Surface treatment of steel pipes is a crucial step in pipeline corrosion prevention, directly impacting the lifespan of the pipe's protective coating and its effectiveness. Whether in industrial pipeline systems, construction, or the oil and gas industry, corrosion prevention measures play a pivotal role in ensuring the long-term safe operation of pipelines. Research shows that the quality of surface treatment affects the durability of the corrosion protection coating by as much as 50%. Therefore, it cannot be overlooked. This article will explore common steel pipe surface treatment methods and their impact on corrosion protection effectiveness, aiming to assist you in making the most appropriate decisions when selecting and implementing corrosion prevention measures.
Surface treatment of steel pipes is one of the key factors determining the service life of a pipeline's corrosion protection. It is the prerequisite for a strong bond between the protective coating and the steel pipe. According to research, the lifespan of a protective coating depends not only on the type of coating, coating quality, and construction environment but also on the surface treatment of the steel pipe, which accounts for about 50% of the coating's lifespan. This means that if surface treatment is inadequate, even the best corrosion protection coating will fail to perform as expected, and the pipeline's corrosion protection lifespan will be significantly reduced. Therefore, it is essential to strictly follow surface treatment standards for steel pipes and continuously explore, summarize, and improve treatment methods.
Now let's take a detailed look at several common surface treatment methods. Each method has its own principles and applications. Understanding their characteristics, advantages, and disadvantages can help us better choose the most suitable treatment method to achieve the best corrosion protection results.
Cleaning the steel surface with solvents or emulsions removes oils, grease, dust, lubricants, and similar organic substances. This method is relatively simple and cost-effective, but it has a clear limitation; it cannot remove rust, oxide scales, or welding flux from the steel surface. Therefore, cleaning is only used as an auxiliary measure in corrosion protection and cannot achieve the ideal corrosion protection effect on its own.
This method involves using tools such as wire brushes to grind the steel surface, removing loose or raised oxide scales, rust, welding slag, etc. Manual rust removal can achieve an Sa2 grade, while power tool rust removal can achieve an Sa3 grade. However, if the steel surface has firmly attached oxide scales, the effectiveness of tool-based rust removal is insufficient, failing to meet the required anchor profile depth for corrosion protection. The anchor profile depth refers to the roughness formed on the steel surface after rust removal, which directly affects the adhesion of the protective coating. If the anchor profile is not deep enough, the protective coating will not adhere firmly to the steel pipe surface, leading to problems such as peeling.
Acid pickling can be performed using chemical or electrolytic methods, but for pipeline corrosion protection, chemical acid pickling is usually used. Chemical acid pickling removes oxide scales, rust, and old coatings, and sometimes serves as a post-sandblasting treatment. The acid pickling process dissolves rust on the steel surface through a chemical reaction, making the surface relatively clean. However, while chemical cleaning can achieve a certain level of cleanliness and roughness, its anchor profile is shallow, and it may cause environmental pollution. As environmental regulations become stricter, the use of chemical acid pickling has been increasingly restricted.
Shot blasting is currently one of the most ideal methods for rust removal in pipeline corrosion protection. It involves using a high-powered motor to drive blades that rotate at high speeds, causing abrasives such as steel grit, steel shot, wire segments, and minerals to be projected at the steel pipe surface under centrifugal force. This method not only completely removes rust, oxides, and contaminants but also achieves the required uniform roughness on the steel pipe surface due to the intense impact and friction of the abrasives. After shot blasting, the steel pipe surface experiences enhanced physical adsorption and mechanical adhesion to the protective coating. Typically, sandblasting is used for the inner surface of pipes, while shot blasting is used for the outer surface.
Steel pipes have different corrosion protection requirements depending on their operating conditions, and selecting the appropriate corrosion protection coating is crucial. Some of the commonly used corrosion protection coatings in the market include epoxy resins, polyurethane, and three-layer PE coatings. Users can choose the most suitable coating based on their specific needs.
Before applying the protective coating, the steel pipe surface must be thoroughly cleaned to remove oil, rust, and other contaminants, ensuring proper adhesion and curing of the protective coating. If the surface treatment is inadequate, even high-quality corrosion protection coatings may not achieve the desired effect. This is similar to painting a dirty wall: no matter how good the paint is, it will be difficult to achieve a durable and attractive finish. Therefore, steel pipe surface pretreatment must be strictly carried out before construction.
Electrophoretic coating technology can form a dense corrosion-resistant film on the steel pipe surface, providing excellent corrosion resistance and wear resistance. However, the downside of electrophoretic coating is its high cost, although it is highly effective and worth considering for environments with extremely high corrosion protection requirements. Galvanizing involves applying a layer of zinc to the steel pipe surface, enhancing the corrosion resistance, wear resistance, and oxidation resistance of the pipe. In addition, galvanized steel pipes are also aesthetically pleasing and easy to process, making them widely used.
Once we understand the basic methods of surface treatment, we can delve deeper into the details and key points of coating-based corrosion protection. Coating corrosion protection is one of the most basic and widely used methods for preventing steel pipe corrosion. The core principle is to apply one or more layers of protective coatings to the steel pipe surface, forming a protective membrane that isolates the steel pipe from external corrosive media, thereby extending the pipe's service life.
Coating corrosion protection involves evenly applying a coating to the rust-treated steel pipe surface to isolate it from various corrosive substances. This is one of the most fundamental methods of steel pipe corrosion protection. By forming a protective film on the steel pipe surface, it isolates the pipe from water, oxygen, acids, alkalis, and other corrosive substances, thereby achieving corrosion protection.
Steel pipe corrosion protection coatings are increasingly being made from composite materials or structures. These materials and structures must have good dielectric properties, physical performance, stable chemical characteristics, and a wide temperature adaptability. For example, amine-cured epoxy resins and polyamide epoxy resins are commonly used as coatings for the inner wall of pipes. These coatings not only offer good corrosion resistance but also enhance pipeline flow and reduce energy consumption.
In medium and small-diameter pipelines for the transport of heated oil or fuel, composite layers of insulation and corrosion protection are applied to the outside of the pipeline to reduce heat loss to the surrounding soil. A commonly used insulation material is rigid polyurethane foam, which can withstand temperatures ranging from -185°C to 95°C. To enhance its strength, a layer of high-density polyethylene is applied to the outside of the insulation, forming a composite structure to prevent groundwater from infiltrating the insulation layer. Polyethylene is odorless, non-toxic, and has excellent low-temperature characteristics (with a minimum operating temperature range of -70°C to -100°C). It is chemically stable and resistant to most acids and alkalis (though not resistant to oxidizing acids). Polyethylene is not soluble in common solvents at room temperature, has low water absorption, and, as a linear polymer, can slowly dissolve in some organic solvents without swelling. This composite material structure leverages the advantages of each material, ensuring both corrosion protection and insulation, making it an effective and practical anti-corrosion method.
Steel pipe surface treatment is not just a technical step; it is vital to the long-term stability and safety of pipeline systems. By selecting the appropriate surface treatment methods and strictly following construction standards, the adhesion of the protective coating can be significantly enhanced, extending the service life of the pipeline. With increasing environmental requirements and continuous technological advancements, new treatment techniques and coating applications are emerging, providing more options for steel pipe corrosion protection. In future applications, choosing the right surface treatment method and corrosion-resistant materials will provide stronger protection for the long-term safe operation of pipeline systems.