Pinhole leaks in copper pipes are caused by corrosion that starts on the inside wall and eats outward at one small spot. The usual drivers are water chemistry, water moving too fast through the pipe, and installation faults such as leftover flux or rough cut edges. Most leaks come from several of these working together.
Water chemistry comes first. The Copper Development Association’s Copper Tube Handbook names two opposite problems: aggressive, hard well water that causes pitting, and soft, acidic water that never lets a protective film form inside the tube. Dissolved oxygen, carbon dioxide and disinfectant levels shift that balance, which is why pinholes can cluster in one neighborhood or one water system.
Speed is the second cause. The same handbook says flow should not exceed 8 feet per second in cold lines and 5 in hot lines up to about 140°F, to avoid erosion corrosion. Undersized pipe, an oversized recirculating pump or a run with many sharp turns pushes water past that, and the wall wears thin just past elbows and tees, most often on hot lines.
Workmanship adds the rest. Excess flux left inside a soldered joint keeps attacking the copper, and a cut end that was never reamed leaves a burr that churns the water and wears a hole downstream of it. Under a concrete floor, the same pinhole is how many slab leaks begin, which is why a slab leak is serious even when it starts small.
The exact cause of one leak is often hard to prove, because these factors overlap, and a single pinhole can mean the same conditions are working on the rest of the line. Locating every weak spot starts with water leak detection rather than patching the one you can see.