External coating

Types of external coating
Asphalt coating
Asphalt is a traditional anti-corrosion coating layer, has a long history, which consists of petroleum asphalt layer and strengthening glass cloth and PVC outer protective membrane, with excellent waterproof performance, coupled with cheap, weather resistance and for better adhesion to a variety of objects, simple construction, the technology is mature, it is still used extensively for metal corrosion. The disadvantage is that the temperature of the asphalt coating in accordance great patience, softening temperature, easy to melt at low temperature brittle, prone to crack upon impact, easy to aging, rocks buried in the soil, the pipeline to increase the mechanical protective layer or in the ditch filled with sand, increases the cost. Susceptible to bacterial attack, poor working conditions, loss, especially the problem of environmental pollution during production and buried, so that in use in densely populated urban areas is limited.

Coal tar epoxy coating
Coal tar epoxy based epoxy resin and coal tar pitch as the base material, heating miscible, adding some solvents, anti-rust paint, filler and hardener mix abrasive material. Has excellent water resistance, chemical resistance, corrosion resistance and good adhesion, mechanical strength and insulation advantages can bring warm and cold coating, thus representing petroleum asphalt, coal tar pitch facilitate production. Disadvantage of coal tar epoxy coatings require longer curing time is after coating, in order to achieve the minimum strength requirements for transportation, even in summer, but also 1 to 2 days. During curing, sand, rain, frost and snow have had an adverse effect on the coating surface at low temperatures when unable construction. Also used in the primer, paint, curing agents, diluents should be classified storage, sun arson high cost.

Epoxy powder coating
Epoxy powder coating by electrostatic spraying method, powder is sprayed onto the pre-treated and preheated pipe surface, the pipe surface curing reaction, form a dense coating epoxy powder coating The main disadvantage is that the film is thin, vulnerable to injury, prone to blisters, produce technically demanding, but also more complex equipment, field construction management difficult. Different manufacturers, or even different batches of product performance differences, the choice should focus on its mechanical properties and adhesion, etc. In addition to the heat resistance outside, either as anti-corrosion materials or preservative systems are inferior to polyethylene. Study abroad focused on its impact resistance at low temperatures, bending resistance and electrical insulation; high temperature water resistance, adhesion and resistance to cathodic peeling resistance; and to protect its high cost of materials, construction difficult.

Polyethylene anti-corrosion coating
Polyethylene has good impact resistance, high hardness physical and mechanical properties. Before polyethylene extrusion coating layer, the tube should be coated with a layer sealing layer. Primer performance indicators are based on currently used asphalt hot melt dependent. GB in the softening point of the primer, heat loss, penetration, thermal decomposition temperature, there are provisions of self-repair, shear strength, peel strength. But confirmed that the rubber production practice, there are still many shortcomings, such as low temperature levels, poor bonding properties.

Polyethylene tape coating
Since the beginning of the tape and packed with polyethylene inner band played the role of anti-corrosion insulation, packed within the protective tape effects that make it from damage. When using polyethylene tape as a coating, the hierarchical structure of its coating according to specification requirements are divided into general level, to strengthen the level and especially to strengthen the level (newly revised specification has canceled the special strengthen level). This design is more convenient, according to the different environment with different corrosion level, and can and petroleum asphalt coating, coal tar epoxy coating corresponding (approximate correspondence), to facilitate comparison choice. Depending on the corrosion requirements, different construction methods to select different thickness, different width within the band and packed (especially for pipe corrosion).

Polyethylene coating layer structure
Polyethylene three-layer structure of the protective layer, also known as PE, in the three-tier structure, the main effect of epoxy primer is: to form a continuous coating, bonded directly to the pipe surface and has good chemical resistance and anti-cathode stripping performance; and the middle layer of adhesive to form a chemically reactive group bonding, ensure the overall coating has excellent adhesion at higher temperatures. The intermediate layer is typically a copolymer adhesive, whose main component is a polyolefin, is now widely used vinyl copolymer adhesive. Polar functional group of the copolymer with an epoxy adhesive primer epoxy groups can react hydrogen or a chemical bond, the intermediate layer and the bottom layer to form a good bond; nonpolar ethylene portion and a surface layer of polyethylene having good cooperation with the pro, so the middle layer and the surface layer also has good adhesion properties. The main role of the polyethylene surface layer is from mechanical protection and anti-corrosion effect, with the traditional two-story structure polyethylene coating has the same effect.