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Zinc plating problem

2005

Q. Hello!
I am a metallurgy engineer. I have a cyanide zinc electroplating industry.my process stages is:
oil cleaning-acid - washing (sulfuric acid) - cold rinse - zinc bath - cold rinse - chromating bath (blue-yellow-green-black). My bath brightener I buy from Canning company.

My bath composition: zinc cyanide-sodium cyanide-NAOH.the anode is zinc(99.99%)
1. why are the anodes color changed to iridescent?
2. the problem is: my specimens are very good after cyanide zinc bath but in the rinse stage the pieces are dark brown and the brightening is lost -- what is reason?
I tried a dip of 1% HNO3 after zinc plating and before chromating, with rinse before and after, but the problem isn't solved.

Hamed Khakzad
- Tehran

A. Please Check your bath composition. It should be ZN:CN:NaOH = 1:2.7:2 (i.e., 30:85:65).

Since the articles are becoming dark after rinsing/bright dip there may be copper contamination or very thin zinc deposit.

K.Ramesh
- Mumbai, India
2005

A. Hamed
Please analyse the bath for Cyanide. It seems to be on the higher side as you anodes are shining.
Your bath may be contaminated with metallic impurities like copper. Inspect the anode hooks if it is getting immersed in the bath.
Better if you call the tech service personal from the supply house and get proper guidelines set to run the bath.

T.K. Mohan
plating process supplier - Mumbai, India
2005

A. If after making up a new nitric dip the deposit darkens ex the nitric it is most likely copper contamination in your zinc bath. Plate out or treat with zinc dust [on eBay or Amazon] . Possibly worth adding polysulfide in case other heavy metal. If it darkens in the chromate then contamination is in the dip.

Geoffrey Whitelaw
- Port Melbourne, Australia
2005

A. There might be impurities of copper or cad in your rinse waters. Try rinsing with clean water from the tap on one article and see if that makes a difference. Your local water supply might have heavy metals in the water and could cause heavy metals to enter your rinse or in your tank while raising levels.

Gary Joseph
Johannesburg,
South Africa
2005

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