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topic 45368

Can Trivalent Chromium degrade into Hexavalent Chromium?

A discussion started in 2007 but continuing through 2020

2007

Q. I am currently doing a RoHS compliancy assessment for our products and this question came up today, could a RoHS compliant finish with trivalent chromium change in some way into hexavalent chromium? We use a multitude of acids and other nasty chemicals I really know nothing about (I'm an electrical engineer) We probably won't have to worry about it but it would be nice to know anyway.

Dylan Gifford
RoHS compliancy Intern - Winooski, Vermont, USA


2007

A. It takes very, aggressive oxidizing conditions to convert trivalent to hexavalent chromium. If the parts were exposed to such, to the extent that the RoHS hexchrome limits were exceeded, there's little doubt in my mind that it would also degrade the coating. To the point that they would be so ugly, that no customer in his right mind would accept them.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


2007

A. Hexavalent chromium tends to be oxidising in nature, having a tendency to head to the trivalent state. To convert trivalent chromium to hexavalent you will need quite forcing conditions, ones you won't find in a normal treatment shop or electronics business. So I am pretty certain you will not have to worry about RoHS compliance.

Brian Terry
Aerospace - Yeovil, Somerset, UK


2007

A. From a theoretical viewpoint trivalent chrome can be quite easily oxidized to Cr6 with peroxide in alkaline conditions.
From a practical view point it is highly unlikely that these conditions would lead to Cr6 being left in any chromate coating left on the plated part after exposure to the above solution, that is the Cr6 would be in solution.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia



Trivalent Chromate Conversion Coating found to contain Hexavalent Chromium

2007

Q. Can hexavalent chromium be produced in a chromate conversion coating that states it is purely made with trivalent chromium compounds such as chromium (III) nitrate?

If pH or temperature is not maintained, can hexavalent chromium ions form?

Are there any process parameters that if not controlled can cause hexavalent chromium to be formed in trivalent chromium tank?

These questions are related to fastener plating of chromate conversion coating over zinc electroplating over bare steel.

One further question, is there any way to effectively remove hexavalent chromium conversion coatings and re-plate with trivalent chromium conversion coatings from small fasteners in barrel?

Jeremy Lynn
Engineer - Newnan, Georgia, USA


2007

A. Hi Jeremy. First question last: Yes, it is generally easy to strip the zinc plating and chromate conversion coating and start over. If contamination of your tanks weren't an issue, it's as easy as re-running the parts because the chromate will come off in the electrocleaner and any remaining zinc will come off in the acid dip.

I get the sense that you are soliciting comment while being somewhat guarded about the actual facts -- like whether you are the supplier of the conversion coating chemistry, the shop applying it, or the buyer of the parts :-)

It's theoretically possible to convert trivalent chromium to hexavalent through a chemical or electrochemical reaction, but probably quite unlikely that poor operation of a trivalent chromating bath will cause it to generate hexavalent chromium, although you might read Geoff Whitelaw's very interesting comment about chromating baths that contain hydrogen peroxide in letter 10026.

I get the sense that someone is holding parts that test positive for hex chrome and you are asking how this most likely happened -- but probabilities of alternative scenarios are difficult to compute when we have little of the background information to go on. Good luck.

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


2007

Q. I appreciate your response. This is just a preliminary inquiry - homework if you will. I have found hexavalent chrome in parts that are being sold as hex-free and solely using a trivalent chemical conversion coating (Chromium Nitrate) over zinc. So, where could it be coming from? If the pH gets high, what happens in a trichrome bath? Likewise, if the temperature gets high? Are there any parameters that if not controlled produce hexavalent chrome? Does anyone have the chemical equations?

Jeremy Lynn [returning]
- Newnan, Georgia, USA


2007

A. The equation itself isn't really complicated, Jeremy:

Cr+3 - 3e- --> Cr+6

But you need an oxidizing force (electricity or a fairly powerful chemical oxidizing agent) to remove those electrons that way. Second opinions are welcome but I can't see temperature or pH causing this.

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


2007

A. Got to say that I agree with Ted and that merely temperature and/or pH would not give you the conditions to form hex chrome from tri chrome.
It sounds to me more like a cross-contamination problem. You don't say what level of hex chrome you are finding, are we in the low ppm, high ppm or even higher? This will give us a clue to the extent of your problem.
Are there more than one type of chromate conversion on the line? If there is then it would sound like there has been drag in to the tri chrome conversion coating. Just an idea.

Brian Terry
Aerospace - Yeovil, Somerset, UK


2007

A. You don't say how you determined that there is hex in the tri chromate coating. One type of widely used test, generally said to be described in GMW 3044, can give a false positive in the presence of iron, cadmium, zinc (yes zinc) and certain organic compounds (sometimes found in zinc brighteners). A more accurate test is one which is done on an ICP to check for hexavalent chromium.

Gene Packman
process supplier - Great Neck, New York

2007

A. Dear Pals at finishing.com,

Just for your information, I have been through a complicated situation like this with a company that used inox. [stainless] steel tanks instead of the plastic recommended by us and the tanks have been releasing hexavalent chrome into the liquid and contaminating the parts.

Regards and good luck!

Celio Santos
- São Paulo, Brazil



Hi Celio. Thanks greatly for this info. Can any other readers relate a similar experience?

Regards,

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



Does Trivalent Plating Revert Back to Hexavalent Plating

2007 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am a mechanical design engineer, and not an expert on zinc plating and chromate conversion. I specify my products per ASTM B633 [affil. link to spec at Techstreet] . I am aware of the RoHS compliance and avoiding the use of Hexavalent chromate conversion. The solution to this has been to apply a Trivalent chrome conversion. Recently a plating supplier suggested that the trivalent conversion coating gets converted to hexavalent coating over time and during shipping and thus rendering the part unfit. Does this phenomenon occur? Please advise? Thank You.

Bhaumik Shah
Engineer - Buffalo, New York


2007

A. Time will tell, Bhaumik, and the richness of experience in a thousand different applications -- but the current belief is that we can count on it not happening.

Readers may be interested in the somewhat parallel thoughts expressed in thread 10026, "Trivalent conversion coatings with hydrogen peroxide added ... why?"

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


June 5, 2020

A. I believe there is an equilibrium reaction between CR+6 and Cr+3 that is dependent on pH, but when in a solution like a conversion coating certain alloys (2024) will have reactions occur on the surface where some Cr+3 is converted into Cr+6. In general though the concentration is extremely low, and very difficult to measure.

Bob Hagerty
- Patuxent River, Maryland USA


June 8, 2020

A. As a direct answer to your query - most of the trivalent chromium in a trivalent passivate remains in the trivalent state. But most of the chromium in a hexavalent passivate is also trivalent.

In October, 2007, Plating & Surface Finishing published an article by me and Zachary W. Kennedy entitled "Unexpected Results from Corrosion Testing of Trivalent Passivates" (Plating & Surface Finishing, October 2007, Volume 94, Number 10, pp 14-18). No one, to the best of my knowledge, has refuted our results, which are that trivalent passivates generate hexavalent chromium during the corrosion process. In my opinion, that is how they work.

If the generation of hexavalent chromium is suppressed (as in, for example, a Kesternich Test) trivalent passivates offer no increased protection in accelerated corrosion testing. (This I reported at Sur/Fin in 2009).) At this point in time, IMHO, the accepted position is that if the articles in question are free from hexavalent chromium at the conclusion of the plating process, they are arguably RoHS-compliant.

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


June 2020

opinion! Thanks again, Tom. I'm very much in favor of metal finishing having a lower environmental impact, but am also bothered by the almost universal tendency to legislate and then try to stay blind to the consequences. So I appreciate it when you try to help us see that just because we wish something were true doesn't always make it so!

- Item: We all know that a child can remove virtually all of the chromate from a zinc plated item with a rubber eraser, whereas a skilled machinist can't remove the zinc plating from the steel item, yet the European Union says that plating cannot be considered "homogeneous" with the steel, but the chromate must be considered "homogeneous" with the zinc. Words lose all meaning when you're on a mission :-)

- Item: I get frustrated with the many threads here where shops are constantly pressured to stop phosphating, when the truth is that the elimination of phosphating before painting is an environmental travesty. Outdoor items used to last decades. My expanded steel patio set was zinc phosphated, electrocoated, and powder coated -- and is going strong after being outside in the sun, rain, and snow 365 days a year for over 20 years, 15 of it on a deck over a salt water lagoon, and being submerged in brackish water for a week or more during superstorm Sandy. In the meanwhile I keep replacing my other outdoor stuff every 3 years or less. -- tons of ore mined, transported, smelted, hot rolled, fabricated, painted, packaged, shipped, sold, and landfilled again & again & again to save a pound of phosphate once :-(

- Item: The most devastating environmental discharge in the plating industry in my memory (and I'm getting up there) was the White River fish kill in Indiana, caused not by a plating solution but by DTC, a dangerous chemical that no shop wants on hand, but which is required to be used as a 'scavenger' after conventional wastewater treatment has already been done. I worked for Dr. Leslie Lancy, who brought plating wastewater treatment to the US and remember him ceaselessly *begging* the regulators not to "split milligrams" and thereby force shops to use these unconventional treatments. While the discharge in question was to a river, many shops must use DTC to meet standards for discharge to their sanitary sewers :-(

I'm not claiming there isn't another side to the things I've point out -- just that there IS the side I point out.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
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