What is passivation of stainless steel?

L.S,
I am working in the biotech industries and in this working field for production equipment stainless steel is used. In my department people work
with 316L or 304L SS micro mesh filter screens. These screens are ordered at an external company. Before we use these screens in our production process they are treated with Sodium Hydroxide solution and after the base rinse they are treated with Nitric Acid solution. Does anybody in this newsgroup knows what is the sense of this process? We call it a passivation process...
Regards, Maurice.
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Maurice Zuidgeest wrote:

You should lokk to an undergraduate materials textbook.
oxygen-based corrosion in usual electolytes produces oxides and/or hydroxodes. If the oxides/hydroxides are dense and adhesive to the base material, they protect from further corrosion, i.e. the corrosion prcess can only go on by diffusion through the surface layer. Passivation means making a dense layer.
Michael Dahms
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Maurice,
It is not a ceramic layer on the surface as a later poster wrote, but a chromium oxide layer that gives stainless steel its corrosion resistance. My guess is that the NaOH treatment degreases and cleans the metal nicely, and the nitric acid is what does the passivation. Nitric is a strong oxidizer, and it will increase the chromium oxide layer, making the surface more passive, ie more corrosion resistant. Once a tough oxide layer is formed, the metal is resistant to further oxidation [corrosion].
Watch out for chloride ions, these can break down the passive layer. -Jay entr0pyf0e
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Entropyfoe wrote:

^^^^^^^^^^^^^^^ Rubbish! The passive layer contains chromium and oxygen, but more than that.
Michael Dahms
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Isn't chromic oxide a ceramic? If not, what is it?
Tom
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Tom Walz wrote:

The passive layer is not chromium oxide.
Michael Dahms
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All,
Thanks for the answers to my question, someone told me the passivation process also occurs spontenious in the air, is this true and how long does it take before a stainless steel piece of equipment is passivated spontaniously?
Regards, Maurice.
-- http://home.wanadoo.nl/zuidgeestm
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Maurice:
The passive film formation process will occur spontaneously in air, provided that you first manage to clean off the surface all of the other thicker junk that could interfere. For a discussion of this, go read the paper "Specifying Stainless Steel Surface Treatments," by Tuthill and Avery, which can be downloaded at the Nickel Development Institute web site, www.nidi.org as Publication No. 10068.
Nitric acid passivation is the final step in a sequence of processes that may need to be preceded by degreasing and more agressive cleaning ("pickling").
Pittsburgh Pete
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Pittsburgh Pete wrote:

Also available from the Nickel Development Institute, Beer Barrels: from Roman times to the Present Day
http://www.nidi.org//index.cfm/ci_id/12606.htm
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