The high heat generated by the chemical reaction can damage or deform certain types of plastic piping, particularly older PVC. It can also corrode or pit metallic finishes like chrome, stainless steel, and aluminum.

Do not use sulfuric acid if:

While modern schedule 40 PVC is resistant to acid, the heat generated is the killer. If the clog is 4 feet down the pipe, the acid reaction generates enough heat to soften PVC to the point of sagging or collapsing. Furthermore, if the acid sits in the trap for an hour, it can leach the plasticizers out of the pipe, making it brittle.