47037

Chromate conversion on cadmium not adhering [New York] 

November 14, 2007

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, NY

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, P.E.  finishing.com Brick, NJ

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 - Bronx, NY, USA

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 exhiting 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. Anoplate Corp Syracuse, NY, USA

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
- Great Neck, NY

No, I haven't heard of a trivalent chromate designed for cadmium, Gene; I guess I was suggesting the possibility that 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, P.E. 
finishing.com
Brick, NJ

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

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

Khozema Vahanwala Automaxx   Bangalore, Karnataka, India