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Plumber found (what he called) a pin leak at the elbow. It has done considerable damage.

I want to know how this happens and if more of my pipes might be affected.

NOTES:

House is 20 years old.

I don't often use the bathtub this was connected to.

Pipe has a 10' horizontal run before the elbow redirects water up.

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Edit1: Original setup with jacuzzi pump grounding.

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There are several reasons why copper pipes develop leaks:

  • When fittings are sweated with a torch, too much heat and flux can corrode and weaken the pipe walls.

  • Lately the copper that is being produced is subpar (in my opinion) to copper produced 15 years ago. Not sure if the forging recipe changed or if the wall thickness has been reduce, but it doesn't hold up well.

  • Hard water has a detrimental effect on the soft copper metal. Over time it will wear away the inside walls.

It's impossible to predict where a leak will occur. When a similar situation happened to a customer, I installed CPVC pipe to replace the leaking copper. Also after checking the water pressure, I found it to be very high (90 psi) and installed a reducing valve, which so far stopped any more leaks.

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    I'd vote for pex, but if the op wants to stick with copper, at least go with class L (thicker), rather than class M (cheap and thin). – George Anderson Feb 7 at 0:28
  • Hard water! As determined by a plumber – Marinaio Feb 22 at 22:11
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That is corrosion due to flux residue from the soldered joints not being wiped from the outside or flushed from the inside.

There are several grades of flux and some are more aggressive than others - I use a very "soft" one for all tasks but make sure to clean both the pipe and the fitting, reserving the aggressive one for the very few joints that don't take.

Timeline is also about right - I had one go on me a few years ago. it weeped into a concrete floor for about 6 months until the evidence became visible.

Based on the evidence, there may be other joints being affected - check the ones that are visible. Note that some may have been cleaned on the outside as part of normal house cleaning, so find some under floor or behind cupboards etc to check.

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    Pinhole leaks in copper pipes have also been caused by chemicals added at water treatment plants. Northern Virginia suffered from this phenomenon for decades until a UVA professor determined the cause, after which the chemical was discontinued. – Phil Freedenberg Feb 6 at 21:59
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    If cleaners containing ammonia or amines could have gotten on the outer surface ,they will cause corrosion. – blacksmith37 Feb 6 at 22:14
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    "Water safe flux is mandated under Chicago code." – Flux induced corrosion in copper pipes, plbg.com - that explains why I've never heard of this. Y'all are using what? Yuck. – Mazura Feb 6 at 22:36
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    It really depends upon the water source. A friend of mine in an upscale development here in western Washington started developing pin hole leaks in copper plumbing in the piping as it entered the home. Several other neighbors had similar problems. Turned out the water was acidic and after about 10 years, ate thru the copper. He ended up replacing the supply part of the plumbing with PEX and installed a water softener to protect the rest of the copper. – George Anderson Feb 6 at 22:54
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    Similar problems in Maryland 10 - 20 years ago, resolved by utility changing chemicals, etc. We were lucky - most of the problems in open areas. Neighbor across the street (and others) major damage from pinhole leaks inside finished walls. In this ares the pipes were anywhere from brand new to 50 years old - it was clear (in the end) that it was a water quality problem, not bad construction or pipes. – manassehkatz-Moving 2 Codidact Feb 7 at 1:26
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Mazura added this as a comment, but should be a top answer: galvanic corrosion is an extremely common cause of pin-hole leaks in copper pipe, caused by direct contact between iron and copper. See: What is causing these water heater pipes to rust/corrode?

Normal fixes to prevent this are brass or plastic-lined dielectric coupling nipples.

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Another possibility is Erosion Corrosion caused by disturbance to the flow in the pipe at the elbow. You have a large blob of solder that could cause disturbance to the laminar flow inside the pipe causing cavitation and erosion of the inside surface of the elbow. Subject to how much flow and how frequent, and whether the blob is upstream of the pinhole, you should consider the possibility.

For more information see the following link

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