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topic 10026

Trivalent conversion coatings with hydrogen peroxide added ... why?


(2001)

Q. My question concerns trivalent chromate conversion coatings over zinc. I have noticed that many companies have developed their own trivalent chromate products. We have heard that trivalent chromates do not offer the 'self-healing' properties that hexavalent chromates do, however, some companies offer trivalent chromates that do offer self healing properties. Of those, we noticed that hydrogen peroxide is one part of the bath ingredients. In speaking with a chemist, we learned that hydrogen peroxide will oxidize some of the trivalent back to hexavalent. Should we avoid these products altogether? Do they meet regulations or is this an attempt to work around the regulations that will eventually be uncovered in the future?

Bruce F [last name deleted for privacy by Editor]
- Columbia, Missouri


(2001)

A. Hi, Bruce.

The ingredients in the bath itself are irrelevant to RoHS compliance because it's the finished part, not the chemicals in the bath, that must comply. This peroxide in chromate thing is unknown to me, but can there really be sufficient hydrogen peroxide left in the finished product to oxidize trivalent chrome to hexavalent off into the future? I thought peroxide needed to be kept in sealed brown bottles because it became water and oxygen so quickly out in the atmosphere. Sorry, but I'm not a chemist either :-)

I am a recent convert to believing in trivalent chrome though, and from what I've recently read in some journal articles, it's really Cr+2 that provides the protective value anyway. My present limited and perhaps faulty understanding is that hex chromating is just an easier path to getting the Cr+2 onto the article than trivalent chrome is.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2001)

Q. Ted, Now I'm curious. What articles are you talking about? Journals, dates, etc.?

Bruce F [returning]
- Columbia, Missouri


(2001) 

Q. Ted,

I'm interested in this whole trivalent vs. hexavalent thing going on. You mentioned journal articles. Could you please post the journals and dates for us?

Thanks,

Brian L [last name deleted for privacy by Editor]
- Wolcott, Connecticut


(2001) 

A. Without doing a lit search to find the numerous articles about modern trivalent chromates, the most illuminating thing I've seen on this subject is "Clear Chromates - Theory and Praxis" by Dr. Klaus Peter Klos of Elektro-Brite GmbH & Co., Trebur, West Germany. Everyone seems to have xeroxs of this article, without attribution of what conference it was presented at, and I certainly hope it is a public document rather than a pilfered internal one! In any event, try to reach Dr. Klos for a copy of this genuine eye-opener which authoritatively challenges a lot of earlier erroneous assumptions in the literature.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2001) 

A. I use and would recommend contacting SurTec and seeing the article on trivalent passivation posted in June 2001.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

saify logo



(2001) 

I have been introduced to trivalent products which utilize hydrogen peroxide. The peroxide is not a component in the proprietary product you purchase, generally the supplier wants you to add a certain percentage of hydrogen peroxide on makeup.

With all due respect to Mr. Mooney. If one is ultimately trying to get to Cr(II) why would there be a need to supply an oxidizing agent to the make-up in the first place. In conclusion hydrogen peroxide is a powerful oxidizing agent, its longevity at elevated temperature is diminished but can easily oxidize Cr(III) to Cr(VI) especially at elevated temperatures.

David B [last name deleted for privacy by Editor]
- Columbus, Ohio


December 2013

Hi again. This is getting very confusing to me, but I thought we were discussing a proposition about the role hydrogen peroxide might play in "self-healing of chromate films" -- and whether it could and does oxidize trivalent to hexavalent chromate in order to provide that self-healing power. I was not questioning whether hydrogen peroxide can oxidize trivalent chrome to hexavalent in a vat of solution, but whether a chromate film in use in the field can actually retain hydrogen peroxide in order to provide that oxidizing power on a chromated surface.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 (2001)

A. Mr. David B,

Langford and Parker, "Analysis of Electroplating and Related Solutions" [linked by editor to info about book at AbeBooks] 1971, page 72, does not mention hydrogen peroxide, mentioning ammonium persulphate or sodium peroxide.

When I oxidized trichrome to hexchrome for analysis in Chromic acid plating solutions, we used sodium peroxide in boiling, acid solution. I believe I was told that in acid solution, hydrogen peroxide would actually act as a reducing agent. The use of hydrogen peroxide would have been much preferred because of it's lower cost, safer storage, and ready availability in the laboratory. Sodium peroxide costs about $1.00 per gram, the amount used in our analysis.

The addition of hydrogen peroxide may make sense then, if we want to stabilize an acid solution against oxidation of +2 to +3 chrome ion. And adding hydrogen peroxide upon recommendation of the supplier is one way to keep the cost to the consumer down, and eliminate the problem of packaging (and venting) a volatile, inexpensive component into the rest of the proprietary solution.

tom pullizzi portrait
Tom Pullizzi
Falls Township, Pennsylvania


(2001)

Just a quick note for Tom on why he used sodium peroxide.

You are looking for trivalent chrome right?

You first find the hex chrome answer.

Then you oxidise the trivalent to hex by an excess of powerful oxidising agent. You now have a problem that if you test this sample as is you will find out basically how much excess powerful oxidiser you have added.

To remove the powerful oxidiser but not breakdown the hex chrome produced from the trivalent is the problem.

Sodium peroxide is not temperature stable, hex chrome is at the 100 °C mark.

That's why you used to boil the sample once the peroxide was thrown in - to remove the peroxide and leave the hex chrome produced from the tri chrome.

You could also use ammonium persulfate.

Martin Trigg-Hogarth
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England


(2001)

Hydrogen peroxide HAS the ability to oxidize Cr3 to Cr6 in alkaline conditions and reduce hex to trivalent in fairly acid conditions (the pH needs to be lower than for bisulphite). Try adding peroxide to a dilute acid Cr6 solution. It goes through a very dark blue complex which then changes to the green trivalent form.(This is also a spot test for Cr6) If it is used as a reductant in say waste treatment then it is imperative to totally destroy the peroxide (e.g., with Fe) or, when pH is raised it will reoxidise to hex. Re: sodium peroxide when analysing for Cr, it is alkaline so oxidizes Cr3 to Cr6. If you don't boil long enough to destroy ALL peroxide then you will get false results as some, or all Cr6 can be reduced. It is often the reason for false trivalent readings. The peroxide is added to chromate dips to reduce the Cr6 to Cr3. This is one way of making a trivalent clear dip.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia


(2001)

!! I think there are/were two different discussions interleaved here. Bruce F seemed to be asking if he needed to be concerned about the possibility of the chromate on his components initially passing the standards for hex-free chromate at shipment time, and then later turning into hex chromate in service because there was H2O2 in the processing solution. It struck me (with my limited chemistry knowledge) as farfetched that any small amount of H2O2 remaining on the parts or within the chromate film could have the stability to persist on the parts in service and oxidize the chromate in service after it was found hex-free at shipment.

Geoff's very interesting explanation of bath chemistry seems to resolve the real reason for peroxide in the bath -- not to oxidize trivalent chromate, but to reduce hexavalent! Others may have been fooled, like I was, into thinking that it's purpose was to oxidize trivalent.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



(2001)

Q. The company I work for has just started to get into the picture regarding the removal of any Hex 6 compounds in products. My question actually has to do with the process of plating this kind of treatment over a zinc based material and we would like to know whether if the trivalent treatment compound is to be used does an intermediate layer need to be applied? How can we increase the amount of corrosion protection that our part can withstand.

Your help would be greatly appreciated.

Regards,

Gregory J [last name deleted for privacy by Editor]
- Malta


December 2013

A. Hi Gregory. Years have passed since this thread started, and trivalent chromating is now the rule rather than the exception. Reasonably well developed proprietary trivalent chromates are widely available. They do not need a pre-coat before the chromate but many need a topcoat.

Readers: If this topic is close to what you were looking for, but not 'spot on', you may be interested in letter 45368, "Can Trivalent Chromium degrade into Hexavalent Chromium?"

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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