3PE anti-corrosion steel pipes are currently one of the corrosion protection technologies used in oil and gas transmission, water supply, and drainage pipeline projects worldwide. The following outlines the entire manufacturing process for 3PE pipes—from the arrival of the steel pipes at the factory to the final product—to give you an understanding of the technology behind them.
Step 1: Steel Pipe Preheating and Surface Treatment
Surface treatment is a critical step that determines the quality of the anti-corrosion coating. If the steel pipe surface contains rust or impurities, the subsequent coating will not adhere firmly, leading to peeling later on.
- Preheating: After entering the production line, the steel pipes are first preheated to 40°C–60°C in an induction heating furnace. This is primarily to remove moisture and humidity from the pipe surface.
- Shot Blasting for Rust Removal: After preheating, the steel pipes are fed into a shot blasting machine. A high-speed rotating impeller violently propels steel shot and grit onto the outer surface of the steel pipes.
- Purpose: To remove scale, rust, and old paint layers from the surface of the steel pipes, achieving a surface cleanliness level of Sa2.5.
- Surface Texturing: Shot blasting also creates a micro-rough surface with minute irregularities (known as anchor pattern depth, typically ranging from 40 to 100 micrometers), which increases the contact area between the steel pipe and the coating, as well as the mechanical interlock.
- Dust Removal: After shot blasting, residual fine steel grit and dust on the steel pipe surface are removed using high-pressure air and a dust extraction system.
Step 2: Reheating the Steel Pipe
Before applying the first layer of powder, the steel pipe must be heated to a specific reaction temperature.
- The steel pipe passes through a medium-frequency induction heater, which rapidly raises its overall temperature to 200°C–240°C.
- This temperature is determined by the curing characteristics of the first layer of epoxy powder. If the temperature is too low, the powder will not melt and cure; if it is too high, the epoxy powder will age and degrade.
Step 3: Continuous Application of Three Coating Layers
This is the core part of the 3PE process. These three coating layers are applied sequentially on the same continuous production line within a very short period of time, ensuring that their molecules interpenetrate tightly while in a molten state.
1. First Layer: Fused Epoxy Powder
- Coating Method: Electrostatic spraying is used. Epoxy powder charged with static electricity is evenly sprayed onto the surface of the high-temperature steel pipe as it rotates and moves forward.
- Thickness: Typically greater than or equal to 80 micrometers (μm).
- Function: Upon contact with the high-temperature steel pipe, the epoxy powder instantly melts, flows, and cures. This layer serves as the core corrosion-resistant layer; it forms a strong chemical bond with the steel pipe surface and exhibits excellent resistance to cathodic delamination.
2. Second Layer: Polymer Adhesive
- Coating Method: While the first layer of epoxy powder is in a molten state, the adhesive is melted and extruded via a side-mounted extruder, wrapped around the steel pipe in a ribbon-like pattern, and compacted using pressure rollers.
- Thickness: Typically between 170 and 250 micrometers (μm).
- Function: It serves as a transitional bridge. Since the epoxy powder and the outer layer of polyethylene (PE) cannot bond directly, one end of the adhesive chemically reacts and bonds with the epoxy powder, while the other end thermally blends with the polyethylene, thereby firmly and securely bonding the first and third layers together.
3. Third Layer: High-Density Polyethylene
- Application Method: Immediately following the adhesive application, another extruder extrudes molten high-density polyethylene into a sheet, which is then wrapped around the pipe’s outer surface using the winding or sleeve method and compacted with rollers.
- Thickness: Depending on the pipe diameter, this layer is the thickest, typically ranging from 1.8 to 3.7 mm.
- Function: This serves as a mechanical protective layer. Polyethylene provides waterproofing, insulation, and resistance to mechanical damage. It isolates the pipe from external soil moisture and acidic or alkaline substances, and protects the pipe from impacts and scratches during transportation and underground installation.
Step 4: Water-Cooling Curing
- After the three layers have been applied, the coating on the steel pipe is still in a softened, thermoplastic state.
- The steel pipe then enters a water-cooling spray tunnel, where it is rapidly cooled using a large volume of cold water.
- Purpose: To rapidly harden and set the adhesive and polyethylene, while ensuring that the epoxy powder is fully cured. The temperature of the cooled pipe should be reduced to below 60°C to facilitate subsequent handling and inspection.
Step 5: Pipe End Preparation
- To facilitate butt welding between steel pipes at the construction site later on, the ends of the pipes must be free of anti-corrosion coatings.
- After the pipes have cooled, the pipe ends are fed into a flying-knife beveling machine or a grinding machine.
- Workers remove the polyethylene and epoxy layers from a section of approximately 100–150 mm at the pipe ends, exposing the clean steel surface, and grind the ends to form a standard welding bevel.
- The edges of the removed coating are typically chamfered at an angle of approximately 30 degrees to prevent the coating from curling during subsequent transportation.
Step 6: Inspection and Finished Product Storage
The quality of corrosion-resistant pipes directly affects the safety of underground projects for decades or even centuries; therefore, they must undergo rigorous quality inspections before leaving the factory:
- Visual Inspection: The coating surface should be smooth and uniform in color, with no bubbles, cracks, or visible scratches.
- Thickness Gauge Inspection: An electromagnetic thickness gauge is used to take measurements at multiple points along the circumference and axis of the steel pipe to ensure that the total thickness of the three layers and the thickness of each individual layer fully meet the specifications.
- Electrospark Leak Detection: A high-voltage electrospark detector (typically set to a voltage of around 25 kV) is used to scan the entire surface. If there are even minute pinholes or areas where the coating is missing, the electrospark will break through at that point and trigger an alarm.
- Peel Strength Test: Samples are taken periodically to test the peel strength between the polyethylene layer and the steel pipe at a fixed temperature, ensuring there is no delamination among the three protective layers.
Qualified steel pipes are labeled, transported to the finished goods area for storage, and await shipment to major energy, water supply, and drainage construction sites.