47049

Please explain theory of trivalent passivation [Indonesia] 

November 16, 2007

Is there someone can help me to expalin the detail theory of hexavalent cr free treatment? also the comparison to alodine #1000?
I have a project to improve the surface quality of honda's motorcycle part, with one of those kind of treatment.

Roynindra Malaon
process engineer - Jakarta, Indonesia

Many are merely cosmetic - there's little or no passivation. Sometimes, the application of a silicate based sealer provides the real protection over what is just a blue or gold dye.

Some employ molybdenum, or other elements that form coatings that polymerize like the hydrous chromium/zinc compounds that comprise standard chromate coatings.

Dave Wichern - Bronx, NY, USA

You should read the article Zachary Kennedy and I wrote in Plating and Surface Finishing [link is to product info at Amazon], October, 2007, entitled "Unexpected Results from Corrosion Testing of Trivalent Passivates." As a summary: During the corrosion process, trivalent passivates generate hexavalent chromium, which acts as an inhibitor, slowing the corrosion of the underlying zinc. It is my personal opinion that the coating thickness (100 to 900 nanometers) is much too thin to act as a barrier coating.

Tom Rochester Plating Systems & Technologies, Inc.   Jackson, MI, USA

Ed. note: you can also listen to our podcast interview with Tom at finishing.com/podcast/071112.mp3

Hi,

Where can I read this article? Can I access it online?

I am currently doing a project on chromate free conversion coating replacements and am looking at trivalent chrome as a possible replacement. So any information on the subject would be much appreciated.

Cheers

Tom

Thomas Kelly
Materials Engineer - Birmingham, England

You can access it on line at www.nasf.org if you are a member of the National Association for Surface Finishing, Tom. If you are not, please consider joining; it's a vital organization and membership is only about the price of a subscription to the journal.

Ted Mooney, P.E.  finishing.com Brick, NJ

Is it really true that the trivalent passivates create a hexavalent protective compound? How does this sit with the RoHS compliance commonly required for European supplied components? I'm only asking out of curiosity as I have just started looking at trivalent passivates for various materials.

Brian Terry
Aerospace - Yeovil, Somerset, UK

Well, so far we have Tom Rochester's test results and his interpretation of them, and a strangely silent world. I am told that there has been no rebuttal to date (which I consider astounding considering the billions that have been spent on conversion to trivalent chromating). If anyone anywhere wishes to rebut his findings or interpret them we would certainly be pleased to immediately do a podcast interview presenting that rebuttal or interpretation. Tom's findings will be mentioned again in the January issue of Products Finishing [link is to product info at Amazon], and I believe that at some point soon the 800-pound gorilla in the room will be acknowledged :-)

Ted Mooney, P.E. 
finishing.com
Brick, NJ

I read the article by Tom Rochester which was published in Plating & Surface Finishing and found the results interesting. Since trivalent chromates are highly proprietary,is it possible that the tri to hex oxidation reaction is happening with only a certain type of trivalent chromate and not all trivalent chromates? Was this observed with more than one type of trivalent chromates?

Thanks

Prat George
- Baton Rouge, LA

Zachary and I tried a fairly large number of trivalent passivates and found the same general behavior in all of them. Some generated more hex chrome and some less. We found no correlation between the amount of cobalt in the formulation and the amount of hex chrome generated. It is, of course, possible that there is a trivalent passivate being manufactured that does not generate hex chrome in the ASTM B-117 [link is to info about spec at TechStreet] Salt Spray Test. If there is one, it would raise serious questions about our theory that trivalent passivates protect zinc surfaces by generating hexavalent chromium. If any supplier had one, you would think that by now they would advertise that fact.

Tom Rochester
Plating Systems & Technologies, Inc.  
Jackson, MI, USA

Ted,

I will be presenting a paper at Sur/Fin Indy 2008 in June on Tuesday on the Automotive Track 2-5PM I am session chair of the track as well. Originally, when I started my paper it was going only to be on the improvemnets on the trivalent yellow we are working on. Tom Rochester's articles brings up a true challenge to the industry. Therfore, I will be discussing some theory on the diphenyl carbazide challenge and offer some possible alternative test methods for Cr+6 that are based on an ISO method. We have all of our trivalent chromates verfired for no Cr+6 by an outside metallurigical lab that uses this ISO test method. We did conduct testing after salt spray and those results will be presented in this paper. Eric

Eric Olander Electrochemical Products Inc. New Berlin, WI

Thanks, Eric. There is another article questioning trivalent chromating in this month's Metal Finishing magazine. John Bibber of Sanchem includes pictures and test results as a trivalent coating allegedly converts itself to hexavalent in a humidity test.

We're certainly looking forward to your paper and the session, as this is potentially a very serious issue, and it will be great to see it thoroughly talked up from all sides. Thanks again.

Regards,

Ted Mooney, P.E. 
finishing.com
Brick, NJ

Ted,

I will also have test results using humidity as well. I heard in Tom's pod cast that he used phosphoric acid with his DC test. I did not use that method with phosphoric acid. I assume that everyone is embracing the ISO method for accompishing this test. Maybe this ISO method has an issue.
Eric

Eric Olander
Electrochemical Products Inc.
New Berlin, WI