Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

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Ductile Iron Zinc Plating

Our shop is trying to switch from a malleable to a ductile iron base metal and then zinc plate. When the ductile iron parts are plated using the same chemistry we use to plate the malleable parts the recessed crevices have a black soot that I suspect is graphite. Any ideas on how to address this. I think a more aggressive electroclean will solve part of the problem, but do I need to change anything on the HCl acid etch/pickle?

David A. McWilliams
- Columbus, Nebraska, USA

You didn't mention whether you use acid zinc or alkaline zinc. The bad news is that alkaline zinc is probably unsuitable for these parts. It has to do with a fairly complex phenomenon called 'hydrogen overvoltage', but in brief, zinc will not deposit on carbon-rich areas.

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

We had the same problem. I switched the pickle to 5% sulfuric acid, and the problem was solved.

Michael Brewington
- Salisbury, Maryland, USA

Our shop uses an acid zinc process. Ammonium Chloride Zinc bath.

David A. McWilliams
- Columbus, Nebraska, USA

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