trevormagee 
Come back to it in ten years when the steel has rusted off and the price of copper has doubled.
But as the others said: what is it worth as is?
Do the numbers: how much do you stand to gain by getting it clean?
If it is substantial, then it is worth giving it some thought...
I wonder if you drill down through the steel and pour acid in the holes, would there be a galvanic action that would encourage corrosion at the boundary between the metals? This should begin to create a layer of rust between the metals and maybe allow the acid to creep in by capillary action.
Possibly a silly idea, but it is late over this side.
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