Papers by Keyword: Hot Corrosion

Abstract: The paper presents results of research into isothermal oxidation test of thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were deposited by overaluminizing of MeCrAlY–type coating deposited by LPPS method. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized.. The isothermal oxidation test at 1100^oC for 1000h shown that thickness of the TGO layer in overaluminized bondcoat was significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats.

Abstract: The paper presents results of research into thermal barrier coatings characterized by high oxidation resistance and hot corrosion. Bondcoats were formed by overaluminizing of an MeCrAlY–type coating deposited by low pressure plasma spraying. The outer ceramic layer of yttrium oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally LPPS-sprayed were MeCrAlY bondcoats, which were not subsequently aluminized. Used as base material was Rene 80 nickel superalloy. The research has shown that as a result of aluminizing by the CVD method there was formed in the bondcoat a deposit zone built of the β-NiAl phase which protects from oxidation. Preserved below increased chromium content ensures resistance to hot corrosion. The outer ceramic layer was characterized by columnar structure similar to that obtained in the EB-PVD process.

Abstract: The Ni-Cr-Co based superalloy, Inconel 740, was corroded between 700 and 900°C for 20 h in 75% Na₂SO₄-25% NaCl salt. The scale consisted primarily of Cr₂O₃, (Ni,Co)Cr₂O₄, NiO, and Ni₃S₂. During corrosion, oxidation dominated rather than sulfidation. The scale was neither dense nor compact owing to the formation of the (oxides, sulfides)-mixed scale where Na₂SO₄ and NaCl were incoporated.

Abstract: The microstructure and corrosion behaviour was studied for a diffusion β-NiAl and Si modified β-NiAl coatings formed on the superalloy MAR-M 247. First type, β-NiAl coating was applied with the help of method “out-of-pack”. Second type, Si modified β-NiAl coating was applied by method “pack-cementation”. Diffusion coatings created protective, heat-activated layer which separated superalloy from aggressive environment. Corrosive environment was created by tablets Na₂SO₄ at 900°C. Technique of scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDS) was used to characterize the corrosion products. Experiment confirmed the advantages relating to the application of diffusion coating in aggressive environment which imitated environmental condition during operation of turbine engine. This experiment was made in cooperation with company PBS Velká Bíteš a. s., Velká Bíteš, Czech Republic and The Silesian University of Technology, Faculty of Materials Science and Metallurgy, Katowice, Poland.

Abstract: Corrosion behavior of polymer-derived ceramics (PDCs) was investigated in aqueous hydrofluoric acid (HF) and sodium salts (NaCl or Na₂SO₄). Two oxides (SiCO and SiCNO-(Hf)) and two non-oxide PDCs (SiCN and SiCN-(B)) were examined in this study. The HF acid corroded the oxide PDCs, whereas non-oxide PDCs resisted acid corrosion. Nevertheless, the degradation is slow in some cases to extend the engineering ceramic materials lifespans. The PDCs composites were hot corroded by NaCl or Na₂SO₄. The Na-salt attacked the PDCs, producing corrosion layers. The cross-sectional X-ray elemental analysis and microstructure surveillance exhibited that the corroded layers comprised of distinct regions. The corrosion mechanism is discussed in line with the experimental discoveries.

Abstract: The aim of this research is to improve the hot corrosion resistance of In718 superalloy by laser shock processing. Specimens were treated by laser shock processing on the whole surface. The hot corrosion resistance beneath molten Na₂SO₄ under 900 °C was investigated. The effects of laser shock processing impacts on residual stress, surface microstructure and microstructure after hot corrosion test were investigated. Results show that 3 times laser shock processing can induce-400 MPa compressive residual stress. The microstructure was characterized by highly tangled and dense dislocations and a large number of twins. The average corrosion rate of In718 superalloy treated by laser shock processing is about 50 % lower than that of the untreated superalloy.

Abstract: Solar Salt is the best hot transfer medium molten salt of solar thermal power generation.In order to study the hot corrosion behavior of four graphite is in the molten Solar Salt.The graphite immersed equipped with molten salt crucible placed in a muffle furnace,each period of time remove the sample and weight the weight gain until the sample is completely destroyed.The destruction forming of graphite is analyzed by XRD, SEM and OP.The study found that the order of hot resistant corrosion of molten Solar Salt of four graphite materials are the fine structuregraphite,isostatic graphite,9# graphite and cold pressing graphite.The principal factors can affect hot corrosion resistant of graphite is the graphitization degree. The hot corrosion resistance of the graphite can be enhanced with the increase of graphitization degree.

Abstract: The T122 steel (ferritic Fe-Cr-W alloy) and 347HFG stainless steel (austenitic Fe-Cr-Ni alloy) were corroded in the Na₂SO₄ salt at 800 and at 900 °C, and their hot corrosion behavior was studied. They displayed good corrosion resistance owing to the formation of the protective Fe₃O₄ and (Fe_0.6Cr_0.4)₂O₃ scales. These oxides effectively suppressed sulfidation by the Na₂SO₄ salt. The oxide scales that formed were thin and loosely adherent. The displayed good corrosion resistance was mainly attributed to the large amount of Cr in the alloys.

Abstract: Three NiCoCrAlY cladded coatings reinforced by different contents of nano- Al2O3p and one without nano-Al2O3p were prepared on a Ni-base super alloy using laser cladding technique. Hot corrosion of the NiCoCrAlY coatings in Na2SO4/K2SO4 (75:25, wt. /wt.) mixture was performed at 1050 °C in static air. The effects and influencing mechanism of nano-Al2O3p on the hot corrosion behavior of the coatings were discussed. Results indicate that the hot corrosion resistance of the coatings with nano-Al2O3p is better than that of the one without nano-Al2O3p, among which the one with 1.0 wt. % nano-Al2O3p presented the best hot corrosion resistance. The improvement of the hot corrosion resistance is largely due to the refinement of the microstructure after adding nano-Al2O3p.

Abstract: The hot components of gas turbine are loaded by high stresses in presence of high-temperature oxidation and hot corrosion environment during service process. It is important to simulate the interaction of stress and hot corrosion on the components in real service situation. Effect of the ratios of salt mixtures of Na₂SO₄/NaCl and the coating amounts of salt mixtures on the microstructure and the high temperature stress-rupture properties of a corrosion resistant directionally solidified superalloy at 850/500MPa were investigated. The results demonstrated that the stress-rupture life decreased with the coated salt mixture ratio of Na₂SO₄/NaCl changing from 3:1 to 9:1. Stress-rupture lives of the alloy decreased from 63 h to 42 h, while the coated salt mixtures amount increased from 1 mg/cm² to 4 mg/cm², hereby the salt mixture contained 75% Na₂SO₄ and 25% NaCl. When the coating ratio of Na₂SO₄/NaCl was 9:1, the stratified corrosion products layers were composed of the (Ni, Co)O, Ni (Al, Cr)₂O₄ layer, bright grey phase that be identified as CrWO₄, and the underlying dispersed particle shape phase CrSx, the underlying layer forms the sulfides eutectic and leads to catastrophic destruction for the alloy, which decreased the stress-rupture lives dramatically.