What is used to join copper pipe to steel pipe?

The correct choice for joining copper pipe to steel pipe is the dielectric union. This type of fitting is specifically designed to connect dissimilar metals, which is crucial for preventing galvanic corrosion. When copper and steel are directly connected, an electrochemical reaction can occur, leading to corrosion of the metal that is less noble in terms of electrochemical potential, which, in this case, is generally the copper.

Dielectric unions contain an insulating material that separates the two metals, effectively hindering the electrical conductivity that facilitates corrosion. This ensures that both pipes can maintain their integrity over time while still being securely joined.

In contrast, other options, such as compression fittings, socket fittings, and flared fittings, are not intended to prevent corrosion between dissimilar metals and are primarily suitable for joining pipes made of the same material. Thus, using dielectric unions is critical for maintaining the longevity of plumbing systems that involve various pipe materials.