Problems in Acid Chloride Zinc Plating

A. Hi Sanjay, my first thoughts are:

1. your zinc plating tank is too hot, and too variable. What does the technical data sheet from the supplier say the temperature should be? Oil-out of brighteners sounds like a possibility.
2. your dryer is too hot; 65-70 °C sounds more appropriate to me.

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi again. I remain concerned with your zinc plating temperature because in my very limited experience, 25 °C is a bit higher than I am used to seeing, 30 °C is a lot higher, and a looseness which allows a 5 °C range seems like insufficient control.

Do you have Hull Cell panels indicating that things are okay? The very obvious and noticeable reaction you are seeing in your nitric acid dip seems to be an indication that things are not right as the work exits the plating tank. And the fact that the chromating doesn't work without nitric acid seems to be yet another indication of a problem with the zinc tank.

A. If you just did a batch treatment I can't see TOC being a problem.
B. Trivalent chromates are highly proprietary and I would not be able to hazard a guess for the right concentration for the one you are using. The supplier must tell you. C. 10-20 ml/l of nitric acid does not sound excessive at all, but surely 15 minutes is a typo and you mean 15 seconds? D. Again, addition agents are proprietary and aside from supplier instruction, Hull Cell panels are probably your only indication whether the additives are functioning appropriately.

A final thought that arose from paging through Geduld's "Zinc Plating" [on Amazon, AbeBooks affil links] is the possibility of lead or cadmium contamination. If a friend or university library has this book it has a 29-page chapter on "Hull Cell Control of Bright Zinc Plating Baths".

Hopefully someone with more hands-on acid zinc plating experience than me will be more perceptive about the likely cause :-)
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. The bath is out of recommended range. First, pH should be 5.0-6.0. The crap on top of the tank is probably the wetter (carrier) portion that has "kicked" out at pH 4.5. Normally it won't at 4.5, right on the edge. When adjusting pH it is possible it was lowered to 4.0 or less. As you have found, it will not re-solubilize. Keep running and changing the filter.

The zinc metal content should be 3.0-5.0 for a rack bath. When the zinc rises above 6.0, the efficiency goes down ,i.e., when the bath is right, plate thickness is 6:1, high CD to low CD on the part. Metal above 6.0 may be 12:1 or higher high to low. You may be experiencing brittle plate with high zinc. You should decant the bath to reduce zinc.

To check for brittleness, run a 2 amp-30 minute Hull Cell, no agitation, do not nitric bright dip it. Just dry, and bend a corner over and back to see if it cracks.

Boric should be 4.0-6.0 oz/gal. When the zinc metal gets to 5.0-5.5 keep the boric at 6.0. No benefit to have boric higher.

Iron is good at 0-50 ppm. Is still OK at 50-100 ppm.

Chloride should be 17.0-19.0 oz/gal. When zinc is close to 6.0 oz/gal., chloride of 20.0-21.0 is OK.

The bath is most efficient at temperature 70-90 degrees F, 80 degrees is the best.

I don't know what brightener system (company) you use, but I am sure the salesman is smiling at a brightener add of 1 Gal. every 8,000 amp hrs.

Recommend you discuss with your vendor all the above. They should check for total organics, (wetter-carrier), this should be 4-6 %/vol.

A. Hello Travis

It is possible that with the increase of Chloride in solution the wetter part of your brightener system is unable to " HOLD" in all the brightener that you have now started adding.

This phenomenon is sometimes called " Brightener Kick Out or Oiling out '

This question cannot be completely answered by this forum. Connect with your Brightener supplier and ask for service.
The only answer is to use a stable Brightener system that can work under your conditions mentioned .

Regards,

A. The temperature of 80° will cause "oiling out" in older standard baths.

You could also be overloading the wetter side of your brightener system.

A. There was a great deal of information in the previous replies that is true and some which may not be quite so.
Firstly, zinc metal does tend to rise in chloride zinc solutions since the drag out is less that the drag in and the high anode efficiency leads to increasing metal. Most platers have to decant solution periodically. Secondly, it would seem that for a rack bath you are using an excessive amount of brightener from any vendor. Reduce brightener additions by at least half, treat the bath with potassium permanganate ⇦ this on eBay or Amazon [affil links] and carbon filter. The material you see on top of the bath is probably unsolubilized brightener. With most brightener systems you need to increase the quantity of carrier (aka starter, wetter) to provide enough solubilizer for the higher amount of brightener. This situation would be readily apparent if you ran a cloud point on your bath. Cloud point is a good indicator of low carrier and/or high total organics. You also should be having problems with yellow chromate adhesion at this high brightener loading. Finally, the suggested level of 4 to 6 oz/gal of Boric Acid is high since I believe this is beyond the solubility of boric acid at this operating temperature. If you wish to run boric acid at the high end of solubility (to help prevent burning and for improved conductivity) take an extra anode bag and fill it with boric acid, it will dissolve as much as possible for the temperature you are operating at. One caveat: if the temperature drops, the now insoluble boric acid will precipitate out and if not picked up by the filter will cause roughness.

One last thought, I hope you are running continuous filtration with a turn over rate (real time) of a minimum of 2.

A. Dear Danish,

- The advised optimum temperature for acid zinc potassium electroplating bath is 22 °C. Over 28 °C there are (commonly) problems of over-consumption of brighteners, under 15 °C there are problems to level, low conductivity and low solubility of potassium chloride and boric acid (addition of these may be troublesome).

For 30 kg of material, 500 lt of bath seems good. If you have some small refrigeration equipment you may have no problems.

Best regards!

A. Hi Dharmendra. It is normal for the pH to rise, and it is expected that you will need to add acid to maintain it. The plating is not 100% efficient, but releases some amount of hydrogen gassing at the cathode. When hydrogen gas is released from water, it leaves OH- in solution, so the pH rises.

Regards,

A. Titrate the chloride via the silver nitrate method with chromate indicator. Determine the zinc content via EDTA titration. Calculate the ZnCl2 concentration by simple math...then get to the KCl concentration by subtraction. Helpful hint: run that KCl as high as you can get away with. (limitation, the KCl solubility) It helps improve the "throw" of the bath.

A. Hi Naval. As you see, we appended your inquiry to a thread where it is answered. If you have additional questions, please continue the discussion. But please tell us who you are and your situation (rack, barrel, or continuous plating; approx. tank size; what method you've used to date, etc.) so people can more immediately help you rather than having to concern themselves with all of the possibilities raised by abstract questions. Thanks!

Regards,

A. Dear mohd solehin,

According to your plant working period, you must refine the solution after max. 3 months by adding Hydrogen peroxide H2O2 0.1-0.5% of volume of tank, and give 48 hours for settling dissolved solids.

A. Dear Louis,

You said zinc chloride composition is 28 g/L. It's very low composition. Your zinc anode, you might notice, will highly melt down or transfer high current, and solution temperature is also going to increase. This is causing foaming in solution. You should enhance composition of zinc chloride up to 60 g/l and boric acid composition up to 20 g/L. Your Foaming problem will be solved and finishing of product enhanced as well. Also maintain the composition of brighteners according to such composition.

Warm Regards

A. You might consider having a lab do IR spectroscopy on the mystery tar.

A. I would investigate possible gross overdosing with organic additive (brightener)

A. Hi,
I think it is too much of breakdown products from the the additive in the acid zinc and following rinses.

I suggest to do a analysis of these breakdown products.

Regards,