Passivation of Stainless Steel 1

[structural1]

Dear Friends,
While working on passivation, (65% nitric acid bath for 30 minutes), for the stainless steel finished works, i faced the following problem: some of the products changed color and turned to be like dark color on the surface.
If you can help me i would be glaful.
Regards.
Wissam Khoury.

 

[kenvlach]

What is the SS alloy? Download QQ-P-35C to check whether you are using an appropriate passivation solution for your alloy. Some free machining SS require Type II solution (dichromate + nitric).

http://assist2.daps.dla.mil/quicksearch/

Another possible problem is with organics (lube/cutting oil)embedded in the surface as the result of machining. These organics combine with the nitric acid to create a powerful etchant. For further explanation of this and other passivation problems, see

http://www.finishing.com/faqs/sstpass.html