RoHS Zinc Plating ASTM B633 Type III - are trivalent chromates ready for prime-time
A discussion started in 2013 but continuing through 2018July 15, 2013
Q. We are converting zinc-plated part specs to RoHS (to ensure no hexavalent chromium).
I would like to maintain equivalent salt-spray corrosion resistance.
Looking through the forum I see people claiming that the trivalent chromates are:
much worse than..
almost as good as..
hexavalent chromate. Since these answers span several years, it wasn't clear whether there was a consensus developing or even whether the answer had changed as the processes and formulations have evolved.
So - can I get a RoHS compliant ASTM B633 [link by ed. to spec at TechStreet] Type III finish that is [corrosion] equivalent to the non-compliant hex-chrome version and if so, how do I clearly state that on a spec?
Design Engineer - Lowell, Massachusetts, USA
July 17, 2013
A. Hello Paul,
The equivalent is ASTM B633 Type V. You have the same thickness, you can have the same corrosion resistance. For clear blue chromates, trivalent are (in my opinion) far better than hexavalent ones. Take a try with a few in "lab scale" (2-3 gallons, a few parts) and have they tested for corrosion resistance.
For yellow chromates (B633 Type II), the equivalent is Type VI. But yellow hexavalent chromates are far better than yellow-colored trivalents, in my experience.
Well, I hope you can change to trivalent and improve your results (process cost, quality, and water treatment costs). Good luck!
- Canuelas, Buenos Aires, Argentina
July 22, 2013
Thanks for your response, Daniel. I talked to a local plater who also suggested the type V (and said that it was rated better than the old (non-RoHS) process. Your explanation about the yellow trivalents (type VI) not being as good as the yellow hex was also helpful. I think that may have explained why some folks on the forum were indicating that the trivalent wasn't up to par. In any case, I am going to move forward with a clear/blue type V and will run some tests. Once again - thanks for your input.Paul Maier [returning]
- Lowell, Massachusetts, USA
November 20, 2013
A. The equivalent for yellow hexavalent chromate is a thick trivalent chromate, and the equivalent for clear hexavalent chromate is a thin trivalent chromate.
You can improve the corrosion resistance by applying a sealer on top of the trivalent chromate.
chemical process supplier
November 20, 2013
A. Perhaps my experience is dated. But I believe that a trivalent "chromate" is merely a decorative finish. Somebody show me a part with that on it, with no sealer, that will stand up to a salt spray, and I'll become a believer.
I question the environmental benefit of using some "eco/green" finish when corrosion resistance, and hence the useful life of the part is lowered.
Consultant - The Bronx, New York
November 21, 2013
A. Hi David. You are extremely knowledgable in metal finishing, but in this specific area I do think recent progress in trivalent chromates may be substantial. Although today's trivalent chromates are not the equal of hexavalent in ALL ways, they are not the trivalents of the old days, and I believe they are the equal of hexavalents in salt spray resistance. My understanding, subject to correction by someone more knowledgable, is that the thin film trivalents generally require a sealer but the thick film chromates do not.
Ted Mooney, P.E.
Pine Beach, New Jersey
January 3, 2014
A. Hello Ted,
I fully agree with your view.I have been connected with this finish for auto parts for multi national companies for the last 6 years ever since the arrival of trivalent chromate.They have been so remarkable in Salt-spray testing,one customer company has revised the hours for white corrosion from 96 to 140 hours now.
August 31, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. We have a print that calls out for zinc ASTM B633 [link by ed. to spec at TechStreet] fe/zn 5 sci type V. Our plating sources are unclear of what the V is:Cathy Delbridge
- Sterling Heights, Michigan usa
A. Hi Cathy. You and/or your plating shops need to spring for recent additions of ASTM B633, rather than outdated versions. Type V is clear trivalent chromate.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
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