Help with Thin Chrome Deposit
In Sept. 96 our trivalent Cr. Bath started to leave a brown pattern in the high to medium current density areas. Sometimes accompanied with a black swirl pattern. The parts are hand dryer covers (found in rest rooms to dry hands). Each cover is 3.5 s.f. with 2 parts/rack. The covers are buffed and polished, then vapor degreased. The plating process is as follows:
Electro cleaned for 30 seconds followed by acid salt etch.
Then plated in Copper Cyanide strike for 5 minutes.
Then a min. of 0.2 mils acid copper followed by a min. 1.2 mils of nickel.
Then a trivalent chrome plate in the 40-50 ASF range for 3-5 minutes
Each bath is followed by a counter flow rinse.
We have tried the following to solve this problem:
1. Lower pH to 1.8 and then raised it back to 3.5
2. Both low current and high current dummying.
3. Cleaned lead anodes and replaced ionic membranes
4. Daily H2O2 treatments
5. 2 lb. carbon filter change every 2 days
6. Alkaline rinse before chrome
7. New anolyte solution in anode boxes
8. Peroxide treated nickel bath and nickel rinse
9. Checked tank and rectifier for stray current and ripple
What has given some favorable results is when I put one cover on a rack or put 2 covers on a rack, plating it for 2 minutes then break contact for 5-10 seconds and plate again for 2-3 more minutes. Both these have given me limited success.
This problem also occurs on parts with less square footage.
Bath analysis: Chrome 1.0 OZ/GAL Boric 12.6 OZ/GAL Specific Gravity 1.15 Baume 31.1 Carrier 1.0% pH 3.5
ANY INPUT WOULD BE APPRECIATED.Craig McCarl
- - Excel Dryer Inc.
Lived in the next town over from you in the 70's and have used your product innumerable times.
To start with, I am a hex chrome plater not decorative or trivalent, but would like to take a guess.
The swirl pattern would lead me to question the buffing. New operator, or new lot of polish, new wheel, different dispenser. Something is probably different here and it is so subtle that it is overlooked.
How about the vapor degreaser? Is it going acid? Have you changed vendor or type solution?
Coming out of the vapor degreaser and cooled, will it water break before you caustic clean? After caustic clean?
I would run a test panel with no polish to see if you got the black. If that is bad, I would put about 30 seconds nickel on the next panel and see if it was black. If that failed, I would caustic clean, acid etch and go directly into the chrome skipping copper and nickel. Somewhere, you should be able to isolate the problem tank or area.
Finally forget everything you know and walk the line backwards checking every item. No joke, it sometimes works when all else fails.
Good luck, Jim.James Watts
- Navarre, Florida
Brown stains used to be caused by metallic impurities like iron, nickel, copper, zinc. Has the bath been analyzed for these lately?
There is a purifier that gloms up all of these impurities, I believe Harshaw had the patent. Do you use purifier?
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