How can you repair a pin hole in a copper pipe?
Cut the pipe, clean it, and sweat in a coupler.
They make clamps with rubber seals in them to fix this, but it is only a band-aid solution. Use the clamp only until you can fix it correctly.
Fixing the pin hole is nothing; the cause is something else.
Copper pipe corrosion can be caused by many things and many people have had the electric power company, every electrician, water treatment company and plumber they could find try to solve their problem with no luck. It is not caused by defective pipes. The fact that you indicate that you indicate that you are using your home every few weeks may actually be aggravating the problem by not having a regular flushing of the pipes.
The known causes are: 1.) Water condition - PH, mineral content or lack of, dissolve gases, microorganisms etc. 2.) Excessive flux used in the soldering of the pipes. 3.) Water velocity in the pipes - Copper gets a thin green coating this actually inhibits further corrosion. If the velocity of the water is too high due to undersized pipes being used the green coating may be eroded causing further corrosion. 4.) Electrolysis - you are correct in assuming that the stray millivolts may have something to do with your problem. Stray voltages may be from improper grounding, of electrical, cable tv, phone. Or, leakages of current from water heaters, circulator pumps, well water pumps, boilers etc. 5.) dissimilar metals used on the piping or in contact with piping can also be a problem we all have seen where a lazy hack has used steel strap to hang a copper pipe or used a black iron nipple because he was to lazy to head for the supply house when he was 1 short for the job.
Treatments out there include: Sacrificial Anodes for underground copper pipes. Epoxy coatings inside the pipe (I'm a skeptic on that one). Finding the right water treatment for your water (Do have it tested). Finding something electrically to cure your problem Repiping the house with a pipe such as Pex tubing.
So the wife calls me this afternoon at work and says there's a water spot on the ceiling downstairs in my split foyer house. After work, I come home and knock a hole in the ceiling, and find that a 1/2" copper pipe supplying cold water to the upstairs has cracked on the inside of an elbow joint, and the hot water line is damp and sticky but not dripping yet. I Google, and decide to head to Lowe's to see whether I can use epoxy putty, or I should replace the elbow joints entirely.
I get to Lowe's, open an epoxy putty tube and read the documentation. It claims to result in a sandable, drillable, hard-as-steel patch, but warns against using it on pressurized lines. Sounds dubious. The plumbing hardware specialist agrees with me. It might be an acceptable risk if I were patching a pipe, say, in the garage, where the pipe is exposed and I can see whether further repair is needed in a year or two.
I tell her I'd rather fix it right if it's going to be covered by drywall again, but I'm hesitant to stick a butane torch between the ceiling and floor. After all, setting my house on fire somewhat defeats the purpose of fixing a leaking pipe.
So she suggests that I try a new gizmo they recently started stocking, called a GatorBITE. She says professional plumbers rave about it. It's basically a copper elbow (I guess they have straight couplers as well, but I didn't really look), that has teeth inside that allow copper pipe to be crammed in, but not slip back out without some sort of removal tool. The GatorBITE was somewhere between 6 and 7 bucks. I got two + a pipe cutter for 20 bucks with tax. No solder and no torch needed.
I bring them home, spend about 10 minutes contorting myself and cut out the damaged joint in the cold water line. The next 60 seconds are devoted to cramming the ends of the pipe into this GatorBITE. It's mind-numbingly easy -- too easy. I tell my wife there's no way this'll work.
However, I turn the main back on, all the fawcets in the house are already on full blast, yet the seams around the GatorBITE stay dry as a bone. I'mma install the second one on the hot water line in a couple of days after I'm convinced the first was not too good to be true. I'll leave the ceiling open for a month or two with a bucket underneath and keep checking for drips. From what I've seen tonight, though, the GatorBITE is a viable solution.