The three-layer polyethylene anti-corrosion layer is referred to as 3PE. Its bottom layer is epoxy powder coating, the middle layer is adhesive, and the outer layer is polyethylene coating.

Performance

The first layer of 3PE is epoxy coating, the second layer is adhesive, and the third layer is extruded polyethylene. The layers are tightly bonded to each other to form a composite structure that complements each other. It uses the strong bonding force between epoxy powder and the surface of the steel pipe to improve the bonding; it uses the excellent mechanical strength, chemical stability, insulation, resistance to plant root penetration, and resistance to water penetration of extruded polyethylene to improve its overall performance, making the overall performance of the three-layer PE anti-corrosion coating more prominent and more comprehensive. It is suitable for harsh environments that require high mechanical properties, soil stress resistance and water resistance of the covering layer, such as gravel soil, stone sections, high soil moisture content, and areas with developed plant roots.

Coating

Before applying the anti-corrosion layer, remove the grease, dirt and other attachments on the surface of the steel pipe, and preheat the steel pipe for surface pretreatment. The preheating temperature of the steel pipe is 40-60¡æ. The quality of surface pretreatment should meet the requirements of Sa2.5 specified in GB/T 8923 "Surface Rust Grade and Rust Removal Grade of Steel Materials Before Coating", and the anchor pattern depth should reach 50¦Ìm-75¦Ìm. Welding slag, burrs, etc. on the surface of the steel pipe should be cleaned.

The steel pipe should be heated to a suitable coating temperature using a pollution-free heat source, and the epoxy powder coating should be evenly applied to the surface of the steel pipe; the application of the adhesive must be carried out during the gelling process of the epoxy powder; the coating of the polyethylene layer can be carried out by longitudinal extrusion process or lateral winding process. For steel pipes with a nominal diameter greater than 500mm, the lateral winding process should be adopted. When the lateral winding process is used, the polyethylene of the overlapping part and the polyethylene on both sides of the weld should be completely rolled and compacted, and the surface of the polyethylene layer should not be crushed; when the longitudinal extrusion process is used, there should be no voids on both sides of the weld. After the polyethylene layer is applied, ensure that the fused epoxy coating is completely cured, and then cool it with water until the temperature of the steel pipe is not higher than 60¡æ.

Repair

When defects are found on 3PE pipes, for damage less than or equal to 30mm, it is advisable to use radiation cross-linked polyethylene patch to repair. When repairing, first remove the dirt on the damaged part and roughen the polyethylene layer there. Then cut the polyethylene layer at the damaged part into a circle, and the edge should be chamfered into an obtuse angle. Fill the hole with the adhesive that matches the patch, and then apply the patch. The size of the patch should ensure that its edge is not less than 100mm from the edge of the hole in the polyethylene layer. When patching, roll it with a roller while heating or squeeze it by hand with heat-resistant gloves to expel air until the adhesive overflows evenly around the patch.

For damages larger than 30mm, first remove the dirt from the damaged area, roughen the polyethylene layer at the damaged area, and cut the polyethylene layer at the damaged area into a round shape with the edges chamfered into an obtuse angle. Fill the hole with the adhesive that matches the patch and stick the patch on. Finally, wrap a heat shrink tape around the repaired area, and the wrapping width should be at least 50mm wider than the two sides of the patch.

When patching, you can also clean the surface first, then use a two-component liquid epoxy paint for corrosion protection, with the dry film thickness the same as the main pipe, and then stick the patch or heat the shrink tape.

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