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ZN CYANIDE BATH PROBLEM

 

I'M A PLATING SUPERVISOR OF A SMALL PLATING SHOP. WE'RE HAVING MULTIPLE PROBLEMS IN OUR ZINC CYANIDE BATH WHICH ARE AS FOLLOWS: EXCESSIVE SLUDGE BUILD UP, HULL CELL PANELS SHOWING IRIDITE COLORS, PLATED PARTS LOOKING GREY BLUE, CHEMICAL ADDS GOING IN HARDER THAN NORMAL, CHEMICAL ADDS NOT STAYING IN SOLUTION, BUT PRECIPITATING TO THE BOTTOM.ALSO PLATED PARTS AFTER CHROMATE SHOWS BRIGHT GREEN,BLUE, PURPLE AND PINKS. ALSO I'm UNABLE TO COMPLETE AN ANALYSIS ON THE ZN METAL (SAMPLE TURNS GREEN) BATH ALSO LOOKS YELLOW INSTEAD OF THE NORMAL YELLOW BROWNISH COLOR. CURRENT DOESN'T STAY CONSISTENT, IT DROPS MORE RAPIDLY THAN NORMAL WHEN RUNNING PARTS. SAMPLE COUPONS ARE BROWNISH IN COLOR AFTER DRYING.WE'RE ATTEMPTING TO DUMMY OUT THE BATH, FREEZING A 1 GALLON SAMPLE, SENT OUT A SAMPLE FOR AN ATOMIC ABSORBTION [on eBay or Amazon] TEST AND SENT A SLUDGE SAMPLE FOR ANALYSIS. WE DILUTED A SAMPLE BY 25% AND RERAN OUR ANALYSIS BUT WERE STILL UNABLE TO COMPLETE A ZN METAL ANALYSIS. THE HULL CELL TEST DID COME OUT CLEARER AND WE HAVEN'T SEEN THE SETTLING OF THE CHEMICALS AS IN THE ORIGINAL SAMPLES.

CYNTHIA C [last name deleted for privacy by Editor]
- SARATOGA SPRINGS, NEW YORK U.S.A.

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Are you not adding a proprietary brightener? If you are buying through a distributor, tell him to contact the proprietary company. They EXPECT to be service intensive.Why you cannot run the zinc analysis is puzzling. Very simple. Maybe reagents got contaminated.

1. 2 ml sample of zinc bath.
2. Add 30 mls distilled water.
3. Add 10 ml concentrated ammonium hydroxide.
4. add a pinch of Eriochrome Black T [affil links] (so you can read a newspaper article through it, too dark, you won't see color change).
5. add 10 ml of 8 or 10 % formaldehyde.
6. Titrate with 0.0575 M E.D.T.A. solution to blue endpoint.
7. Mls titrated x .25 = oz/gal

A cyanide zinc bath does sludge. You also cannot just "dump" the cyanide and caustic adds into the bath. You should not be using zinc cyanide powder for adds. This won't dissolve into the bath. Has to be dissolved off-line with caustic soda [affil links] (nasty, heat generated). Sodium carbonates build-up, creates excess heat because rectifier has to be cranked way up, still won't cover in the low CD. There is a ratio of zinc metal to cyanide that is critical. Caustic soda concentration is critical, and temperature. It all works together.

As I said you better find a knowledgeable zinc brightener vendor. Many vendors support this site. Why not consider non-cyanide zinc plating? Very few platers in the USA have cyanide zinc in their shops.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan

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To restate your description of problems:
1. Deposits are grey/blue in color
2. Solution is yellow in color
3. Additions are hard to dissolve
4. Having trouble analyzing for zinc metal
5. Current fluctuations
6. Iridescence after chromating.

Now have I got the symptoms right?

The information you have provided is somewhat sketchy. First, I will give the time honored answer: Call your supplier (isn't it always the brightener that's at fault). Second, are the grey blue deposits bright or dull, smooth or rough. Is this condition all over the part or localized in a particular current density? Additions of what are hard to dissolve? You do not describe the method you are using for your zinc metal analysis but if it is the traditional one using Eriochrome Black T [affil links] indicator, I suggest that you shine a light through the bottom of the flask when you are titrating, the color should go from a reddish something (I am the color blind chemist) to one that is lacking in reddish tones. It is important to titrate immediately after adding all chemicals to the flask. If this still does not work, double your sample size and divide you final answer by 50%. This is typically effective if you have low metal. The color of the solution does not have much to do with how it plates. A yellow cast is frequent, dark brown is not usual. Are you using a sulfide type purifier? Use of this type of material will cause some sludge to form in the bottom of the tank. If this is a barrel plating operation greater quantities of carbonates are prone to form.

All of the above said, I have a gut feeling that you are high in carbonates. You did not state what the result was after you tried freezing out some of the solution, but if sludge did form and the hull cell was improved, then it is probably true. Are you analyzing the bath? If so, what is the analysis.

I apologize for the rambling answer, but this frequently happens when I'm "under the weather".

If you can give some of the details I have mentioned perhaps I or someone else would be better able to help you.

Gene Packman
process supplier - Great Neck, New York