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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Can light polishing remove passivation from stainless steel
Q. Dear Sir,
I repair and refurbish surgical instruments for a living. Almost all of the instruments I work on are stainless steel. When I encounter an instrument, let's say a hemostat, that has become discolored by repeated autoclaving or hard water stains I'll refurbish it. I bead blast the inside of the jaws and boxlock. I use the finest glass bead media available, and blast the instrument at 60 psi. Next I will polish the instrument using a polishing wheel spinning at 3,600 rpm. For a high gloss finish I'll use a soft cloth wheel dress with polishing compound. For a satin finish I'll use a wheel made of Scoth-Brite material. Finally I'll ultrasonically clean the instrument in a mixture of water and stainless steel cleaner. My question is can this process remove the passivation coating applied by the instrument manufacturer? Should I re-passivate my instruments after refurbishing? Does it depend upon the level of passivation? Please advise
Thanks,
Vince
product designer - New York, New York
January 8, 2008
A. Mr Tarantella.
The treatments described can remove some of the effects of Passivation.
The Passivation treatment itself consists of selective removal of iron from the surface and the leaving behind of Oxides of chrome and nickel that protect the base from further corrosion.
You do treat the steel with abrasive material that can remove the oxides of Nickel and Chrome.
Hence I would suggest, as a matter of abundant precaution, that you re passivate the parts after processing them.
Regards,
Asif Nurie [deceased]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016
January 16, 2008
First of two simultaneous responses --
Quite so.
Passivation is always the final step in the manufacture of stainless steel parts for this very reason, all the other stages, including forming, blasting, grinding, polishing, etc., exposes new free iron on the surface.
As Mr. Nurie suggested, you should regard your refurbished parts as unpassivated. Fortunately with citric acid based formulations, passivation is very safe and easy do to. Let us know if we can help.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
January 17, 2008
Second of two simultaneous responses --
Q. Dear Mr. Nurie,
Thank you very much for your response and advise. I also contacted the maker of the stainless steel cleaner that I use during my refurbishing process. I spoke to a chemist that explained to me that his cleaner contained acids that were designed to replace some of the oxides removed from surgical instruments during cleaning, sterilization, refurbishing,etc. If you would be so kind, I'd be curious to hear thoughts. I bathe my instruments in the cleaning solution for an half an hour after refurbishing.
Thanks again,
product designer - New York, New York
January 18, 2008
A. You may want to look into having your instruments electropolished as a last step for a superior form of passivation. This will also micro-deburr, smooth, brighten and polish the surface. An ultra-clean surface is an added benefit. You may be able to complete processing your parts faster by eliminating and or replacing certain parts of your processing with electropolishing.
Electropolishing is widely used in the medical instrument manufacturing/refurbishing field
electropolishing shop - North Vancouver, British Columbia, Canada
September 23, 2009
Q. We apply solid film lubricants to a variety of materials that include stainless steel. The surface preparation for stainless steel occasionally requires a "light grit blast using 120 - 220 aluminum oxide grit blast media". I have read on this sight that the dipping of stainless steel in a nitric acid bath removes the free iron from the surface and upon removal from the tank passive film is produced and that film is regenerative, for example if the part was scratched (and iron was not re-introduced to the surface). Would the "light grit blasting" surface prep allow the passive film to regenerate?
Dan Marini- Enfield, Connecticut, USA
October 4, 2012
A. Hi Dan. The stainless steel must be re-passivated even if your abrasive is completely iron-free. A passivated stainless steel surface is chrome-enriched, and abrasive blasting will remove that enrichment.
Although it is 'kinda/sorta' true that stainless steel 'passivates itself' after citric acid treatment, don't let multiple casual meanings of "passivate" lead us like a will-o'-the-wisp into a semantics swamp. Aluminum kinda/sorta "anodizes itself" on exposure to the atmosphere, too, but just leaving bare stainless steel or aluminum bare in the air is no substitute for passivation or anodization. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 5, 2012
A. Iron on the surface of stainless steel generally comes from two sources, contamination and the alloy itself. Now, the latter depends highly on which alloy is being used, 316 is far better off than something like 420. For that matter, unpassivated 316 is better off than passivated 420.
According to the principles of the passivation process though, if you disturb the surface though blasting, polishing, bending, welding, scratching, etc. etc., you have exposed raw alloy rather than the "chromium enriched" (I like to call it iron depleted) surface.
Does stainless self passivate? To a certain extent, sure. Take 316 for example once more. That's a combined chromium and nickel content of 26-32%. So a self passivated 316 surface you would expect around 30% Cr and Ni oxide. That's pretty good. Good enough to pass the copper sulphate
⇦ this on
eBay or
Amazon [affil links] test usually. Run it through a passivation bath (nitric or citric) to strip iron away and now you have a surface that's at least 80% Cr and Ni, and once the air hits it you have a surface that's over 80% Cr and Ni oxide. That's excellent.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
October 24, 2012
Q. Interesting topic. Does a color buff remove the passivated surface of a cobalt chrome implant? If after passivation, a small area needs to be touched up, is a color buff gentle enough to retain the passivated surface? We have checked the surface with copper sulphate and there does not appear to be any free iron.
Sal EmmaOrthopedic Implants - Shirley Massachusetts USA
February 3, 2015
A. Sal,
Since you are talking about a non-ferrous alloy, it's a totally different animal.
With non-ferrous alloys, there can only be external sources of iron. If you have a manufacturing process which is known to or suspected of depositing contaminant iron, then you need to use a passivation bath to remove it. If your process could not possibly be adding iron, there is no problem. I often have to remind people that they don't need to passivate after laser marking titanium the way they would if it were stainless steel.
As long as your buffing equipment isn't used on any carbon steel, I think you don't have to worry.
Now, technically, yes, you may be removing the "passive" oxide layer, but it will reform rather quickly, and you have not exposed iron by doing so because there is none (or very, very little) in the alloy.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
February 10, 2015
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