Anything free is good obviously but I would not pay for copper as it corrodes very quickly.
That's weird - copper is one of the metals that occasionally shows up as metal amongst ore deposits millions of years old. Any soil you have that's acid enough to etch it would grow virtually nothing.
There's always black/brown oxide (or green carbonate) on the surface but it is very shallow - that's why it's used for roofing and can last centuries - at least until acid rain appeared.
It is a form of corrosion without damage... on the other hand iron/steel rebar will last a couple of decades OK but it will corrode to flakes fairly soon after (unless you use stainless steel).
On the same subject, you may notice that you sometimes get supermarket food sold in aluminium trays, and they seem sturdy enough to use under plants etc, but aluminium is notorious for corroding into 'pits' - within a season they start to leak like sieves. That's why they use zinc for 'silvery' roofs instead...
Cheers.
