Electropolishing and Chemical Etching Processes for Cobalt Chromium

A. Hi Ricardo,
Bruce's posting was from 6 years ago so he may not be reachable, but Bruce's numbers total 100% without any water, and water is rarely added to any electropolishing solution.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Ricardo,
Just so you're aware, acids are almost always used in an aqueous solution, meaning that water is already there. (Very pure acids that are not a dry powder form are generally extra hazardous or otherwise more difficult to work with, and therefore are most often instead handled in a high-concentration solution with water.)

Also, water itself does not conduct electricity. Extremely low conductivity is one of the common tests for very pure water. Rather, it's the ions dissolved in water that allow it to conduct electricity. Ions, such as... acid. So if you have an acid solution where the conductivity is lower than you want, you should add... more acid!

A. Hi Ricardo.
Bruce doesn't seem responsive to postings here or e-mail to the address we have anymore, but I would suspect that "EF 200" is just a brand name.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Those are carbides that do not dissolve in regular CoCr electrolyte. Our way to overcome them in CoCr hip implants is with a final touch of brief electropolishing in plasma.

As for CoCr stents - it is more complicated. Prior to electropolishing the delicate coronary stents need to be gently treated by softening and removing laser oxides. And only then a very brief electropolishing in electrolyte that does not leave those hard carbides on the surface.

A. More words please. Sorry, but it's just not clear whether you want to etch a cobalt-chrome alloy . . . or whether you are suffering etching when trying to electropolish it . . . or quite what your situation is. Do you require an ambient temperature bath, or you are saying that the bath you are using which is causing the etching is ambient temperature.

The most common causes of etching are excess water (and at ambient temperature you have no evaporation to deal with the drag-in) and insufficient current density.

The Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] has a good chapter on electropolishing, and Metal Coating Process Corporation runs an electropolishing school. Good luck.

Regards,

A. Hi Tassy.

Although I am not personally familiar with the specifics of electropolishing of CoCr alloys compared to other alloys, the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] has an excellent 20+ page chapter on Electropolishing which explains both the whys and the hows of the process.

Metal Finishing Information Service / Finishing Publications Ltd. in Stevenage, Herts, England has a specialty computerized literature search database for the metal finishing industry which can be accessed online, or is available on CD; they also can perform the literature search for you if you prefer; and scholar.google.com can usually find such stuff.

Russamer Lab [a finishing.com supporting advertiser] has done a lot of similar work, I believe.

Good luck.

Regards,

A. In order to electropolish cobalt you either use strong acids with conventional method or environmentally safe salty electrolyte with special high-voltage equipment. Look at the pictures on our web site to see sample of such equipment and some polished cobalt parts.

Q. Anna, a question, What do you mean with high voltage? Do you think 20 volts is high enough for electropolishing cobalt chromium? I have tried till 16 volts but I have not succeeded.
Best regards

Isabel Pérez
medical device - Spain
July 2, 2010

A. Sorry Isabel, but I cannot reveal this technological process since it has been licensed exclusively to one of the medical implants manufacturing companies.

Anna Berkovich
Russamer Lab

Pittsburgh, Pennsylvania

July 6, 2010

A. In the dusty archives we see this response in Spanish from Tom Pullizzi's mother, Vijay:

Your inquiry will probably enthusiasm if you briefly describe your experience with stainless steel electropolishing, including the electrolyte you use. Thanks!

? Dear Mr. Mooney,

An english translation Please? expectantly,

Poems lose a lot in translation, but it basically says, "Yes, I'm asking for help but I wouldn't presume to do so unless I was willing to help in return." The point being that postings which ask for help but are cast in a way which offers nothing in return often don't draw a response; but those which convey help are usually enthusiastically replied to.

For example:

I am unfortunately unable to help this particular poster technically myself, so I am trying to get him help based on my experience with moderating the site :-)

A. Hi Devesh. Again, I don't know the idiosyncrasies of Cobalt Chromium alloys, but there is an old U.S. Patent, No. 2,607,772 which explains a sulfuric-glycolic mix for electropolishing stainless steel which may be applicable for you.
I believe that a plating process distributor may be able to offer you a proven proprietary. Alternately you may wish to contact an electropolishing specialist which offer training in the specialized chemistry.

Hi Scott. Surely Cobalt Chromium alloys can be electropolished. Obviously your second question about cross contamination is difficult to answer conclusively but considering the mechanism and polarity of the operation, I don't think I foresee any contamination in the electropolishing step per se.

Best of luck.

Regards,

A. Hi Nancy. Electropolishing selectively dissolves the asperities because the highest current density is there. If the surface was reasonably smooth before electropolishing, I think very little metal needs to be consumed to achieve the smoothing. Here's what Dean Ward had to say about that in the Electroplating Engineering Handbook:

Regards,

A. The possible cause of this problem is bad preparation of the surface before electropolishing. After welding there were developed oxide layers of different thickness. In locations where oxide layer thickness is max (and oxide does not conduct electrical current), electropolishing does not take place, but electropolishing does go on around such locations. That is why you see creation of the mushroom shape.
You need to add etching step after welding and before electropolishing.

A. Most electrolytes for Cr Co alloys are composed of:

85% EF 200 Ethylene Glycol,
10& Sulfuric Acid,
5% Hydrochloric acid. Brilliant Shine. Some corrosive fumes.
(The hydrochloric acid can be optional).

-OR-

85% EF 200,
15% Phosphoric acid (Shine varies from Brilliant to matte)

This particular formula has good effect on most CrCo alloys and has a 70 year history in industry.

A. Hello,

Use an electrolytic etchant (10%, 2 sec, 3 mv) such as: oxalic, phosphoric, etc.

adv.
Russamer Lab licenses electropolishing of the Cobalt Co-Cr stents to interested companies. Contact us for testing samples arrangements.

Hi Kinjal. Sorry, I don't understand you. Do you mean: "why not use Cobalt cathodes?" Please explain your situation. Thanks.

Regards,

A. Final finishing of Co-Cr stents faces the following challenges:

1. The surface must not be saturated by hydrogen, in order to prevent embrittlement;

2. Since Co-Cr stents are usually very thin, special holder design is required to distribute electrical current evenly;

3. Depending on the alloy composition there may be various hard-to remove molybdenum or other carbides, that do not disappear during electropolishing.

We work with cobalt-chrome material for almost 15 years, mostly with knee implants.

adv.
Our ultrapolishing method allows to remove all of the peaks of hard-to-remove carbides. However for such delicate stents the ultrapolishing method does not work.

Recently we developed two-step processing. During the first step the surface is smooth electrochemically --

(image of the stent)

The second step dissolves all carbides on the stent surface.

(SEM image)

A. Hi Dhavel; Hi, Shrikant.
Please try to phrase your question in terms of the previously posted answers to keep the dialog moving forward.

Thanks & Regards,

? Hi Vickysh. Unfortunately, "the sticking problem" doesn't mean much to me. Are you saying that you performed an annealing process, and the parts fused together? If these are stents or other medical or dental parts, I'd find it hard to believe that they could be salvageable after they have fused :-(

Luck and Regards,

A. Hi Aref. The average current density is easy: it's just the current divided by the surface area of the parts.

But the specific current density at different locations in the tank is quite a bit more difficult. There were immersible current density monitors available from companies like QCI but I'm not sure if anyone manufactures them anymore. You could try Material Testing Technology in Wheeling, Illinois. Stresstabs may help too. You can also research "electrochemical modeling software". Good luck.

Regards,

Hi Bao! Bruce Dean has taken a first cut at that. Please ask for clarification, or describe in what way it didn't help.

We might also suggest that you contact Russamer Lab [a finishing.com supporting advertiser] if you'd like to purchase appropriate chemistry or consulting services; and if you want technical references on the subject, we've listed books, computer databases, and patents covering it on the above page.

Good luck, and Regards,

A. There are many ways to electropolish CoCr, the effectiveness depends on the exact alloy composition and alloy supplier. Another problem is that even after regular electropolishing, there are pits produced by undissolvable carbides. We remove them by final plasma-like electropolishing, with duration of few seconds.

adv.
Contact me for more information and testing possibility. I have attached SEM images before and after plasma treatment

A. Hi Manisha. A solution which does not involve changing the current and time may or may not be possible. But please start by telling us the temperature and time that you are employing, and the solution that you are using. Thanks.

Regards,

A. Dental style Chrome Cobalt alloys can be, and have been, electropolished for over 70 years.

Pre-Made solutions can be obtained from ESMA Chemicals, Nobilium.com, Dentsply.com (Sirona Division), Dentaruim.com. For small quantities, try ESMAINC.COM Chemicals (about US $70 per gallon plus shipping). As well as making them yourself.

60% Ethylene Glycol
40% Phosphoric Acid (85%)

6 amps/sq ft DC

Thanks Bruce, but can you double check your units? I don't think you mean 6 Amps/sq ft; to me that sounds low by a factor of more than 10 to 1. Thanks.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Co-Cr stents electropolishing is different from other parts electropolishing. Usually if electropolished by standard solution, the outer stents surface is electropolished well. However the inner surface is not polished property.

adv.
We have developed the unique technological process that electropolishes the inner and the outer surface equally on tiny cobalt stents. See image below:

Also our electropolishing is not using hazardous ethylene glycol. We also develop special holders for each PARTICULAR stent design. We can do outsourcing or deliver processing and equipment.

A. Hi Brijesh. Anna seems to be claiming that achieving good internal polishing without internal anodes will require proprietary solutions. Are you using internal anodes to get some electropolishing current to that area? If so, please describe what those anodes look like.

But several mixes have already been suggested, have you tried any of them like those described by Bruce Dean and achieved a satisfactory finish on the outside, but not on the inside? Is the inside etched, or shows no effect, or what? What solution & current density have you been using? Are you being very careful that water is not being dragged in? Thanks!

Regards,

A. Removing oxides from cobalt-chrome laser-cut stents is a very important step prior to electropolishing. These oxides are more like carbides of various chemical composition. Traditional oxide-removal methods:
a) mechanical blasting - can leave oxide spots that will produce areas that cannot be electropolished;
b) alkali- or acid-based chemical etching - damage the original metal surface underneath;
c) high-voltage plasma electropolishing works fine, however coronary stents are very delicate, and plasma eats up the stent in seconds.

adv.
Our electrochemical proprietary processing completely eliminates oxides without impacting the stent's surface.