Don't know how bad the tarnish is, but your average HVAC guy would snap that up in a minute, depending on size. I might try a piece in a bath/wipe with CLR for a bit and see how it looks. I highly doubt you have lost any measurable alloy content
What size/shape is it?
A silver guy might give you 3-500$ for it, seems like you could sell it easily for $30-40/lb