Several common anti-corrosion processes

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Nickel-based alloy thin sheet lining (commonly known as the “wallpapering” process) is currently used primarily at the dry-wet interface at the inlet of the flue gas in WFGD systems. When high-temperature flue gas enters the absorption tower, it comes into contact with the slurry sprayed from above, thereby creating a highly corrosive, acidic dry-wet interface at this location. Additionally, CL- ions are released from the slurry in this area. In areas such as gaps or welds in the metal structure, as well as on surfaces with defects, oxygen deficiency, difficulties in hydrolysis and ion diffusion lead to a potential difference between different parts of the metal matrix, resulting in the formation of galvanic cells. Corrosion in these gaps and pitting caused by CL- ions arise for these reasons, and such localized corrosion is more harmful than the uniform corrosion that can occur within the absorption tower. Numerous engineering practices of WFGD systems abroad have proven that nickel-based alloys are highly effective in the harsh corrosive environments of WFGD systems. Nickel-based alloys are generally defined as materials in which nickel is the main component or primary element. Their excellent corrosion resistance stems from a thin passivation film rich in chromium and oxygen on their surface; when this passivation film is damaged, it will re-form automatically if there is sufficient oxygen available. The composition of this passivation film is determined by the alloy’s components. Due to the addition of other alloying elements, nickel-based alloys have a lower rate of dissolution compared to ordinary steel, which enables them to maintain good chemical resistance even when their passivation film is damaged. The alloying elements added to improve the corrosion resistance of nickel-based alloys are typically nickel, molybdenum, and chromium; nickel helps to restore the damaged protective film and improves processing properties, especially welding properties ； Chromium forms a protective passivation film ； Molybdenum can enhance the alloy’s resistance to local corrosion such as pitting corrosion and crevice corrosion. In wet FGD systems, the use of nickel-based alloy materials mainly takes several forms: solid nickel-based alloys, explosive bonding of nickel-based alloys with the substrate, rolling bonding of nickel-based alloys with the substrate, and lining with thin nickel-based alloy sheets (commonly known as the \"wallpapering\" process). Compared with several other processes, the technology of lining with nickel-based alloy thin sheets is favored for its simplicity, low cost, and ease of implementation ； No special tools, equipment, or highly skilled trainers are required ； It can be carried out either in a workshop or on-site, which gives it wide applications; it is particularly the preferred method in the renovation of old systems. To date, this technology has been successfully used in wet FGD installations around the world for nearly 20 years, and according to foreign surveys, the average lifespan of such lining systems is 15 years. The so-called backing plate technique involves using thinner backing plates (with a thickness usually ranging from 1.6 mm to 3.2 mm) that are overlapped on top of the substrate, and welding them together to create a sealed corrosion-resistant system. 2. Glass flake lining: Due to its excellent corrosion resistance, superior resistance to medium penetration, low thermal expansion coefficient, relatively low cost, and good construction properties, glass flake lining is widely used in FGD systems. The glass flake lining used in WFGD systems is primarily composed of vinyl ester resin (55%–65%), micro glass flakes (about 30%), and other functional additives. The vinyl ester resins used in FGD systems mainly include standard bisphenol A epoxy vinyl ester resin and phenolic epoxy vinyl ester resin. Glass flakes refer to flaky thin glass products made from a certain type of (silicate) glass material through specific manufacturing processes; when combined with resin, they form anti-corrosion coatings. The vinyl ester resin itself possesses excellent corrosion resistance, while microglass flakes have outstanding acid resistance. By adding microglass flakes to the vinyl ester resin, a labyrinthine sealing structure composed of discontinuous flaky fillers is formed within this anti-corrosion lining (see Figure 2). This structure effectively prolongs the pathways through which corrosive agents can penetrate, and it also reduces the residual stresses generated during the curing process of the lining layer, thanks to the orderly distribution of the flakes. In this way, it overcomes the two main problems associated with traditional lining technologies: media penetration and corrosion caused by residual stresses. 3. Fiberglass Reinforced Plastic: Fiberglass reinforced plastic is a composite material that was developed early on and combines functional and structural properties. It possesses very notable performance characteristics; its corrosion resistance depends primarily on the resin used. As synthetic technology has continued to advance, the performance of resins has also improved. In particular, the emergence of vinyl ester resins in the 1960s further enhanced the corrosion resistance, physical properties, and heat resistance of fiberglass reinforced plastic. When properly designed, manufactured, and installed, fiberglass-reinforced plastic offers significant technical, economic, and social benefits when used in WFGD systems. Abroad, fiberglass-reinforced plastic components made from vinyl ester resin have been successfully used in more corrosive environments than those encountered in WFGD systems for a long time now. The temperature range at the flue gas inlet of the absorption tower in a WFGD system is 160°C to 180°C, and there are situations of sudden high-temperature cooling. Due to the risk of thermal damage and the formation of highly corrosive by-products, expensive alloy materials such as high-nickel alloy C-276 are typically used in this area to meet the required standards. Fiberglass reinforced with phenolic epoxy vinyl resin possesses excellent thermal shock resistance, which prevents delamination under thermal shock conditions at temperatures up to 200°C; it can therefore operate continuously for extended periods in flue gases at around 200°C. In corrosive environments, the wear resistance of fiberglass-reinforced plastic is superior to that of steel. To improve its wear resistance, appropriate fillers can be added to the resin matrix. Abroad, FRP desulfurization towers have been in use for a long time with great success. Desulfurization towers made of vinyl ester resin fiberglass can operate at higher temperatures, have a longer lifespan, and are more reliable. Currently, Dow Chemical Company in the United States has developed and produced FGD desulfurization towers that can be used at temperatures of 220°C. According to foreign sources, fiberglass has been successfully applied in chemical corrosion-resistant equipment such as desulfurization towers, slurry tanks, mist eliminators, slurry transport pipelines, flues, and chimneys in WFGD systems. 4. Rubber: Butyl rubber is the most commonly used in WFGD systems. Theoretically, rubber linings have lower heat resistance compared to glass flake linings, but better wear resistance and impermeability. Therefore, rubber linings are generally used in areas subject to high mechanical stress, such as inside absorption towers, limestone slurry systems, gypsum dewatering systems, and flues with lower temperatures. 5. Sprayed polyurea elastomer (SUPA) is a new type of solvent-free, pollution-free green construction technique that has been developed abroad over the past decade to meet environmental protection requirements. Its main raw material is amino-terminated polyoxypropylene ether developed by the American company Texaco/Huntsman. This process belongs to the category of fast-reacting spray systems; the raw materials contain no solvents, the curing speed is fast, and the process is simple. Since its introduction, this technology has seen rapid development. At present, its use in FGD systems in China is still in its initial stages, but there are already some successful applications on a small scale.