Pipe nipples are connectors. Threaded on both ends, they allow plumbing to be connected to a water heater or other plumbing. They can be made of brass, galvanized steel, or plastic-lined -- that is, galvanized steel with dielectric inserts.

You can see a few examples at right. The big one is a 1 1/2-inch plastic-lined galvanized nipple for use on commercial water heaters. The others are 3/4-inch for residential tanks.

The one upright is another plastic-lined galvanized; the two directly behind it are plain galvanized steel. You'll notice that they appear quite rusty.If you looked straight through them, you'd see them to be quite clogged with rust. We're not fond of plain galvanized nipples.

The one between the rusty ones is made of brass. It won't rust, but it will react with steel, such as that from which the water heater is made of. Plain galvanized nipples are the cheapest. Brass and plastic-lined are on a par. We prefer the latter. The plastic sleeve will prevent electrolysis with the connecting plumbing, while the steel will not react with the tank.