Black zinc plating turns green

A. Hi Paul. I haven't seen any published studies on it, but I haven't been really looking. What I HAVE seen are black chromated parts which, over time, turned chocolate brown, mottled green, and every which color but black. I believe it happens because of the light sensitivity of the silver-based black chromate.

I'm not saying it's the only solution, but one thing I have seen work excellently for some platers is switching the alkaline zinc bath to an alkaline zinc-iron alloy bath and silver-free black chromate. I'm not heavy into the chemistry of it, but I believe it has to do with being able to draw the black color from the iron as opposed to needing silver. If your supplier proposes it, listen to him/her; my experience is that it works!

A. Standard Black Chromates over zinc usually contain silver. Silver chromate is deposited onto the zinc as a gel like layer. Silver makes the black color. However it is photo-sensitive, thus turning green when exposed to light. There are some Black Chromates commercially available that are less sensitive. These should be Phosphoric-acid based. Acetic-acid based ones are said to be more sensitive.

When using Zinc-Iron Alloy instead of Zinc as the metal coating, the chromate formulations can be silver-free due to the darker color formed while chromating. Here you can obtain real black color without changing to green even after long exposure to light.

Regards,
Rudolf

A. Typically, the Black Chromates that have better salt spray results are Olive Drabs with a silver additive. Without the silver, you get green. You may be using up the silver because of chloride drag-in from a chloride zinc, in which case add more silver or rinse more effectively. Also, tap water can have chlorides which would tie up silver ions.

Take a beaker [beakers on eBay or Amazon [affil links] of your "green" Chromate and add a big dose of the silver additive. Try chromating a part. This will tell you if what I've mentioned is indeed the problem.

A. Black chromates contain simple salts of silver, which are light sensitive. Many shops have switched from zinc plating to zinc-iron in order to be able to use a non light-sensitive black chromate.

A. You can decrease the tendency of the black chromate to turn green by improving the post rinsing but remember that this coating is sensitive to UV.

As Ted mentioned, switch to zinc-iron. The black chromate of zinc-iron is silver free and not sensitive to light.

A. I have to disagree with the previous response. Black Zinc Chromate can and usually is photosensitive. Exposure to UV lighting will turn it GREEN. I have experimented with this and have seen the results for myself. Notice how the variation YOU are experiencing relates to light exposure! Bill.

A. The other area to investigate besides the chromate solution that Asif discussed is the Zinc Plate, it must be pure and thick enough. Some Zinc plate (especially from a cyanide bath) may have significant amounts of Iron, Copper, etc. that will interfere with black chromates more than they will with a clear or yellow finish. Black chromates tend to remove more zinc plate than other chromates, so more plating thickness is required initially - .0003" minimum is usually safe. Check the distribution of your color variations, if some areas of the parts are always good, then the solution is probably not to blame (it can't "know" that it should work here, but not there!).

A. Hi, Rajan. This is a very common problem due to the light sensitivity of the silver salts used in black chromating. Please consider switching to zinc-iron plating which will permit a silver-free black chromate that is color stable. Good luck.

A. Hi, Christopher. Black chromates are often silver based and tend toward this problem. Poor plating can accentuate the problem as can excessively bright light. My understanding is that the best answer is to switch the plating to nickel-iron^^ZINC-IRON plating because the presence of the iron allows it to be chromated to a stable black color.

P.S.: I was thinking of zinc-iron and meant to say zinc-iron but didn't recognize my slip until Mr. Nurie's reply below :-(

A. I'm a big fan of using sealers for black zinc plating. We started this and have greatly reduced the fading and premature corrosion problems. Also, the pH is critical. I keep it under 2 and check it regularly, with frequent small additions of both components.

A. Hi,

There are post dips after black chromate which enhance the jet black colour and also the brightness (chromic base); and if you go for zinc iron with a silver and non-silver passivation, iron ratio to zinc is most important factor to obtain best results -- practically if you consider in production level and a sealant coating to minimise this greenish appearance.

A. The black chromate on zinc-iron is NOT silver based so it does not tend to get greenish with light. These chromates are not even based on hex-chrome but on trivalent chromium. And yes, for a perfect appearance you need a sealer.

A. Hi, Avinash. The black chromate that you are using is apparently based on silver, and these are light sensitive. In my limited experience, the best resolution is to switch from zinc cobalt to zinc iron plating because there are chromates that can take advantage of its iron content to build a stable black color without silver.

Regards,

Q. Sir,
We are doing hexavalent black passivation on Zinc Cobalt Plating for automobile spare parts and exporting it Europe countries. Our black passivated jobs are changing in colour from black to military green. Customer is rejecting the parts, and we are not finding out the exact reason for it. The discolouration occurs within one month to three months period.
Our bath condition is as follows.
Zinc 42 gm/lit
Chloride 165 gm/lit
Boric 37 gm/lit
Cobalt 1.2 gm/lit
We work within pH 4.8 -5.2
can you please guide us?

A. Dear Avinash,

Your question was repeated. Please read the answer provided above. That is all you need to resolve your problem.

A. Hi, Milt. It's probably nothing to do with your processing; it happens because silver is light sensitive. I think no matter how fastidious you are, you cannot completely prevent it from happening. If you need black, please consider zinc-iron plating with a black chromate. The iron content allows silver-free dye-free black chromating, and thus a more stable color.

A. Following Ted's recommendation, you can use a hex chrome free black on zinc-iron to comply with RoHS

A. Hi Ashish. It may be dangerous to extrapolate from little information, but as you can see from this thread, black chromate turning green is so strongly associated with the photosensitivity of silver-bearing black chromate that it would be my first suspicion. Can you find out from your supplier if your chromate contains silver and whether they have any suggestions about making the color more stable? Good luck.

Regards,