Chromate conversion on cadmium not adhering

A. I think you'll need to make contact with the plating shop and have them help figure out what's wrong, because:
• It could be just excessive brightener or allowing two thick chromate, and the next batch could be just fine; but
• It could be a move to a trivalent chromating which meets salt spray requirements but introduces new adhesion issues, and perhaps the problem won't go away until they make a major change.

Unfortunately there's a lot of possibilities :-)

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 Consultant - The Bronx, New York 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. Syracuse, New York 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 Saify Ind Bengaluru, Karnataka, India

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!

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A. Hi Chip. I am guessing that when you talk about 'gold dip' you are referring to a yellow or gold colored chromate conversion coating designed for cadmium plating. So we appended your inquiry to a thread about that problem.

Daniel has given you two good possibilities, and your statement that "the cad seems to be coming out nice and bright" might be a clue that you have too much brightener (organics). But it is not quite clear to me whether you are attempting to chromate conversion coat freshly plated parts and it no longer works, or whether the problem might be that after you got a replacement container and replacement solution and made up the process, you are now attempting to chromate conversion coat cadmium plated parts which you plated some time ago (before the leak); if that is the case, the surface may no longer be active enough for reliable chromating anymore. One other thing you may realize but we'll say for the benefit of newbies who (You didn't mention catching the leaked solution in a secondary containment vessel; I hope you have researched the toxicity of cadmium and hexavalent chromium; they're bad ones. A bucket of chromate solution can do quite a bit of environmental damage, and you should definitely have secondary containment).

Luck and Regards,

A. Hi Larry,

I have used a 10% caustic solution to remove the yellow chromate from cad in the past when I needed the parts dyed green, blue or red, so I'd guess that this approach should work for black dyeing. The one thing you can't do is try to apply the dye over the already chromated part, it just doesn't work.

A. The plater did not neutralize off the cyanide before entering the yellow chromate.

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A. Hi Leong. There's a good chance that Mr. Probert is right. But there are other possible causes including the chromate being dried at high temperature (above 140 °F / 50 °C) before it has fully cured.

The components are not satisfactory in this condition, but figuring out what an unknown plating shop is doing wrong without benefit of walking the line, chemical analyses, examining the records, or holding the actual parts is problematical.

Your postings and effort at learning are very welcome, and I'm not trying to discourage you from it ... I'm just forewarning that trying to solve plating problems that way is hard and may be wasted effort. Good luck.

Regards,

A. Hi again Leong. I am not quite following ...
Are you saying that as long as the components are not cleaned the chromate discoloration does not occur? If that is true, that's helpful data! The large splotch of missing chromate does in fact look like poorly adhered chromate that was pressure washed off, and pressure washing a poor coating can certainly partially strip it.

But if you're saying there is no data available about whether the discoloration occurs before or after that final cleaning, guessing about whether the final cleaning is the cause probably isn't helpful :-)

Regards,

A. Hi Leong. You must solve the defects that arise before the post-cleaning process before you worry about whether the post-cleaning process causes additional defects. Yes, the pressure washing is apparently knocking some of the chromate off, but until you are producing good parts that have sufficient reliability & adhesion that the parts do not spontaneously fail, it's just a waste of time to speculate about whether the post-cleaning is making it worse.

Have you been able to find out from the plating shop about neutralizing the cyanide before chromating, and about the drying temperature? Thanks! And good luck.

Regards,

A. Hi Irem. Please try to send better pics; unfortunately, those are are not very good :-(

At first I thought you were concerned about all those dark red blotches and dots until I noticed that they are everywhere, not just on the parts. But it does look like you have some areas with no cadmium plating coverage. My guess is that this is not a problem with the geometry, but something about the processing in that flared area, or perhaps a skim of oil on your cleaning tank that catches in the flared area. After your usual cleaning, please try to scrub that area very well with a scrub brush ⇦this on eBay or Amazon [affil links] and pumice ⇦this on eBay or Amazon [affil links] & water, then rinse and proceed to your acid activation step and tell us / show us the result. Good luck.

Regards,

A. Hello Pankaj,
Once cadmium has been chromate passivated the surface remains fragile for up to 24 hrs after dipping. The coating needs time to stablise before it can be handled. We give our cadmium plating a 24 hr hold once done before handling and or painting etc. AMSQQP416 details this, as do other specs.

A. Hi Pankaj.

It sounds like Hull Cell panel testing might help you see if the chromate adheres.

Luck & Regards,

A. Aloha,

Did you check your pH on the rinses after chromate?
the pH of the rinse affects by two ways:
1)pH acid, chromate coming off.
2)pH alkaline, Color tends to be dull and with discoloration.