Summary: With the development of science and technology and urban modernization, more and more steel pipes are used in various pipelines and surface coating materials of steel pipes are also constantly improving. This article discusses the development status of anti-corrosion coatings on the outer surface of steel pipes, polyethylene anti-corrosion coatings and their advantages and disadvantages, some common problems and causes of coating materials, and compares petroleum asphalt anti-corrosion coatings, epoxy coal tar anti-corrosion coatings and epoxy powder anti-corrosion coatings.
 
1. The role of coating materials
The outer surface of the steel pipe is coated in order to prevent rust. The rust on the surface of the steel pipe will affect the actual use effect, quality and its appearance. Therefore, the coating process of the steel pipe has a great influence on quality of the steel pipe product.

2. Requirements for coating materials
According to the American Petroleum Institute standard, the steel pipe needs to be free from corrosion within three months, so the anti-corrosion time of the steel pipe after coating needs to meet the requirements for this standard. However, nowadays, users have higher and higher requirements for the anti-rust time of steel pipes. Many users require steel pipes not to rust when they are put outdoors for 3 to 6 months. Users have put forward their requirements for the anti-rust time of steel pipes. In addition to the anti-rust time requirement, they also have certain requirements for the appearance quality of the coating, which is mainly reflected in the need for a smooth surface after the coating of the steel pipe, and the preservatives being uniformly distributed on the surface of the steel pipe and no leakage or dripping affecting the appearance quality.
 
3. Types, advantages and disadvantages of coating materials
At present, more and more steel pipes are used in urban underground pipelines. As a pipeline used to transport gas, petroleum, water, etc., its outer surface preservative covering material has gradually developed from the early asphalt materials to polyethylene resin and epoxy resin materials. Polyethylene coating on the surface of steel pipes has been applied since the 1980s. According to different use, the composition and coating process of polyethylene resin are gradually improved.
 
3.1 Petroleum asphalt anticorrosion coatings
Petroleum asphalt coating is a traditional anti-corrosion layer with a long history. It is composed of a petroleum asphalt layer, a reinforced glass cloth and a polythene film for external protection. It has excellent waterproof performance, cheap prices, weather resistance, good adhesion to many objects, simple construction and mature technology, so it is still widely used for metal anticorrosion. The disadvantage of the petroleum asphalt coating is that it extremely relies on temperature, softening and being easy to melt at high temperature and being brittle at low temperature. In addition, the asphalt coating is easy to crack and age when impacted. When the petroleum asphalt coating is buried in rocky soil, the mechanical protection layer needs to be added or sandy soil should be filled in the ditch for the pipeline, which increases the cost. The petroleum asphalt coating is easy to be eroded by bacterium and has poor working conditions and large losses. Environmental pollution during production and installation especially restricts use of the petroleum asphalt coating in densely populated urban areas.