company to truck the waste to their site to do this task, also at considerable cost, but the savings in labor balance this out.

Dear Sir:

Thank you for this information. Have you heard of stainless steel companies using this method?

I don't know what your requirements are, but there is a much more environmental friendly method available now , It is called electrical passivation whereby a brush made from carbon filaments of some sort is wiped across the weld with an electrolyte and a current passed.

There are a few company's making these at the moment. try these links

http://www.hss.com/imagshop/guides/og_HW858.pdf

http://www.anopol.co.uk/weldkleen.htm

The terms "pickling" and "passivation" are NOT the same thing. (notice in my original post I asked if pickling or passivation was the desired effect)

Pickling removes layers of the object submersed in the acid bath. All metals in the outer matrix are treated as equal, and the end result is a standard stainless steel matrix on the surface. The finish is similar to sand blasted. My company offers pickling as a surface finish, as well as a means to remove heat weld marks. The surface is not really passivated yet, and a bath in nitric sometimes follows.

Passivation removes "free iron" (iron contaminations) and leaves a chromium enriched matrix on the surface. The finish, if it was 10 Ra, will still be 10 Ra. However, a not so desireable "mill finish", will still be mill finish.

The method you describe appears to be passivation, not pickling as requested.

I did say I was not sure what his requirement was, the terms are both used in weld cleaning ie

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 austentitic 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.

I was not picking an argument with you mearly offering a suggestion.

Not picking a fight - I think we all should know the difference between the two processes, and have described the differences twice before in similar lines on CR4. Yes, both terms are used in welding (I am in the tank/heat exchanger/filter housing fabrication business). Both apply to processes applied to welded (or raw stock) stainless steel. They still should be used correctly. A bit like myself calling a TIG operation a MIG operation - that would bring a corrective response from you, wouldn't it?

The response was also to verify that the original post did indeed mean pickling, not passivating. I assume he will read both your post and my reply.