Papers by Keyword: High Temperature Corrosion

Abstract: Laboratory corrosion test of Ni-base alloys and austenitic stainless steels was performed in 0.1/0.5% SO₂ -5%O₂ -10%H₂O -15%CO₂ –N₂ at 650-750°C for 20 h. Mixture of 1.5m/o Na₂SO₄ -1.5m/o K₂SO₄ -1m/o Fe₂O₃ was used as a corrosive. The effect of SO₃ concentration on both the onset temperature of melting and the thermal stability of the synthetic coal-ash mixture was investigated by differential scanning calorimetry. Concentration of SO₂/SO₃ affected the corrosion of steels and alloys at 650°C and 700°C drastically.

Abstract: The grain boundary oxidation mechanism in hot rolled chromium – manganese steels during heat exposure at 700 °C was mathematically modeled. Given a fixed exposure time, the migration of the atomic species (iron, oxygen, chromium and manganese) has been calculated with the parabolic rate equation for diffusion. After each small time step, the data was transferred into the database ChemApp (GTT-Technologies, Germany) to calculate the oxide composition for each point in thermodynamic equilibrium. The concentration for each phase was illustrated in a phase map, similar to a cross section polish of the respective specimen. Total element concentration is shown as density plot to give a better comparison with experimental pictures from EDX or AES measurements. The obtained results are in good agreement with experimental data for alloyed steel samples with an element concentration below the critical concentration of protective oxide scale formation.

Abstract: This study was carried out to determine the minimum Al content needed to form an Al₂O₃ scale on creep resistant ferritic steels at 650 °C. Two steels differing mainly in Al content were oxidized in air at 650 °C for 3000 h. One of the steels contained 2.3 wt% Al and the other 1.9 wt% Al. Oxidation resistance of the two steels was also compared with that of the commercial P92 steel at the same temperature. The oxidation was monitored by weight gain measurement. XRD, SEM and EDS techniques were used to analyze the scale formed on the surface of the steels. For the steel containing 2.3 wt% Al, a continuous Al₂O₃ scale was observed after 3000 h of oxidation and growth of the scale was parabolic with an extremely low rate constant of 0.00058 mg cm^-2 h^-1/2. For the steel containing 1.9 wt% Al, however, only a non-protective scale was formed, which exhibited a layer structure that consisted of an outermost porous Fe₂O₃ layer, followed by a relatively dense intermixed Fe₂O₃ and FeCr₂O₄ inner layer and then by an internal oxidation layer containing voids, Al₂O₃ and un-reacted metal particles in addition to Fe and Cr oxides; growth of this type of non-protective scale followed the logarithmic kinetics Δm_t = k_lln(αt + 1) for oxidation times up to 3000 h.

Abstract: The corrosion behavior of X33CrNiMn23-8, X50CrMnNiNbN21-9 and X53CrMnNiN20-8 steels utilized in automobile industry has been studied in oxidizing atmosphere, containing water vapor and acetic acid, usually present in combustion gases of biofuels in car engines. It has been found that the mechanism of corrosion under these conditions is rather complex. After early stages of the reaction, not exceeding 40 hours, the process follows approximately parabolic kinetics, being thus diffusion controlled. It has been shown that the presence of acetic acid highly increases corrosion rate of X33CrNiMn23-8 steel, containing highest chromium content, but has virtually no influence on the corrosion rate of X50CrMnNiNbN21-9 steel with lowest chromium concentration. These differences have been explained in terms of phase and chemical composition of corrosion products.

Abstract: The increasing use of biomass and waste derived fuels in combustion challenges the chemical durability of refractories. Durability of an alumina refractory was studied in a chemically aggressive environment. A mixture of potassium chloride and carbonate (molar ratio 1:9) was placed on the sample and heated at 700-1000°C in an electric laboratory furnace in air for one week. Cross-sections of the samples were studied by SEM-EDXA to determine penetration of potassium in the refractory. Potassium was found only in the silicate matrix phase of the alumina refractory. Penetration of potassium decreased steeply from the surface to 1 mm, after which the decrease was linear but varied with temperature. At 700 and 800°C the thickness of the matrix layer that had reacted with potassium was 3 mm, while the layer was thinner at 900 and 1000°C. At the higher temperatures a glassy layer consisting of K2O, Na2O, CaO and SiO2 formed on the refractory surface. At 900°C the thickness of the surface layer was of 10μm, while a 200μm layer was measured at 1000°C. The procedure used in this work can be used to develop a laboratory scale method to be used to study corrosion of refractories in biomass combustion devices.

Abstract: The determination of the corrosion processes of metallic materials in glass melts is of great interest for glass makers. Our attention has been specifically focussed on Cr-bearing alloys that form Cr2O3 layers when they are immersed in a silicate melt and offer a good resistance to melt corrosion. The comprehension of the corrosion processes has been extensively studied in the last 10 years using stationary electrochemical techniques. Results relative to the thermodynamic state were described. Complex impedance spectroscopy offers the possibility to determine the reaction kinetics in terms of limiting processes. Three chromium rods were directly immersed in molten glass at T=1050°C, maintained respectively in the active, passive and transpassive state and studied using electrochemical techniques. The results that are reported in this paper show a good agreement between complex impedance spectroscopy data, scanning electron microphotographies of the glass/metal interface and previous results obtained using stationary electrochemical techniques.

Abstract: The present study focuses on the high temperature oxidation of a AISI 304 chromiaforming nitrided alloy. Isothermal oxidations were performed in air, at 800°C. The effect of nitridation on the steel surface depends on the temperature of the treatment. It leads whether to a γN solid solution formation or to CrN formation. In situ X-ray diffraction has been used to follow the oxides formation. Results show the concomitant growth of CrN and Fe2O3 at the beginning of the test. Then, Cr2O3 quickly appears which leads to the formation of a protective oxide scale (a parabolic rate law is observed). Our conclusions suggest that nitridation increases the high temperature oxidation resistance of 304 steels at 800°C.

Abstract: It is well known that water vapour accelerates oxidation; however different gas conditions and material compositions affect the mechanism. The paper addresses this issue from two different application areas; biomass and kraft recovery boilers. In these applications water vapour and sulphur are simultaneously affecting the corrosion mechanism, though the mechanisms are different. Low-alloyed steels were exposed to an atmosphere containing different amounts of water vapour at temperatures of 420, 550 and 600°C. Under oxidising conditions increasing water content generally accelerates oxidation. However, presence of SO2 in moist atmosphere retards oxidation at high temperatures. The phenomenon is seen at low temperatures with higher chromium contents. Stainless steel 304L was tested in an atmosphere containing hydrogen sulphide and carbon monoxide with and without water vapour at a temperature of 440°C to simulate elevated kraft recovery boiler furnace conditions. The tests showed that water vapour in the test atmosphere produces a protective spinel oxide on the metal surface. In tests without water vapour, the initial scales at metal surface were different sulphur compounds and intensive sulphidation occurred. The effect of water vapour on the sulphidation mechanism is addressed in the paper through the described tests and thermodynamic modelling.

Abstract: The SiMo cast iron is a spheroidal graphite cast iron which major alloying elements are silicon and molybdenum. This alloy is currently used in diesel engines as part of exhaust gas system like exhaust manifold or turbocharger housing components. It is then designed to work at high temperature under corrosive atmosphere. However, this alloy presents sometimes unusual high corrosion rates, and it is thus advisable to define the critical conditions leading to this type of problem. In the present study, the SiMo cast iron is tested in complex atmospheres containing N2, O2, SO2, CO2 and H2O in a temperature range lying between 300°C and 600°C. SEM observations and EDX and XRD analyses allow to qualify the nature of the corrosion products. Some thermodynamic calculations complete the experimental results. Three experimental parameters are especially studied in this work: the temperature of the isothermal corrosion tests, the presence of water vapour and the SO2 content in the gas mixture. The main results show that the corrosion products are metallic sulphides, sulfates and oxides. When sulphides and / or sulfates are present, the corrosion scale does not ensure a sufficient protection of the substrate and it is necessary to find conditions where only oxides are present. Such conditions are obtained for the highest temperature used in this study (600°C), or with highly oxidant atmospheres.

Abstract: This paper presents the corrosion of potassic glasses by SO2 gas at high-temperature. The corrosion phenomenon of potassic glass is well-known for medieval stained-glass windows under natural conditions of weathering [1]. Nevertheless the corrosion is also possible without humidity at high temperature in presence of polluting gas such as SO2 gas in the furnace. All source of SO2 gas can thermodynamically involve the main formation of K2SO4 and other alkaline salts such as Na2SO4. This corrosion manifests itself by the presence of “white flakes” in the bulk glass and this defect leads to discard of the piece for a crystal-maker like Daum. Alkali sulphate formation was determined by using scanning electron microscopy and presents a very particular morphology. This study allowed underlining on, one hand the influence of the temperature and on the other hand the role of gas concentration on the formation of the potassium sulphate. Thermal analysis technique (TGA) was carried out in order to understand the corrosion chemical kinetic of potassic glasses by SO2 gas.