3LPE Anti-corrosion Layer Structure and Material Requirements:
1. Three-layer composite structure:
Bottom Layer (Epoxy Powder FBE): Thickness ≥ 80μm (standard grade) or 100-400μm (reinforced grade). It forms a chemical bond with the steel pipe surface through electrostatic spraying, providing excellent adhesion and cathodic disbondment resistance.
Middle Layer (Adhesive AD): Thickness 170-250μm, using copolymer materials, density 0.92-0.94g/cm³, melt flow index 1.5-3.0g/10min, ensuring the bonding strength between the epoxy layer and the polyethylene layer.
Outer Layer (Polyethylene PE): Thickness adjusted according to the anti-corrosion grade: standard grade 1.8-2.0mm, reinforced grade 2.2-2.4mm, extra reinforced grade 2.9-3.7mm; using high-density polyethylene (HDPE), Vicat softening point ≥ 110℃, carbon black content 2.5±0.5% to enhance weather resistance.
2. Material Performance Indicators:
Epoxy Powder: Curing temperature 230±10℃, gelation time ≤30 seconds (at 200℃);
Polyethylene Layer: Elongation at break ≥600%, environmental stress cracking resistance (F50) ≥1000h;
Adhesive: Peel strength ≥70N/cm (23℃) or ≥40N/cm (70℃).
Production Process Technical Requirements:
1. Steel Pipe Pretreatment: Shot blasting to Sa2.5 cleanliness level, anchor pattern depth 50-100um, surface salt content ≤20mg/m² (≤10mg/m² for subsea pipelines).
2. Coating Process: Medium-frequency induction heating: Steel pipe preheating temperature 180-220℃ (epoxy powder coating requires 230±10℃); Three-layer synchronous coating: Production line speed 8-12 m/min, using winding or circular mold coating process; Water cooling system: Segmented gradient cooling, cooling rate ≤30℃/min.
Laboratory Performance Testing:
Adhesion test: Cathodic peel radius ≤8mm (23℃) or ≤15mm (70℃) after 48 hours;
Impact strength: ≥10J at 23℃, low-temperature impact retention rate ≥85% at -30℃;
Chemical resistance: No bubbling after immersion in 10% HCl, 5% NaCl, or saturated Ca(OH)₂ solution for 30 days (online testing);
Electrical spark leak detection: Ordinary grade 9kV, reinforced grade 15kV, extra reinforced grade 20kV;
Thickness measurement: Allowable deviation ±10%, minimum local thickness not less than 80% of nominal value.
Applicable Standards:
GB/T 23257-2017 "Polyethylene Anticorrosion Coating for Buried Steel Pipelines" (Current National Standard); SY/T0413-2002 "Technical Standard for Polyethylene Anticorrosion Coating for Buried Steel Pipelines" (Some contents still applicable); GB/T9711-2017 "Steel Pipes for Pipeline Transportation Systems in the Oil and Gas Industry" (Standard for Working Steel Pipes).