Is this pipe corrosion a problem that needs to be fixed? This is one of the water connection for our hot water tank (gas powered).


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    And that, kids, is why we have those funky things called dielectric unions. Yikes!
    – Ecnerwal
    Commented Feb 10, 2022 at 1:54
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    Short answer: YES! and soon.
    – JACK
    Commented Feb 10, 2022 at 2:19
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    The answer is yes, Ecnerwal+ a dielectric union needs to be installed ASAP! Why the steel pipe is getting eroded from galvanic corrosion, wait long think wrong in this case, you want fix it because even the Nobel metal or copper can be comprised but the steel and zinc is the first to go. Solid zinc sacrificial’s are what protect a boat motor looks like the zinc may have have been consumed. Clean it and prepare for a short service life because of no dielectric unions.
    – Ed Beal
    Commented Feb 10, 2022 at 2:28
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    If the tank is over 15 years old...consider replacing it. You don't want to spend money on a repair and then end up replacing it shortly after. Commented Feb 10, 2022 at 16:39

2 Answers 2


To wrap all the comments up into an answer:

Yes, this is a problem and Yes, it should be taken care of immediately!

There is a steel pipe nipple coming out of the top of your water heater. It was connected to a copper pipe. This causes galvanic corrosion from the two dissimilar metals being in contact with each other and exposed to water.

When you replace it, take a good, long, hard look at the steel nipple coming out of the water heater. If there is serious damage, you may want to look at replacing that nipple if possible, or the whole heater, as it's likely to continue to corrode and will fail sooner rather than later.

If the nipple looks serviceable (or you replace it or the whole heater), you'll have to cut back the copper pipe to make room for the installation of a dielectric union* to prevent this from happening with the new heater. Here's a web site** that sells them.

If the new heater has a copper nipple coming out of it you can connect it directly to the existing copper plumbing. However, since the fitting was screwed onto the heater then soldered onto the pipe, you're going to have to cut the pipe no matter what just to get it off.

*No affiliation with this site, just the first I found that had a reasonable explanation of what and why. Ignore the ads.

**No affiliation or recommendation whatsoever! Just as an example of what you're looking for.


Objection your honor!

This corrosion seems to be an outside oxidation. The corroded nipple (assuming steel) has rust all over the surface, even in 1 to 3 cm distance from the copper tube.

It is very likely, that it has nothing to do with the combination of those materials, since the golden rule is applied, if the flow direction is upward in the photo, i.e. from less noble metal (nipple) to the copper tube.

Anyway the galvanic type of corrosion would appear inside and close to the connection between those 2 different metals, f.e. at the thread.

It is more likely that this outside corrosion is caused by external corrosive damps or fluids, f.e. if detergents or chemicals are stored nearby.

Or if the big exhaust gas safety opening is releasing some corrosive gases from time to time, f.e. sulphur which may be part of the exhaust gas. If the connection between the copper tube/heater nipple is leaking, this water would build sulphuric acid and contribute to the corrosion.

Also the copper-copper connection has the typical green copper oxidation, which is another indicator of outside factors.

A frequent mistake after soldering is to not clean the tubes from the soldering paste/acid, that way a long term corrosion can start inside and outside of the tubes, especially in steel parts nearby as in this case.

But this soldered connection could also be leaking (maybe only in the past, the oxidized material has more volume thus stopping the leaking with some probability) and contributing to the corrosion.

Anyway a dielectric union would stop main galvanic corrosion, but it would not stop pit corrosion, if the golden rule is broken, since it is a matter of tiny metal bits traveling with the water to the less noble metal, where they settle and build tiny local galvanic elements - independent on any dielectric union upstream.

So yes, surely it is a problem, but the cause seems to be an external matter.

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