- Author: Liu Yukun, Wu Di, Cui Zhigang
Langfang Aiguma Newli Material Technology Co., Ltd.
Summary:Epoxy powder coatings exhibit excellent anti-corrosion properties and are widely used in pipeline corrosion protection. Geothermal pipelines have characteristics such as high temperature, corrosion resistance, and fouling, therefore, epoxy powder coatings used for geothermal pipelines require targeted design research. This article discusses the heat resistance, corrosion resistance, and fouling resistance of epoxy powder coatings from multiple aspects, providing guidance for the formulation design of epoxy powder coatings for hot water pipelines.
Keywords:- Curing epoxy powder coating; geothermal water; heat resistance; corrosion resistance;
Research on Anticorrosive and Antiscaling Epoxy Powder Coatings with High Temperature Resistance for Geothermal Water Pipelines
Liu Yukun, Wu Di, Cui Zhigang
(Langfang AGM-KINLIN Material Technology Co., Ltd., Langfang, Hebei 065001, China)
Abstract: Epoxy powder coating exhibits excellent anti-corrosion performance and is widely used in pipeline anti-corrosion applications. Geothermal water is characterized by high temperature, high corrosion potential, and scaling, therefore, epoxy powder coating for geothermal water pipelines needs to be specifically designed. This paper discusses the heat resistance, anti-corrosion, and anti-scaling properties of epoxy powder coating, providing suggestions for the formulation design of epoxy powder coating for geothermal water pipelines.
Keywords: fusion-bonded epoxy powder coating; geothermal water; heat resistance; anticorrosion; anti-scaling
0. Introduction
In recent years, China has continuously increased its efforts to address climate change and promote the transition to clean energy, aiming to achieve peak carbon emissions by 2030 and carbon neutrality by 2060. This demonstrates that the transition to low-carbon energy is an unstoppable trend. Geothermal energy is a clean and renewable energy source that can be widely used in various fields, such as power generation, heating, healthcare, agriculture, animal husbandry, and refrigeration (using heat pump technology). The scientific and rational development and utilization of geothermal resources are of great importance for reducing carbon emissions and developing a low-carbon economy. Geothermal water contains SiO2、Cl-、SO42-、CO2、Ca2+、Mg2+、Ba2+、H+、H2S, O2Due to the presence of corrosive media and easily crystallizing salts, as well as the high water temperature, it exhibits high corrosion and fouling characteristics.[1-2]"This requires the protective layer to have excellent water resistance and anti-corrosion and anti-scaling properties. While fused epoxy powder is a mature technology for pipe corrosion protection, specific research and development are necessary due to the unique characteristics of geothermal environments.
1. The detrimental effects of geothermal water on organic coatings.
Organic coatings, upon absorbing water, undergo swelling, significantly increasing the space for molecular motion and altering the intermolecular forces. Furthermore, due to the presence of numerous polar groups in the coating, hydrogen bonding occurs between these groups after film formation, further enhancing the density of the coating. However, the ingress of water and oxygen largely disrupts this effect, causing the coating to soften [3]. The adhesion between the coating and the substrate involves both physical bonding and chemical bonding. The softening of the coating and the destructive effects of water and oxygen directly lead to a decrease in adhesion. Higher ground temperatures will undoubtedly exacerbate this destructive effect. Swelling provides a larger volume for molecular movement, and higher temperatures accelerate the movement of molecules and chain segments, combined with the interaction between water and oxygen with polar groups. When the molecular kinetic energy exceeds the van der Waals forces and the bond dissociation energy, it directly leads to a decrease in adhesion, coating aging and degradation, and the failure of protection.
2 Formulation Design for Eutectic Epoxy Powder for Geothermal Wells
The formulation for epoxy powder used in geothermal water pipelines requires careful consideration of various aspects during design, including:* Coating's thermal resistance, water resistance, adhesion, integrity, and anti-scaling properties.The following details specific considerations for designing formulations to achieve each performance characteristic.
2.1 Glass Transition Temperature of the Coating
The thermal resistance of organic coatings is directly related to their glass transition temperature. When the temperature of the environment exceeds the glass transition temperature of the coating, the polymer chains within the coating begin to move, leading to a decrease in adhesion. For water pipelines, it is necessary to consider the destructive effects of water on the coating. Experiments have shown that after the coating absorbs water, its glass transition temperature decreases significantly.[4]Table 1 lists the glass transition temperatures of six pure epoxy systems, for both dry and wet films.
As shown in Table 1, different formulations exhibit varying degrees of reduction in the wet glass transition temperature of the coating film. However, all formulations show a significant reduction, generally ranging from 20 to 30°C. Therefore, when designing formulations, it is recommended that the wet glass transition temperature of the coating film be at least 30°C higher than the highest water temperature, and that the wet glass transition temperature of the coating film should not be lower than the highest water temperature.
2.2 Adhesion between coating and substrate
Good adhesion is the prerequisite for corrosion resistance. Pre-treatment of the substrate plays a decisive role in adhesion, which will not be discussed here. This article focuses on analyzing the factors that affect the coating itself. The bonding between the coating and the substrate is mainly achieved through physical bonding and hydrogen bonding. Regardless of the bonding strength, good wetting is essential. Therefore, appropriately reducing the viscosity and surface tension of the coating system, and extending the curing time, can effectively improve the wetting condition. During the coating process, the wetting condition of the powder coating can also be improved by adjusting the preheating and curing temperature.[5]。Methods for assessing adhesion typically include: peeling test, cross-hatch test, and pull-off test. This article selects epoxy resins with different Bisphenol A softening points, uses phenolic curing agents, and uses barium sulfate as filler, to produce three types of epoxy powder coatings without using other additives to enhance adhesion. According to the test methods and standards specified in SY/T0442-2018 "Technical Standard for Epoxy Powder Coating Anti-Corrosion Layer on Steel Pipes," 100mm x 100mm x 6mm carbon steel plates are sandblasted, and epoxy powder coatings are applied using thermal curing. Different pre-heating temperatures are selected, and experimental samples are produced. The adhesion is tested after immersing the samples in 95°C water for 24 hours. The results on the influence of resin softening point and pre-heating temperature are shown in Table 3 and Table 4.
As shown in Table 3 and Table 4, the water resistance adhesion decreases with the increase of the epoxy softening point, and increases with the increase of the preheating temperature. This reflects, to some extent, that as the melt viscosity increases, the wetting between the coating and the substrate deteriorates, leading to a decrease in adhesion.
Furthermore, there are many additives available on the market that enhance coating adhesion. These additives typically have a high hydroxyl content, which forms hydrogen bonds between the hydroxyl groups and the polar groups on the substrate surface to improve coating adhesion. However, high hydroxyl content can also lead to decreased water resistance. Therefore, it is important to select the appropriate additive and verify the optimal addition amount through experiments to achieve the best results.[6]。
2.3 Film Integrity
Pores in the coating often provide pathways for the penetration of water and oxygen, negatively impacting the coating's corrosion resistance. Several factors influence the porosity of the coating, including the volatile components in the powder coating, the curing speed of the powder coating, the viscosity of the system, the coating thickness, the cleanliness of the substrate, the anchor pattern depth, and the cleanliness of the air during application. For geothermal pipelines, the impact of coating porosity on its protective ability is particularly significant and can even significantly accelerate the aging of the coating. To achieve the ideal porosity, the powder coating formulation, production process, and application process must be comprehensively combined. Excluding the influence of material volatility and moisture absorption during production and storage, the viscosity and curing speed of the powder coating often have the greatest impact on porosity.[7]According to the "Technical Specifications for Anti-Corrosion Pipe Coating Bonding Surface Porosity and Cross-Sectional Porosity Determination Method" (SY/T0315-2013), this article tests the differences in porosity of epoxy resins and epoxy powders with varying softening points and curing times, as shown in Table 5. The method for determining the porosity and cross-sectional porosity of anti-corrosion pipe coatings according to the specified standards is illustrated in Figure 1 (from 1st to 5th level, from top to bottom).
As shown in Table 5, with the increase in the softening point, the porosity of the bonding surface also gradually increases, primarily due to the increased viscosity of the resin, which leads to poorer wetting of the substrate, and the high viscosity hinders the release of gases. Furthermore, as the curing time increases, the porosity gradually decreases, which is due to the increased time for the system to achieve a fluid state, which promotes better wetting and gas release.
Please note that many coatings with low porosity can also exhibit corrosion during actual protection, which often occurs in areas where the coating is poorly applied, too thin, or contains impurities and pinholes. These are all instances of coating defects. After applying the protective coating on the pipes, an electrical leak test must be performed to check for any defects in the coating. These defects are the weakest points in the protective coating and provide pathways for corrosive agents to penetrate. Therefore, it is essential to identify and eliminate any defects before the pipes are put into use. Figure 2 shows the corrosion that occurs due to defects in the protective coating.
2.4 Coating Anti-Fouling Ability
Geothermal water contains a large amount of minerals, which can easily form scale. As shown in Figure 3. Scale not only significantly increases the flow resistance, but also can potentially cause pipe blockage and accelerate corrosion over time. Therefore, a protective layer with good anti-scaling properties is required for the inner wall of the pipes. When designing the formulation, the anti-scaling and anti-fouling ability of the coating film is often improved by reducing the surface tension of the coating, for example, adding a certain amount of fluor wax powder can greatly reduce the surface tension and friction coefficient of the coating film.[8]Additionally, a smooth and uniform coating finish can effectively reduce conveying resistance and prevent the accumulation of dirt, which requires the coordination of epoxy powder preparation processes and customer coating techniques.
2.5 Material Selection
To generally increase the glass transition temperature of the coating, the following three methods are typically employed: ① Select a curing system with a higher crosslinking density, such as using phenolic modified epoxy resins; ② Modify the resin structure to increase rigid groups such as biphenyl and naphthalene rings; ③ Select a curing agent to improve the system's heat resistance, such as phenolic resins, polyamines, or aromatic amines. When selecting a curing system, it is also important to consider the system's water resistance. A higher crosslinking density in the system results in a smaller internal free volume within the crosslinks, which can effectively reduce the rate of water penetration and improve the water resistance of the coating. However, numerous studies have shown that the final water absorption rate of the coating is not directly related to the crosslinking density. Many high-crosslinking density resin systems also have high water absorption rates. This is because coatings with high crosslinking densities often contain more polar groups, which increase the hydrophilicity of the coating. Therefore, when designing the formulation, it is necessary to consider the performance of the coating in a water environment.[9]。
Due to limitations in coating properties and cost, the selection of epoxy resins is restricted. Furthermore, to achieve good adhesion, the resin itself needs to contain a large number of polar groups. Therefore, adjusting the water absorption rate through different types of epoxy resins is limited. Powder manufacturers typically improve the system's water absorption rate by adjusting the types of hardeners and fillers.[10]"It is worth noting that when using epoxy powder coatings for hot water pipes, it is advisable to avoid using dicyandiamide as a curing agent. Dicyandiamide provides good heat resistance due to the reactive hydrogen in -NH2, which reacts with the epoxy group, and also the -C≡N group and the tertiary amine generated from the reaction, which can catalyze the self-etherification of the epoxy resin. This results in a higher cross-linking density and a higher glass transition temperature compared to general amine curing agents. However, dicyandiamide is water-soluble, especially at temperatures above 80°C, which can cause dicyandiamide to decompose. Additionally, dicyandiamide has a high melting point (approximately 210°C), and it remains solid during extrusion or curing, so only the surface groups of the dicyandiamide particles react with the epoxy resin. If it comes into contact with water, it will dissolve or decompose, leading to a decrease in the mechanical strength of the coating. The epoxy powder in Table 1, with a glass transition temperature of 152°C, uses dicyandiamide as a curing agent."
The selection of filler material also significantly impacts the water resistance of the coating film. Spherical fillers help to wet and encapsulate the resin, improving the density of the coating film. Plate-shaped fillers can form an effective barrier within the coating film, preventing the penetration of water. Epoxy powder coatings commonly use silicon microspheres, silica powder, and plate-shaped mica powder to improve the density and water resistance of the coating film.
3. Experimental Comparison
This document describes the design and production of four epoxy powder coatings with different glass transition temperatures. The performance of each coating was tested using high-pressure pot water boiling and alternating hot and cold water experiments. The results are shown in Table 6. The high-pressure pot was boiled with water (120°C) for 8 hours per day, and the coating was allowed to stand and insulate (80°C) for 16 hours. The experiment lasted for 120 days.
The cold and hot water alternating test method, based on the reference standard EN877, is primarily used to assess the aging resistance of epoxy coatings under alternating cold and hot water conditions. The specific method is as follows:
① (30±1) L (93±2) °C water, flowing through the piping system for 1 minute.
② Empty and allow to stand for 1 minute;
③ (30±1) L (15±5) °C water, with a flow rate of 1 min passing through the piping system;
④ Empty and allow to stand for 1 minute.
The above steps are repeated 1500 times.
Tg=126°C epoxy powder is made by combining phenolic modified epoxy with phenolic curing agents, and it performs excellently in high-pressure cooker experiments and alternating hot and cold water tests. However, due to its glass transition temperature, it is not recommended for use in environments where water temperature > 100°C. Tg=152°C epoxy powder is made by combining phenolic modified epoxy with dicyandiamide, and from high-pressure cooker experiments and alternating hot and cold water tests, it can be seen that even with a high glass transition temperature, its resistance to hot water is still poor. Tg=164°C epoxy powder is made by combining phenolic modified epoxy with fatty amine curing agents, and it performs well in high-pressure cooker and alternating hot and cold water tests. Tg=188°C epoxy powder is made by combining a special modified epoxy and a special modified curing agent, and its experimental performance is also good.
Please note that phenolic-modified epoxy resins can significantly enhance the coating's temperature resistance, but the resulting coating is often brittle. When designing the formulation, it is important to also consider the coating's mechanical properties. During pipeline installation, there is a risk of bending and deformation, so it is essential to ensure that the coating does not crack or otherwise deteriorate under these conditions.
The highest temperature achieved in the experiment was only 120°C. Subsequently, this article conducted experiments in collaboration with the client, simulating the composition of geothermal water at 150°C, and subjected the coating to high-pressure boiling for one month. The coating performance is shown in Table 7 and Figure 6.
Under 150°C geothermal conditions, the differences in the heat resistance of the coatings are more apparent. The epoxy coating with a Tg of 164°C exhibited the best performance, with no changes observed after testing. The epoxy coating with a Tg of 126°C showed numerous small bubbles after testing. The bisphenol A-based epoxy coating with a Tg of 152°C exhibited the worst performance, producing a large number of large bubbles after testing. All three coatings showed no significant fouling.
4. Conclusion
Fused epoxy powder coating is an ideal corrosion and fouling protection coating for geothermal pipelines. When designing the formulation, it is essential to fully consider the destructive effects of hot water on the protective coating, and to reasonably balance thermal resistance, corrosion resistance, fouling resistance, and mechanical properties, as well as comprehensively evaluate the changes of the material in the geothermal environment. This will enable the design of appropriate and applicable epoxy powder coatings to achieve long-term corrosion and fouling protection.
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This article is reprinted from "Coatings and Protective Materials", Volume 43, Issue 10.