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Letter 10026 Trivalent conversion coatings over Zn and hydrogen peroxide
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 Flint
Personally, I can't imagine hydrogen peroxide in the finished product having the longevity to oxidize anything. I think it would be water and oxygen before you can blink your eyes. And as for the ingredients in the bath itself, that's irrelevant. I, for one, am a recent convert to the idea of trivalent chrome because--from what I've recently read in some journal articles--it's really Cr+2 that provides the protective value anyway. My present understanding is that hex chrome is just an easier path to getting the Cr+2 on the article than trivalent chrome is.
Ted, Now I'm curious. What articles are you talking about? Journals, dates, etc...? Bruce Flint
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 Lucas
I'm a little too busy to do a lit search this morning but, to me, the most illuminating thing I've ever 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.
I use and would recommend contacting SurTec and seeing the article on trivalent passivation posted in June 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 J. Boogaard
Mr. Boogaard, Langford and Parker,
"Analysis
of Electroplating and Related Solutions [link is to book info at Amazon; also avail. from
this link at this link at Abebooks 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
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 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.
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 (eg 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.
I think there's two different discussions interleaved here. Mr. Flint 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. While I have no trouble accepting Geoff's explanation of bath chemistry, it strikes me 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.
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 Joslin Ed. note Oct. 2005: Four years have passed since the first posting 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 precoat before the chromate but many need a topcoat, Mr. Joslin.
Dear Reader: please choose what you want to do.
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