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Acid Chloride Zinc
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We have made a few changes to our acid chloride zinc rack plating line. Increased our choride to 20 oz/gal from 17 oz/gal. Increased our brightener additions from 1 gal every 25,000 amp hours to 1 gal every 8,000 amp hours based off of hull cell testing. Lowered our pH from 5.0 to 4.5 with diluted hydrochloric acid. Increased our bath temperature from 60 deg. F to 80 deg. F. Added more uniform air agitation. Our boric acid level is 3.3 oz/gal. Our zinc has increased from 5.6 oz/gal to 6.6 oz/gal in the last 7 months. Iron usually runs between 20 - 35ppm. However, our parts look very good in apperance with no rejects since all of our changes. We conduct "non-air agitated" Hull Cells daily.
My question: Initially we noticed a very "dark" brown material across the surface of our bath - this stuff is extremely sticky - what could it be? Also, just recently the "dark" brown material seems to have changed to a lighter brown material.
mfg - Robinson, Illinois, USA
First of two simultaneous responses -- ++++++
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-soluablize. 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.
tech svc. w/ chemical supplier - Grand Rapids, Michigan
Second of two simultaneous responses -- ++++++
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,
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Asif Nurie Atlanta Global  New Delhi, India |
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First of two simultaneous responses -- ++++++
Thanks for the comments - Our supplier has been notified - they always stress "consistency" and keeping the bath in "balance." We have had some different employees in and out of our shop which might have contributed to what's happening. Bill made mention of the pH maybe lower than the 4.5. I would have to agree that this is a strong possiblity. We only add acid at the beginning of each day.
In regards to pH - Can you purchase a controller to constantly monitor the bath pH & automtically feed in acid to maintain a set pH? I've been told not to check pH while work is being plated - then I've been told it is ok?
Again, thanks for your comments.
- Robinson, Illinois, USA
Second of two simultaneous responses -- ++++++
The temperture of 80 degrees with cause "oiling out" in older std baths
you could also be overloading the wetter side of your brightener system
plater - Charlotte, North Carolina
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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 in is less that the drag out 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 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 enought 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 opg 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 (realtime) of a minimum of 2.
- Great Neck, NY