Letter 3571

Passivation problems with T304 stainless steel 

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We have been passivating Type 304 stainless steel according to ASTM standards and have questionable results. We have our vendor degrease the parts and then wash them in an alkaline bath for 10 minutes. After 10 minutes the parts are rinsed in clean water before being immersed in a Nitric 2 solution. 20 to 45 volume percent of nitric acid. The parts are then immersed for 30 minutes at a temperature range of 70 to 90 degrees F. They are then rinsed in water left to air dry. after they are dry they are then tested with a solution of potassium ferricyanite nitric acid and they still appear to be contaminated with free iron.

Does anyone else experience these same problems?

Mark Early
Lititz, PA

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Your test is the Ferroxyl Test and is very sensitive. I doubt if you will have many parts pass it with your standard cycle. A much longer caustic clean followed by a much much longer passivation step will help. You will have to be exceptionally careful that any machine that these parts are worked on are surgically clean to avoid iron particles being forced into the surface.

My bet is you will end up electropolishing them to pass this test. It is hard to pass with 316, let alone 304. (If you are honest)

James Watts
- Navarre, Florida

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Hi Mark ,
I reckon everyone who tries these nitric passivation processes is kidding themselves that they work , if all who did it were like you and did the tests they'd soon find out. In my opinion there is only one way to be sure of removing the Iron and thus enriching the surface with nickel & chrome to achieve that sought after Oxide film and the answer is Electropolishing! Go to Dan Weaver , he has a process you just might be interested in , he markets it under the name of "Global Stainless Technologies" Tell him I sent you

Best regards

John Tenison - Woods
- Victoria Australia

Mark: I agree with all those who recommended electropolishing. Look up Metal Coating Process Corporation in the Suppliers' Directory on this web site. There is a link that will take you to our own site where you will find background information on the process. Regards,

Ed Bayha Metal Coating Process Corporation Charlotte, North Carolina, USA

Maybe you're water is not démineralised or you wait too much time with the ferroxyl test on. After a minute it is normal that the test turn blue cause of the air.

Jean Lambert
- Boucherville, Québec, Canada