【Haichuan Pipeline Installation】Why is internal anti-corrosion treatment necessary for ductile iron pipes? What are the main types?
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Ductile iron pipes are a type of cast iron pipe. In terms of quality, the spheroidization grade of cast iron pipes is required to be controlled at levels 1–3 (with a spheroidization rate of >80%), which results in improved mechanical properties of the material itself; it possesses the characteristics of iron along with the properties of steel. The annealed ductile iron pipes have a microstructure consisting of ferrite with a small amount of pearlite; they exhibit good mechanical properties, excellent corrosion resistance, good ductility, and good sealing performance. They are easy to install and are primarily used for water supply, gas transmission, and oil transportation in municipal and industrial facilities. A certain amount of spherical graphite is distributed within a ferrite and pearlite matrix; depending on the nominal diameter and the requirements regarding elongation, the ratio of ferrite to pearlite in the matrix varies. In cases of smaller diameters, the pearlite proportion is generally no more than 20%, while in larger diameters it is usually kept around 25%. Ductile iron pipes possess the properties of iron and the strength of steel; they have excellent corrosion resistance, good ductility, strong sealing capabilities, and are easy to install. They are primarily used in municipal systems as well as in industrial and mining enterprises for water supply, gas transmission, and oil transportation. It is the preferred choice for water supply pipes, offering excellent cost-performance. Compared to PE pipes, ductile iron pipes are simpler and faster to install, and they offer better internal and external pressure resistance after installation. In terms of airtightness and corrosion resistance, ductile iron pipes provide better airtightness after installation, and their corrosion resistance can be enhanced through various anti-corrosion methods. Regarding hydraulic performance, since the specifications for ductile iron pipes generally refer to the inner diameter while those for PE pipes refer to the outer diameter, ductile iron pipes enable a higher flow rate under the same specifications. Overall, in terms of installation and maintenance costs, ductile iron pipes offer a better cost-performance ratio. Zinc plating on the inner wall, cement mortar as a corrosion-resistant material, etc.
Cement mortar lining: The cement used in cement mortar linings falls into three categories – ordinary Portland cement, high-alumina cement, and sulfate-resistant cement. Ordinary Portland cement mortar lining: This is a hydraulic binding material made by grinding Portland cement clinker, 5%-20% of supplementary materials, and an appropriate amount of gypsum. It features high strength, high heat of hydration, good frost resistance, low shrinkage upon drying, good wear resistance, good resistance to carbonation, poor corrosion resistance, and poor tolerance to high temperatures. It is suitable for most raw water and drinking water, and is widely used.
High-alumina cement lining: High-alumina cement (formerly known as bauxite cement) is a hydraulic cementing material whose main mineral component is aluminate; it is produced by using bauxite and lime as raw materials, mixing them in certain proportions, then calcining and grinding them. It is also referred to as aluminate cement. Under normal curing conditions, aluminate cement exhibits strong resistance to sulfate corrosion due to the absence of tricalcium aluminate and calcium hydroxide in the cement paste, as well as its high density. It is widely used in sewage-related projects.
High-sulfur-resistant cement lining: Sulfate-resistant cement is a type of Portland cement series. As the name implies, this type of cement must have a certain resistance to sulfate attack. In Portland cement, the hydration products most susceptible to sulfate attack originate from the cement mineral tricalcium aluminate (C3A); therefore, sulfate-resistant cement primarily limits the amount of C3A in the cement, and to a lesser extent, it also limits the amount of tricalcium silicate (C3S). Furthermore, calcium hydroxide produced by the hydration of calcium silicate minerals can also easily react with sulfates and be eroded; sulfates are the second most abundant ion in seawater after chloride. Therefore, high-sulfate-resistant cement-lined ductile iron pipes are generally used in projects involving the transport of seawater.
Polyurethane coating: The polyurethane coating on ductile iron pipes is applied by spraying polyurethane paint. This urethane coating has the following advantages: ① Good chemical stability. ② Good resistance to water penetration. ③ Higher adhesion and mechanical strength. ④ Polyurethane coatings exhibit good resistance to bending and ovalization. In addition to being suitable for water with high corrosivity, polyurethane lining coatings can also be used to transport industrial wastewater with a high content of solid particles.
The epoxy-ceramic anti-corrosion coating is composed of epoxy resin and quartz powder (with a weight ratio of over 50%). This coating features high mechanical strength, excellent adhesion, good anti-corrosion properties, high hardness, as well as smoothness and wear resistance. It also possesses a certain degree of toughness, making it an excellent anti-corrosion coating for pipe linings that can **increase the service life of cast pipes in corrosive environments. Epoxy ceramic linings are also suitable for gas pipelines and sewage pipes. However, due to their high cost, complex manufacturing process, and stringent production requirements, they have certain limitations in terms of application. Epoxy ceramics are divided into black ceramic (GH101 type) and white ceramic (CH102 type)
White porcelain-coated cast iron pipes are used to transport water that is suitable for drinking or reusing. Epoxy ceramic products can also be used for internal anti-corrosion in wastewater treatment plants and chemical plant equipment.
Cement + epoxy resin lining: A cement + epoxy resin lining refers to the application of a special epoxy resin coating of a certain thickness on the surface of the cement mortar lining as a sealing layer, thereby providing a physical barrier on the inner wall of the ductile iron pipe to prevent water from coming into contact with the inner surface of the pipe. ①It can promote the hydration reaction of cement mortar, enabling uniform curing and thus ensuring stable quality; ② It prevents the precipitation of alkaline substances in the cement mortar lining, which not only extends the lifespan of this lining but also avoids an increase in the pH value of the water during water flow.
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