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topic 6781

Correct Procedure for 303 Passivation


(2000)

We have a machining supplier that has not followed the procedure on our drawing calling for sodium dichromate to be added to the nitric acid bath for passivating our 303 St Stl part. If sodium dichromate is left out of the bath, will this effect the corrosion resistance of the part?

Becky Stufft
- Plymouth, Minnesota USA


(2000)

Sodium dichromate is an oxidizer that will enhance the passive oxide film that forms during the passivation process. The passive oxide film should be more stable when formed with sodium dichromate that for passivation with straight nitric acid, therefore absolute corrosion resistance should be better when the sodium dichromate is used. Whether it will be significant for your parts will depend on the component and the service application.

larry hanke
Larry Hanke
materials testing laboratory
Minneapolis, Minnesota




(2000)

Becky:

For 303 stainless it can be critical to have the dichromate in nitric acid. It not only enhances the oxidation of the surface, but it can prevent problems with the surface of the steel turning dark.

You can also use products like CitriSurf 2250, which works well with 303ss without the problems of nitric acid.

Let us know if we can help.

Regards,

lee kremer
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Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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(2001)

The passivation spec AMS-QQ-P-35 [link by ed. to spec at TechStreet] gives an option for PH materials of using type II or type VIII passivation. The type II uses sodium dichromate and a weaker solution of nitric acid. Type VIII uses only a stronger nitric acid solution for a longer time.

Is one method better than the other?

Thank you for the help,

Jay Shapiro
- Valencia, California, USA



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