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Dave Wichern |
You should read the article Zachary Kennedy and I wrote in Plating and Surface Finishing, 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.
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Tom Rochester |
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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.
Alternately you can listen to our podcast interview with Tom at finishing.com/podcast/071112.mp3
Good luck.
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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 :-)
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 Ted Mooney finishing.com Brick, New Jersey |
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 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 2-5 PM I am session chair of the track as
well. Originally, when I started my paper it was going only to be on
the improvements on the trivalent yellow we are working on. Tom
Rochester's articles brings up a true challenge to the industry.
Therefore, 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 verified for no Cr+6 by an outside metallurgical lab that
uses this ISO test method. We did conduct testing after salt spray
and those results will be presented in this paper. Eric
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Eric Olander |
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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,
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Ted Mooney finishing.com Brick, New Jersey |
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 accomplishing this test. Maybe this ISO
method has an issue.
Eric
Eric Olander
Electrochemical
Products Inc.
New Berlin, WI
Hello Ted,
The discussions on trivalent passivates are very interesting.I only
request you to publish the abstract or the salient points of
Rochester et al's paper on the subject in this forum so that it is
helpful to those who cannot access the original article.Thanks.
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Subramanian
Ramajayam |
Hi, Subramanian. The salient point was that Rochester et al put trivalent chromated components into a salt fog chamber and, upon looking at the parts after a period of exposure, believed them to now contain hexavalent chromium, i.e., that in the course of offering protection to the substrate, the coating oxidizes to hexavalent chromium anyway -- perhaps calling into question the whole idea of RoHS mandating this more expensive and yet inferior substitute :-)
Please understand that although Rochester's observation is terribly important, it may not be the definitive word on the issue -- thus the importance of tracking the issue as studied by Mr. Olander and others.
For the 2nd time on this page I urge all professionals to join the National Association for Surface Finishing (www.nasf.org), which will insure the article and followups are available to them, and to remind everyone that you can listen to our interview with Tom Rochester at finishing.com/podcast/071112.mp3
Regards,
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Ted Mooney finishing.com Brick, New Jersey |
Hello Ted,
Thank you for the reply giving the conclusions of Rochester et al on
trivalent passivates.My initial reaction is one of disbelief for the
simple reason that under Neutral SS Test conditions(pH=6.5 to
7.2)formation of hexavalent chromium ions looks out of question.The
stable valence state of Chromium ions is pH dependent.In acidic
conditions trivalent Chromium is stabilized while under alkaline
conditions it is the hexavalent state.The reduction potentials also
indicate the same(E0=-0.13 v for the equilibrium,
CrO4--+4H2O+3e-->Cr(OH)3|s| + 5OH- at pH=14
<---
indicates that alkaline media favor stabilisation of hexavalent
Chromium.In acidic solution(pH = 1)Cr+3 is stabilized as indicated by
the reduction potential,E0 = +1.2 v.).We do not encounter a pH
>2.0 for trivalent passivation solution.Hence the presence of Cr+6
may be ruled out in the solution.
I have conducted DPC Test on trivalent and hexavalent passivated
parts.If the test solution does not contain H2O2 or NaOCl then the
distinction is very clear.Trivalent passivate gives a black color and
the hexavalent passivate gives bright pink color and these results
are obtained after SS test also.While testing for trivalent Cr the
ISO test solution gives instant pink color with hexa Cr and there is
a delayed appearance of pink tinge on Trivalent Cr,though initially a
black color appears may be due to in situ oxidation of Cr+3 by
NaOCl.A relook on the ISO test standard seems necessary for testing
the absence of Cr+6 on trivalent passivates.If I find any other
literature or exptl evidence supporting or contrary to the above
observation I will be happy to share with you.Thanks,
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Subramanian
Ramajayam |
I believe that you should read (or re-read) the article carefully. The experimental results seem to be unequivocal, and as Ted describes them. As a chemist, I think that the way another chemist needs to look at this is that both hexavalent chromium and trivalent chromium exist in some form of equilibrium; that equilibrium can be pushed in one way or the other by oxidizing agents and reducing agents, by temperature and humidity. I, too, was surprised by the presence of hexavalent chromium in the salt spray chamber because I am, of course, familiar with the oxidation potentials, electrochemical potentials, and EMF's for these species. I have also run the standard titration for tri chrome in which it is oxidized (with much difficulty, according to my experience) to hex and titrated therefore. That is exactly why we were surprised by the results we published. I hope to give a paper at Sur/Fin in Louisville (if it is accepted, of course) in which my coauthors and I disprove the existence of the purported barrier phenomenon attributed to trivalent passivates in what I consider to be an elegant test. (Some readers may already be familiar with these results which have been reported informally.)
Tom Rochester
Plating
Systems & Technologies, Inc.
Jackson, MI, USA
As for the Cr+3 and Cr+6 controversy I am trying to start a discussion that will happen at Louisville, KY June 15-17. If you would like to attend this forum send e-mail erico@epi.com (phone 262-786-9330). We will be getting a date set soon and a table of users and technical reps.
Eric Olander
Electrochemical
Products Inc.
New Berlin, WI
I have a few general comments... but I can not argue against
specifics and laboratory data etc...
My understanding is that Toyota has performed testing and has also
found that the Cr3+ oxidizes to Cr6+.
However, it seems to be a very small amount. If you have a room full
of Cr3+... some will oxidize to Cr6+.
But my general feeling is that the argument is a lost cause... at
least as far as Europe is concerned. Europe will continue to use and
to improve the Cr3+ technologies.
As far as Salt Spray Testing is concerned the Cr3+ performs well and
comparable, and in some cases with Thick-Film Passivates that include
nano-particles the results are amazing. Cobalt is often used to
improve corrosion resistance (note that Cobalt also has carcinogenic
properties as well).
One significant difference that is often talked about is the
self-healing properties of the Cr6+. Nano-particles may create
self-healing properties, but the stabilization and incorporation of
the nano-particles into the layer is difficult.
My last statement... at least for Europe... Cr6+ is a "dead"
technology and Europe will continue to go forward with Cr3+. No need
to waste extra breath.
Thomas E. Kidd
- Budapest, Hungary
If the Oxidation of Cr+3 is negligible as Mr Thomas Kidd has explained ,a small amount of impurity is always tolerated considering the prevention cost.Further research is called for to improve the process so that oxidation can be prevented 100% so that the purpose of introducing the Cr+3 process is not negated.It is also necessary to replace a carcinogenic Cobalt by a harmless corrosion inhibitor.I request Mr Olander to share the views expressed on the subject at the Symposium he has organised in this Forum for the benefit of everyone.
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Subramanian
Ramajayam |
It is possible to suppress the formation of hex chrome from trivalent passivates. But then they provide no incremental corrosion protection.
Tom Rochester
Plating
Systems & Technologies, Inc.
Jackson, MI, USA
August 7, 2009
We have heard that Trivalent Chromate will change to hexavalent chromate under humidity and other environmental conditions. Can someone please elaborate?
Jeff Owens
Security - Winston Salem, NC, USA
Hi, folks.
Thomas Kidd: You are probably correct that hexavalent chromates are a dead issue, and the world will go forward with trivalent chromates. But that does not make any of this "wasted breath". Trivalent chromates are proprietary, widely differing one from the other, and all exhibiting various weaknesses -- in many cases to the shock of the customer. Paint and powder will not stick to some. Others will not survive a simple cleaning cycle, such as when chromated parts must be electropainted. Others require a topcoat that wrecks the expectation of conductivity. Perhaps some vary in how much hex they generate in service, etc. And, of course, the purveyors will not broadcast the weaknesses of their proprietary. We are a long way from having a universally accepted more or less generic approach to trivalent chromating; until then, all of these issues require light shined upon them.
Subramanian Ramajayan: Although the internet generation foolishly thinks there is now such a thing as a free lunch, you and I know better. The proceedings of technical conferences cannot be available for free or there will be no such technical conferences. The proceedings from Sur/Fin 2009 are available, for a fee, at www.nasf.org.
Regards,
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Ted Mooney finishing.com Brick, New Jersey |
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