Copper Wire Corrosion at Outlet?

Am fixing up basement rooms, and had reason to remove a double gang electrical outlet.  Removed wires from the terminals on the outlets and found a bunch of the green copper corrosion on the terminal screws and wires.  These are solid copper #12 wires and standard NON-aluminum outlets.  Wires were not loose on the terminal screws, all the wires were screwed down TIGHT.  I am going to replace both outlets in the box, but this corrosion issue has me wondering what the cause was.    

Any input would be welcomed.

In my experience, the green has been copper oxide and it comes from moisture.

If this receptacle was mounted against the block wall of the basement, or if there has been a history of humidity or water problems in the basement, that could have formed the copper oxide.

If this is a possible recurring problem, consider seeking out the new "Weather Resistant" receptacles.  Their metallurgy is improved and designed to resist moisture incursions.  For the interior of a normal dwelling, this is an optional requirement above the minimum of the National Electrical Code.

Also remember, when replacing existing receptacles, if the old one was a plain old one, the replacement must now be Tamper Resistant.

Condensation will do that every time. If I were you I'd look and see where the moisture could be coming from and fix that issue before replacing any electrical parts. They may be new and improved, but when it comes to water and electrical wiring, why take a chance?  

I personally think it has to do with moisture too. Fix a screen around the electric socket to 'block' out any condensation and have it drained elsewhere to prevent it from damaging the wiring. Try that for a few days and observe on the screen for any water droplets or residue presence.