Please explain theory of trivalent passivation

A. Hi Tom. You can access it on line at www.nasf.org if you are a member of the National Association for Surface Finishing. 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 and immediately, you can listen to our podcast interview with Tom Rochester at www.finishing.com/podcast/071112.mp3

Good luck.

A. 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", and I believe that at some point soon the 800-pound gorilla in the room will be acknowledged :-)

A. 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 B117 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.

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,

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

Electrochemical Products Inc.

New Berlin, Wisconsin

A. 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,

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 the 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 experimental evidence supporting or contrary to the above observation I will be happy to share with you.Thanks,

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.)

A. It is possible to suppress the formation of hex chrome from trivalent passivates. But then they provide no incremental corrosion protection.

Q. We have heard that Trivalent Chromate will change to hexavalent chromate under humidity and other environmental conditions. Can someone please elaborate?

Hi, folks.

Thomas Kidd: You are surely correct that hexavalent chromates are a dead issue, and the world will go forward with trivalent chromates irrespective of facts -- that's just the way governments function. But that does not in the slightest way make any of this "wasted breath"!

The trivalent chromates are all proprietary and differ widely one from the other. Each exhibits various weaknesses which will not be revealed to the user by the vendor -- and in many cases they come as a shock to the customer: Paint and powder will not adhere properly to some trivalent chromates. Others won't survive even a simple cleaning cycle, such as when chromated parts must be electropainted. Others require a topcoat which destroys the customer's expectation of conductivity. Perhaps some may vary in how much hex they generate in service, etc. We are still a very long way from having a universally accepted more or less "generic" approach to trivalent chromating; until then, each of these issues requires light shined upon it. The exclusive focus until now has been "salt spray hours", whereas chromates are called upon to do much more that simply deter white rust, and we are seeing many failed installations as the unsung weaknesses of different trivalent chromate weaknesses start surfacing.

Subramanian Ramajayan: Although the internet generation thinks knowledge wants to be free, you and I know better. The proceedings of past technical conferences cannot be available for free or there will be no future technical conferences. The proceedings from Sur/Fin 2009 are available for a fee at www.nasf.org.

Regards,

Trivalent passivates (sometimes called trivalent chromates) function by generating hexavalent chromium during the corrosion process. This has been known since 2007 or perhaps earlier. There has been widespread commercial acceptance of trivalent passivates as RoHS-compliant, if they are applied from a trivalent passivating bath that is free from hexavalent chromium. Trivalent passivates applied to parts or articles will generate detectable quantities of hexavalent chromium under ambient conditions within 30 days and much sooner at higher temperatures and/or humidity (for example, detectable quantities are obtained in a salt spray cabinet within a day or so.)

Why should these passivates be considered RoHS-compliant? The logic goes back to the EU restriction on hexavalent chromates on zinc plate. The EU felt that the trivalent passivates offered an acceptable alternative to the hexavalent chromates previously in use, and banned hexavalent chromium as a post-plate treatment. They therefore accepted trivalent passivates as RoHS-compliant since they knew (or should have known) that they function by generating hexavalent chromium.

The only way to determine if the coatings are RoHS-compliant under this definition is to audit the applicator and test the bath itself on-site for hexavalent chromium using the 1,5 diphenylcarbohydrazide [affil links] test.

The views above represent my views and those of Plating Systems and Technologies, Inc. and not necessarily of any other people or organizations.

(I am appending this to the above thread, since that seems to be where it is most relevant.)

An interesting article appeared recently in the Journal of Hazardous Materials. The authors found that hexavalent chromium was generated on the surfaces of trivalent-passivated zinc plated articles at ambient conditions in just a few days. Here is the link:

http://www.sciencedirect.com/science/article/pii/S0304389412003822

Happy Holidays!