Effect of coating on large diameter spiral steel pipe

Coatings play a crucial role in enhancing the durability and performance of large-diameter spiral steel pipes, protecting them from external and internal corrosion, reducing heat loss, and improving operational efficiency.

Polyethylene Outer Protective Coating:
For large-diameter spiral steel pipes (SSAW pipes), if the outer protective layer is made of polyethylene, there is no need for additional anti-corrosion treatment. Polyethylene is odorless, non-toxic, and has a wax-like feel. It offers excellent low-temperature resistance, with an operating range from -70°C to -100°C. Its chemical stability is also high, as it is resistant to most acids and bases (except for oxidizing acids). Polyethylene is insoluble in common solvents at room temperature and has low water absorption. However, due to its linear molecular structure, it may slowly dissolve in certain organic solvents, though without swelling. Additionally, polyethylene provides excellent electrical insulation properties.

Coating for Anti-Corrosion:
The primary purpose of applying anti-corrosion coatings to large-diameter spiral steel pipes is to uniformly cover the pipe surface with a dense layer of paint that isolates the metal from corrosive media. This is one of the most fundamental methods for protecting steel pipes from corrosion. To meet growing needs, anti-corrosion coatings are increasingly using composite materials or layered structures. These materials must possess excellent dielectric properties, robust physical characteristics, stable chemical properties, and be suitable for a wide range of temperatures.

Outer Wall Anti-Corrosion Coating:
The coatings used on the outer walls of large-diameter spiral steel pipes vary depending on the application. These coatings must provide corrosion resistance and protection from environmental factors.

- Inner Wall Anti-Corrosion Coating: A coating applied to the interior surface of the pipe helps prevent internal corrosion, reduces friction, and increases throughput. Commonly used coatings include amine-cured epoxy resin and polyamide epoxy resin, with a coating thickness ranging from 0.038 mm to 0.2 mm. Proper surface treatment is required to ensure strong adhesion between the coating and the pipe wall. Since the 1970s, the same material has been used for both the inner and outer wall coatings, allowing simultaneous application of the coatings during the production process.

- Thermal Insulation and Anti-Corrosion Coating: For medium and small-diameter pipes used in thermally transporting crude oil or fuel, a composite layer of anti-corrosion and thermal insulation is applied to reduce heat dissipation to the surrounding soil. Rigid polyurethane foam is commonly used for insulation, with an operating temperature range of -185°C to 95°C. This material is relatively soft, so a layer of high-density polyethylene is added outside the insulation to enhance its strength and prevent groundwater from penetrating the insulation layer. This composite structure offers both thermal protection and corrosion resistance.