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Machine shop inadvertently copper plates steel bolts

While cleansing a copper manifold in hydrochloric acid, I decided to throw in a few stainless steel and mild steel bolts that had accumulated some rust. When I pulled them out, they were all copper coated? Why?

Mike Shilling
machine shop - Fort Walton Beach, FL, USA
2003

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2003

What happened is called immersion plating, and what you saw is similar to what high school chemistry students see when they do the experiment of putting steel nails into a copper sulphate [on eBay or Amazon]solution.

Hydrochloric acid will dissolve copper (more accurately it will dissolve copper oxide tarnishes). This leaves copper ions dissolved in the acid. The acid will also dissolve iron, and as soon as iron ions are available to go into solution, copper ions will come out of solution as copper metal. Copper is more 'noble' than iron; given the opportunity the copper ions will always force an iron ion into solution to replace them. As soon as the iron is covered with a very thin layer of copper there is no more exposed iron and the reaction is over.

There are a few limited instances where immersion platings are useful, but they are too thin and non-adherent for general purpose applications.

I was told that long ago scrap autos were placed in streams to remove copper contamination from the water by this method.

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