Thermal power plants used low grades fuels containing S, V etc contents these reacts with NaCl either presents in fuel or in air and forms molten deposits at combustion which disrupt the protective oxide particularly due to uneven-expansion and contraction of oxide scale during shutdowns. In this study selected T22 tubes joined together by shielded metal arc welding technique using basic coated electrode AWS-A5.5-E9018-B3. Preheated specimens (250 oC) of different-regions of weldment were coated with 3 to 5 mg/cm2 salt of Na2SO4-60% V2O5 paste and then subjected to fifty hot corrosion cycles at 900oC. After every cycle the weight gain was measured. The base-metal oxidized at higher rates than those of weld-metal and of HAZ due to intense spallation and cracking in oxide-scale. The oxide scales produced in the three regions were compared by scanning electron-microscopy with energy dispersive-spectroscopy (SEM/EDS), X-ray diffraction profiles. Result of SEM/EDS investigates the formation of inner scales with Cr on the HAZ than the weld metal.
Keywords: Hot Corrosion; Shielded Metal Arc Welding (SMAW); Boiler Tube Steel; Weldment; Molten salt.
The fossil-fuels based thermal power-plants especially coal and oil are the chief sources of electricity-generation in the world, and substantial losses of materials in these most-significant industries have been accounted because of severe corrosion problems. Since the energy crisis during the 1970s, the efficiency of conventional boiler/steam turbine fossil power-plants is a strong function of the steam-temperature and pressure. In the increasing of both, research has been sought after around the world. The superheater-tubes in the boiler are probably going to experience the most serious service-conditions and ought to meet rigid prerequisites concerning fire-side-corrosion, steam-side-oxidation, creep-rupture-strength and fabricability.
Increased in temperature, reduction in hardness values of the tube metal and growth of oxide scale on the inward surface of boiler tubes are usual problems in power industries over prolonged period of time. In a typical power plant, It is well known that critical components (i.e., reheater, superheater etc.) involve large numbers of welding joints in tube–to-tube or tube-to-sheet forms, which are frequently cycled between room temperature and above 650°C particularly in boiler during power-plants shutdowns, which might be because of unplanned-maintenance.
During welding the high heat-input results of unnecessary grain-growth and precipitation of micro-alloying elements, both of which lead to reduced the mechanical-properties. Moreover, the corrosion failures of industrial components are commonly associated through welding. Welding may reduce the resistance to-corrosion and environmentally assisted cracking by altering composition & microstructure. Corrosion at elevated-temperature is dominating damage mechanism in various industrial components which prompts an untimely failure of components. It was seen, the higher rate of corrosion of Cr-Mo steel weldment was mainly ascribed to the least protective internal scale of Cr2O3 with least Cr Content.
The materials utilized for high-temperature applications are exposed to hot-corrosion and high-temperature wear. Due to thermodynamics-stability of Na2SO4, power-generation equipments, other energy-conversion and chemical process industries are facing hot corrosion problems (Otero et al, 1992). The accelerated corrosion at high temperatures occurs on the heating surfaces of the superheaters or reheaters of power-plant boilers, because of deposition of ashes during combustion processes. Ashes usually contains high concentrations of compounds of vanadium, sodium and sulphur, mainly as Na2SO4 –V2O5 complex and sodium-vanadates mixtures. Vanadium, sulphur and sodium are often present as impurities in residual oils used as fuel (Harada and Kawamura, 1980 and Harada et al, 1981).
Along these lines, hot corrosion is an accelerated form of oxidation, which occurs when metals are heated in the temperature-ranges 700oC-900oC within the presence of sulphate-deposits produced because of the reaction between sodium-chloride and sulphur-compounds in the gas phase about the metals (Hancock, 1987 and Eliaz et al, 2002). Fused Na2SO4, in hot-corrosion, is an ionic-conductor, so the corrosion-mechanism is absolutely electrochemical in-nature. It has been seen that pitting in tube weldment was caused by high temperature hot-corrosion Type I and type II corrosion created by Na2SO4 [k a guan 15]. In spite of the fact that, when the corrosion layer isolates from the base materials, Fe and Cr presents in the corrosion products will react with the mixtures and accelerate the dissolution into molten-salts. In this procedure, the mass of the specimen reduces quickly as time goes on.
The ferritic 2.25Cr-1Mo steels tubes are widely used as superheaters and reheaters tubes in fossil fuel fired power plant, pulp and paper industry, because these steels also provide the good combination of weldability, and resistance to oxidation at elevated temperature. Corrosion problems in boiler tubes occurred because of overheating are very common. This type of failure is predominantly found in superheaters, reheaters, and water wall tubes, and in the consequence of operating conditions in which tube metal temperature exceeds the design limits for periods ranging from days-to-years. Present study aims to investigate and compare thermocyclic hot corrosion behviour of base-metal, weld-metal and HAZ regions of SMA weldment in Cr-Mo boiler tube steel in Na2SO4 -60% V2O5 at 900oC. Oxide scales formed over the different regions of weldment have been characterized using the techniques of X-ray diffraction (XRD, and scanning-electron microscopy/energy-dispersive analysis (SEM/EDS) and SEM back-scattered electron image analysis.