How to select materials for heavy anti-corrosion coatings in thermal power plants?

China is a coal-fired country, the characteristics of energy structure lead to more serious corrosion conditions, forming acid rain and other pollution conditions. The air pollution caused by coal-fired thermal power is quite serious, which is soot-type pollution, and dust, sulfur dioxide (SO2) and nitrogen oxide (NOx) are the main pollution in the atmosphere.
thermal power plant is a complex corrosion environment, in addition to the serious atmospheric corrosion caused by coal, it also involves all kinds of tank internal wall corrosion, circulating water pipe outer wall and inner wall, high temperature insulated parts, and the most severe corrosion environment flue gas desulfurization system (FGD) .
structure is not caused by atmosphericThe fuel used in thermal power stations, such as heavy oil and coal, releases a large amount of corrosive gases such as sulphides during operation and combines with water vapor or rainwater in the air to form an acidic solution. The deposition of coal ash on the steel structure will form electrolytes and accelerate the electrochemical corrosion of the steel structure. Therefore, the anti-corrosion problem of thermal power station has become one of the important contents of production and maintenance.
thermal power plant, the steel structure in the atmospheric corrosion environment mainly involves:
(1) boiler steel structure
(2) main plant steel structure
(3) coal transmission system: coal remover, reactor, crane and coal stack bridge
(4) pipe exterior
(5) Tank outer wall
These atmospheric environment corrosion grade, due to the corrosion environment of thermal power plants, according to the international standard "steel structure corrosion protection of anti-corrosion coating system" ISO12944-2, can be identified as a high-grade environmental C4 level, in accordance with the requirements of long-lasting corrosion protection for more than 15 years, can be used in long-lasting corrosion-resistant coating system.
Water-based heavy anti-corrosion coating system, in recent years has been very good technical development, the corresponding coating system passed a fairly harsh corrosion test test, but also in practice has been satisfied with the results, therefore, here in addition to the introduction of solvent-based coating system, but also introduced the corresponding water-based heavy anti-corrosion coating system.
of a solvent-based coating system can be used in two categories: epoxy-rich zinc-rich primer and inorganic zinc-rich primer. In contrast, when using epoxy zinc-rich primer, the construction performance is better, when using inorganic zinc-rich primer, we should pay attention to the thickness of dry film can not exceed a certain range (usually about 150 microns), otherwise it will cause the paint film cracking, and inorganic zinc-rich primer surface is porous structure, coating the rear channel intermediate paint, need to use fog spray / spray techniques Otherwise, the solution is prone to pinhole bubbles, solvent inorganic zinc-rich primer curing requirements of higher relative humidity, R.H.65% or more, at 23 degrees C, the time to apply the back-channel paint to more than 16 hours;
Water-based heavy anti-corrosion coating system in addition to environmental protection and the health of construction personnel, in the steel structure processing and production process, do not need to avoid the surrounding fire industry, and paint film drying is quite fast, which means that the coating process can be accelerated, usually in a day can be completed three coating systems, which for steel structure processing units to speed up the production process, saving a lot of time and money.
the inner wall of theThe tank anti-corrosion coating system of thermal power plant involves two parts: the outer wall and the inner wall. The coating system on the outer wall of the tank can refer to the corrosion-resistant coating system in the atmospheric environment. The storage tanks of power plants are more complex, mainly involving drinking water tanks, fire water tanks, fire filter tanks, condensate, deionized water tanks, fuel, deionized water supply system tanks and so on. The condensate tank is made of stainless steel.
coal-fired power plant storage tanks do not have a special corrosion medium, the coating system can be used pure epoxy resin coating and phenolic epoxy coating. Coatings used on the inner walls of drinking water tanks require hygienic standards for drinking water quality. The coating of the deionized water tank must not contain any ions to reduce the corrosion of water on the boiler's water-cooled walls and steam turbines. Fuel tank inner wall, if the design requires the use of anti-static coating, it must be required to use the corresponding anti-static coating system, inorganic zinc-rich coating because zinc powder is a conductive pigment, can also be used as a fuel tank inner wall coating.
a recyclingThe circulating water pipes of thermal power plants are also serious corrosion sites, and corrosion prevention requires 20 years. In addition to high-performance corrosion-resistant coatings, cathode protection is also combined. In order to save fresh water resources, seawater is used as cooling water in coastal thermal power plants. According to ISO 12944-2, the outer wall of the circulating pipe belongs to the Im3 buried corrosion environment and the inner wall belongs to the im1 fresh water or Im2 seawater corrosion environment. The anti-corrosion coating film in the circulating water pipe requires excellent adhesion in the sea water, and in the case of long-term pipe through the sea water, the paint film maintains its excellent performance without falling off. Paint film has a very strong wear resistance, in order to resist the pipe backfilling process caused by friction and scratching, paint film good flexibility performance to resist the impact and collision during the construction process, to ensure the good paint film. The coating system should be compatible with the cathode protection system.
Traditionally used in pipelines are epoxy coal asphalt coatings, according to domestic and foreign experience in large-caliber water pipe corrosion prevention, epoxy coal asphalt coatings have many shortcomings, such as paint film is black, has a "stealing light" effect, is not conducive to the quality of construction control and maintenance, but also not conducive to future pipeline maintenance operations. Asphalt also has a carcinogenic effect, and has begun to limit its use in Europe and the United States. The pressure water pipe in the first phase of the Three Gorges water conservancy hub project is epoxy coal asphalt paint, in the third phase of the project, after strict testing and demonstration, changed to asphalt-free modified epoxy coating and solvent-free epoxy coating. Because the construction of solvent-free coatings for painting equipment and construction personnel have very high requirements, so high solid part modified epoxy coatings are more suitable coating products, can be used in accordance with conventional construction methods and procedures for construction. High solids modified epoxy coatings have a successful use record in pressure pipes and circulating pipes of thermal power plants in many large hydropower stations abroad.
high temperature and insulation of theCarbon steel pipes in thermal power plants involve high-temperature areas and need to be protected with high-temperature coatings. Carbon steel pipes for new thermal power plants may involve different temperature ranges, so pipes with different temperature ranges may use appropriate high temperature coatings.
chemical curing coatings, such as epoxy coatings and polyurethane coating products, can withstand up to 120 degrees C of high temperature, therefore, in the temperature range below 120 degrees C, can be used in the usual corrosion-resistant coating system, it is important to note that acrylic polyurethane paint at more than 100 degrees C, light paint film will yellow, but this does not affect its corrosion resistance.
alkalic acid powder heat-resistant paint and siliconic acid modified alkyd resin paint can withstand high temperatures of 200 degrees C.
the coating system in the thermal insulation layer must consider this situation, it is important to note that once the insulation layer is broken, there will be a build-up of water vapor, acidic condensate is the main corrosion factor, using heat-resistant phenolic epoxy resin coating, inorganic zinc silicate coating It can withstand high temperatures of 400 degrees C, so as long as the pipe temperature is within 400 degrees C, the inorganic zinc silicate primer can be used as a universal rust-proof primer, which can reduce the application of coating varieties, so that in addition to the design of simple, but also to reduce the actual processing and painting process trouble. Because a large number of pipes stacked on the site, in the early stage of construction to be very good to separate different media different temperature ranges, is very complex, construction personnel at the same time to frequently replace paint varieties and construction equipment, the actual operation is more troublesome. If you use only one primer, you can greatly simplify the work procedures and workload.
according to NACE' research, zinc reverses polarity in sodium chloride solutions (70 to 80 degrees C), in which case an inorganic zinc-rich primer is used, which in turn makes steel an anode to protect zinc powder. And zinc powder in the acidic or alkaline environment, easy to dissolve in warm water environment, zinc powder consumption, it also lost the protective effect. To avoid this, a silicone aluminum paint can be covered with an inorganic zinc-rich primer surface.
silicone aluminum powder heat-resistant coatings can be used at high temperatures of 500 to 600 degrees C. Paint film thickness is usually only 20 to 30 microns thick, can be applied two, but can not be applied too thick, so as not to affect heat conductivity.
flue gas desulfurization systemCoal used in coal-fired power plants contains sulfur, which is produced in the production process and seriously affects the atmospheric environment. Under pressure from environmental protection, flue gas desulfurization ("FGD") has become the main means of sulfur dioxide control in thermal power plants. Therefore, FGD flue gas desulfurization device in many power plants has become an essential supporting project, construction did not, and now also began to smoke desulfurization device.
flue gas desulfurization technology of coal-fired power plants, the most important process is the wet limestone method. In this process, the flue gas containing SO2 is dusted and then entered into the heat exchanger to cool down, and then into the absorption tower and limestone slurry contact with desulfurization after heating emissions. In the whole process, the flue gas desulfurization device, because of the large number of SO2 and other corrosive media in the flue gas, the flue gas desulfurization device is subject to a variety of chemical media erosion. The flue gas desulfurization device system is huge and difficult to maintain. In order to ensure the long-term normal operation of flue gas desulfurization device, the corrosion-resistant materials used are stainless steel, high alloy composite steel plate, rubber and glass scale lining coatings.
chlorinated butyl rubber or brominated butyl rubber lining technology is one of the main anti-corrosion technologies of flue gas desulfurization system, its outstanding advantages are strong binding force with steel, dense lining, strong permeability, and has a certain degree of elasticity and wear resistance. The defect of rubber lining is that the quality of lining is greatly affected by bonding operation, and the de-layering of drums is a common problem.
is also an effective method to prevent corrosion by using paint for flue gas desulfurization device. In general, corrosion-proof failure of the corrosion-resistant layer is mainly the erosion of the resin substitum, the substation resin first produces weight loss, color change and so on, and then causes the material bubble, layering, stripping or cracking, and finally leads to the failure of the corrosion-resistant layer, especially the latter, due to osmosis and other factors, accelerates the corrosion of chemical media into the interior of the protective layer. Therefore, while selecting a resin substate with good corrosion resistance, effective measures should be taken to reduce and slow the osmosis of corrosive medium or water vapor. General coatings are usually not resistant to high temperature and strong acid flue gas desulfurization harsh corrosion environment, the best coating is the use of strong chemical resistance, but also with temperature resistance of ethylene ester glass scale coatings. Ethylene ester resin, especially Novolek ethylene resin, has a strong resistance to chemicals and permeability, long-term resistance to 180 degrees C.