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topic 61226

Acid zinc barrel plating is dark grey and black

March 22, 2021

Q. I have started 5 new acid zinc barrel in one of these barrels I got dull grey finishing I have tried every thing. I do chemical analysis but all things are on the mark my analysis report shows: zinc - 35 g/l, Chloride -145 g/l, boric - 32 g/l, pH -4.8.

acid zinc plating is black

Please let know what is this problem when I performed Hull Cell it forms black layer on high current density.

ABHISHEK KALIA
- Ludhiana Punjab
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March 2021

A. Hi Abhishek. Just so we don't misunderstand your situation, that looks to me like a brass Hull Cell test panel with no plating at all at extreme low CD end, normal zinc plating at low current density, dark black plating in the midrange, and grey-black, gritty, burnt plating at the high CD end?

There is obviously something in the plating solution which should not be there. My guess is that it's organic (a brightener or other addition agent) rather than a metal. But that's only a guess. It looks like the brightener is not merely blocking zinc deposition at higher CD, but is being electrolytically converted and deposited on the surface.

Is the plating solution purchased from a major supplier who would have analytical equipment your shop would not have and would be able to help analyze this? Hopefully another reader has actually seen a similar situation.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey
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March 25, 2021

thumbs up sign Hey sir, I got solution of my problem. As you mentioned it was organic chemical imbalance. So I treated my solution first with potassium permanganate and then treated with activated carbon, and my problem is gone and I got clean and shining Hull Cell panel. Thanks for your help and suggestions sir.

ABHISHEK KALIA [returning]
- Ludhiana Punjab
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