Common defects of three-layer PE anti-corrosion steel pipe

Three-layer PE anti-corrosion steel pipes are widely used for their durability and excellent protective properties. However, several common defects can occur during the production or long-term use of these pipes. Below are some of the most frequently observed issues.

Coating Warping at Pipe Ends
Coating warping can be caused either during production or as a result of long-term stacking. The three main types of warping are:

Overall warping: The epoxy powder layer at the bottom is peeled off from the steel pipe surface.

Debonding between layers: The adhesive layer in the middle separates from the epoxy powder, causing warped edges.

Debonding of the outer polyethylene coating: The polyethylene layer separates from the middle adhesive layer, resulting in lifting of the surface.

Surface Pitting
Pitting defects appear as hemispherical or semi-ellipsoidal bulges with diameters around 2-5 mm. These pits create an uneven surface, which severely impacts the overall appearance and protective quality of the coating.

Thinning or Cracking of the Coating at the Weld
In spiral-welded or straight seam-welded steel pipes, the coating at the weld area may become thinner or cracked. This is often due to an excessive weld reinforcement that causes the coating to be too thin. When cooling and residual stresses build up in this thin zone, it can lead to a decrease in coating strength. A rough weld surface exacerbates this defect.

Drum Bag Defect
This defect occurs when the protective layer bulges out, reducing the adhesion and cathodic protection of the steel pipe. It is typically caused by insufficient cooling water during the coating process. If residual heat inside the steel pipe exceeds the softening point of the coating, it can cause bulging, which compromises the integrity of the coating.

Surface Wrinkles
Wrinkles often form when an air layer between the primer, polyethylene film, and the steel pipe is not removed before coating. This can result in bubbles, which collapse and form wrinkles during rolling or water cooling. The winding method, which requires special silicon rubber rollers to remove air bubbles, can also lead to defects if not properly executed.

Adhesion and Cathodic Peeling
Adhesion failures occur when the coating peels off during the adhesion or cathodic peel tests. This is usually due to poor bonding between the primer and the powder layer, caused by factors such as insufficient curing or rapid cooling of the powder before curing. Another cause can be improper cleaning of the steel pipe surface before coating.

Raw Material Issues

Moisture absorption or excessive additives: If the polyethylene raw materials absorb moisture or contain excessive recycled materials, it can lead to defects like fishnet-like membrane formation during extrusion.

Low primer melt flow rate: If the primer melt has too low a flow rate, the resulting primer film becomes overly thick with reduced ductility, impairing adhesion to both the powder layer and polyethylene.

Improper particle size distribution of epoxy powder: A non-standard particle size can result in poor powder loading rates and reduced bonding between the layers, leading to weak adhesion.

These defects, if not carefully controlled, can compromise the long-term performance and protective capabilities of three-layer PE anti-corrosion steel pipes.