Why is my plumbing work all corroded and rusty???

Five years ago, I plumbed in a new 3/4" copper supply line for my well water. I 'thought' I did a pretty good job, sweating in all the fittings. It looked beautiful at the time. But now it looks like crap (see pic below). There is corrosion at all my sweat joints, and it's even rusting at the hanger too, jeesh. This area of my basement is pretty dry, with my hot-air furnace very close, but the basement is big and I do have two sump pumps collecting water, so there is humidity in the air.

I have 4 questions:
1) What's causing all this corrosion?
2) Is there any danger of leakage?
3) How can I prevent this in future work?
4) Can I fix this?

Many thanks, Dave F.