3.2 Epoxy coal tar preservative coatings
Epoxy coal tar pitch is made of epoxy resin and coal tar bitumen as the base material which is miscible by heating. And then an appropriate amount of solvents, anti-repulsive pigments, fillers and curing agents are mixed and ground. Epoxy coal tar pitch has advantages of being water proof, drug resistant, anti corrosive and having good adhesion, mechanical strength and insulation. It is easier to produce coal tar pitch compared with petroleum asphalt. Epoxy coal tar pitch has a disadvantage of needing a long time for solidification after coating to reach the minimum strength requirement for transportation. Even in summer, it takes one to two days for solidification. During the solidification, wind, sand, rain, frost and snow all have an adverse effect on the surface of the anti-corrosion layer, and it cannot be constructed when the temperature is low. In addition, the primers, finishing coats and thinners used should be stored separately, and the cost of sun and fire protection is high.
 
3.3 The preservative coating of epoxy resin powder
The preservative layer of epoxy resin powder is an anti-corrosion layer that appeared in the 60s of the last century. It is sprayed on the surface of the pre-treated pipe body by electrostatic spraying, and a curing reaction occurs on the surface of the pipe body to form a dense layer. The anti-corrosion layer has good corrosion resistance and low environmental pollution. When the operating temperature range is between 100℃ to -60 ℃, the preservative layer of epoxy resin powder is used more than PE. The adhesion of the preservative layer to the pipe body is good, and has cathodic peeling resistance. The preservative layer of epoxy resin powder has advantages of good impact resistance, toughness, temperature resistance, small welding damage, waterproof, good electrical insulation and the damage being easy to find and repair. The preservative layer has advantages of thin foot membranes, easy damage, blowholes, high production technology requirements, complex equipment and difficult field construction management. The performance of the product will be different with different manufacturers and even different batches. The preservative layer should be chosen based on its mechanical properties and tightness. Besides heat resistance, the epoxy resin powder layer is worse than polyethylene either as an anti-corrosion material or anti-corrosion system. Foreign research on the epoxy resin powder layer mainly focuses on impact resistance, bending resistance and electrical insulation at low temperatures, water resistance, adhesion and cathodic peeling resistance at high temperatures as well as the high cost of its protective materials and difficult construction.
 
3.4 Polyethylene preservative coatings
Polyethylene has good impact resistance, good hardness and other physical and mechanical properties. And its temperature range is relatively wide, between 60℃ to -40℃ for adhesion and between 80℃ to 45℃ for tightness. Polyethylene has better flexibility and impact resistance than other materials especially under low temperature conditions. Therefore, it is widely used in pipelines in cold regions such as Russia and Western Europe. Encapsulated polyethylene coating is made by an extruder extruding hot melt polyethylene onto the surface of the steel pipe to form a plastic shell to prevent corrosion. It is also called polyethylene thermoplastic coating, which is a new product developed abroad in the 1950s and has developed rapidly. It used to monopolize in the use of various anti-corrosion coatings. At present, almost all oil and gas pipelines in Germany use such anti-corrosion coatings. China started to use the polyethylene coating in the 1980s.