The thing homeowners get told, and why it is wrong
When a copper pipe pin-holes, the explanation offered is often that it was a bad bit of pipe. The National Physical Laboratory, whose guidance on corrosion in plumbing systems is the standard UK reference, puts it the other way round: defects in manufacture are very rare these days and nearly always the fault lies elsewhere. Copper has been used in plumbing for over 200 years and in the vast majority of cases it gives a long trouble-free life. When it does not, it is a combination of materials, design, installation and water quality.
That matters to you for one reason. If the cause was the pipe, one repair ends it. If the cause was the installation or the water, the repair is a patch and the next pinhole is already forming somewhere you have not looked.
What is actually happening inside the pipe
Fresh copper in aerated water corrodes slowly, releasing copper ions, and then the rate drops sharply over the first few weeks as protective films build on the bore: a thin brown or black oxide layer overlaid with a thicker green or turquoise basic copper carbonate. These are not true passive layers like the invisible film on stainless steel. They are a coating that can fail to form properly in the first place, or be disrupted later. Every form of pitting is a story about that film going wrong in one small spot while the rest of the pipe is fine.
One consequence is worth holding on to. Corrosion rate depends heavily on dissolved oxygen, and a sealed heating circuit has very little of it because the water is not renewed. Corrosion damage that occurs in a domestic hot and cold system is much less likely in a closed heating system. So a pinhole on a heating pipe is the less expected result, and it usually points to something specific: repeated topping up putting fresh oxygenated water in, flux left at installation, or contact with something on the outside of the pipe.
Flux attack, which is the one to suspect on recent work
Soldering flux works by stripping the oxide off the copper so the solder can wet it. Fluxes commonly contain ammonium or zinc chlorides, and those are aggressive to copper in their own right. Use too much and a run of flux is carried downstream of the joint, where it sits and eats.
The signature is specific enough to diagnose from the pipe once it is out. Pin-holing usually within a few months of installation, the hole a few centimetres downstream of a soldered fitting rather than at it, and high levels of chloride in the deposits around the pit. NPL call it what it is: the result of poor installation practice, avoidable by not using excessive flux and by flushing the system to BS EN 806-4 afterwards.
If your leak is on a bathroom or kitchen refit that is under two years old, and it is just past a solder joint, this is the first hypothesis. It also changes the advice: the same installer used the same technique on every joint in that refit.
Type 1 pitting, and why London is relevant
Type 1 pitting was traditionally seen in cold water pipes on hard borehole supplies where a carbon film residue from the extrusion process had been left in the bore, and only where the water had a pH between 7 and 8.2 with an unhelpful mix of inorganic constituents. BS EN 1057 in 1996 set strict limits on carbon residues after manufacture and the problem largely went away in the UK.
It has not entirely gone. NPL record similar pitting more recently in hard water areas in copper with little or no carbon residue, where analysis of the water and the deposits showed high levels of phosphate, which appears to play a role. Phosphate is in a lot of UK supply water because companies dose it to inhibit corrosion of old lead pipes. London is a hard water region: Thames Water describe all the water in their region as hard, because it passes through chalk and limestone, with hard water classified at 200 to 300 mg/l of calcium carbonate and very hard above that.
What Type 1 looks like: hemispherical pits capped with a blue-green mound of malachite, distributed randomly along a length of pipe, with the deeper pits nearly always close to the bottom of horizontal runs. That last detail is the useful one on site. If you are looking for the second pinhole, look along the underside of the same horizontal run.
Stagnation, which is a builder problem that becomes your problem
Both Type 1 pitting and the blue water phenomenon are made much more likely by water standing still in new pipework between installation and the system being brought into use. The mitigation NPL give is blunt: if there is a long period between filling and handover, flush the system at least once a week to bring in fresh aerated water and build a stable protective film.
In practice this is the empty flat, the renovation that stalled, and the new bathroom plumbed in October and used in March. Pinholes appearing in the first year or two of a property that sat empty are consistent with this, and there is nothing wrong with the pipe.
Deposits and bacteria
Debris brought in during construction, or particulates from unfiltered water, settle out under low flow and sit in the bottom of horizontal tubes. Copper corrodes under deposits by differential aeration, slowly, but eventually through the wall. NPL's design answer is to size pipework so flow rates are at least 0.5 m/s and to flush to BS EN 806-4.
Deposits also shelter sulphate reducing bacteria, which need anaerobic conditions under a deposit to multiply and which produce sulphuric acid and hydrogen sulphide locally. The result is a high density of tiny pits, often called pepper-pot corrosion, which coalesce into larger pits and eventual pinholes, usually over several years. It is favoured by stagnation and by temperatures between 22 and 45 degrees, which is why it turns up in buildings with long dead legs and poorly lagged hot pipes running alongside cold ones.
Erosion corrosion, and why it is probably not your house
This one gets blamed a lot and deserves it rarely. Erosion corrosion is turbulent flow releasing air bubbles that strip the protective patina, and the damage it leaves is unmistakable: grooves or horseshoe shapes in the bore, with the horseshoes appearing to have walked upstream, usually just downstream of an elbow or a change of section. In severe cases the wall perforates within weeks.
NPL are direct that it is very rarely found in the average size house and is more likely in large buildings with recirculating hot water systems, such as hotels, commercial buildings and large country houses, with hot water return lines particularly prone. The velocity guidance is below 3 m/s cold and 2 m/s hot as the classic critical figures, with a design recommendation of under 2 m/s cold, under 1.2 m/s hot, and under 1 m/s in fully softened water. Burrs left on cut ends and sharp changes of section make it more likely, which is a good argument for deburring even when nobody will ever see the cut.
If you are in a block with a circulating hot water riser and repeated failures downstream of bends, this is worth raising with the freeholder as a design issue rather than treating each leak as an event. That is a commercial repair conversation.
Two things that are not the water and not the pipe
External corrosion is the first. Water Regs UK guidance on installing pipework says concealed pipework in walls should not be embedded or come into contact with other materials such as plaster or cement, and should be run in a chase, duct or void, with insulation wrapping explicitly not an acceptable substitute. Copper chased into a wall and plastered straight over, or laid into screed unsleeved, can corrode from the outside in. If the pit is on the outside of the pipe rather than the inside, stop looking at water chemistry.
The second is brass, not copper. Dezincification leaves a porous copper skeleton where a brass fitting used to be, and Schedule 2 paragraph 7(3) of the Water Supply (Water Fittings) Regulations 1999 requires concealed fittings to be gunmetal or another dezincification-resistant material, marked CR or DRA. NPL also record stress corrosion cracking of over-tightened brass fittings under phenolic foam lagging where condensation got to them.
What to do with one pinhole
Cut it out and keep it. The piece of pipe tells you which of the above it was, and whether to plan for more. Then decide honestly: a single flux pit on a two-year-old refit is a repair plus a conversation about the rest of that refit. Randomly distributed pits along the underside of a horizontal cold run is a re-run of that section, because you will be back.
What not to do is fit a repair clamp and forget it. A clamp on a corroding run buys months, and it hides the evidence that would have told you what was happening.
When not to call us
If the pinhole is visible, accessible and spraying at you, that is a plumber and a section of new pipe, not a detection visit. You do not need anyone to find a leak you are looking at. Turn the water off at the stopcock and get it cut out.
Where we are the right call is the one you cannot see: pressure that keeps dropping, a stain arriving through a ceiling, a warm patch underfoot. Detection is £150 per hour, fixed before we attend, with no find, no fee. We then repair the pipe and make the floor or ceiling good, which is the half most detection firms hand back to you. If you are claiming, see trace and access.
If the pipe has a crack along it rather than a pinhole through it, the cause and the repair are different: see what to do about a cracked pipe.
