Copper Tubing Pinholes
Pinhole leaks with pitting initiating on the exterior surface of the pipe can occur if copper piping is improperly grounded or bonded. The phenomenon is known technically as stray current corrosion or electrolytic pitting. Pin-holing due to poor grounding or poor bonding occurs typically in homes where the original plumbing has been modified; homeowners may find that a new plastic water filtration device or plastic repair union has interrupted the water pipe’s electrical continuity to ground, when they start seeing pinhole water leaks after a recent install. Damage occurs rapidly, usually becoming obvious about six months after the ground interruption. Correctly installed plumbing appliances will have a copper bonding jumper cable connecting the interrupted pipe sections. Pinhole leaks from stray current corrosion can result in high plumbing bills and require the replacement of the entire water line. The cause is fundamentally an electrical defect, not a plumbing defect; once the plumbing damage is repaired, an electrician should promptly be consulted to evaluate the grounding and bonding of the entire plumbing and electrical systems.
The difference between a ground and a bond is subtle. See Ground, for a complete description.
Stray current corrosion occurs because: 1) the piping system has been connected accidentally or intentionally to a DC voltage source; 2) the piping does not have metal-to-metal electrical continuity throughout its length; or 3) if the voltage source is AC, one or more naturally occurring minerals coating the pipe interior may act as a rectifier, converting AC current to DC. The DC voltage forces the water within the piping to act as an electrical conductor (an electrolyte). Electric current leaves the copper pipe, moves through the water across the nonconductive section (a plastic filter housing, for example), and reenters the pipe on the opposite side. Pitting occurs at the electrically negative side (the cathode), which may happen to be either upstream or downstream with respect to the water flow direction. Pitting occurs because the electrical voltage ionizes the pipe’s interior copper metal, which reacts chemically with dissolved minerals in the water, creating copper salts; these copper salts are soluble in water and wash away. Microscopic pits eventually grow and consolidate to form pin holes. When one is discovered, there are almost certainly more that have not yet leaked. A complete discussion of stray current corrosion can be found in chapter 11, section 11.4.3, of Handbook of Corrosion Engineering, by Pierre Roberge. Copper Tubing Pinholes
Detecting and eliminating poor bonding is relatively straightforward. Detection is accomplished using a simple DC voltmeter, with test probe leads placed in various locations in the plumbing. Typically, a probe on a hot pipe and a probe on a cold pipe will tell the user if there is improper grounding. Anything beyond a few millivolts is significant, and potentials of 200 mV are common. A missing bond will show up best in the area of the gap, as the measured electrical potential dissipates over distance. The missing bond is usually located near the cold water inlet to the building, as filtration and treatment equipment are usually added there, but pinhole leaks can occur anywhere downstream or upstream from the interruption of electrical continuity. Copper Tubing Pinholes
Correcting the problem is a simple matter of either purchasing a copper bonding jumper kit, composed of copper cable at least #6 AWG in diameter and two bronze ground clamps for affixing it the plumbing. See NFPA 70, the U.S. National Electrical Code Handbook (NEC), section on bonding and ground for details on selecting the correct bonding conductor wire size. Copper Tubing Pinholes
A similar bonding jumper wire can also be seen crossing gas meters, but for a different reason.
However, if building occupants are experiencing shocks or large sparks from plumbing fixtures or pipes, it is more serious than a missing bond. Larger voltages may be caused by a live electrical wire bridging to the plumbing, and improper or missing plumbing system grounding. Such a situation poses an electrical shock hazard and potential fire danger; an electrician should be consulted immediately.