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topic 54869

Medical Implant and Instrument Passivation

A discussion started in 2008 & continuing through 2017

April 12, 2008

Q. I work for a large corporation that manufactures surgical instrumentation. We currently use citric acid to passivate because the instruments are not implants. We validated the citric on every type of stainless that we use (17-4, 420, 465, 455, 316 and 317) and had no failure with a boil test or immersion test. We are about to start making implants out of cobalt chrome and from previous experience I think we have to use nitric acid to passivate. If this is true what percentage should we use and what would the specific gravity be for that percentage? If I use nitric how do I know when it is spent or when does it need to be changed?

Billy McCulley
Supervisor manufacturing - Bartlett, TN, United States

April 16, 2008

A. Citric acid based passivation can and is used for implants as well as instruments. It is also used on many non ferrous alloys just as it is with stainless steel.

On what basis do you expect you must use nitric? If you refer to ASTM F86 [link by ed. to spec at TechStreet] spec on implants, there is a clause there that allows any effective treatment as long as you document it. If your customer is specifically requiring nitric, it may be worth the effort to convince them of the benefits in allowing you to use citric.

If you still go with nitric acid, you can refer to a stainless passivation spec such as ASTM A967 [link by ed. to spec at TechStreet] and follow the formula and procedure recommended for 316 SS.

Let us know if we can be of help.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

Verification Tests for Cobalt Chrome Passivation

May 12, 2009

Q. Hi All,

I work for a manufacturer. We currently passivate a range of stainless steels in Citric or Nitric solution (depending on customer requirements) and use Copper Sulphate and Boil tests as verification methods.

We are now looking to passivate some cobalt chrome implants in Nitric Acid. Can anyone suggest any verification methods for testing the effect of the passivation on cobalt?


James Dodds
Quality Engineer - Sheffield, United Kingdom

June 2, 2009

A. James,
Passivation tests for stainless steel are checks for the presence of free iron on the surface, and you would want a passivation test for non-ferrous alloys such as your cobalt chrome to do the exact same thing. Go ahead and use copper sulfate and/or the boil test.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

April 21, 2010

Q. Hello,

I am a quality engineer for a contract manufacturer of medical implants and instruments. Historically, we have always passivated our stainless steel instruments in Nitric Acid. However, we are now embarking on end-to-end projects which will require we passivate our Titanium and Cobalt Chrome implants also.

I've been advised that Stainless Steel passivation should have its own line and be kept separate from Ti and CoCr. I'm assuming this is because it is a ferrous material. Is this correct? I've also been advised that Ti and CoCr can be passivated in the same line.

Is any of this laid out in a spec, or would anyone be able to advice me on what is considered good practice within the industry?


James Dodds
Quality Engineer - Sheffield, S.Yorks, United Kingdom

April 28, 2010

A. Refer to standards like ASTM F86 [link by ed. to spec at TechStreet] Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants and ASTM B600 [link by ed. to spec at TechStreet] Standard Guide for Descaling and Cleaning Titanium and Titanium Alloy Surfaces.

Toby Padfield
automotive component manufacturer - Michigan

April 29, 2010

A. The titanium line definitely needs to be separate or you will have a much shorter tank life.
What alloy are you talking about with CoCr?

James Watts
- Navarre, Florida

May 6, 2010

thumbs up signThanks for the responses.

We are processing CoCrMo per ISO 5832-12

James Dodds [returning]
- Sheffield, S.Yorks, United Kingdom

May 11, 2010

A. James,
In this situation I presume "passivation" for the Ti and CoCr means removal of any actual or potential surface contaminant iron. As such, this is the same thing that a passivation treatment of stainless steel does and the same procedures can be used.

Perhaps with nitric acid passivation baths there is a need to keep separate non-ferrous and stainless lines. With citric acid based passivation it can all be done in the same bath.

Most of the specs and standards refer to pickling (scale removal) of Ti and other non-ferrous alloys (usually with something utilizing nitric acid) but there is little about just passivation (iron removal) for these metals.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

Passivating Cobalt Chrome implants ... nitric vs. citric acid

October 7, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. My shop performs hand finishing of Cobalt Chrome Femoral implants and I am about to validate a new clean line/passivation line. I would like to make the switch from Nitric to Citric if possible. What are the pros and cons for Cobalt Chrome? I know this is the age old debate, and I have done the searches, just don't see much on Citric in regards to Cobalt Chrome specifically.

Jason Bradshaw
Production Manager - Wallingford, Connecticut USA

October 18, 2016

A. Passivation of CoCr Femoral implants will not remove the carbides on the surface of the implants. Sooner or later such during constant friction these carbides will destroy the opposite surface of the implant construction. Carbides can be removed by special final procedures. Only later Nitric acid final passivation is applied.

54869-1a 54869-1b

See images before and after.

Anna Berkovich
Russamer Labs
supporting advertiser 
Pittsburgh, Pennsylvania
russamer labs banner

October 25, 2016

A. When it comes to non-ferrous alloys I usually try to be specific about what we are talking about here. This type of passivation treatment, the one used for stainless steel, is for removing iron from the surface. If that's what you need, nitric will do it for you, and citric will do it with more safety and with less etching of other materials. This counts for CoCr, titanium, aluminum, Hastalloy, inconel, etc. etc. and so on.

If you can be certain there is no surface iron contamination, then you don't need passivation. If you need something different than iron removal, then you want some other process.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

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