The corrosion structure of concrete chimneys and the application of anti-corrosion coatings

China Coatings Online News Information
1. Foreword
With the development of China's industry, chimney anti-corrosion, flue gas environmental protection management is more and more social and owners. The early chimneys were mostly reinforced concrete structures constructed by sliding mold process. The flue gas produced by combustion of fuel in boilers mainly contains SO2, CO2, NOx, H2S, H2O and other components, especially the coal produced in Guizhou will also contain fluoride, so flue gas is often acidic. The chimney of reinforced concrete structure has been gradually corroded under the action of flue gas acid medium for a long time, and the chimney has been destroyed, so it is necessary to protect against corrosionthe inner wall of the chimney.
traditional practice of lining fire-resistant acid-resistant bricks, the method is high cost, difficult to construct. In the 1980s, China began to promote the application of soft steel glue to chimney line anti-corrosion, but the material in addition to the complex construction process, but also fragile, poor adhesion, often pieces fall off.
2. Analysis of the flue gas corrosion of reinforced concrete chimneys from chemical thermodynamics
commonly used common silicate cement slag silicate cement is mainly silicate cement clycerine and part of the mixed material and a small amount of stone Paste mixed and milled, its cement stone is mainly hydrated products are hydrated calcium silicate gel (C-S-H gel), followed by calcium hydroxide, calcium sulfate, calcium aluminate and calcium hydrate. If the high aluminum cement (commonly known as refrainable cement) made of concrete its hydration products are mainly aluminized aluminum acid calcium, aluminized aluminum acid calcium, aluminized aluminum acid tricalculmb and aluminum glue, when the maintenance temperature exceeds 30 degrees C, the hydration product is mainly aluminized aluminum acid tricalcalculum.
When studying the corrosion reaction of chimney reinforced concrete, the chemical thermodynamic activity of the eroded media ions in the gas receding solution can be determined the relative stability of some compounds can be determined. The calculated value of the equilibrium pressure calculation of so2, CO2, H2S and other acidic gases in flue gas that destroy the various parts and materials of cement stone structure is shown in Table 1.
table 1 acidic gas non-concrete corrosion reaction equilibrium pressure
table shows that the cement components after solidification of acidic gas stability order for calcium hydrated silicate >calhydrated sulfuricate> calcium hydrate >calculum hydrate>>
Coal-fired, heavy-oil coal-fired power plants, coking plants, cement plants, ceramic plants, sulphuric acid plants, smelters and other industrial chimneys to stop coal-fired civil boiler chimney emissions of flue gas, SO2, CO2, H2S content is not used coal-free heavy oil sulfur content. Such as the composition of a soot, combustible base element content Cr-85.32%, Hr-4.50%, Or-4.07%, Nr-1.86%, Sr-4.25%, also known as Aq (dry-based ash) -7.78%, WY (applied base moisture) -3.0%. The theoretical values of acidic gas content in flue gas after full combustion are: SO2-0.38%, CO2-17.42%, table 2 shows some plant flue gas SO2 emissions.
so2 emissions from various types of plants in Table 2
compared with Table 1, the super-acidic gases contained in these flues are bound to cause serious corrosion damage to the concrete materials in chimneys. Even in accordance with environmental requirements, various sulfur-lowering technologies are used to reduce SO2 emissions from the acidic gases of flue gas to 100 ppm (equivalent to PSO2≈10-4 flue gas pressure). For example, the U.S. EPA (U.S. Environmental Protection Agency) has a fixed value of 200-220 ppm ≈2-2.2×10-4 flue gas in coal-fired boiler flue gas. This is also 54 orders of magnitude greater than the most corrosion-resistant calcium hydrated silicate equilibrium pressure in cement after curing.
considering the lowest amount of flue gas emissions, both SO2 and CO2 and H2S are hundreds of millions of times greater than the equilibrium pressure, there is a great balance of forces. Therefore, the action of concrete parts on acidic gases in the flue gas environment is chemical thermodynamically unstable.
the action of Ca (OH) 2 in acidic gas and cured cement, the pH of the phase will be reduced sharply, when the pH value is out of balance, it will make calcium hydrated silicate in cement become unstable, and the decrease of pH will also affect the stability of the steel passivation protective film in reinforced concrete.
In summary, concrete in the flue gas environment is chemical thermodynamic instability, under the action of an over amount of acidic media, reinforced concrete will be corroded, layer by layer crisp, powdered, reinforced exposed, rust and even the entire structure damaged. Therefore, the inner walls of reinforced concrete chimneys must bethe line. Through the use ofglass steel rubber mud ring glass scale coating line protection, the coating hard and brittle, under the heat impact is very easy to produce peeling, once the local occurrence of peeling water vapor
this requires that the material used for chimney corrosion protection must have a certain elasticity and have a suitable coefficient of thermal expansion (line expansion coefficient). The material must be wear-resistant and permeable. Commonly usedglass scale-type anti-corrosion paint,glass steel glue and other film after the film is relatively hard, difficult to effectively resist frequent temperature changes, large temperature change occurs due to the insistency of the coefficient of line expansion finally appear peeling, brittle cracking phenomenon.
3. From the physics analysis of reinforced concrete chimneys by flue gas corrosion of theThe maximum temperature of flue gas in the chimney can reach about 500 degrees C, because of the process requirements, the amount of fuel injection is changed, so that for the chimney without smoke temperature control equipment, there is a change in the smoke temperature. In the event of discontinued maintenance, the temperature in the chimney can be regulated as atmospheric temperature. A large temperature change will bring about a large temperature difference deformation of reinforced concrete structure (hot expansion and cold shrinkage), if there is no telescopic seam, the concrete structure is very easy to be destroyed.
addition to gas-fired boilers, dust is present in other furnaces, with a particle size of ∅≤0.3mm and a dust concentration of about 10%. And the flow rate of flue gas is about 5-20m/s, the smoke wrapped in dust will wash the chimney wallrows, concrete structure by wind erosion, erosion.
, concrete in the flue gas environment will exist thermal deformation corrosion, wind erosion, dust washing corrosion.
4. Application of chimney anticorrosive coatings
4.1 Characteristics necessary for coatings
a. acid-resistant corrosion; b. anti-condensation sulphuric acid corrosion; c. high temperature (not less than 600 degrees C) ;d. Heat shock resistance, dense impermeability; e. Abrasion resistance at high temperatures; f. deflection resistance, heat-resistant expansion resistance; g. temperature-resistant rapid degeneration; h. Good adhesion; i. ease of construction.
4.2 A company in Beijing produces ZS-1041 flue gas anti-corrosion coating performance index (Table 3)
Table 3 Beijing company produces ZS-1041 flue gas anti-corrosion coating technical parameter table
corresponding to the corrosion structure of concrete chimneys and above parameters, the coating is theoretically suitable for the anti-corrosion of concrete chimneys.
4.3 Practical application and effect acceptance
In September 2013, a large cement production enterprise in Heilongjiang adopted ZS-1041 flue gas anti-corrosion coating -line mixing
anti-corrosion transformation of the inner wall of the solidified chimney. After ensuring that the originalglass scale coating in the chimney wall is clean and oil-free, the floating ash has a certain roughness, the construction of ZS-1041 flue gas anti-corrosion coating is carried out by 3 spray methods Dry film thickness of about 300 m, natural conservation 48h after the boiler opened normally.
in December 2015, the furnace maintenance, paint film integrity is basically unchanged, local dust washing more severe paint film has a slight trend of thinning, after a little repair equipment normal operation.
, the paint construction is relatively simple, after the application of anti-corrosion effect is good, no peeling, no powdering, no drum. Effectively ensure the normal operation of the equipment.
5.ZS-1041 flue gas anti-corrosion coating function analysisThe coating is mainly film-forming silicon-containing inorganic polymer
-based inter-perforated network polymer, the polymer at high temperature non-organic polymer grafting chelation, forming a multi-inlay alternating mesh arrangement inorganic-organic chelation copolymer. The polymer has permeability, can penetrate into the substrate during construction, after the film paint film dense, excellent adhesion.
coatings using nano-silicon micro-powder, silicon carbide, boron nitride, fine crystal alumina, graphite, ultra-fine zinc oxide, titanium oxide, micro-powder zirconium oxide as functional pigments, change the potentiometer of the coating, improve the corrosion reaction balance of the paint film pressure, improve the corrosion resistance of the coating.
also added inorderable ceramic fibers to the coating, the coating inline cross-strength has been greatly improved, so that the coating has excellent impact resistance, temperature deformation, anti-abrasive and other characteristics.
6. ConclusionsThrough the study of the corrosion-proofing structure of concrete chimneys, the comparison of the current anti-corrosion coatings of concrete chimneys is not sampled, which greatly opens up our research ideas, and in this -step era when benevolent people see the wisdom of the people to see the great science and technology of the birds, the author hopes to see more and more effective new materials appear for China's energy conservation and emission reduction to contribute their own strength.
Refester

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Author Profile:
Zhang Jun: (1974-), Male, Senior Engineer, Dr. Zhou Guohong: (1980-), Male, Engineer, First Class Builder