What problem may occur if a black steel pipe connects with copper pipe?

When a black steel pipe connects with a copper pipe, a galvanic reaction can occur due to the difference in electrochemical potentials between the two metals. This reaction is known as galvanic corrosion, where one metal, in this case, the black steel, becomes the anode and corrodes more rapidly in the presence of an electrolyte, which could be water or moisture.

The black steel, being less noble than copper in the galvanic series, will corrode preferentially, leading to rust and damaging the structural integrity of the pipe over time. This phenomenon is a critical consideration in plumbing and HVAC systems, as it can lead to leaks and system failures if not properly managed.

Preventative measures, such as using dielectric unions, can help to mitigate this issue by separating the two metals and preventing direct contact, thus stopping the galvanic reaction from occurring.