Corrosion is a phenomenon in which a material, generally a metal, or its properties deteriorate due to chemical reactions with the surrounding environment. Protecting pipelines against corrosion and chemical damage requires not only knowledge of the fluids being transported, in order to prevent corrosion caused by the conveyed materials, but also an understanding of the environment surrounding the pipeline. Pipes may be buried in different types of soil or water. The chemical conditions surrounding transmission pipelines may be industrial, saline, or subject to significant temperature fluctuations during the day or across different seasons. Pipelines buried in seas and oceans may also be exposed to abrasion caused by sand, shells, and other sediments that can penetrate certain protective coatings.
Considering the above factors and the significant costs resulting from pipeline corrosion, reliable protection systems must be selected to control corrosion. Choosing an appropriate method for protecting transmission pipelines can increase pipeline service life, reduce shutdowns for repairs, and lower the costs associated with repairing or replacing protection systems.
Various organic coatings are used to protect the external surfaces of steel pipes against corrosion, complementing the protection provided by cathodic protection systems. The selection of a particular coating depends on the considerations mentioned above, as well as the external stresses that the pipe must withstand, compatibility with cathodic protection and its required current, soil characteristics, and other operating conditions.
The essential properties of an external coating system for steel pipes include:
- Impact resistance
- Resistance to penetration
- Resistance to cathodic disbondment
- Resistance to high temperatures
- 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 possible damage during handling and transportation
- Adequate health and safety performance
Types of External Coatings for Steel Pipes
A coating is a layer that prevents direct contact between an object, such as a pipe, and its surrounding environment. Steel pipe coatings may consist of one, two, or three layers, and in some applications, the pipe coating is additionally protected with cementitious materials.
Single- and Dual-Layer Epoxy Coatings
Epoxy powder is a solid thermosetting polymer that bonds to the heated metal surface through chemical reactions. Fusion-bonded epoxy powder formulations include hardeners, pigments, flow-control additives, and stabilizers. To apply epoxy powder to the pipe surface, the pipe is first cleaned by sandblasting or shot blasting and then heated. The epoxy powder is sprayed onto the surface using an electrostatic spray gun. Upon contact with the heated pipe, the powder melts and flows, then gels and cures to form a tough protective coating against corrosion. This type of coating has been commercially used since 1960.
In a dual-layer epoxy system, the first layer consists of epoxy powder used for corrosion protection, while the second layer is also made of fusion-bonded epoxy and is generally applied to provide mechanical protection for the pipe. The second epoxy layer is sprayed immediately after the first layer, while the first layer is still active, allowing chemical bonds to form between the two layers.
Two-Layer Polyolefin Coating
Extruded plastic coatings containing an adhesive or sealant have been used for corrosion protection since the 1950s. Two-layer coatings are available in various forms, such as butyl-based adhesives and butyl coatings, or asphalt-based mastics used as the first layer, with a polyolefin—most commonly polyethylene or polypropylene—used as the second layer. During application, the adhesive is heated until it reaches optimum flow and forms a uniform layer on the pipe surface. It is then immediately covered with a polyolefin layer of the required thickness.
Three-Layer Polyolefin Coatings
Three-layer coatings have been used since the 1980s. The polyolefin used as the outer coating is usually polyethylene or polypropylene.
First Layer: The epoxy powder used in the first layer plays the main role in protecting the pipe against corrosion. The epoxy powder is applied by an electrostatic spray gun at a thickness of 80 to 250 micrometers onto the pipe surface, which has previously been cleaned by sandblasting and heated for the coating process. The heat of the pipe melts the powder and cures the resin, forming a strong epoxy network.
Second Layer: The second layer is a copolymer adhesive applied over the epoxy layer by side extrusion at a thickness of approximately 250 micrometers. The adhesive must provide good chemical adhesion to both the underlying epoxy layer and the external coating.
Third Layer (Polyethylene): The third layer is applied by extrusion at a thickness of 1.2 to 3.5 mm to protect the pipe against physical damage. Polyethylene coatings are used for applications where the service fluid temperature is approximately 80°C.
Third Layer (Polypropylene): This layer has a thickness of 1.2 to 3.5 mm and, in some cases, up to 10 mm. It is applied over the previous two layers and can withstand service fluid temperatures of up to approximately 120°C.
Comparison of Polyethylene and Polypropylene in Three-Layer Coatings
When comparing polyethylene and polypropylene coatings, polypropylene has a higher service temperature and is a suitable option for transporting fluids at temperatures of up to 120°C. It also provides better impact resistance than polyethylene and, due to its higher abrasion resistance, is more suitable for projects where the coating is more likely to be damaged, such as rocky terrain, mountainous areas, and drilling operations.
In the next article, we will discuss other types of external coatings for steel pipes.
