It is the galvanizing and any loose weld that is the problem. When you get a new aviary or new wire you have to either let it age or scrub it with vinegar solution to remove the galvanizing. Seeing you have bought a second hand aviary I would just give it a scrub down with vinegar solution at 10% dilution rate, that is 1 part vinegar to 9 parts water and use a strong wire brush, making sure to remove any loose bits of weld. Scrub the rust and you could use a rust killer/converter to treat that. The rust is more dangerous than the oxidized wire. If you wanted to you could paint the wire using a water based paint. Either dark green (Colorbond Cottage Green) or black and let it sit for a couple of weeks for the paint to hard dry. Those two colours will highlight the birds better and blend in with the surroundings. Most painted aviarys are in the green.