I just put about 20 layers of teflon tape on each end of a nipple and tightened one into female galv an one end into female copper. I then used a multimeter to measure the resistance from one fitting to the other, through the two teflon-tape joints. The resistance was less than a tenth of an ohm (i.e. short circuit).

When I did the same across a dielectric union, I got about 2 mega-ohms (i.e. there's just a tiny bit of conduction through the water, but the metals aren't touching)

So the business about teflon tape being just as good as a dielectric union is complete nonsense. The threads just cut through the teflon. It may work in some cases, but it won't work in others and should not be recommended.

What's amazing to me is that there are all these "old plumbers' tales" out there, so people are arguing about whether brass or teflon or stainless are good or bad. Shouldn't this be scientifically determined? I realize that corrosion happens over many years, but still, there must be ways of measuring corrosion in the lab. It shouldn't be a matter of opinion or first-person stories (anecdotal evidence).



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I appluad your efforts to confirm or deny the reasons for the corrosion process between brass or copper and galvinized steel. My opinion(non-scientific)is the galvinization has little or no effect preserving the meatal the metal under certain conditions. In gas installations, a brass gas valve connected to galvinized or black metal there isn't a problem especially when a drip leg is installed. A drip leg is a tee with a nipple and cap pointing downward instead of a ninety degree elbow turning horizontally out of the wall, it collects moisture inside the pipe. The problem I have encountered in my twenty-two years of experience is when yo use dis-similar metals(keyword) in water systems. A water heater is a good example. Copper flex connectors with brass nuts connected to galvinized nipples that are sometimes included from water heater manufactuers always corrode, usually more so on the hot side. Brass nipples installed into water heaters that use black iron FIP connections will also corrode after years. So... is it the water? High total dissolved solids? Why is hot water corrosion more common than cold? And I agree with your opinion of teflon tape. Although a great innovation, it isn't enough to prevent corrosion in dis-similar metals. I would love to hear more about your findings.

I never heard this argument? so you guys are saying some plumbers say teflon tape will act as an insulator stopping corrosion between 2 different metals?....nah, I can't agree with that. I do like the way you went about testing your theory.

Teflon tape is a great invention for its intended use, as long as that use doesn't involve electricity!

Though untested, I always felt some corrosion between different metals came from some kind of reaction from the moving water inside the pipe. Then again I've seen some wacky build up on pipes under a kitchen sink from all the different chemicals stored there so who knows?

Gerry

Gerry in PA.

You can always do it right the second time.....free!

I think the main talk of teflon insulating electically a connection was on the use of teflon tape on the anode threads on a water heater. Bear in mind this connection between the threads of the anode and the steel tank needs to be direct as even 1 Ohm of resistance is too high. We are concerned on millivolts going through this connection to control galvanic corrosion in the water heater. Hence the word from the manufacturers of water heaters "Do not use teflon tape on the anode connection."

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Teflon tape was never designed to be an isolator. In fact, its only purpose is to allow the two pieces to turn easily until they do reach the point of metal to metal contact. At that point all the teflon tape is doing is filling any voids caused by a mismatch between the two threads, which is also the purpose for joint comounds. And you are missing the point that electrolysis occurs through the water not through the metal, which is why an anode rod can protect the entire tank as long as it has water in it. Unless the dielectric union is able to provide adequate physical separation distance between the two metals, the dielectric union is cosmetic.



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I don't know how you even screwed it together with 20 layers of tape! A single layer of tape might actually survive and not be cut by threads, but certainly this is never suggested as a means to provide dielectric isolation .

As to the scientific aspect of galvanic corrosion, this is well studied. Google "galvanic series" or "nobility series of metals". The basic galvanic reaction is the electric current through two electrodes connected by an electrlyte solution, such as water. The nobility series describes which of the two metals gives up electrons (corrodes) thereby sacrificing itself and protecting the other metal. The reason dielectric nipples work is because of the plastic lining, WATER never TOUCHES the nipple. The fact that the nipple touches the WH and the copper fitting is not relevant. There can be no corrosion due to no path for current flow.

There are numerous examples of this in practical applications. The anode rod in WH is one. On submarines, there are sacrificial zinc blocs attached all over the inside of ballast tanks and free flood areas to protect the hull and structures. All of the museum ships here in San Diego have a cathodic protection system installed. An electric current of proper polarity and ampacity is passed from ground to the hull, to neutralize the galvanic current. I believe this same system is used in industrial applications like pipelines, tanks, etc.

"And you are missing the point that electrolysis occurs through the water not through the metal"

Current always goes in a closed loop and you must have a return path through the metal. Current cannot go through the water and never come back. Dielectric unions are not cosmetic.

And your point is? Too much time on you hands?

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Around here, dielectrics are cosmetic. The best bet to join galvanised and copper is stainless steel.

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