How it is built
| Layer | Material | Function |
|---|---|---|
| 1 | Fusion-bonded epoxy | Bond to the steel, corrosion resistance, resistance to cathodic disbondment |
| 2 | Copolymer adhesive | Chemical link between the polar epoxy and the non-polar polyethylene |
| 3 | Polyethylene | Impact, abrasion and indentation resistance, low moisture permeability |
The layers are applied in one continuous line. The pipe is blast cleaned and induction heated, the FBE powder is sprayed, and the adhesive is applied while the epoxy is still reactive so the two bond chemically. The polyethylene is then extruded over the adhesive, by side wrapping or crosshead extrusion, and the pipe is water cooled. Timing and temperature between layers govern interlayer adhesion, which is why plant process control and peel testing matter.
Where it is used
3LPE is chosen where the coating must survive more than corrosion: long transport and repeated handling, rocky terrain, rough backfill, large-diameter lines. The epoxy alone would carry the corrosion protection, but it is thin; the polyethylene takes the mechanical abuse. For higher operating temperatures, three-layer polypropylene (3LPP) follows the same architecture with a polypropylene outer layer. Temperature limits for each system come from the governing standard and the manufacturer's qualification.
How it is specified
In Canada, plant-applied polyethylene coatings fall under CSA Z245.21, where the FBE, adhesive and polyethylene construction is designated system B2. CSA Z662 requires external corrosion protection on buried pipe and refers to the Z245 series. Internationally, ISO 21809-1 covers three-layer polyethylene and polypropylene coatings, with qualification and production tests such as peel strength, impact, indentation and cathodic disbondment.
Field joints and limitations
As with FBE, the pipe ends are left bare for welding, and the girth weld zone is coated in the field under CSA Z245.30 — most often with heat-shrink sleeves designed to bond to polyethylene. The joint, not the mill coating, is usually the weaker link.
The thick, highly resistive polyethylene layer is excellent while bonded. If it disbonds and water enters underneath, it can shield the steel from cathodic protection current, so adhesion at the plant and at the field joint is the property to verify. The pipeline coatings and field joints article covers the records an owner should ask for.