Papers by Keyword: Hot Corrosion

Abstract: In this work Inconel 600 alloy was coated with two different types of coatings, Crmodified aluminide coating and Y- doped chromium modified aluminide coating . Diffusion coating was carried at 1050 oC for 8 hrs under Ar atmosphere by single step aluminizing- chromizing process and by single step aluminizing- chromizing- yttriumizing process. The cyclic hot corrosion tests of IN 600 and its coated systems deposited with 2 mg / cm2 NaCl / Na2SO4 (100/0, 50/50, and 0/100 wt. %) deposits were conducted at 900 oC in air for 105 hrs at 15 hrs cycle. The hot corrosion kinetic of uncoated Inconel 600 alloy follows parabolic rate law when oxidized with 100% Na2SO4 deposits, whereas it follows a linear rate law when oxidized with 100% NaCl deposits and with (50% NaCl + 50% Na2SO4) deposits. In cyclic hot corrosion tests, the parabolic rate constant (kP) values for Cr- modified aluminide coating when oxidized with NaCl / Na2SO4 concentrations (100/0, 50/50, and 0/100 wt. %) deposits are: 2.67x10-6, 2.73x10-6, and 8.34x10-7 (mg2/cm4)/s. respectively. But for Y- doped chromium modified aluminide coating are: 2.10x10-6, 1.51x10-6, and 6.66x10-7 (mg2/cm4)/s. respectively, under the same test conditions. The kP values for both coated systems oxidized with 100% Na2SO4 deposits are one order of magnitude lower than that for 100% NaCl, and for (50% NaCl + 50% Na2SO4) deposits under the same test conditions.

Abstract: The most common metallic coatings used in today’s gas turbine engines are MCrAlX (where M is Ni and/or Co and X is one or more reactive elements such as Y, Hf, etc.) type overlay coatings. However, overlay coating techniques (plasma and flame spraying or physical vapor deposition) are line-of-site processes, and so, it is possible not to deposit coating on some surface of the complex turbine components. The diffusion aluminide coatings can solve this problem. A NiCoCrAlY and diffusion aluminide coating were prepared on K38G cast alloy by multi-arc ion plating and low pressure chemical vapor deposition (LP-CVD) techniques, respectively. The isothermal oxidation behavior of K38G and the coatings was studied in air at 900 and 1000 oC. Their hot corrosion behaviors in the presence of 75 wt.% Na2SO4+K2SO4 and 75wt.%Na2SO4+NaCl film at 900oC were studied. The results showed that the two kinds coatings exhibited low oxidation rate at 900 and 1000oC and the presence of salt accelerated the oxidation rate. The NiCoCrAlY coating showed the better hot corrosion resistance than the aluminide coating.

Abstract: Modified zirconia thermal barrier coatings (TBCs) with segmentation cracks were sprayed onto a TMS 82+ single crystalline substrate. The thermal cycling lifetime of the modified TBC was improved by 10 times compared to that of the traditional non-segmented TBC. Also, the modified coating showed much better resistance to high temperature cyclic hot-corrosion.

Abstract: The behavior of three different materials with respect to their Fe, Cr, Ni contents have been studied between 450°C and 700°C, in waste incinerators combustion conditions, where it occurs a main influence of chlorine, sulfur and water vapor. This works analyzes the behavior of these materials which depends on their capability to form melted compounds and on the nature of these melted phases. Also as the corrosion mechanism is influenced by the presence or missing of any corrosive species, the study focalizes on the specific influence of SO2 and alkaline chorine. Effect of additive ashes on the corrosion behavior of tested materials is also studied. The mechanism of corrosion in waste incinerators is supported by thermodynamic calculations performed with the “Thermocalc” software.

Abstract: The thermal sprayed coatings are widely used in waste incineration boilers and fossil fuel-fired boilers. However, the defects, such as porosity, cracks and unmelted particles, in these coatings are detrimental to corrosion performance. In this study, the nickel based self fluxing alloy coating was fused by YAG laser to improve hot corrosion resistance of the coating. Under appropriate laser parameters, the nonporous, crack-free coating was produced. Hot corrosion test, conducted in the presence of a mixed salt of Na2SO4/NaCl/KCl at 550
Гshowed that the modified coating exhibited excellent corrosion resistance compared with the as sprayed and gas fused coatings.

Abstract: In order to clarify the failure behavior of plasma sprayed thermal barrier coating (TBC) systems under the complicated modes of thermal-mechanical-chemical loadings, the stress rupture property evaluation and failure analysis were conducted for Y2O3-ZrO2 (YSZ) and CaO-SiO2-ZrO2 (C2S-CZ) TBC systems in air and two kinds of high-temperature corrosive environments. Static creep loading was found to bring about the typical creep failure for TBC systems even in the aggressive environment so called hot corrosion almost in similar manner to the case in air. On the contrary, it was revealed that the dynamic fatigue loading tends to cause a significant failure life reduction of TBC systems both in air and in corrosive environments. For YSZ TBC system, the penetration crack preexisting through the top-coat layer tends to provide a nucleation site for the fatigue crack even in air, and more significantly a short circuit path for the corrosive species in hot corrosive environment. For C2S-CZ system, on the contrary, the top-coat / bond-coat interface tends to provide easily the nucleation site for a main crack to propagate thereafter toward both the alloy interior and outer surface. Under lower stress level at 950°C, however, the oxide-induced crack closure together with crack tip blunting attributed mainly to the high reactivity of Ca compounds as a major constituent of the TC is effective to suppress substantially the crack propagation, so as to cause the prolonged failure life as compared to YSZ system even in aggressive gaseous environment.

Abstract: Achieving higher plant efficiency in thermal power plants is one of the major global challenges from the viewpoint of reducing carbon dioxide emission levels, particularly in coal-fired boilers, irrespective of the type of coal being burned. In recent times, it has been possible to increase the steam temperature in coal fired ultra supercritical (USC) plants without too much of a cost impact. The temperature has already been increased to 600
for main steam and 610
for reheat steam. The main enabling technology is the development of stronger high temperature materials such as newly developed high Cr ferritic steels and austenitic steels, capable of operating under high stresses at increasing high temperatures. Other key demands for those materials are hot corrosion resistance such as coal ash corrosion in superheater and reheater tubes and sulfidation of waterwall tubes, and steam oxidation resistance. This paper will mainly present the hot corrosion and steam oxidation properties of newly developed high strength heat resistant steels for their application to USC boilers and long-term experience in an actual plant.

Abstract: The thermal sprayed coatings are widely used in waste incineration boilers and fossil fuel-fired boilers. However, the defects, such as porosity, cracks and unmelted particles, in these coatings are detrimental to corrosion performance. In this study, the nickel based self fluxing alloy coating was fused by YAG laser to improve performance of the coating. Under appropriate laser parameters, the nonporous, crack-free coating was produced. The rubber wheel type abrasion wear test and hot corrosion test conducted in the presence of a mixed salt of Na2SO4/NaCl/KCl at 650°C showed that the modified coating exhibited excellent wear and corrosion resistances compared with the as sprayed and gas fused coatings.

Abstract: Homogeneous and FGM environmental barrier coatings (EBC) made of alumina – NiCr on the steel substrates by high-velocity oxygen fuel (HVOF) spraying technique were studied. At room temperature, these coatings were characterized by DC polarization measurements and by electrochemical impedance spectroscopy (EIS) after various exposure times in highly aggressive basic solutions. At high temperatures (400-750°C) corrosion resistance was tested in respect to sulfide – sulfate – chloride – carbonate melts in air, which simulates environments in incinerators, kraft recovery boilers and turbines. After testing, the samples were examined by SEM and EDS and their corrosion resistance was compared.

Abstract: Strength degradations of porous silicon carbide hot gas filters bonded with clay and calcium carbonate are investigated in the simulated condition of the pressurized fluidized bed combustion (PFBC). Thermal cycling, static fatigue, thermal shock, and hot corrosion tests are conducted to differentiate the independent parameter that affects the strength degradation during the filtration using silicon carbide filter in PFBC condition. The results indicate that hot corrosive gas mainly causes the strength reduction because of the degradation of grain boundary region.