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Zinc plating is coming off as "dust". Why?

May 21, 2014

Q. Hi,
I am the production manager of a machine shop.
Six months ago we made some items from two different grades of similar steel (mild steel / structural steel used in most machine shops); the parts were machined all over, washed and sent to be zinc plated.
They came back looking okay; we coated them in hydraulic oil and wrapped them in plastic.

Now, six months later, we unwrap the plastic and the zinc is coming off like fine dust and exposing the metal underneath on both parts. If you run your hands over the item they will have tiny silver specks over them. It is not even over the part, but is only on certain faces and patchy. I can't see any blisters or discolouration, just silver particles and bare metal.
What could cause this?

Thanks
Richard
Australia

Richard Birks
Production manager - Australia