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  • #31
    Re: (dielectric) unions VS brass fittings

    Originally posted by Dr. House View Post
    im loving this thread its what made me join this board... it doesnt have to be a union, it can be something brass or even a waterway nipple, just something to separate the steel and copper
    Not really. If metal-to-metal contact occurs between the brass and the copper, the brass corrodes (very slowly, it's only a tiny bit electro-negative in comparsion to copper). But if metal-to-metal contact occurs between the brass and steel, the steel corrodes-- quickly. All those insulating bits are meant to keep the metals apart, because galvanic corrosion only happens when the metals have direct contact AND water in the pipe serving as the electrolyte.

    As long as the metals are separated you're safe. But way to often, incompetent "installers" mash 'em back together without keeping the insulators lined up, or they torque them so hard that the insulators crack, or they burn up the insulators up attrocious solder technique. Because of these installation errors, direct contact between the metals often does occur. And in that case, something always will start to corrode. But if that happens, I'd prefer to replace a $5.00 zinc-galvanized Union, rather than a $900 water heater tank.

    I just joined here too, because of this thread-- just like you did some months ago.
    Last edited by rickst29; 10-29-2009, 03:11 AM.

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    • #32
      Re: (dielectric) unions VS brass fittings

      Has anyone heard of the dielectric corrosion being reduced in combined black steel and copper fire sprinkler systems, because of a no flow condition? "the stagnant water is a very poor conductor" And also, no dielectric unions would be warranted, where connecting steel to copper. I have read that this also has to do with the ratio of copper to steel in the system. steel ring hangers could be used on copper pipe as long as there was no moisture present, and also where copper pipe was resting on steel studs etc. I personally would use a brass coupling as a buffer between dissimilar metals. If you think about it, all steel fire sprinkler systems use brass sprinklers and valves and have no problems with corrosion.

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      • #33
        Re: (dielectric) unions VS brass fittings

        I was installing a Comercial water heater 80 Gal. Don't remember manufacturer at the moment but it had a BIG sticker on the side that said DO NOT INSTALL DIE ELECTRIC UNIONS. I have always been instructed to and done so because of the code here and installed die electric unions so i found it odd that it said not to. It will void tank warantee if u installed the D-E unions. I think the Tank after i started reading had a stainless steel tank in it.

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        • #34
          Re: (dielectric) unions VS brass fittings

          Interesting discussion. I'm not a plumber but I've done a lot of plumbing in 53 years... and I totally agree that the rubber gaskets on the dielectric unions are pretty cruddy. But, galvanic corrosion ("electrolysis") occurs when you have dissimilar metals in contact, with moisture. It's generally worse at high temps (hence more corrosion on the hot water side). A DEU electrically isolates the copper from the steel pipe, removing a necessary condition (metal to metal contact) for galvanic corrosion.

          Of the two dissimilar metals, the relatively more anodic one will corrode. Steel (the galvanization is machined away when the threads are cut) is anodic relative to copper, so the galvanized pipe will rust. If you're connecting a water heater to a house copper pipe system, it's worse because the rate of corrosion is strongly affected by the relative area of the materials. A lot of the cathodic material (copper) and a little of the anodic metal (galvy) is very bad news for the steel pipe. You might not see much problem if you connected a section of new copper piping into an old galvanized pipe system -- say a repair or limited renovation -- as long as the amount of galvy is much greater than the amount of copper. But I wouldn't rely on this.... too dangerous, and corrosion is unpredictable.

          The dielectric union works by insulating the steel from the copper. I suspect that failed, corroded unions are due to leaks - again, the rubber gaskets are crap. The DEU has a gasket and a nylon ferrule that will, if installed correctly, electrically insulate the joint (you can check it with a voltmeter). If your dissimilar metals aren't in electrical contact... no galvanic corrosion. The water IS the electrolyte, but as chemeng mentioned, it's too resistive. The potentials generated by dissimilar metals are on the order of microvolts to millivolts - not much. So you need a direct metal-to-metal joint, with some moisture in the micro-gaps, to get galvanic corrosion.

          As to the bonding, one reason for the bonding of the water pipes is so that the fixtures in the home will be connected to the safety ground. In my area, you have to bond your safety ground wire to the water supply within 5 feet of it entering the structure. if you put DEUs in the water heater lines, you just isolated the hot water side of the system, but the cold water side is still bonded... good enough in my area at least. If you bond to both sides of the copper-galvy joint, do you still need the DEU? Yes! Just because you bonded doesn't mean that there isn't electron flow at the dissimilar metal junction, and that's what causing the corrosion.

          Moving water is often considered to be more corrosive because it contains higher levels of dissoved oxygen. I've not heard that it is more or less resistive, but I'm no expert in that area.

          Many have said that they use 6" brass nipples instead of DEUs and it seems to work. I've tried it, it does seem to work. You can't argue with experience, but the mechanism isn't obvious. Copper and brass are very close on the electropotential series, with brass being just slightly more anodic. Brass is thus still very far from steel, which is relatively anodic compared to brass or copper. Based on this, with the other conditions being the same, you would expect the steel pipe to corrode about the same whether there's a brass nipple installed or the galvy pipe is screwed directly into the copper. But it doesn't. So why does it work? The galvanic series says it shouldn't be much better than straight copper to galvy. I'm guessing that it could be related to the composition of brass. Brass is an alloy of about 30% zinc and 70% copper. Zinc is very anodic compared to copper, but its standard electrode potential is close to steel. So I think that brass being an alloy of copper and zinc has a lot to do with the effectiveness of brass as a corrosion preventative coupling. It could also be related to the cathode to anode area - the anode (galvy) is now connected to brass, which is cathodic relative to galvy but its area is very small. I'm far from an expert in this area, so these are just speculations. Maybe a corrosion control engineer will chime in.
          Last edited by Andy_M; 11-04-2009, 02:37 AM.

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          • #35
            Re: (dielectric) unions VS brass fittings

            I have used di-electric fiber-inserted galvanized couplings with brass npt x compression fittings threaded together and these have lasted over 20 years with zero problems but I have had to replace zinc plated rubber gasketed unions after only 5 years,pure junk!
            I have mentioned this to the engineers at the manufacturers but they just give me that "deer in the headlights" look.
            Last edited by sunworksco; 11-12-2009, 12:22 AM.

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            • #36
              Re: (dielectric) unions VS brass fittings

              I agree dielectrics are junk. Every time I replace a water heater, that has copper piping connected to it, the heater nipples are perfectly clear, the piping above dielectrics is clear, but the actual dielectric is full of corrosion because it has a galvanized body. But they require dielectic unions at water heater connection, and at any galvanized to copper connection, here in Mich. Brass would be my choice.

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