Study on corrosion prevention and wear resistance

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Research on corrosion and wear resistance of boiler water-cooled wall tubes in thermal power plants

Abstract: the mechanism of high-temperature corrosion and wear of boiler water-cooled wall tubes in thermal power plants is complex, which is closely related to furnace flame temperature, sulfur content of coal, and erosion of flue gas and ash particles. There are two common methods to prevent high temperature corrosion and wear of water walls, namely, non surface protection methods and surface protection methods. In this paper, for the 3# boiler of Shajiao B power plant in Guangdong Province, the company has adopted supersonic arc spraying 45ct alloy coating with a working life of nearly four years. It is considered that the active use of thermal spraying technology is the most reliable solution for the high-temperature anti-corrosion and wear-resistant coating of boiler water walls in thermal power plants

key words: high temperature corrosion and wear of boiler water wall supersonic arc spraying

I. Introduction

high temperature corrosion and wear of boiler water wall tubes has always been a serious problem in the power system, and its direct harm is mainly manifested in the following two aspects:

(1) thinning the tube wall. According to statistics, the annual thinning amount is generally about 1mm, and the serious one can reach 5~6mm years, forming a serious hidden danger of safe operation, It increases the workload of temporary maintenance and overhaul of the power plant, causing great economic losses to the power plant

(2) sudden tube explosion accident of water-cooled wall caused emergency shutdown and emergency repair, which not only disrupted the normal power generation order of the power plant, reduced the output value of convenient and fast power generation, but also increased the labor intensity of workers and additional maintenance costs, which directly affected the enterprise benefits, but also interfered with the normal dispatching of regional electricity, affected the local industrial and agricultural production, and thus caused a great social impact

during the operation of the boiler, due to the existence of sulfur and other harmful impurities in the combustion coal, the water wall is corroded at high temperature. This phenomenon is common in all coal-fired boilers. We often encounter it in the periodic inspection of boilers in thermal power plants, but to varying degrees. Due to the high sulfur content of coal in Shajiao B power plant in Guangdong Province, the high-temperature corrosion of water walls is particularly serious, resulting in heavy losses in tube explosion and tube replacement. At the same time, a large amount of ash powder produced during coal combustion, in the complex dynamic process of boiler internal combustion, violently impacts the water wall, severely cuts the working surface of the water wall, and causes the working surface of the water wall tube to be worn into small platforms to varying degrees, resulting in the actual thinning of the wall thickness of the water wall, which is easy to cause the water wall tube to burst due to insufficient strength at high temperature, and its harmful effect is as serious as high temperature corrosion. Therefore, we need to find a technical solution to increase the wear resistance of the water-cooled wall, so as to prolong the service life of the water-cooled wall

second, the mechanism of high temperature corrosion and wear of water wall tubes

the mechanism of high temperature corrosion and wear of water wall tubes is very complex. In short, it is related to the following factors: (1) furnace flame temperature; (2) Sulfur content of coal; (3) Erosion of smoke and ash particles. During the operation of the boiler, the furnace temperature can be as high as 1600 ℃. Due to the existence of sulfur and other harmful impurities in the combustion coal, the water-cooled wall is generally subject to high-temperature corrosion. The hazards involved in high-temperature corrosion include SO2, SO3, H2S, HC 1, alkali metal salts and vanadium salts produced in the combustion process, which is a complex dynamic corrosion process of a variety of chemicals on the pipe wall at various temperatures. Among them, sulfide is the main factor of boiler high-temperature corrosion. The first is the corrosion caused by the interaction between hydrogen sulfide in flue gas and pipe wall metal. Sulfur compounds produce monatomic sulfur at high temperature, and sulfur reacts with iron in the pipe to produce iron sulfide (fe+s → FES); Second, the sulfate generated by nonflammable sulfur under the action of high temperature is mixed with ash and deposited on the surface of the pipe wall, but it no longer has all kinds of good high-temperature mechanical properties required by the water-cooled wall pipe, which actually leads to the thinning of the useful wall thickness of the water-cooled wall pipe, thus its effective bearing capacity continues to decline, resulting in corrosion. In addition, the high-temperature flue gas can impact the pipe wall at a speed greater than 8 m/s. The corrosion of flue gas and the erosion of ash particles alternate on the metal surface. When choosing them, they need to be determined according to the actual situation, resulting in the thinning of the pipe wall

III. ways to prevent high temperature corrosion and wear of water wall

this selection is to implement the spirit of the national intellectual property strategic program and the national patent development strategy (2011 (2) 020). Accidental tube explosion will cause great economic losses. In order to reduce tube explosion, the power plant has invested a lot of manpower and material resources to strengthen the monitoring and replacement of water-cooled walls, but the monitoring has not achieved any substantive effect, and tube replacement will greatly increase production costs and maintenance costs. The best way is to nip in the bud. After analyzing the causes of high temperature corrosion of water walls, effective methods can be used to prevent it. The common methods can be divided into two categories, namely, non surface protection methods and surface protection methods. Non surface protection methods include:

A. low oxygen combustion technology is adopted

b. try to make the pulverized coal concentration in each combustion room uniform

c. reasonable air distribution and strengthening turbulent mixing in the furnace

d. control the appropriate fineness of pulverized coal

e. avoid local overheating of the wall temperature of the heating surface

f. spray air protective film near the wall

g. add additives

h. control reasonable flue gas temperature at furnace outlet

i. adopt corrosion-resistant Superalloys for coals prone to high-temperature corrosion

j. adopt flue gas recirculation

k. the heating surface shall be designed and arranged reasonably to avoid high smoke temperature zone and high wall temperature zone

l. add furnace lining protection to the corrosive area

the common feature of non surface protection method is that it can reduce the corrosion of water wall to a certain extent, but it can not really prevent its corrosion. Moreover, some methods cannot be effectively implemented in actual operation for various reasons, and even some methods are controversial, such as lining protection, which not only affects the heat absorption capacity of the combustion chamber, but also complicates corrosion. Therefore, it is necessary to seek other better surface protection methods

covering the corrosion-resistant isolation layer on the surface of corroded components is the most direct and effective anti-corrosion measure, which belongs to the surface protection method of high-temperature corrosion. The main methods are

(1) brushing method: the plasticity and thermal expansion of the painted coating can not adapt to the internal environment of the boiler and the desulfurization device, so it is easy to produce delamination in use, which is difficult to be applied in practice

(2) electroplating and hot infiltration plating: the coating has good coverage and adhesion, but due to the size limitation of the workpiece, the coating will also appear weak links in the field welding, which will reduce the service performance. The existing facilities cannot be protected against corrosion again

(3) thermal spraying: it is suitable for on-site operation, with a wide range of coating materials and multiple combinations. It can provide a variety of performance coatings, provide additional protection for the unprotected part of the existing facilities, and protect the protected part again

super makes titanium alloy have good ductility and plasticity. Sonic arc spraying technology uses the "Laval principle" in fluid mechanics, so that the particle velocity during spraying really exceeds the sonic velocity. Through the introduction of this technology, especially in view of the shortcomings of the heavy and bulky spraying equipment, which is not conducive to the on-site construction, we boldly carried out the structural transformation and functional improvement of the equipment, and achieved the prominent characteristics of strong on-site practicality, good spraying performance and significantly improved coating quality. Compared with ordinary arc spraying and flame spraying, its technical indicators are as follows:

due to the progressiveness of supersonic arc spraying technology - the ion spraying speed is fast (4 times faster than ordinary arc spraying), its technical performance is essentially improved compared with ordinary arc spraying, which improves the porosity, particle size, bonding strength and other properties of the coating, and increases the adjustment range of coating thickness and the selection range of coating materials available, It can greatly expand the application range of arc spraying

however, the original application of supersonic arc spraying is still almost based on the ordinary arc spraying technology, which simply uses the improvement of its performance, and is basically in a passive application situation. For example, the spraying materials still use the original wire materials for arc spraying, and there is no special development of new coating materials for the progressiveness of supersonic arc spraying technology. We have invested a lot of human, material and financial resources to further study them, and developed the MC boiler four tube special coating, nicr/cr 3C 2, nicraife/cr 3C 2, 45ct series spraying materials to increase their field applicability in the power system, Make full use of the progressiveness of this technology to achieve a good surface coating

IV. application of 45ct alloy by supersonic arc spraying

considering various methods of high-temperature corrosion and wear prevention of water-cooled walls, we believe that the ideal method is to use thermal spraying technology. After a series of preparatory work, in May, 1999, during the overhaul of the 3# boiler of Shajiao B power plant in Guangdong, we carried out anti-corrosion and wear-resistant supersonic arc spraying on its water-cooled wall. The surface protective coatings of steel materials are divided into two categories. One is isolation coating, such as chromium plating, paint and organic paint; The other is anode coating, such as galvanizing, hot dipping or spraying 45ct alloy as sacrificial anode plate, which still has anti-corrosion protection effect on the steel on the surface, avoiding pore corrosion, corrosion under the protective layer and the falling off of the coating caused by it. The anode coating also has the function of isolation coating

in addition to considering its anode property (i.e. selecting metal components with anode potential lower than that of steel), the selection of anti-corrosion coating materials should also require its thermal expansion coefficient to be close to that of steel materials and have good plasticity to avoid delamination, and the materials should also have certain erosion resistance. Spray test, performance test and comparative test shall be carried out for one or more selected materials

for high-temperature corrosion and wear prevention spraying of water-cooled walls, the spraying material we developed is self heating material 45ct. After spraying, atomic diffusion occurs between the coating and the surface of the base metal, forming metallurgical bonding, and the coating forms a rough surface, which is convenient to connect with the working layer and improve the bonding strength of the coating. The expansion coefficient of the spray coating material is close to that of the water wall tube material, so that the coating will not fall off under the action of alternating thermal stress. It has the characteristics of corrosion resistance, oxidation resistance, high temperature resistance, good ductility, strong erosion resistance and low price. The coating thickness is 0.8~1.0mm. The spraying process is as follows:

A. sand blasting and grinding:

the substrate surface must be clean, oil-free, and meet the cleaning and texturing requirements before spraying. The purpose of sand blasting and polishing is to make the surface of water-cooled wall tube appear gray white metal appearance and uniform coarsening. After sand blasting, the surface roughness of the substrate shall reach rz40~80um. And dry, no dust, no oil, no oxide skin, no rust. When selecting abrasives, it depends on the type of base metal and the thickness of the coating. The abrasives must be clean and angular to ensure a good combination between the coating and the substrate. We choose brown corundum, chilled cast iron sand and other abrasives that meet the relevant regulations for grinding. The abrasives are angular, clean, dry, free of oil, soluble salt free substances and feldspar, so as to carefully clean the surface and effectively roughen the surface, so as to improve the bonding strength of spraying

b. anti corrosion spraying:

the water-cooled wall is sprayed with supersonic arc spraying technology. The spraying material is special 45ct, and the design spraying thickness is 0.8~1.0mm. The sprayed surface shall be uniform and dense. The water-cooled wall pipes after sandblasting shall be sprayed as soon as possible, and the shorter the interval, the better. In sunny or non humid weather, the interval shall not exceed 12 hours, and in rainy days

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