3LPE three-layer polyethylene anti-corrosion coating consists of a bottom layer of fusion-bonded epoxy powder, a middle layer of copolymer adhesive, and an outer layer of high-density polyethylene. It is the mainstream anti-corrosion solution for buried pipelines, possessing excellent corrosion resistance and impact resistance, and can guarantee a pipeline life of over 30 years. Its core operation process follows the logic of "pretreatment-heating-coating-cooling-inspection," with each process closely linked, directly determining the coating quality.
Pretreatment is fundamental to the process and directly affects coating adhesion. First, qualified steel pipes are hoisted to the loading platform, their specifications are checked, and defective pipes are rejected. Then, they enter the shot blasting stage, where high-speed steel shot removes surface oxide scale, rust, and other impurities, achieving a surface cleanliness level of Sa2.5 and a roughness controlled within Ra40–80μm. After rust removal, loose dust is blown away with high-pressure air. In humid environments, subsequent coating must be completed within 4 hours to prevent secondary oxidation.
The rust-removed steel pipes are fed into the medium-frequency heating zone, where they are uniformly heated to 190–230℃ via electromagnetic induction. An infrared thermometer is used to monitor the temperature and ensure the difference does not exceed 15℃. Too low a temperature will result in incomplete coating curing, while too high a temperature will cause material aging. The heating system uses closed-loop control to adjust the power in real time to ensure temperature stability.
The heated steel pipe enters the spraying chamber, where 60kV high-voltage electrostatic spraying of fused epoxy powder forms a dense underlayer of 120–300μm thickness, serving as the core anti-corrosion defense line. Subsequently, an extrusion winding process is used to coat a 150–300μm thick layer of maleic anhydride-grafted modified adhesive, achieving a tight bond between the underlayer and the outer layer. Finally, a 1.8–3.5mm thick outer layer is wrapped with specialized HDPE material at an extrusion temperature of 210–230℃, and air bubbles are eliminated by pressure rollers, forming a mechanical protective layer.
After coating, a gradient cooling process of "air pre-cooling + spray water cooling" is used to lower the temperature to below 60℃, with a cooling rate not exceeding 30℃/min to prevent coating cracking. After cooling, 100-150mm of the anti-corrosion layer is removed from the pipe end, ensuring a clean, burr-free cut and proper end protection to facilitate subsequent welding. Finished products must pass core inspections including appearance, thickness, adhesion, and spark leak detection to ensure no defects and that parameters meet standards. Qualified products are labeled, packaged, and stored.
