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Product of Non-Cyanide Alkaline Zinc Plating

2003

Quickly, my question is:
What material is plated from a non-cyanide alkaline zinc bath (caustic soda [affil links] ) - pure Zn metal, ZnO, or ZnO2?

Brief Background Info:
Solution is high pH (14+), medium concentration of NaOH (60-100 g/L), and high chemistry (9-20 g/L Zn).

Long-winded Background Info:
I have been making the solutions with ZnO and not Zn powder because I cannot get the Zn to dissolve in even 50% caustic at temps exceeding 150C.

I am running experiments on control solutions in a 1L Kocour Hull cell in order to establish a baseline (for comparison with other solutions with additives, etc...basically building a database). Both electrodes are low carbon steel, and I have kept the bath temperature around 27-30C. I am running 3A for 45-50 minutes.

There has been some debate (inter-company) about the product of alkaline zinc plating without the presence of cyanide. I scraped and ground the plated product then tested it empirically by dropping H2SO4 on it. The powder sizzled and bubbled tremendously and dried out fairly quickly, which supports the theory that this is actually pure Zn.

I have ordered the Zinc Plating text from AESF but it is (and has been for almost 2 weeks) out of stock. Please help me. I am stuck. Thank you very much for any guidance, reference, or assistance.

Sincerely

Elizabeth Sendil
- Pacifica, California, USA

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I would say it is pure zinc too. Once the zinc is dissolved into the caustic I don't think it much matters whether it was zinc metal or zinc oxide [affil links] before it became dissolved.

Zinc can be plated out of alkaline non-cyanide solutions; in fact this is probably the fastest growing zinc plating method.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

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2003

Elizabeth,

The deposit you obtained is is Zinc. The solution you used is a zincate bath.

Zinc metal will dissolve in caustic soda [affil links] if it is contained in a mild steel basket . Or Mild steel wire is wrapped around the zinc piece at the lab. Then immersed in caustic soda . Warm solution temperature 35 to 40 °C helps, so does some stirring.

This sets up an electrochemical cell ( Between the zinc and steel ) which results in dissolution of zinc in caustic thereby creating a zincate solution.

Possibly for Environmental reasons you are evaluating Alkaline zincate Cyanide-Free baths, and this is indeed the right direction.

Asif Nurie [deceased]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016

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Thank you both very much for your responses. We did an "empirical" test of the plated material based on the significant difference in melting point between zinc and zinc oxide. The substance still had not melted by 1500F when the control of pure zinc metal had melted at the expected 787F. This would indicated that the plated material is not zinc but zinc oxide. Any thoughts?

Elizabeth Sendil
- Pacifica, California, USA
2003

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Interesting observation, Elizabeth. Sorry but I don't see, in the usual books like Safranek's "Properties of Electrodeposited Metals and Alloys".

and Jack Dini's "Electrodeposition -- The Materials Science of Coatings and Substrates" .

any observations about melting points.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003