These nipples provide no usefull purpose at all. The water has an electrical path through the center of the nipple and is in contact with the inside of the copper tubing on one end of the nipple and is also in contact with the inside of the steel pipe or steel water heaterat the other. At one end of this nipple there is a hard thread to thread connection of steel to copper and the electrical path would now be complete. If the water is acidic or salty it attacks the less noble metal (the steel), pure distilled water is an insulator and would not allow current flow. The plastic liner in the nipple doesn't insulate anything because the water is touching both pipes at the end of the nipple.

The nipple is not designed to prevent dissimilar metals from making contact. That is not what causes electrolysis. The plastic liner within the nipple prevents WATER from making contact to ONE of the dissimilar metals (steel nipple) thereby preventing flow of current from one dissimilar metal to the other.

THESE NIPPLES WORK PERFECTLY!!!!