High Temperature Corrosion and Wear of Boiler and Waste Incinerator Tubes
HVAF Thermal Spray Chromium Carbide
Boiler coating is extremely important to mitigate corrosion and wear in boilers and waste incinerators. HVAF chromium carbide has proven to be the excellent coating for the application.
Circulating Fluidized Bed Erosive Wear
Circulating fluidized bed (CFB) boilers are used widely in coal-fired power plants because of their advantages of highly efficient combustion of a wide variety of solid fuels, in-bed sulfur capture, and relatively low NOx emission. Hard coal gangue is the primary cause of erosive failures in economizer boiler tubes at high temperature. Erosive wear resistance of the boiler tubes’ surface is necessary to lengthen the continuous operation lifetime. The size of the coal gangue particles in CFB is on the millimeter scale, and the particle velocity is from several to tens of meters per second.
Boiler Cold End Sulfuric Acid Dew Point Corrosion
Using fuels containing sulfur yields a potential hazard of sulfur corrosion at the cold end of the boiler. The severity depends on many factors like percentage of sulfur in the fuel, excess air, moisture in the flue gas, etc. The problem is most severe for waste incinerators.
Steam generating boilers use different types of fuels containing sulphur. The higher the percentage of sulfur, the higher will be the risk of cold-end corrosion in the boiler. The sulphur converts to sulphur dioxide during combustion. Depending upon the other impurities present in the fuel and excess air levels, some portion of the sulphur dioxide gets converted to sulphur trioxide. Sulphur dioxide and trioxide combine with moisture and forms sulphurous and sulphuric acids.
The basic reactions taking place are:
S + O2 → SO2
SO2 + O2 ↔ SO3
H2O + SO2 ↔ H2SO3
H2O + SO3 → H2SO4
Condensation of these acids results in metal wastage and boiler tube failure, air preheater corrosion and flue gas duct corrosion.
The amount of SO3 produced in boiler flue gas increases with an increase of excess air, gas temperature, residence time available, a number of catalysts like vanadium pentoxide, nickel, ferric oxide, etc. and the sulfur level in fuel. The flue gas dew point temperature increases steeply from 90°C to 135°C (194 to 275°F) with the sulfur percentage increasing up to 1%. A further increase in the sulfur percentage in the fuel gradually increases the dew point temperature of 135°C to 165°C (275 to 329°F) at 3.5% sulfur in the fuel.
The industry uses wear and corrosion resistant coatings widely to mitigate these risks.
High Velocity Thermal Spray Boiler Tube Coatings
Kermetico HVAF Chromium Carbide Coating Properties
Thermally sprayed Cr3C2-NiCr coatings combat high-temperature wear due to the high wear resistance imparted by the hard carbide particles and the high temperature oxidation resistant nature of the Cr2O3 oxide formed over both phases.
While WCCoCr 86/10/4 provides better wear resistance when temperatures are under 510°C (950°F), high velocity sprayed chromium carbide ceramic coatings mitigate abrasive and erosive wear at temperatures up to 750°C (1,380°F). The corrosion resistance is provided by the NiCr matrix while the wear resistance is provided mainly by the carbide ceramic phase.
A Typical Microstructure of a Kermetico HVAF Sprayed Chromium Carbide Coating.
HVAF and HVOF Sprayed Inconel-Type Coatings
Despite less wear resistant than chromium carbide, an Inconel-type coating is a popular choice for dew-point corrosion protection of boilers.
HVAF Inconel-type coatings provide excellent hot corrosion resistance due to Cr2O3 formation, high density and high bond strength.
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