Ge Hua Fei
Shanghai Municipal Engineering Design and Research Institute (Group) Co., Ltd., Shanghai 200092
Summary:A detailed analysis of the main characteristics, process requirements, usage conditions, and unit costs of six anti-corrosion methods was conducted, along with a comprehensive comparative study of both internal and external anti-corrosion applications. Appropriate selection recommendations are provided, including: epoxy coal tar and FBE for general corrosive environments, and 3PE for harsh corrosive environments. Additionally, cement mortar for internal anti-corrosion requires low pretreatment of steel pipes and possesses self-healing properties, while epoxy coatings and FBE for internal anti-corrosion require high pretreatment of steel pipes. Epoxy coatings are widely applicable, and FBE for internal anti-corrosion is generally recommended for simultaneous use with FBE for external anti-corrosion.
Keywords:- Steel pipe; corrosion protection; coal tar epoxy; epoxy powder for fusion welding; three-layer polyethylene; cement mortar
On selecting common corrosion prevention methods for large-diameter water supply steel pipes
Ge Huafei
(Shanghai Municipal Engineering Design Institute (Group) Co., Ltd., Shanghai 200092, China)
Abstract: This paper analyzes the main features, construction requirements, working conditions, and unit cost of six corrosion prevention methods, primarily focusing on comprehensive comparative research on internal and external corrosion prevention. It provides recommendations for selecting methods based on the specific environment, such as: epoxy and coal tar external corrosion prevention can be used in common corrosive environments; 3PE external corrosion prevention is suitable for highly corrosive environments; cement mortar internal corrosion prevention requires less pre-treatment of steel pipes due to its self-healing properties; liquid epoxy coating and FBE internal corrosion prevention require more stringent pre-treatment of steel pipes. The paper also clarifies that liquid epoxy coatings have extensive applications, while FBE internal corrosion prevention can be used in conjunction with FBE external corrosion prevention.
Key words: steel pipe; corrosion prevention; epoxy coal tar; fusion bonded epoxy (FBE); 3-layer PE (3PE); cement mortar
Introduction
Steel pipes offer high strength and ductility, excellent adaptability to various geological conditions, and ease of processing, making them a common choice for large-diameter water pipelines. However, steel pipes have poor corrosion resistance, and the quality of the internal and external corrosion protection layers directly affects their service life. Currently, common external corrosion protection methods for large-diameter steel water pipes include coal tar epoxy, fused epoxy powder (FBE), and three-layer polyethylene (3PE) corrosion protection. Common internal corrosion protection methods include cement mortar, liquid epoxy coatings, and FBE. Each corrosion protection method has different characteristics and application conditions, so it is essential to select the appropriate method based on the project's specific requirements.
01、External corrosion protection
1.1 Coal Tar Epoxy
Epoxy coal tar coatings, made from coal tar pitch, high molecular epoxy resin, and fillers, exhibit high strength, insulation, water resistance, high temperature resistance, corrosion resistance, and antibacterial properties. The structure consists of a base coat and multiple layers, with glass fiber fabric interlaced for reinforcement, categorized into three grades: general, reinforced, and ultra-reinforced. Detailed information can be found in Table 1.[1]"The 'six oil and two cloth' method is the most commonly used."
Before applying the anti-corrosion layer, first treat the steel pipe surface by using shot blasting and air blasting to meet the specified rust removal and dust removal standards. After the surface treatment is qualified, apply the anti-corrosion layer within 4 hours using high-pressure airless spraying or manual application. Each layer should be applied evenly, without any gaps or air bubbles. The next layer should be applied before the previous layer has fully cured. After application, allow it to naturally cure.[2].
Epoxy coal tar anti-corrosion has a 60-year history of use, with low cost. Currently, the market price for specialized high-grade anti-corrosion agents is approximately 60 yuan/m.2～70 yuan/m2It exhibits excellent corrosion resistance, low production requirements, and easy quality control, making it widely used today, such as in the Guangzhou Xijiang Water Diversion Project DN1600～DN3600 water pipelines and the Nanjing Jiangbei Emergency Water Supply Project DN1600～DN2600 water pipelines, all of which utilize "six oils and two fabrics" epoxy coal tar anti-corrosion.
1.2 Molten Bond Epoxy Powder (FBE)
FBE uses a gas as a carrier to apply solid coating onto preheated steel pipe surfaces, forming a uniform protective film through melting, leveling, and curing. The FBE technology was initially developed abroad in 1960 and introduced to China in 1980, where it has been widely used. The high-activity and high-polarity epoxy resins in FBE form strong chemical bonds with the metal surface, and the epoxy resin, hardener, and other components are evenly distributed in each FBE particle. During melting and curing, the coating undergoes complete crosslinking. The FBE coating has good mechanical properties and can withstand the stress, impact, friction, and bending requirements of steel pipes during toping operations. Additionally, it is corrosion-resistant and durable. FBE external anti-corrosion coatings typically consist of single-layer and double-layer structures, each with ordinary and reinforced grades. See Table 2 for details.[3].
Before applying the anti-corrosion layer, the steel pipe surface must be pre-treated. After the surface treatment meets the requirements, the steel pipe is uniformly heated to the temperature recommended by the coating manufacturer using a pollution-free heat source, typically between 180°C and 250°C. The FBE process involves thorough fluidizedization within a fluidized bed, followed by electrostatic spraying onto the steel pipe surface. The resulting mixture is then heated and melted, leveling out, and after a solidification period of 1 to 1.5 minutes, the steel pipe is cooled using air or water. The quality of the FBE coating is affected by factors such as the surface treatment effect, material properties, temperature control, uniformity of the coating, and solidification performance. The process conditions require high precision.
FBE anti-corrosion has been used in China for over 30 years. Its unit price is similar to that of coal tar epoxy coatings, with a market price of approximately 60 yuan/m.2~70 yuan/m2Compared to epoxy coal tar, it is more environmentally friendly and offers superior mechanical properties, with excellent strength and abrasion resistance, making it suitable for top-handling applications. Its wide range of applications include DN2200～DN2400 water supply pipelines for the new water source in Yancheng and the water intake project, as well as DN3600～DN4000 water supply pipelines (top-handling) for the upstream water source connection project on the Huangpu River, and DN1600～DN3600 water supply pipelines (top-handling) for the Qingcao Sha Yuan water project in Shanghai. All these applications utilize 400μm thick FBE coatings. Due to the high production requirements of FBE coatings, strict quality control is essential. Poor-quality FBE coatings are prone to detachment after a period of use.
1.3 Three-layer Polyethylene Anti-Corrosion (3PE)
The FBE primer application is identical to that of 1.2 mm, and the FBE bonding process involves applying bonding agent, where polyethylene is wrapped laterally around the bonding agent, followed by rolling and compaction, water cooling, to ensure complete curing of the FBE layer. The quality of the 3PE coating is primarily influenced by the quality of the FBE primer, and the process conditions require high precision.
3PE anti-corrosion has been used in China for over 30 years, with good application effects, few reported defects, and suitability for various harsh environments, including highly corrosive conditions. However, it has a higher cost, with a current market price of approximately 100 yuan/m.2～120 yuan/m2It is commonly used in national key projects, such as the West-to-East Gas Transmission Project and the South-to-North Water Transfer Project.
The 3PE anti-corrosion layer consists of a fused epoxy powder bottom layer, a polymer adhesive intermediate layer, and a polyethylene top layer. The FBE bottom layer provides adhesion and corrosion resistance, while the adhesive layer bonds with the FBE layer and the polyethylene layer. The polyethylene layer provides weather resistance and impact resistance. It is available in two grades: standard and enhanced, as shown in Table 3.[4].
2、Recommended external anti-corrosion measures
The comprehensive comparison of epoxy coal tar, FBE, and 3PE for external corrosion protection is shown in Table 4.
For pipelines in general corrosive environments, direct burial pipelines can be protected with epoxy coal tar or FBE (Fusion Bonded Epoxy) coatings. For pipelines in harsh corrosive environments or those requiring high levels of protection, 3PE (Polyethylene) coatings can be used. For the exterior of the pipe, FBE or epoxy glass flake coatings are typically used. Epoxy coal tar coatings are not resistant to mechanical abrasion, while 3PE coatings in pipe systems can be affected by soil friction and shear forces at the pipe head, leading to the detachment of the polyethylene. However, there are also engineering case studies of 3PE-coated pipes used in pipe systems.
3、Internal Corrosion Protection
3.1 Cement Mortar
Internal corrosion protection with cement mortar is achieved by spraying cement mortar onto the inner wall of the steel pipe using a centrifuge or spray gun. This creates a dense protective layer on the inner wall of the steel pipe, isolating it from water or air. Due to the similar thermal expansion rates of cement mortar and steel, the cement mortar forms a secure bond with the steel pipe surface under the following three forces: 1) The cohesive force generated by cement hydration; 2) The frictional force generated by the volume shrinkage of cement; and 3) The mechanical interlocking force between the cement mortar and the uneven inner wall of the pipe. In addition to the physical isolation effect, the cement mortar can generate an alkaline passivation zone with a pH of up to 12 on the inner wall of the pipe, providing chemical protection. The cement mortar lining is smooth and uniform, forming a slippery manganese-based layer, and the roughness coefficient can be maintained at around 0.012. As the cement hydrates in water, it generates calcium carbonate, which can repair cracks.
During on-site construction, the concrete mortar should be applied after the steel pipe has been laid, tested, and compacted with backfill, and the steel pipe should remain in a stable state. During factory prefabrication, the pipes with completed linings should remain stable during stacking, transportation, installation, and backfilling. Before construction, the inner walls of the steel pipes should be cleaned to remove surface rust and contaminants, and the height of any weld protrusions should not exceed 1/3 of the lining thickness. The pipes should be cleaned with water and allowed to dry before application. When using mechanical spraying, the lining thickness and tolerance requirements are shown in Table 5. When applying by hand, the thickness should be increased by 2mm. After the lining has set, it should be promptly watered and maintained. The maintenance temperature should not be less than 10°C, and the duration should not be less than 7 days. For mineral ash-silicate cement, the duration should not be less than 14 days.[5].
Concrete mortar anti-corrosion has been used in China for nearly a century, with a low cost, currently priced at approximately 50 yuan/m2. Compared to epoxy coatings and FBE anti-corrosion, it offers simple processing, low substrate surface pretreatment requirements, and reliable quality, making it a commonly used anti-corrosion method for large-diameter water pipelines. Recent projects, such as the DN1600～DN3600 water pipelines in the Shanghai Qingcao Sha Yuan project, the DN2200～DN2400 water pipelines in the Yancheng New Water Source and Irrigation project, the DN3600～DN4000 water pipelines connecting the Huangpu River upstream water source, and the DN3400 water pipelines in the Hangzhou Da Mao Wu – Ren He Avenue water supply pipeline, all utilize concrete mortar lining. During the 2002 renovation of the Shanghai Yangshu Pu Water Plant, a DN600 concrete mortar-lined steel pipe that had been in operation for 120 years was found to still maintain its integrity.
3.2 Liquid Epoxy Coatings
Liquid epoxy coatings are divided into solvent-based and solvent-free types. Solvent-based epoxy coatings have a resin content of approximately 50%～80%, with the remaining portion being volatile solvents, which are generally toxic. Solvent-free epoxy coatings have a resin content close to 100%. Compared to solvent-based epoxy coatings, solvent-free epoxy coatings offer advantages such as smaller usage, fewer coats, and environmental friendliness. Therefore, solvent-free liquid epoxy coatings are generally used for corrosion protection in water pipelines. Liquid epoxy coatings are two-component coatings, primarily composed of epoxy resins, with added curing agents, modifiers, and fillers. The composition is adjusted according to different functional requirements to create primers and topcoats. Primers provide adhesion and corrosion resistance, while topcoats provide decorative, scratch-resistant, weather-resistant, and corrosion-resistant properties. Some coatings also include intermediate coats, such as epoxy zinc-rich primers and commonly used epoxy iron-rich intermediate coats, to increase the coating thickness and adhesion between the primer and topcoat.
Prior to applying the anti-corrosion layer, the steel pipe surface must be treated. The process involves shot blasting and air blowing to achieve the specified rust removal and dust removal standards. After the surface treatment is completed and passes inspection, the anti-corrosion layer should be applied within 4 hours. Generally, mechanical spraying is used. During application, the coating should be delivered and atomized smoothly, with uniform speed. The anti-corrosion layer should be smooth, free from sagging and overspray. When applying multiple layers, the interval time between coats should be controlled. The curing process should be carefully monitored to ensure the correct temperature and time. Application should not be carried out in environments with relative humidity greater than 85% or in adverse weather conditions such as rain, snow, fog, dust, etc. Liquid epoxy coatings have three levels of anti-corrosion performance and thickness: ordinary, enhanced, and super-enhanced. Please refer to Table 6 for details.[6].
Liquid epoxy coatings offer excellent corrosion protection, easy and fast application, and relatively low environmental requirements. The current market price is approximately $50/m.2~60 yuan/m2is one of the most commonly used internal corrosion protection methods for water supply steel pipes, and is also the most commonly used corrosion protection method for repairing corrosion holes in steel pipe fittings and on-site steel pipes. The DN1600～DN3600 water supply steel pipes of the Guangzhou Xijiang Water Diversion Project and the DN1600～DN2600 water supply steel pipes of the Nanjing Jiangbei Emergency Water Source Project all use liquid epoxy coatings for internal corrosion protection.
3.3 FBE
FBE internal and external anti-corrosion properties have similar characteristics, but the main difference lies in the application method: external anti-corrosion uses fast-setting putty powder with a short curing time, typically 1 minute to 1.5 minutes, while internal anti-corrosion uses conventional putty powder with a longer curing time, typically 5 minutes or more. The resulting coating has good leveling properties. The thickness of the FBE internal anti-corrosion layer is shown in Table 7.[7]。 Similar to external anti-corrosion (FBE), the production process for internal anti-corrosion (FBE) requires stringent conditions. However, as the FBE anti-corrosion production process has gradually matured and improved, the application of both internal and external FBE anti-corrosion pipes, or internal FBE-3PE anti-corrosion pipes, is becoming increasingly widespread. This is due to the fact that both internal and external anti-corrosion processes utilize FBE anti-corrosion, which improves equipment utilization and production efficiency, reduces production costs, and offers a promising outlook.
4、Recommended anti-corrosion measures for the interior.
The comparison of concrete mortar, liquid epoxy coatings, and FBE (Fusion Bonded Epoxy) for corrosion protection is shown in Table 8.
The main characteristics of cement mortar for corrosion protection are low pre-treatment requirements for steel pipes, and self-healing properties, which ensure high safety and reliability. Prefabricated cement mortar-protected steel pipes in factories require high stability during construction and cannot be used in jack-and-box construction. The cement mortar lining for jack-and-box construction must be constructed on-site after the jack-and-box construction is complete.Epoxy liquid coatings require high pre-treatment of steel pipes, but are easy and quick to apply, with relatively low environmental requirements, making them widely applicable. In addition to protecting the outer surface of steel pipes, they are also frequently used for internal corrosion protection of pipe fittings and patching FBE (Fusion Bonded Epoxy) internal corrosion.FBE (Fusion Bonded Epoxy) has excellent mechanical properties, is durable and abrasion-resistant, and requires high pre-treatment and process conditions. It is rarely used alone and is typically used in conjunction with FBE external corrosion protection to improve production efficiency.
5、Observations
The selection of corrosion protection methods for large-diameter water pipelines should be based on specific project characteristics, considering factors such as geological conditions, climate conditions, pipeline construction methods, project construction period requirements, manufacturer's production technology level, supply cycle and distance, and overall cost. The chosen method should be suitable for the external environment, easy to construct, have reliable quality, and be economically feasible.
References:
[1] National Housing and Urban-Rural Construction Bureau of the People's Republic of China. Specifications for the Construction and Acceptance of Water Supply and Drainage Pipeline Engineering: GB50268-2008 [S]. Beijing: China Architectural Industry Press, 2008.
[2] National Energy Administration. Technical Standards for Epoxy Coal Tar Anti-Corrosion Layer on Buried Steel Pipelines: SY/T0447—2014[S]. Beijing: China Petroleum Industry Press, 2014.
［3］National Energy Administration. Technical Specifications for the Outer Coating of Steel Pipe Fusion with Epoxy Powder: SY/T0315—2013 [S]. Beijing: China Petroleum Press, 2013.
［4］China National Standards Commission. Corrosion protection of buried steel pipelines with polyethylene coating: GB/T 23257—2017[S]. Beijing: China Standards Press, 2017.
［5］China Engineering Construction Standardization Association. Construction and Inspection Procedures for Concrete Mortar Lining of Buried Water Pipelines: T/CECS10-2019[S]. Beijing: China Architectural Industry Press, 2019.
［6］National Energy Administration. Technical Specifications for Internal Anti-Corrosion Coatings for Steel Pipelines with Epoxy Resin: SY/T0457—2019[S]. Beijing: Petroleum Industry Press, 2019.
［7］National Energy Administration. Technical Standards for Anti-Corrosion Layer Technology on Steel Pipe Encapsulated with Epoxy Powder: SY/T0442—2018[S]. Beijing: Petroleum Industry Press, 2018.
This article is reprinted from "Shanxi Architecture" Magazine, Volume 29, Issue 18, September 2023.