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I have a smart home automated shutoff valve that I want to install in my water main. The shutoff valve is a motorized ball valve on a stainless steel fitting. My water main is copper. I am aware of corrosion issues mixing different types of metal piping.

I am hesitant to use a dielectric union on the water main because I have never used one before, and don't know if these are prone to leaking. I also also do not want corrosion though, so I'm stuck not knowing which is best. I love in Colorado, but am not looking for meeting the minimum code, I want to make sure I don't have to worry about corrosion or water leaks.

Thanks in advance!

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3 Answers 3

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The golden rule for different metals for water tubes is from less noble to more noble metal in flow direction. So from zinc plated steel to copper is much better then those steel tubes after copper. Stainless steel seems to cause less or no problems with copper, i.e. the order does not matter. But there is much less experience with stainless steel and copper tubes for domestic water distribution compared to zinc plated steel tubes.

An insulation/plastic part will prevent direct contact of 2 metals and can avoid contact corrosion. But it can not prevent pit corrosion, if noble metal comes before less noble metal.

If more noble tiny metal atoms/parts flow to and settle on less noble metal, the less noble metal can start a pit corrosion resulting in small holes. The pit corrosion is based on a local chemical element, i.e. the electric current does only flow locally around the settle-zone - any insulation between metal tubes upstream or downstream has little or no effect on this local corrosion.

This is why the golden rule is valid even if insulation devices do separate the different metals. Steel boilers are made for upstream copper tubes since they are protected by a hard coating (f.e. ceramic) and by a sacrificing anode, which should to be maintained/tested regularly (decreasing size, limestone coatings, electric current f.e. 0.5mA).

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You will have no problems directly connecting a stainless steel fitting to a copper pipe.

The corrosion problems come from the galvanic cell created by the junction of copper and iron (or zinc). A copper - iron cell (or, junction) causes corrosion because it has a relatively high voltage. Stainless steel has sufficient nickel content — one of those noble metals — so the resulting cell with the copper has insufficient voltage potential to cause corrosion.

Dielectric unions depend on a plastic insulator — the "dielectric" part — and typically include a gasket made from some type of rubber. Over a long time (50+ years) those parts are more likely to fail than the metal fittings or pipes. The most common dielectric fittings are brass (on the copper side) and galvanized steel on the other, which would introduce problems for your (currently) all copper plumbing.

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They are not prone to leaking. But since you are cutting into the water main inside the house (which I assume is coming into the house) you can go from the main to PVC then to your stainless steel valve if you like. What is the material of the plumbing in the rest of the house? Is it already PVC or PEX?

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    The rest of the house is copper.
    – Jon Ulrich
    Feb 26, 2021 at 3:45

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