i have some wire from a home made extension cord. it is 16 awg i cut a bad section out went to strip a new piece and the outside of both coper wires is black should i just scrap it.

 

Either corrosion or the insulation has started to melt/degrade.. I'd scrap it as it costs pennies to replace.

 

don't relay on corrosion-ed wire. you better buy a new one

 

ok thanks thats kind of what i thought. i was wondering if that was because it got overloaded some point and started smoldering or somthing like that. i will probubly just scrap it and use it for my 1/2 amp solar panel.

 

This is pretty common with older stranded cable (like 16 ga. lamp cord) -- the copper oxidizes to a dark brown. It's best to replace it if you can because it will lead to high resistance joints. However, sometimes you have to make new connections with such old wire and it's worth knowing how -- one way is to use some sandpaper to get rid of the oxide. I usually have lots of 250-400 grit silicon carbide paper around the shop, so that's what I use. Spread the individual wires out and sand, then rotate them and sand again. You want to expose bright copper on as many strands as possible. Be gentle, as it's also easy to break some of the strands. I then try to solder to the wire rather than use a compression joint like a solderless terminal or putting the wire under a screw. If the wire was bare and you knew you could completely clean and neutralize things, using some acid is also a possibility. However, for insulated wire, I would never use acid (or an acid-core solder) because it's likely some of the acid would get under the wire's insulation and cause corrosion down the road.