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

1. the end of the steel pipe is coated with warped edges.
coating warping is divided into the warping produced during the production process and the long-term stacking process. there are three common forms:
①the overall coating is warped, and the epoxy powder of the bottom layer is peeled off the contact surface of the steel pipe;
②the intermediate adhesive layer is debonded from the epoxy powder layer, causing warped edges;
③the outer polyethylene coating is debonded from the middle adhesive layer, causing the surface to lift. there are common factors as well as individual factors in the production of curling.

2. surface pitting.
the surface of the coating presents a hemispherical or semi-ellipsoidal bulge with a diameter of about 2-5mm, forming uneven pitting, which seriously affects the apparent quality of the protective coating.

3. the coating at the weld is thinned or cracked.
spiral welded or straight seam welded steel pipes, after three-layer pe layer forming, the coating at the weld is thinned or torn. the weld reinforcement exceeds the standard. when the plastic with the same width of the extrusion die is coated on the steel pipe, the coating formed in the weld zone is too thin, and its cooling shrinkage residual stress accumulates in the thin zone, due to the heat storage capacity of the weld zone after cooling and forming, the residual heat accumulated in the weld zone will heat the coating in this zone again through thermal conduction, resulting in a decrease in the strength of the coating in this zone. if the weld surface is rough, such defects will be more obvious.

4. drum bag defect.
the bulging of the protective layer not only affects the apparent quality of the steel pipe but also indicates that the adhesion and cathodic dependability of the protective layer and the surface of the steel pipe are reduced. the main reason is that the amount of water in the coating cooling and shaping process is insufficient. after the spray cooling is completed, the residual heat inside the steel pipe is much greater than the vicat softening point of the coating.

5. surface wrinkles.
in the three-layer pe molding process, no matter whether the coating method or the winding method is used, there is an air layer at the interface between the primer and the polyethylene film and the steel pipe. before coating, this air layer must be eliminated, otherwise air bubbles will be formed and passed through the silicone rubber pressure roller. with rolling or water cooling, the bubbles will collapse, form wrinkles, and sometimes burst. in the winding method, the air bubbles need to be driven out by a special silicon rubber roller, but the rolling process can easily cause coating defects.

6. adhesion and cathodic peeling.
in the adhesion or cathodic peel test of the finished tube, the coating part of the whole is peeled off, and the product does not meet the standard at all. in the coating molding process, if the primer has been cured and then the primer is wrapped (covered), the adhesion between the powder and the primer will be drastically reduced; after molding, the rapid water cooling of the powder before curing will cause the material properties to change, it is easy to reduce the bonding force; the surface of the steel pipe is not cleaned, which can also cause such defects.

7. raw material issues.
①polyethylene raw materials absorb moisture, or raw materials contain excessive recycled materials or excessive additives. during the extrusion process, the fishnet-like membrane will be extruded.
②when the flow rate of the primer melt is lower than the standard, the resulting primer film is super thick and has low ductility, which reduces the adhesion to the powder layer and polyethylene.
③the particle size distribution of the epoxy powder exceeds the standard, resulting in a low powder loading rate; the powder's applicable temperature range is narrow, and the gelation and curing time is short, resulting in poor bonding between the powder primer and the primer.