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Copper plating an historic cast iron bridge
We are currently involved in a project that is a small bridge dating from the 1890's that has decorative cast iron railing elements. The cast iron appears to have originally been electroplated with copper. The majority of the copper electroplating appears to be missing or has failed.
We are looking for suggestions for replicating the finish so that it once again will have the appearance of a copper finish. Is electroplating cast iron with an intermediate plate and then copper viable for an exterior application? What standards can be referenced for electroplating the cast iron with an intermediate coat and then copper? What would the typical surface preparation (chemical or mechanical) be for electroplating cast iron?
Conservation - New York, New York
Sounds like a cool bridge.
I was involved with the restoration of Philadelphia City Hall aka the William Penn Tower that you see on Philadelphia newscasts, and TV shows like Cold Case. It ended up being tin-zinc plated and then painted, but when built in about your same time frame, it originally had copper plating on cast iron and then zinc plating (which apparently was claimed to be aluminum plating).
The copper plating, probably deposited from an acid copper (copper sulphate) process, had zero adhesion when we looked at it; but it may have had no adhesion from day one. It was an exceptionally heavy copper plating, of uneven thickness but approximately half the thickness of a penny and was probably held in place by encapsulation or a "shrink wrap" effect rather than adhesion.
For restorations of this sort you need to do some trial runs and experimentation before writing specs -- there's no other way -- but my feeling is that if your bridge railings were to be plated with this exceptionally heavy copper plating (these days it would be better to start with a cyanide copper strike before the acid copper plating) that it would be rather like facing a building with copper sheeting and it should last decades if periodically maintained (it would be essential that if the copper received mechanical damage that it be repaired quickly because the cast iron will corrode from galvanic forces if the coating is breached). Feel free to follow up if you wish.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Try Conservation OnLine website-there You can find very good forum for conservation professionals (conservationDist list).Good luck and hope it helps!
- Zagreb, Croatia