Home /
T.O.C.
FAQs
 
Good
Books
Ref.
Libr.
Advertise
Here
Help
Wanted
Current
Q&A's
Search 🔍
the Site

pub
Free personalized metal finishing help!


Please explain theory of trivalent passivation



(-----) 2007

Q. Is there someone can help me to explain the detail theory of hexavalent chrome-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 kinds of treatment.

Roynindra Malaon
process engineer - Jakarta, Indonesia
^


simultaneous 2007

A. 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
Dave Wichern
Consultant - The Bronx, New York
^


2007

A. You should read the article Zachary Kennedy and I wrote in Plating & 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.

tom_rochester
Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
^


2007

Q. 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
^


November , 2007

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.

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


2007

Q. 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
^


2007

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 :-)

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


February 8, 2008

Q. 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, Louisiana
^


February 12, 2008

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 [affil. link 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
Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
^



April 13, 2008

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

Eric Olander
Electrochemical Products Inc.
supporting advertiser
New Berlin, Wisconsin

epi banner
^


April 13, 2008

thumbs up signThanks, 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, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


April 19, 2008

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.
supporting advertiser
New Berlin, Wisconsin

epi banner
^


January 8, 2009

Q. 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.

Subramanian Ramajayam
Subramanian Ramajayam
consultant - Bangalore, India
^


January 8, 2009

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,

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


January 9, 2009

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,

Subramanian Ramajayam
Subramanian Ramajayam
consultant - Bangalore, India
^


January 18, 2009

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
Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
^


January 21, 2009

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.
supporting advertiser
New Berlin, Wisconsin

epi banner
^


February 24, 2009

! I have a few general comments... but I cannot 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
^


February 25, 2009

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.

Subramanian Ramajayam
Subramanian Ramajayam
consultant - Bangalore, India
^


April 7, 2009

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

tom_rochester
Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
^


August 7, 2009 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. 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, North Carolina, USA
^


August 15, 2009

thumbs up signHi, 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,

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



January 3, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello

Our company has parts made and zinc plated in China and are RoHS compliant (Chrome VI free). Recently, our supplier has informed us that under certain conditions (high temperature or acidity) zinc plate with yellow passivation can change from chrome III to chrome VI. Is this possible and is it a known risk?
They are suggesting to use clear passivation as this is more difficult to convert.

I would be grateful for any help.

Christopher Peacock
Project Engineer - Bishop Auckland, County Durham, United Kingdom
^


March 28, 2012

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 test.

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

tom_rochester
Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
^



December 7, 2012

(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!

tom_rochester
Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
^

none
finishing.com is made possible by ...
this text gets replaced with bannerText
spacer gets replaced with bannerImages

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

 
Jobshops
Capital
Equipment
Chemicals &
Consumables
Consult'g, Train'g
& Software


About/Contact    -    Privacy Policy    -    ©1995-2022 finishing.com, Pine Beach, New Jersey, USA