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The house is a ranch built on slab on grade with a newer furnace in a downflow configuration with A/C. The in-floor clay tile supply lines are in good shape and clean.

The original 60 year old in-floor supply plenum (30in x 22in) shows corrosion. The plenum is directly below the A-Coil & subject to condensate.

I have wire brushed the surface rust and treated it with The Must for Rust. I am looking at various cold galvanizing compounds (GRC/Rustoleum) to apply to the bare metal to restore the zinc protection. Access to the plenum (which is much larger than the 14in x 19in cabinet housing the A-Coil) is tight so this will be a challenge.

My concern is that since there are mixed metals in close proximity, a zinc coating may be important to protect the noble metal of the A-Coil by sacrificing itself to the eventual rusting process. Or does this even apply given that plenums are now sometimes fabricated with duct board?

I am wondering if I can just reline the bottom with a thin rubber mat cut to size that will conform itself to the metal isolating it from moisture from possible condensate from above?

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The zinc coating on the galvanized duct work is not in any way required to control corrosion of the A-coil. Isolating the unit from the duct work is commonly done in the trade to provide vibration isolation.

However, sandwiching corrosion under a sheet of rubber that is not fully adhered may not be a good idea. Moisture may become trapped between the two layers. Also, it is generally not acceptable to place flammable materials such as rubber inside of duct work. You would likely be better off painting or cold galvanizing the duct after applying your phosphotizing chemical. If the condensate drain on the new fan-coil is correctly installed and kept clear, very little condensate should reach the duct.

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