Corrosion is the deterioration of a material, usually a metal, or its properties due to chemical reactions with its surroundings. Protecting pipelines against corrosion and chemical damage requires knowledge of both the transported fluids and the surrounding environment. Pipes may be buried in different soils or laid underwater. Pipeline environments may be industrial or saline and subject to significant daily or seasonal temperature fluctuations. Offshore pipelines may also be exposed to abrasion from gravel, sand, shells, and other sediments capable of penetrating certain protective coatings.
Given these factors and the substantial costs associated with pipeline corrosion, reliable corrosion protection systems must be selected. Choosing an appropriate protection method extends pipeline service life, reduces shutdowns for repairs, and lowers the costs of repairing or replacing protective systems.
Various organic coatings protect the external surfaces of steel pipes against corrosion, complementing cathodic protection. Coating selection depends on the considerations above, the external stresses the pipe must withstand, compatibility with cathodic protection and its current requirements, soil characteristics, and other factors.
Essential properties of external coating systems for steel pipes include:
- Impact resistance
- Penetration resistance
- Resistance to cathodic disbondment
- High-temperature resistance
- Resistance to soil stresses
- Resistance to water absorption
- Chemical resistance to acids and alkalis
- Sufficient flexibility for bending
- Hardness and abrasion resistance
- Adequate resistance to damage during handling and transportation
- Suitable health and safety characteristics
Types of External Coatings for Steel Pipes
A coating forms a barrier between an object, such as a pipe, and its surroundings. Steel pipe coatings may consist of one, two, or three layers. In some applications, the coating is protected by cementitious materials.
Single- and Dual-Layer Epoxy Coatings
Epoxy powder is a solid thermosetting polymer that adheres to a heated metal surface through chemical reactions. Fusion-bonded epoxy (FBE) powder formulations contain hardeners, pigments, flow-control additives, and stabilizers. To apply the powder, a pipe previously cleaned by sandblasting or shot blasting is heated, and the epoxy powder is sprayed onto its surface using an electrostatic gun. Upon contact with the hot pipe, the powder melts and flows, then gels and cures to form a tough corrosion-resistant coating. This type of coating has been commercially available since 1960.
In dual-layer epoxy systems, the first epoxy powder layer provides corrosion protection, while the second layer, also made of FBE, typically provides mechanical protection. The second layer is sprayed immediately after the first, while the first layer remains reactive, allowing chemical bonds to form between the two layers.
Dual-Layer Polyolefin Coatings
Extruded plastic coatings incorporating adhesives or sealants have been used for corrosion protection since 1950. Dual-layer coatings vary widely. The first layer may consist of a butyl-based adhesive, a butyl coating, or an asphalt-based mastic, while the second layer is a polyolefin, usually polyethylene or polypropylene. During application, the adhesive is heated to achieve optimal flow and form a uniform coating over the pipe surface. It is then immediately covered with a polyolefin layer of the appropriate thickness.
Three-Layer Polyolefin Coatings
Three-layer coatings have been used since 1980. The polyolefin topcoat is usually polyethylene or polypropylene.
First layer: The epoxy powder in the first layer plays the primary role in protecting the pipe against corrosion. It is applied using an electrostatic gun at a thickness of 80–250 μm to a pipe surface previously cleaned by sandblasting and heated for coating. The pipe’s heat melts the powder and cures the resin, forming a strong epoxy network.
Second layer: A copolymer adhesive is applied over the epoxy layer by side extrusion at a thickness of 250 μm. The adhesive must provide good chemical adhesion to both the underlying epoxy layer and the outer coating.
Third layer (polyethylene): A polyethylene layer, 1.2–3.5 mm thick, is applied by extrusion to protect the pipe against physical damage. Polyethylene coatings are used when the transported fluid has a service temperature of approximately 80°C.
Third layer (polypropylene): This layer is applied over the previous two layers at a thickness of 1.2–3.5 mm, occasionally reaching 10 mm. It can withstand service fluid temperatures of 120°C.
Comparison of Polyethylene and Polypropylene in Three-Layer Coatings
Compared with polyethylene, polypropylene has a higher service temperature and is suitable for transporting fluids at temperatures up to 120°C. It also offers better impact resistance. Its higher abrasion resistance makes it more suitable for projects where coating damage is more likely, such as rocky terrain, mountainous areas, or drilling operations.
In the next article, we will discuss other types of external coatings for steel pipes.
