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Trouble passivating 17-4 Stainless Steel.

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We have been passivating 17-4 stainless steel used in surgical instruments. The final cleaning test is a IPA wipe with a cotton swab. We preform this operation, pass the parts on as clean. Our customer also wipes the parts and passes them on as clean. The final customer revieves the parts several days after this and will find a black smut on the cotten swab after IPA wipe. This occurance is on the ID and ID threads. The ID is gun drilled and the metal working fluid it turns out is a chlorinated parafin.

Our cleaner is a mild alkaline and the parts are brushed ID and OD. Passivation has been both Type II and Citric, both show the same delayed failures. After passivation the parts are re soak cleaned, briefly to neutralize acidity, and hot DE I rinses.

This is a very interesting problem. SEM analysis is not back yet but we are looking at possible chloride. I am guessing that a nasty vapor solvent may better remove the metal working fluid that is driven into the pores but this is speculation at this point.

Has anyone elese experienced this delayed problem? Can any one share with me resolution on this?

Jon Quirt
- Fridley, Minnesota


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While I like vapor degreasers, your application sounds more like one that is optimally suited for ultrasonics both in the alkaline cleaner/neutralizer and the DI rinse. Since you are doing surgical parts, it would not take a large unit to run your parts unless you had them by the thousands, which I doubt since they are gun drilled (deep and narrow).

James Watts
- Navarre, Florida


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Dear Jon ,

My feeling is that you are right about the chloride copntamination , However the passivation would be a hell of a lot better if the items had been electropolished , the surface will be "enriched" with Chrome & nickel , as the Iron will have been preferentially removed and the resultant oxide film will be as complete as it is possible to get .

I can only recommend that you talk with a company in Sydney Australia Called METAGLO Pty Ltd , they have developed a very modern approach to this problem

regards


John Tenison - Woods
- Victoria Australia


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Jon:

We have had several customers with similar problems caused by chlorinated paraffin inclusions. You might try 5% NaOH at elevated temperatures (up to 180F) before the passivation with CitriSurf. This has worked at other places. Ultrasonics also helps a lot if you can use it.


Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois



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I am concerned with the bio-compatibility of this material. We are making micro instruments with it in a powdered metal blend. I seem to be the only one who has reservations about it. Am I wrong?

Ben Brosch
- fountain valley, California

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