It would eventually rust if it were bare pipe. You can spray paint the inside of it which should get most of the bare metal covered.

I'm not sure if a nut would work for that fitting. I think its pipe thread, so you'd have to go to Home Depot and get a pipe fitting that the sender can thread into, then weld that to your pipe. Make sure to get something long enough that the sender won't hit the other side of the pipe, its kinda long.