Which material is specifically mentioned as being unsuitable for use in metallic pip and tubing applications?

The material specifically mentioned as unsuitable for use in metallic pipe and tubing applications is cast iron. This is primarily because cast iron is brittle and can fracture under impact or stress, which poses safety risks when handling gases like propane. The integrity of the piping system is critical in ensuring safe transport and storage of propane, and materials that cannot withstand the demands of pressure and changes in temperature are not appropriate for this application. Consequently, while other options like PVC, copper, and steel may have their own limitations in certain contexts, cast iron's properties make it particularly unfit for metal pipe and tubing uses due to its structural weaknesses.