Dielectric union V. Dielectric nipple

I installed copper pipe to replace galvenized pipe that had breakdown and leaks. A hot water heater was intsalled without a dielectric union. I went to my local plumbing place and asked for dielectric unions for my pipe. I was given dielectric nipples instead. Are they the same? Or should I have installed the unions instead of nipples. The gentleman I spoke to stated that they are interchangeable and that the nipple is easier to install. I agree with the ease of installation, but wonder if I am still endangering my galvenized pipe by using nipples instead of unions. Any input would be appreciated.

I'm not a plumber (actually I'm an electrician), but I can tell you this:
A union and a nipple are not the same (ok, that was easy).
In order to prevent the "galvonic reaction"(oxidation) between the dissimilar metals, you need either a di-electric break(plastic washer or some other non-conductive element) or a metal that does not contribute to the galvonic reaction, such as brass.
What is the nipple made of?

TonyBagadonutz said:

What is the nipple made of?

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The nipple is made of galvenized pipe and has a rubber/plastic tubing in the center. I used teflon tape on the threads and inserted one end in a female galvenized connection and the other end in a female copper connection. The gentleman I spoke to stated that the water going through the nipple cannot carry a current because of the rubber/plastic insulation inside the nipple. I took him for his word and because the nipple is so much easier to install.

di-electric

Since neither is particularly effective, due to the water carrying the current, it does not make a whole lot of difference.

hj said:

Since neither is particularly effective, due to the water carrying the current, it does not make a whole lot of difference.

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...that would explain why the big orange box store was selling the di-electric unions for 0.03¢.

I don't know much, but I know if you install regular galvanized nipples in a water heater instead of those di-electric nipples between the tank and the copper lines, they will corrode MUCH faster.
Go with the di-electric nipples.
Mike