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topic 47037

Chromate conversion on cadmium not adhering

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A discussion started in 2007 & continuing through 2017

(2007)

Q. We have a steel stamping that is cad plated with chromate conversion- it is then painted

We have problems with the paint coming off and it appears to take the yellow conversion coating with it. Any ideas what could be causing this? It appears the conversion is not adhering to the base cad metal.

Joe Arnold
machine shop - Falconer, New York


(2007)

A. I don't think you can troubleshoot this problem from your position or we from ours, Joe. You need to make contact with the plating shop and have them figure out what's wrong. It could be excessive brightener, and the next batch could be fine; it could be an unsatisfactory attempt to substitute trivalent chromating. Too many possibilities, I think :-)

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


simultaneous (2007)

A. I've seen problems like this arise both from excessive brightener, as Ted suggests, and from poor post plate rinsing.

If it's the latter, a dip in 1 - 2% nitric acid before the part goes in the chromate might help. And, of course, using cleaner water.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


(2007)

A. Joe - Falconer isn't all the far from Syracuse where my shop is located. It would be a whole lot easier to see the actual hardware that is exhibiting the problem (if at all possible before and after painting). I'd be willing to do so if you're willing to send in a couple of parts to review. People like to point their finger at the plater, but contamination post-plating is a likely cause as well. Have you gotten good parts in the past? Have you changed painting houses? How long was the lag between plating and painting? Do the parts require embrittlement baking between plating and chromating? There's a lot more to solving this issue than "why is the paint coming off?"

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

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(2007)

A. Ted,
Trivalent chromate on cad? I am yet to hear of such a product. I have had several people asking about one but know of none. Part of the reason is economics, a small market.

That being said, Joe make sure the plater is actually plating the cadmium you have specified and not substituting zinc. There are not too many platers who still plate cadmium due to lessened demand and the toxicity of the bath components

Gene Packman
process supplier - Great Neck, New York


(2007)

thumbs up sign No, I haven't heard of a trivalent chromate designed for cadmium, Gene; but perhaps the plating shop is trying to apply a trivalent chromate to cadmium that wasn't meant for cad and that's why it's not adhering? :-)

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


(2007)

A. I think there is no need to chromate at all if a topcoat of powder or paint is being used. Cadmium is used because it forms its own oxide topcoat which retards further corrosion, unlike zinc that corrodes away.

It might be worthwhile to see if a light phosphate over the cadmium would work better.

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

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June 13, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. The passivation done after cadmium plating is coming off on simply rubbing with cloth.
Can somebody please suggest remedial measures?

Thanks & Regards

sahil arora
- kanpur, india


August 8, 2017

A. Hello Sahil!

Usually if you can remove the chromate with your fingers or a cloth, there are two big possibilities:

-Organics in your cadmium/zinc bath are too high.
-Concentration of the chromate bath is too high.

This two possibilities have a relative probability that depends on each facility and methods, so please explore them and let us know what you find!

Best regards!

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina



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