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Galvanic corrosion of building

I have a base metal sheet 1.2M x 2.5M Zinc plated and need to cover it with copper sheet finish. Both sheets Tek-screwed to metal frame. Concern is dissimilar metals corrosion as it is subject to weather and is part of a building. Cannot get away from using both materials in such a manner. Have thought of paint creating a insulator between the metals, but is there a better solution? it must last at least 40 years. The other concern is the fastens or screws used and their conductivity. Can anybody advise me of a simple solution.

Thank you.

B. Fuller
supplier of building product - Sydney, NSW, Australia
2004

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Well, I don't think it's going to last the forty years unless you eliminate the metal to metal conductive path, whether through the sheets touching each other or through the screws touching both. You have to insulate one of the two sheets from the other and from the screws, and paint will not be enough.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

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Thank you for your advice Ted, If paint will not do it would changing to a stainless 304 or 316 for the base plate and copper sheet cover be a solution ? If so what material fasteners? To explain further there is a number of these sheets (approx. 1000 square meters) which makes quite a large anode/cathode.This will be a covering to the entrance of a land bridge over a road tunnel entrance, as part of a civil contract, hence the 40 years. I would appreciate your thoughts.

Thanks & regards,

Basil Fuller
Fire Protection Building Products - Sydney, Australia
2004

I'm not competent to suggest materials of construction for the project, Basil. Sorry. All I can say is that you will get galvanic corrosion if metals which are widely separated from each other on the sea-water series are connected to each other by both an ionic path (wetness) and a metallic path (the two metals touching each other or connected to each other via conductive fasteners).

Zinc is very far from copper on the seawater series, but if the coating is thick enough (galvanizing rather than zinc plating), it's perhaps possible for some to be left after 40 years of sacrificial action. 304 and 316 stainless steel are much closer to copper in activity, and can actually be either noble or base with respect to copper, depending on the exact conditions. I think this will work, but you need input from an architect who has done this or a project that is somewhat similar, because it's very easy to miss something on the one hand or to overreact on the other if it's just book knowledge like mine :-)

My only actual experience on a vaguely similar project was on the rebuilding of Philadelphia City Hall tower. This was made of iron and steel, probably lightly zinc plated (although apparently it was claimed to be aluminum plating when it was built), then very heavily copper plated, and in place for 100 years. The copper lasted with no problem but the underlying iron was so badly corroded that almost all of it had to be replaced.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

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Ted, Thank you for your response. Would an alternative be to change the base plate material from Plated steel-to- full stainless steel 304 or 316. How good will that compatibility be with copper sitting directly on it? Also what fasteners would the recommendation be.To put you in the picture. This is a civil government project we wish to supply, and is for the entrance facad of a road tunnel covering some 1000 square meters which must be fire proof. the fire proof structural board (base plate)we are supplying is steel faced. Thank you once again.

Regards,

Basil Fuller
Fire Protection Building Products - Sydney, NSW, Australia
2004

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2004

Dear Sir,

I am appreciated if you could answer my question as follow:

What the problems in direct contact between copper pipe and stainless steel pipe bracket? Is Electrolyze occurred? Disturbance and speed up corrosion to copper or stainless steel?

Thanks a lot,

Jonathan Chan
Building Maintenance - Hong Kong, China