I. Introduction to 3PE Coated Welded Steel Pipe
3PE-coated welded steel pipe is a commonly used anti-corrosion pipe material in underground oil and gas transmission, as well as municipal water supply and drainage projects.
3PE-coated welded steel pipe is a type of anti-corrosion steel pipe with three layers of polyethylene applied to the outer surface of ordinary welded steel pipe. It offers excellent corrosion and weather resistance.
Based on welded steel pipe, it features a composite structure consisting of a primer, an intermediate epoxy resin layer, and an outer polyethylene protective layer. It is widely used in oil, natural gas, water, and chemical pipelines, and is particularly suitable for buried or underwater pipelines.
This pipe offers high strength, excellent pressure resistance, a long service life, and effective protection against external corrosion and mechanical damage.
II. 3PE coated welded steel pipe structure
3PE coated steel pipe is a composite pipe with three layers of polyethylene anti-corrosion structure applied to the outer wall of ordinary welded steel pipe. Its typical structure is as follows:
| Layer | Material | Function Description |
|---|---|---|
| Inner Steel Pipe | Welded steel pipe (Q235 / Q345 or X42–X70) | Provides pressure containment, mechanical support and fluid transport. |
| First Layer (Primer) | Epoxy powder coating | Bonds tightly to the steel to prevent direct corrosion of the base pipe. |
| Second Layer (Intermediate Epoxy) | Epoxy resin layer | Increases anti-corrosion thickness and enhances corrosion and mechanical damage resistance. |
| Third Layer (Outer Protection) | Polyethylene (PE) | Protects inner coatings from mechanical damage; weather- and abrasion-resistant, suitable for buried or subsea pipelines. |
III. Inner steel pipe material of 3PE coated welded steel pipe
| Material Name | Type Description | Suitable Environment |
|---|---|---|
| Q235 | Ordinary carbon structural steel | General water pipelines, low-pressure lines |
| Q345 | Low alloy high-strength steel | Water, oil, and gas pipelines with higher pressure |
| X42 | API 5L low alloy steel pipe | Oil and natural gas transmission under low to medium pressure |
| X46 | API 5L low alloy steel pipe | Oil and gas transmission, moderate pressure resistance |
| X52 | API 5L low alloy steel pipe | Oil and gas transport, medium-pressure wells, and mildly corrosive environments |
| X56 | API 5L low alloy steel pipe | Medium to high-pressure oil and gas pipelines |
| X60 | API 5L low alloy steel pipe | High-pressure oil and gas transmission lines |
| X65 / X70 | API 5L high-strength low alloy steel pipe | High-pressure, high-strength pipelines and special engineering applications |
IV. 3PE anti-corrosion layer construction technology
3PE coating is a three-layer polyethylene anti-corrosion layer, consisting of three parts: epoxy primer (FBE), middle adhesive layer (EPR or PE adhesive layer) and polyethylene outer layer. The specific process is as follows:
(1) Epoxy Powder Coating (FBE)
Steel pipes are heated to 200–250°C.
Epoxy powder is applied via electrostatic spraying.
The powder melts at high temperatures, forming a uniformly adhered primer.
Function: Corrosion protection and enhanced adhesion between steel pipe and outer polyethylene layer.
(2) Intermediate Adhesive Layer
After heating the epoxy-coated steel pipe, an EPR/PE adhesive layer is applied via dip coating or extrusion coating.
Function: Improves impact resistance and cushions external forces.
(3) Outer Polyethylene Layer
The outermost layer consists of an extruded molten polyethylene (PE) coating.
Typical thickness: 2.0–3.0 mm.
Function: Protects inner layers, prevents moisture ingress, and resists mechanical damage.
(4) Cooling
The coated pipe is cooled and set using water or air cooling.
V. 3PE Coated Welded Steel Pipe Testing Standards
i. Appearance and size inspection
| Inspection Item | Technical Requirement / Acceptance Standard | Inspection Purpose |
|---|---|---|
| Surface Appearance | Coating should be smooth, even, without bubbles, cracks, sags, missed spots, or mechanical damage | Evaluate coating application quality |
| Total Coating Thickness | ≥ 2.0 mm (standard pipe) ≥ 2.5 mm (heavy-duty corrosion-resistant type) |
Ensure long-term anti-corrosion performance |
| Epoxy Primer Thickness | ≥ 100 μm (recommended 150–250 μm) | Improve adhesion and corrosion resistance |
| Adhesive Layer Thickness | Approx. 200–400 μm | Ensure stable interlayer bonding |
| Polyethylene Outer Layer Thickness | 1.8–3.0 mm | Protect against external mechanical damage |
ii. Performance testing (mechanical and thermal properties)
| Test Item | Acceptance Standard (Typical Value) | Test Purpose |
|---|---|---|
| Adhesion (Epoxy/Steel Pipe) | ≥ 8 MPa (or ≥ 100 N/cm) | Verify bonding strength between epoxy and steel pipe |
| Adhesion (Interlayer) | Peel test: peel angle ≤ 30°, no delamination | Check interlayer bonding firmness |
| Impact Strength | ≥ 10 J/mm (standard coating) ≥ 15–20 J/mm (heavy-duty coating) |
Simulate construction and transportation impacts |
| Bending Performance | Bending angle ≥ 3°/D (D ≤ 219 mm) ≥ 2.5°/D (D > 219 mm) |
Verify flexibility and coating integrity |
| Heat Aging Performance | No cracking or blistering after 110℃ × 1000 h | Assess high-temperature resistance |
| Cathodic Disbondment | ≤ 10 mm (65℃ × 48 h) | Check coating stability under cathodic protection |
iii. Electrical testing (corrosion integrity)
| Test Item | Standard Requirement | Test Significance |
|---|---|---|
| Spark Test Voltage | 25 kV/mm (coating thickness) | Detect pinholes and coating damage |
| Holiday (Leak) Test | No defects allowed (0 defects) | Ensure no exposed metal areas |
| Insulation Resistance | ≥ 10⁶ Ω·m² | Evaluate overall corrosion insulation performance |
iv. Environmental and corrosion resistance
| Test Item | Standard Requirement | Test Significance |
|---|---|---|
| Salt Spray Test | 1000 h without bubbling or peeling | Assess resistance to salt corrosion |
| Water Immersion Test | Immersed in 65℃ water for 100 days without coating change | Verify long-term moisture resistance |
| Chemical Media Resistance | No bubbling or peeling in acid, alkali, or salt solution for 72 h | Check coating chemical stability |
v. Comprehensive evaluation indicators
| Grade | Recommended Specifications | Suitable Applications |
|---|---|---|
| Anti-Corrosion Grade A (Heavy-Duty) | Thickness ≥ 2.5 mm, Impact ≥ 20 J/mm, Peel ≤ 10 mm | Subsea pipelines, wetlands, saline-alkali areas |
| Anti-Corrosion Grade B (Standard) | Thickness ≥ 2.0 mm, Impact ≥ 10 J/mm, Peel ≤ 15 mm | Urban water supply, gas transportation |
| Anti-Corrosion Grade C (Economical) | Thickness ≥ 1.8 mm, Impact ≥ 8 J/mm, Peel ≤ 20 mm | Ordinary buried pipelines, internal plant networks |
VI. 3PE Coated Welded Steel Pipe Purchasing Recommendations
i. Clarify the use environment and anti-corrosion level
| Service Environment | Recommended Anti-Corrosion Grade | Recommended Coating Thickness | Features |
|---|---|---|---|
| Marine, wetlands, saline-alkali areas, high groundwater regions | Grade A Heavy-Duty Anti-Corrosion | ≥ 2.5 mm | Long service life, strong impermeability |
| Urban gas, municipal water, industrial circulating water networks | Grade B Standard Anti-Corrosion | ≥ 2.0 mm | Cost-effective, suitable for most projects |
| Indoor or short-distance buried pipelines | Grade C Economical | ≥ 1.8 mm | Low cost, suitable for low-corrosion environments |
Tip: Not all three layers of 3PE are created equal. The key factors are the thickness of the epoxy primer and the quality of the outer PE layer. We recommend choosing a product with a primer thickness of 150 μm or greater and a PE layer of 2.0 mm or greater.
ii. Focus on the quality of steel pipe matrix
Base Pipe Types:
Longitudinal Seam Welded Pipe (LSAW/ERW): High dimensional accuracy, suitable for medium and low-pressure pipelines.
Spiral Welded Pipe (SSAW): Can produce large diameters, offers high cost-effectiveness, and is often used for long-distance transportation.
Common Steel Grades: Q235B, Q345B, X42, X52, X60, L245–L415, etc.
Base Pipe Standard: API 5L or GB/T 9711 is recommended.
Purchasing Key Points:
Request a weld nondestructive testing report (UT and RT testing).
Inspect the pipe wall thickness for a deviation of ≤ ±10%.
Require the steel mill to provide chemical composition and mechanical property reports.
iii. Check anti-corrosion process and test report
| Test Item | Reference Standard | Acceptance Criteria |
|---|---|---|
| Total Coating Thickness | SY/T 0413-2012 | ≥ 2.0 mm |
| Spark Test | DIN 30670 | 25 kV/mm thickness |
| Adhesion | ISO 21809-1 | ≥ 8 MPa |
| Impact Strength | ≥ 15 J/mm | No damage |
| Cathodic Disbondment | ≤ 10 mm (65℃ × 48 h) | Pass |
Purchasing Recommendations:
Require the manufacturer to provide a third-party testing report (such as CNAS or SGS).
The coating should be smooth, free of bubbles and mechanical scratches.
The application temperature must be ≥ 200°C to ensure adequate epoxy curing.
iv. Size matches the interface
Common Size Range:
Outer Diameter: φ89 mm–φ1620 mm
Wall Thickness: 3 mm–20 mm
Lengths: 6 m, 9 m, 12 m (customizable)
End Styles: Plain End, Bevel End, Flange, or Socket End.
Interface Welding Requirements: Joints must be treated with on-site anti-corrosion patching (using heat shrink tape or hot melt tape).
Purchasing Recommendations:
If the pipeline is buried deep or has numerous weld points, be sure to request the manufacturer to provide a patching construction process and material matching plan.
v. Storage, transportation and construction precautions
Transportation:
Avoid direct contact between steel pipes during loading and unloading.
It is recommended to use soft straps for lifting; do not allow the wire rope to come into direct contact with the coating surface.
Stacking:
Store neatly and flat with the pipe ends aligned. Place wooden boards or rubber mats underneath.
Store in a dry, well-ventilated environment, away from direct sunlight.
Pre-application Inspection:
Confirm that the coating is intact.
Check spark detection records.
Control the temperature at the joint at approximately 220°C to ensure a secure bond.
vi. Purchase List Reference
| Item | Description |
|---|---|
| Product Name | 3PE Coated Welded Steel Pipe |
| Base Pipe Standard | API 5L / GB/T 9711 |
| Anti-Corrosion Grade | Grade A / B / C |
| OD and Wall Thickness | As per design (e.g., φ219 × 6 mm) |
| Length | 6 / 9 / 12 meters |
| Inspection Requirements | Thickness, spark test, adhesion, impact test reports |
| Delivery Condition | Sealed ends, packaging with labeling |
| Special Requirements | Can indicate "Provide coupling material and construction guidance" |
