Production Process of Internally and Externally Coated Epoxy Resin Steel Pipes

The process begins with pretreatment of the base pipe: grinding the steel pipe (advantages: automatic spraying ensures uniform coating on both inner and outer walls, good leveling, meeting CJ/T120-2008 and GB/T5135.20-2010 standards); preheating the ground steel pipe in an electric oven to 230°C~250°C (generally about 1.5 hours); coating the preheated pipe; and curing the coated pipe in a curing chamber (maintaining a temperature of 200°C for 15 minutes); finished product. Alternatively, after spraying, the pipe is preheated to 180°C to cure. The advantage of this method is that the epoxy resin powder is not fully fused to the inner and outer walls of the pipe during spraying; the 30-minute preheating curing process ensures complete curing of the hardener within the epoxy resin, resulting in stronger coating adhesion on both inner and outer walls.

The process also involves preheating the pipe to 180°C after spraying to cure the coating. In my country, the earliest epoxy resin coated steel pipes were all galvanized steel pipes. Later, it was discovered that the galvanized layer on the inner wall of the steel pipe did not improve the adhesion of the inner coating; in fact, it negatively impacted it. It also increased the cost of the galvanizing process. To improve the adhesion of the inner coating, a zinc removal process was required for the inner galvanized steel pipe, further increasing costs and wasting energy. Therefore, some manufacturers have opted to directly apply the coating to welded steel pipes. Before epoxy resin coating, the base pipe of the internally and externally coated steel pipe should undergo sandblasting to remove rust, oil, and the galvanized layer from the metal surface; this is generally referred to as surface pretreatment. The construction industry has no quantitative requirements for surface pretreatment, while the petroleum industry has relatively more specific requirements.