Papers by Keyword: High Temperature Corrosion

Abstract: The aim of the experimental work was to propose innovative procedures for the formation of renovation layers, to determine suitable material, modify the microgeometry and surface topography of new and renovated shaped parts of moulds for high-pressure casting of aluminium alloys. It has been designed and validated under laboratory and operational conditions a method of modifying the surface of the mould parts of moulds for casting aluminium alloys by forming stochastic texture by low energy laser in combination with duplex PVD coatings on the surfaces of mould parts in contact with the aluminium alloy melt. It has been verified the contact angle measurement methodology for determining the number of spurs by separation lubricant on the surface of the new or refurbished mould part before the first casting cycle. For the formation of the renovation layers, the additive materials were verified Dievar, Dratec, UTPA 702. A TruDisk 4002 solid-state disk laser with BEO D70 focusing optics was used for winding.

Abstract: Corrosion behavior of the alloys 1.7386 (P9), 1.4462, 1.4841, 1.4959 (Alloy 800HT) and 2.4816 (Alloy 600) was tested for 24, 72 and 240 h between 480 – 680 °C. The testing gas atmosphere contained 3.8 vol. % HCl, 200 ppm H₂S and CO, CO₂ and N₂. It simulated conditions present in a thermal cracking process for post-consumer plastics. Samples were analyzed by metallography, SEM/EDX and XRD after corrosion experiments. Additionally, their mass loss during the test was evaluated. A multilayered structure of corrosion products grew on the samples during the corrosion experiments. The composition of the corrosion products depended not on the material, but on the testing temperature. At 680 °C chromium sulphide formed the outer layer, followed by a chromium oxide layer. Below these two layers a chlorine containing layer was observed. At 480 °C mainly nickel sulphide was detected, besides chromium oxide and iron- and chromium chloride. Especially at higher testing temperatures FeCl₂ was not observed directly on the samples, but as colorless crystals at the colder parts of the testing equipment. At 680 °C the mass loss of the samples decreased with increasing nickel content. However, this effect changed entirely at lower testing temperatures. At 480 °C 1.7386 and 2.4816 showed nearly the same mass losses.

Abstract: Hastelloy C-276 coating was deposited on stainless steel 1.4923 using HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology to increase high corrosion resistance. Possible influence of high temperature corrosion on local mechanical properties change of Hastelloy C-276 was also evaluated in this article. High temperature corrosion is corrosion in the molten salts environment. The salt mixture of 59 % Na₂SO₄ with 34.5 % KCl and 6.5 % NaCl was used in this study. The selected exposure temperatures were 525 °C and 575 °C and the tests for both temperatures were conducted in autoclave for the time interval of 168 h. The coating and salt mixture layer was analyzed using scanning electron microscope (SEM), EDS analysis and nanoindentor (MTS Nanoindenter XP). High temperature resistance of Hastelloy C-276 coating was evaluated based on the changes in coating surface and according to the occurrence of various phases created on the coating surface during this test. It can be assumed that Hastelloy C-276 coating deposited by HVOF technology shows selective oxidation under the salt film.

Abstract: High temperature corrosion of 310S austenitic stainless steel in simulated rocket combustion gas at 900 degree Celsius was investigated and discussed in this paper. 310S austenitic stainless steel was chosen because it was used for building some components of a rocket launcher. The corrosive atmosphere was prepared by mixing of hydrochloric acid and distilled water with 5.5 mole per liter then, boiling that solution and feeding into a corrosion testing chamber. The chamber was set up at 900 degree Celsius with duration 210 hrs. After testing, the corroded specimen was microscopically characterized by OM and SEM/EDS techniques. The corrosion layer was classified into three main sublayers: peeling-off scale, external corrosion sublayer, and internal corrosion sublayer. The local chemical information was analyzed by XRD (in case of peeling-off scale) and SEM/EDS (in case of external and internal corrosion sublayers). The peeling off scale mainly comprised Fe₂O₃ and Fe_21.3O₃₂ ferrous oxides because they needed much oxygen consumption to exist. In case of external and internal sublayers, there were a lot of pore tunnels and corrosion products. Chlorine and/or hydrogen chloride would penetrate through a passive film and, then, metal chlorides was formed on both external and internal corrosion sublayers. Metal chlorides would volatile because of their lower evaporation temperature than the testing temperature. Moreover, they were oxidized by oxygen in wet condition and resulted metal oxides mostly remaining on the external corrosion sublayer.

Abstract: In this study, three different alloys Stainless Steel 316 (SS316), Titanium 6Al-4V (Ti6A-l4V) and Hastelloy 276 (H276) were exposed to the wet environment with 12% of water vapor and Ar-20%O₂ for 10 h at 900 °C. The oxidation behavior of the samples was analyzed using weight change measurement technique and optical microscope (OM). The phases of the oxide layer on corroded sample were determined using X-ray diffraction (XRD). The difference in the weight gain indicating the difference in the rate of the oxide growth for the samples. Sample Ti6A-l4V showed the most aggressive oxidation behavior compared to the other samples. OM analysis showed a multilayer oxide scale formed on the Ti6A-l4V and H276 samples but only single layer of oxide forms on SS316. Oxide scale for the samples were analyzed using XRD, the result showed the formation of chromia (Cr₂O₃) on SS316. Titanium oxide (TiO₂ ) and alumina oxide (Al₂O₃) were detected on Ti6Al4V, while H276 showed the formation of nickel manganese oxide (Ni_7.2Mn_0.2O₈) and nichromite (NiCr₂O₄).

Abstract: The oxidation behavior of AlCrN coating deposited on Ti-25Al-12.5Nb-6Mo-0.48V alloy was investigated at 700 °C and 800 °C in static air. Oxidation in air indicated that the uncoated alloy exhibited poor oxidation resistance due to the formation of TiO₂ scale, but the major oxide in the oxidized coating was chromia and alumina and the TiN layers were also observed beneath the coating. No sign of degradation and no scaling of the coating was observed after oxidation.

Abstract: Coating properties determine its behavior in operation. The simulation of future operational conditions is therefore the best quality test. The evaluation during operation is usually not possible to perform, and the coatings are therefore frequently characterized by their physical or mechanical properties. This text deals with the high temperature corrosion of HVOF sprayed Stellite 6 coating and with changes of its local mechanical properties before and after the corrosion testing. High temperature corrosion is defined as a corrosion in the presence of molten salts. In this case, the mixture of salts in composition of 59% Na₂(SO)₄ with 34.5% KCl and 6.5% NaCl was used. Two exposure temperatures 525 °C and 575 °C were selected and the tests for both temperatures were performed in the time interval of 168h in the autoclave. The coating with salt mixture layer was analyzed using scanning electron microscopy and nanoindentation. The high temperature resistance of Stellite 6 coating was evaluated according to the changes in the coating surface and by the occurrence of individual phases formed on the coating surface during the test. Generally, it can be said that the Stellite 6 alloys deposited by HVOF technology show selective oxidation under the salt film. This fact was also proved in this study. Furthermore, the nanoindentation measurements of Stellite 6 coating were performed before and after the corrosion testing. These measurements were used to evaluate the change of local mechanical coating properties.

Abstract: The behavior of Fe-12Cr-2Mo stainless steel exposed isothermally in tube furnace at 700°C for 10 h had been studied in different environments; of mixed environment (Ar-20%O₂ with coated Na₂SO₄-50%NaCl salts), molten salt (Na₂SO₄-50%NaCl) and dry oxygen (Ar-20%O₂) atmospheres. The exposed samples were characterized by using optical microscopy (OM), scanning electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD). The results indicated that, sample exposed in mixed environments undergoes highest corrosion rate compared with other samples. The main phase exists in all samples generally were iron-rich oxides which non-protective and thick. Conversely, EDX analysis on cross-sectional samples revealed the Cr-enrichment developed underneath the iron-rich oxide scales as the chromium concentration increases towards near the substrate. In presence of salt mixture, the oxide scales undergo spallation, however there is no crack observed. The catastrophic corrosion sample was occurred in combined environment due to the oxidation induced by the chloridation and sulfidation phenomena as well.

Abstract: In order to improve the power production efficiency of biomass-fired boilers, power plants must be operated at higher steam temperatures than nowadays. One of the main factors hindering the rise of the steam temperatures is the corrosive nature of the flue gases and fly ash towards the superheaters. In this study, the high-temperature corrosion resistance of three commercial superheater steels exposed to potassium chloride was compared. The focus was on the effect of pre-oxidation on the protective properties of different steels, whereupon various variables were used during the pre-oxidation.

Abstract: In order to solve the problem of high temperature corrosion to water wall of opposed wall fired ultra supercritical boiler, comprehensive study is carried out by combining theoretical analysis and experimental research, obtaining the variation of flue gas composition before and after introduction of near wall wind and effect of primary air velocity, central air opening, swirl intensity of outer secondary air and inner secondary air opening on composition of flue gas in near wall region. The results show that 1) the introduction of near wall wind significantly improves reducing atmosphere in near wall region; 2) among the four parameters listed above, swirl intensity of outer secondary air and inner secondary air opening have greater influence on composition of near wall flue gas; 3) under the comprehensive effect of swirl intensity of outer secondary air and inner secondary air opening, the volume fraction of CO in near wall flue gas can be controlled below 1500ppm, which effectively disturbs the condition for high temperature corrosion.