Application of New Technologies for Thermal Corrosion Wear of Boiler Water Wall Tubes and Arc Spraying Protection

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Application of New Technologies for Thermal Corrosion Wear of Boiler Water Wall Tubes and Arc Spraying Protection – Leiyang Leiyang Comprehensive Utilization Power Generation Co., Ltd., Leiyang, Hunan (421800), China: Li Xinglong, Chai Binlin 【Abstract】 Tube wear in the water wall is the most prominent issue among thermal corrosion wears in the heating surfaces of circulating fluidized bed boilers. This article provides a brief overview of the thermal corrosion and wear of water wall tubes and their hazards. It focuses on the characteristics of high-speed arc thermal spraying technology, which shows significant effectiveness in preventing corrosion and wear, as well as its application in protecting the tubes of power plant circulating fluidized bed boilers from thermal corrosion and wear. 【Keywords】 Boiler; Pipes; Arc spraying; Corrosion protection 0 Introduction Circulating fluidized bed CFB boilers offer advantages such as excellent fuel adaptability, high combustion efficiency, low emissions of gaseous pollutants, a wide range of load regulation capabilities, and the possibility of comprehensive utilization of ash and slag; they have seen rapid development in China over the past 20 years. At present, there are nearly 300 circulating fluidized boilers in use in China with an evaporation capacity of over 10 t/t; more than 2,000 such boilers have an evaporation capacity of over 35 t/h, and nearly 260 have an evaporation capacity between 220 and 480 t/h ； 20 large circulating fluidized bed boilers with an evaporation capacity of 1,025 t/h are already in operation, and over 30 more are under construction. In addition, the development project of 600MW supercritical CFB boilers in our country is progressing smoothly, and China has the largest total number of circulating fluidized bed boilers as well as the highest total evaporation capacity in the world. 　　With the rapid development of circulating fluidized bed boilers, extensive experience has been accumulated in design, manufacturing, installation, commissioning, and operation and maintenance, but some problems have also emerged. During the operation of CFB boiler units, shutdowns caused by factors such as wear of metal materials and detachment of refractory and wear-resistant materials account for 40% of all shutdowns. The main problems include high-temperature corrosion and wear of the boiler’s heating surfaces, tube bursts, as well as wear, cracking, and detachment of the refractory wear-resistant linings ； Leakage of slag and wear of the wind cap ； Combustion shutdown, slagging, and explosions, etc. The existence of these problems affects the continuous, safe, and economical operation of CFB boilers, and also leads to issues such as heavy maintenance workload and high operating costs. 　　The combustion method of CFB boilers determines that the wear on their water wall heating surfaces is more severe than that in coal powder boilers; the specific degree of wear depends on factors such as the boiler type, the type of coal used for combustion, and combustion adjustments. People attempt to adopt reasonable methods to effectively control wear, in order to improve the safety, stability, and economic efficiency of CFB boiler operation. In recent years, thermal spraying technology has developed rapidly in China. Heat and wear-resistant coatings are widely used in various industries, and attention has also been paid to wear protection for the tubes in the heating surfaces of power plant boilers. The anti-wear maintenance technique of directly performing thermal spraying on the water wall tubes inside the furnace offers flexibility in implementation, is time-saving and convenient, requires minimal resources, and brings great convenience to power generation companies. 1 Analysis of the corrosion mechanism of water wall tubes in power station boilers. In the heated surfaces such as the furnace water wall of CFB boilers, the main cause of failure is intense erosion and wear caused by high-speed, high-concentration streams of bed material and fuel gas; severe vortex and cutting effects occur in certain areas, in addition to which there is also high-temperature corrosion caused by the flue gases. 　　The wear of the pipe wall is closely related to factors such as the concentration of solid particles in the gas flow, the velocity of the flue gas, the hardness of the particles, and the geometry of the flow channel. According to relevant research, wear is proportional to the 3.6th power of the flue gas velocity and to the dust concentration. In CFB boilers, the concentration of solid materials is very high, often ranging from several dozen to hundreds of times that in coal-fired boilers. Moreover, these particles are hard and have sharp edges; as a result, driven by the high-speed flue gas, they cause severe erosion and wear on heat-exchanging surfaces such as the water wall ； The eddy current effect occurs in the water-cooled wall tubes located at the four corners of the furnace, within 2 meters of the outlet of the dense-phase zone, and on both sides of the furnace outlet; it is caused by the combined action of the primary flow-induced wind and secondary disturbing winds, resulting in particularly severe local wear on the tube walls ； The cutting effect is manifested in the water wall above the dense phase zone; when solid particles such as coking slag descend to this area at high speeds, they rebound. The portion that rebounds toward the water wall exerts a cutting effect on the water wall tubes, resulting in severe wear. The water wall at the flue gas outlet is subjected to severe erosion and wear due to the inertial effect of solid particles as the flue gas changes direction. 　　It is also susceptible to high-temperature oxidation as well as thermal corrosion by sulfates, sulfur, and sulfides. The water wall tubes are exposed to conditions of high-temperature oxidation and corrosion; the flue gas temperature is high, and oxygen-enriched combustion takes place. Practice has shown that above 300°C, the corrosion rate doubles for every 50°C increase in the surface temperature of the tubes. During operation, the boiler’s heat-exchange surface tubes first undergo high-temperature oxidation, resulting in the formation of Fe2O3 on their surface. Secondly, Na2O and K2O present in the fuel ash combine with SO3 in the flue gases to form sulfates, which capture fly ash and lead to the formation of slag. At this point, SO3 and MgSO4 react with Fe2O3 on the tube walls to produce complex sulfates such as MFe(SO4)2 or M3Fe(SO4)3. These complex sulfates decompose under high temperatures into loose iron oxide and sulfate deposits, which can be easily carried away by the flow of fly ash. The oxidative corrosion then continues to progress deeper into the tube walls ； Furthermore, the sulfur content in the fuel, upon combustion, produces S and H2S, which also cause severe corrosion of the tube walls; they react with Fe to form FeS. 　　Based on the above analysis, it can be determined that the corrosion of the water wall in CFB boilers is relatively complex, mainly involving oxidation and sulfate thermal corrosion, as well as corrosion caused by sulfides and hydrogen sulfide. In addition, there is the erosion wear caused by the fly ash generated by combustion, which, driven by air currents, rushes at high speed and scrapes against the surface of the furnace tubes. 2. Application of arc spraying in the wear and corrosion resistance protection of CFB boiler pipes 2.1 Status of thermal spraying protection for CFB boiler pipes at home and abroad For the protection of boiler pipes, foreign countries generally use oxy-acetylene powder spraying, wire flame spraying, arc spraying, and plasma processes. In the first two construction methods, the low flame temperature results in insufficient melting of the material, and the low speed of the sprayed particles leads to a low impact velocity of these particles; as a result, more oxides form on the surface of the coating, the porosity increases, and the bonding strength decreases. The latter two spraying processes can produce coatings with high bond strength, low porosity, and very few oxides, which is why they are widely used. 　　The United States has recently introduced composite materials based on Ni-Cr alloys, with Cr2C3 cermets added, which are applied using the supersonic flame spraying process specifically for areas in boilers that are subject to severe high-temperature erosion. Japan and the UK use plasma spraying technology to spray iron-based chromium-aluminum and nickel-chromium alloys by flame spraying. Switzerland and Sweden have also adopted arc and flame spraying. The American company Metalspray has also recently used arc spraying of the Fe19Cr15W7Ti6Ni alloy to prevent erosion of the tube walls in boiler combustion chambers. 　　Some units in China’s power industry have conducted research on the use of arc spraying nickel-chromium alloys and nickel-chromium-aluminum alloys to protect the walls of power plant boilers against wear and corrosion, achieving significant economic and social benefits. 2.2 Arc spraying technology and its characteristics Arc spraying is a process that uses an arc as a heat source to melt metal wire, which is then atomized by an air stream; the molten particles are sprayed at high speed onto the surface of the workpiece to form a coating. 　　The main features of arc spraying technology are as follows: ⑴ It is possible to achieve high adhesion strength of the coating without raising the temperature of the workpiece or using expensive base materials; this adhesion strength can reach 20 MPa, and it is 2.5 times higher than that of flame sprayed coatings. 　　 ⑵The high efficiency of arc spraying is reflected in the large amount of metal that can be sprayed per unit of time. The production efficiency of arc spraying is proportional to the arc current, and it is 2 to 6 times higher than that of flame spraying. 　　 ⑶Arc spraying offers a highly significant energy-saving effect; its energy utilization rate is much higher than that of other spraying methods, and energy costs are reduced by more than 50%. In addition to its high energy efficiency, the cost of electricity is far lower than that of oxygen and acetylene, making its costs usually only 1/10 of those associated with flame spraying. 　　⑷Arc spraying technology uses only electricity and compressed air, without flammable gases such as oxygen or acetylene, making it highly safe. 　　Due to the aforementioned advantages of arc spraying, it has seen rapid development over the past 20 years, gradually replacing flame spraying and plasma spraying in some applications on an international scale. 　　High-speed jet arc spraying utilizes a new nozzle design and an improved spraying gun, employing a high-speed jet generated by high-pressure air flow or fuel combustion as the atomization gas. This accelerates the separation of droplets, significantly increases the acceleration of particles, and enhances the stability of the arc. 　　By using high-speed jet arc spraying technology, better spraying quality can be achieved, which results in a significantly longer service life for the sprayed coating compared to conventional arc spray coatings. This enables more effective extension of the component’s service life, thereby further reducing maintenance costs. 2.3 Application of high-speed arc spraying technology in the corrosion protection of water wall tubes in CFB boilers. 　　Our plant is equipped with 2 YG-240/9.8-M8 type circulating fluidized bed boilers. In light of the mechanisms of thermal corrosion and wear degradation in the aforementioned heat-exchanging surfaces, as well as the actual conditions of corrosion and wear encountered in most CFB boilers, we employ supersonic arc spraying. As the base coat, we use CNB-955, a high-temperature composite coating material with excellent resistance to impact wear and high-temperature oxidation; CNB-TB is used as the working layer, and a special high-temperature wear-resistant and corrosion-resistant sealing agent is used for sealing. A transition zone is established at the edges of the coating, with a width of 100–150 mm, to ensure a smooth transition. The composite coating obtained in this way can meet the requirements for bonding strength at the coating edges. 　　After operating the anti-wear coating on CFB boiler tubes for a certain period of time, some of the coating has worn off, but residual coating remains in some areas. To ensure the safe operation of the boiler, spraying must be carried out again. However, it is difficult to completely remove the residual coating and achieve an ideal rough surface during sandblasting. If a wear-resistant working layer is sprayed directly, it tends to peel off easily; therefore, there is a need for a material that can be used for spraying a primer over the residual layer. CNB-955 high-temperature re-spraying primer wire can effectively solve the problem of continuing to spray on residual layers. The CNB-955 primer coating filament releases thermal energy when the flying particles reach the substrate, resulting in microstructural bonding that significantly enhances the strength of the bond with the substrate. At the same time, the transition layer made from this material has a thermal expansion coefficient similar to that of the boiler tubes, so it will not detach during high-temperature operation. The main component of CNB-955 high-temperature re-coating primer wire is nickel-alloyed with aluminum (Ni95Al5), with a small amount of rare earth elements; its bonding strength with carbon steel can reach over 65 MPa. 　　The arc spraying wire specifically designed for CNB-TB boilers consists mainly of oxide and boride-based metal composite ceramics, as well as elements such as Ni, Cr, Mo, and rare earths. It possesses excellent high-temperature hardness and wear resistance, making it particularly suitable for use in areas subject to severe high-temperature abrasive wear, such as the high-temperature sections of coal-fired boilers and CFB boilers, where it can be used to apply anti-wear coatings over large areas affected by abrasive and impact wear. 　　The high-temperature sealing agent is applied after thermal spraying for metal protection is completed, and once it has been verified that the spraying quality of the work surface is satisfactory. 　　Metal thermal spray coatings generally contain pores, and if these pores are not sealed, the corrosive components in the smoke can penetrate and damage the substrate; they may even spread along the boundary between the coating and the substrate, leading to the peeling off of the coating. The main components of this material are inorganic non-metallic ceramics and a ternary adhesive, which endows it with properties such as strong penetration ability, high temperature resistance, and high bonding strength. 　　Main technical specifications of the coating: Hardness HRC: ≥65 ； Porosity: ≤0.5% ； Operating temperature: ≤1200℃ ； Thermal conductivity: 60 W/M*K Spraying particle velocity: ≥380 m/s ； Bonding strength: ≥60Mpa ； Spraying temperature: ≤120℃ ； 　　High-temperature oxidation resistance: 1.63×10-4 mg/(mm2.h) (800℃, 100h) ； Matrix temperature: ≤80°C Thermal expansion coefficient: 12×10-6 °C-1 (0～700°C) Surface roughness Rz≤50μm ； 　　Particle size of powder: 10-55μm ； Oxide layer of coating: ＜2% ； 　　Total coating thickness: 0.6–0.8 mm. Service life: ≥18 months. Erosion resistance: 1.09×10⁻⁴ cm³/h (at 900–400°C, with 1000 g of 200-mesh Al2O3, for 600 seconds). Resistance to chemicals: excellent (compared to stainless steel after 30 days of immersion in HCl, H2SO4, NaOH, engine oil, and water). 　　Technical specifications for the special sealing layer designed for high-temperature, wear-resistant, and corrosion-resistant applications: Operating temperature above 1000°C ； 　　Wear resistance: ≤8mg/1000r ； 　　Bonding strength: ≥6MPa ； 　　Thickness: 0.1–0.2 mm. 3 Conclusions: Our factory employs high-speed arc spraying technology to carry out thermal spraying inside the furnaces of 2 YG-240/9.8-M8 type circulating fluidized bed boilers. This method enables effective protection of the surfaces of the water wall tubes, preventing damage to their outer surfaces. It significantly reduces costs associated with material replacement and maintenance, shortens downtime for repairs, and reduces or eliminates unplanned shutdowns, thereby offering clear economic and social benefits. This technical process can be used for preventive maintenance of boilers in newly built power plants, as well as for maintenance at the sites where existing boilers are being overhauled. 　　High-speed arc spraying technology is an excellent, efficient, and cost-effective surface treatment method for protecting boiler pipes against thermal corrosion and wear. It suits China’s national conditions and holds great value in fields such as power generation, machinery, and metallurgical equipment; it thus has broad application prospects and a huge potential market. 4 References: Xu Binshi, Surface Engineering and Maintenance, Beijing: Mechanical Industry Press, 1996, p. 79; Xu Binshi and Li Changjiu, Surface Engineering and Thermal Spraying Technologies and Their Development, China Surface Engineering, 1998, 11(I); Cao Chunan, Corrosion of Materials in Natural Environments in China, Beijing: Chemical Industry Press, 2005, pp. 112–113; Guo Luyang, Analysis of the Causes of High-Temperature Corrosion in Boiler Water Wall Tubes and Preventive Measures, China Electric Power, 2000, pp. 3317–20