In the polyolefin anticorrosive coating, the failure modes of different kinds of anticorrosive coating are different.
A large number of bonding failure problems have been exposed in the operation of fusion bonded polyethylene. Then, although some reports show that the occurrence of bond failure is very common, there are few reports about obvious corrosion under the failure anticorrosive coating from the owner. A large number of laboratory tests show that the corrosion control is still effective under the FBE coating without bond, which may be due to the FBE allowing part of the protective current to flow to the steel pipe surface. A large number of bonding failure problems of FBE lead to the wide application of other kinds of polyethylene anticorrosive coating.
As for the double-layer polyethylene anticorrosive coating, the 2PE with mastic as the base glue is prone to the problem of mastic flowing down under the warm environment. It is also reported that after 20-30 years, mastic has been oxidized into loose and brittle powder, resulting in the corrosion of water penetrating into the anticorrosive coating. This is due to the insulation property of extruded polyethylene, which results in the cathodic protection current unable to enter the anticorrosive coating without adhesion. The stress cracking of LDPE anticorrosive coating appears, which is easy to be caused by small damage or scratch on the anticorrosive coating. In the most serious case, the whole pipeline is affected by the cracking. However, it is reported that the reason for this extreme situation is due to the lack of material specification. The problem of adhesive losing adhesion to the substrate and delamination with polyethylene also appeared in 2PE with hard adhesive as primer. The anti-corrosion coating also cracks, resulting in water infiltration under the anti-corrosion coating, resulting in corrosion.
The three-layer polyethylene anticorrosive coating pipeline has also exposed the problem of bonding failure within a few years after being put into use. It is reported that several pipelines have been put into use for less than two years. There are different degrees of problems. The lighter one is that the adhesion of the anti-corrosion coating is reduced and it can be easily peeled off, but the base is not corroded. The most serious one is that the corrosion layer and the base are completely separated, and there are corrosion products on the metal surface. However, the cause of the problem is usually the coating quality. Even active corrosion was found under the anticorrosive coating with good appearance on the new pipeline, but these 3PE anticorrosive coatings adopt low-density polyethylene, the corrosion reason can be attributed to high oxygen permeability and water permeability; the three layers of anticorrosive coating also have stress cracking, and the cracking reason of anticorrosive coating is also small scratch on the external anticorrosive coating. The properties related to the aging and failure of PE coating include: 1. Cracking and embrittlement caused by ultraviolet aging. 2. Stress cracking. 3. Permeability of water and oxygen