Copper tubing corrosion in domestic hot water system

[Deejon]

Domestic hot water system is a recirculating system on a city water source that is typically somewhat hard, but not usually corrosive. Tubing has developed thinning walls and pin hole perforations throughput the system (1" through 2" tubing). There does not appear to be any dissimilar metal conections, nor is there high velocity flow conditions. Pipe does not have any scale buildup. System is six years old. The pocking and erosion appears to be the heaviest at fittings and joints. Most pocking shows bright copper, but some areas have oxidized (green patina). Would like to find the cause of the tube errosion.

 

[TBP]

You need to determine if you have corrosion, or erosion. (I suspect erosion.) Are you using a high head pump for circulation? If you are, you don't need to - you just need enough head to keep the water moving. The municipal water system provides the delievry pressure. Was the copper piping reamed properly after it was cut? Copper piping cut with tube cutters leaves a ridge on the inside. This can set up a swirling action that will very nicely erode copper. The fact that the problem is showing up primarily at fittings and joints suggests that failure to ream the pipe is a big factor. If this is combined with a constantly running, high head circ pump, it's bad news.

 

[rustbuster1]

The most common cause of excessive corrosion of copper piping in domestic HOT water systems is the TEMPERATURE of the water.
This is a balancing act - hotter is better for instant warmth when the faucet is opened, i.e. user satisfaction. But lower temperatures are safer (less likely to burn someone) and cause less corrosion. If the temperature is above 60C (142F) corrosion rates can be above 15 mils per year and as high as 25 mpy in turbulent areas. A good way to tell if this is the case is to look for green stains where the hot water drips into sinks , etc. Many institutions like hospitals and hotels have the hot water set above 65C because of sanitizing reasons with dishwashing, etc. It is better to have local circuits to do this with corrosion-resistant pipe - Stainless steel or cupro-nickels.

 

[TBP]

If temperature is a serious contributor to the corrosion of the copper pipe in these systems, why can copper pipe & fintube in closed loop heating applications (often operating in the 180*F range) last for decades? Granted these systems are not in service all of the time, but the fintube sections have wall thicknesses very close to DWV copper, which is to say, pretty thin.

 

[WKTaylor]

Folks,

I investigated copper water-pipe "corrosion" problems [2-5 yr old typical] in an overseas Air Base Apartment complex. The pipe "pin-holed" within a couple of years of installation... and outer-bends in the pipes and elbows were especially vulnerable! We uncovered some interesting factors:

1. The system was built to "low-end" Japanese standards:

a Minimum diameter pipe was used for the water flow rates , which induced turbulent/noisy flow [pipes were noticibly noisy]. Domestic Japanese water demand is different than American water demand for the same size structures... so the contractor put the cheaper/smaller pipe in place.

b The water contained trace ammounts of salt and sand.

c. Metallurgical lab testing confirmed that the fittings and pipe specimens copper alloys were contaminated by "trace/trash" elements that were NOT allowed in typical USA copper pipe. The Study also revealed a surprisingly high rockwell hardness that suggested severe cold working that was NOT stress-relieved at the factory. Conclusion: the material was erosion/corrosion-prone to begin with.

d. The pipe was poorly supported.

2. Overall-Conclusions: Turbulent/abrasive flow, combined with poor [cheap] copper materials and tubing vibration, led to an internal abrasion-erosion problem inherent to the "system"... especially at bends. The "severe corrosion" noted on the outside of the pipes was really superficial... and it occured AFTER the pipes pin-holed!

3. About the only "self- help" available to the USAF was to:

a "filter" the water of sand particles.
b reduce water-flow rates [until occupants "screamed"!!!].
c replace substandard pipe installations when they failed with the best [USA] available materials.

Regards, Wil Taylor

 

[rustbuster1]

So, Deejon, what is the temperature of the system? Are there any green stains at the sinks, etc.?
Perhaps TBP has a better explanation for the symptoms of the damage you describe - leaks from pits throughout the system, but worse corrosion at turbulent locations.

It would be a pretty bad "city source" of potable water to have sand or other suspended particles in the water and presumably more than just this one facility would be in trouble then. So ask the neighbors what they have experienced. Until a better possibility or other non-supporting information is provided, I'd still put my money on excessive temperature as the most likely cause.
Most copper and finned copper tubes in air heaters and industrial dryers, etc. that I know about have steam as the heating medium and corrode where condensation creates a two-phase fluid.

I await further feedback(and challenge by TBP)

 

[TBP]

If nothing else over the years, I've become aware of just how much of an interlocking web of mechanical/chemical weirdness water treatment and corrosion can be. What makes perfect sense in one case, is often totally wrong only a few miles away, because they are on a different water source. Several plumbers I know indicate that there are certain geographic areas where copper pipe in domestic water service fails after only a few years, due to local water conditions. However, this problem affects piping in both hot and cold service.

I can't honestly say I've ever heard of copper in water service failing due to temperatures. That doesn't mean that sometime, somewhere, it hasn't been a factor. I do know that there are residential heating hot water boilers that have copper heat exchangers. Many entire apartment buildings have copper fintube hot water heating throughout. Model engineers often build scaled down steam boilers from copper.

If you've got something like a steam heated air intake coil, the condensate should never be backed up in the coil. The condensate should gravity drain to an atmospheric tank, and if there's a temperature control valve on the steam supply, the coil absolutely must have a vacuum breaker. (Lots of people freeze and split their coils every year, during spring and fall because they're not installed correctly.) Corrosion in this case would most likely be from the contact between the steel headers and copper tubes now longer being exposed to steam, but water and be galvanic corrosion, rather than something caused by two phase flow.

I've worked in a number of powerhouses and industrial plants for a long time now, and I can't remember any copper corriosion problem being laid at the feet of high water temperatures. I've looked through my water treatment books from Betz and Nalco, and I really can't find any reference to temperature being a factor in copper piping corrosion. If you (or anyone) has got something you can point me to, I'd be genuinely interesting in reading it.

 

[Deejon]

Deejon,
The DHW system has a 1075 gallon tank capacity, heats to a temperature of 125F. The system is one of three or four on this campus. The others all have 16 or more years of service without incurring this type of problem. All on same water source. We have designed many domestic hot water systems over the years, as well as closed loop heating systems and have not experienced this type of failure. Several years ago we had a chiller with copper tubes experience erosion/corrosion at one end of the condenser water tube sheet. What we have has some similarities in appearance but occurs in several different size tubes and is pronounced at fittings.

One information source, with a lot of water flow experience, has stated that he had seen a similar result in a couple of cases that had an electrical ground connection. Another stated that one system he encountered had dielectric fittings isolating the heat exchanger from the piping sysem. They neutalized the electrical potential difference by attaching jumper straps across the fittings, which apparently solved the problem. We had the owner of this sysem examine the system for a ground attachment and he did discover an electrical ground connection located about 50' from the heater, and potentially a metal to metal contact at an electrical junction box. We expect to remedy the ground and metal to metal contact, but not sure that this is the solution. Thanks for your suggestions!

 

[Hush]

Corrosion's definately not my strong point so this sound like the musings of an idiot but, if pitting is occurring only at the joints, is it possible there is a galvanic cell between the solder and the pipe.

 

[Deejon]

The pitting occurs throughout. Not only at joints, although it is somwhat more prevelent at the joints. Perhaps whatever is creating the corrosive condition is aggrevated by the turbulance at the fittings.