METALLIC PIPE MULTIPLY-COVERED WITH POLYOLEFIN

PURPOSE:To improve workability in manufacture and piping execution by composing all of a corrosionproof layer, a welding preventive layer and a protective layer of polyolefin resin, and constituting the resins of the welding preventive layer and the corrosionproof layer of mutually different monomers. CONSTITUTION:A steel pipe 1 is preheated by a preheater 2, and pressure- sensitive adhesives 11 are applied by a pressure-sensitive adhesive coater 3. A resin for a corrosionproof layer is extruded from an extruder 4 and covered, and the corrosionproof layer 12 is formed and passed through a cooler 5. Each resin is melt-extruded from an extruder 8 for a protective layer and an extruder 8' for a welding preventive layer after a pinhole detector 6, and fed to a cross- head die 21, and the welding preventive layer 13 and the protective layer 14 are extruded onto the corrosionproof layer 12 of the steel pipe 1. The resins are cooled by a cooler 9. A polyolefin resin, which can be extruded by the extruder 8' as the welding preventive layer 13 and is difficult to be welded to the corrosionproof layer 12, is used, and is co-extruded together with the protective layer 14. Accordingly, the control of film thickness can be varied by changing the conditions of the operation of the extruder and the like.