Stop galvanic corrosion
Whenever you join copper water lines to galvanized steel pipe, you should be concerned about the corrosion caused by joining two dissimilar metals, called “galvanic corrosion.” An electrochemical reaction occurs that causes the steel pipe (in this case) to rust and clog up. The dielectric fittings commonly available in home centers (Photo 1) have a short plastic nipple that keeps the metals from touching, but the metals are still close enough to cause a reaction and corrosion anyway.
The solution is to use a special 3-in. fitting called a “dielectric nipple” (Photo 2). They're available for about $9 at plumbing supply stores. A plastic liner (3 in. long) inside the fitting keeps the metals far enough apart so corrosion won't occur (as much!).