Materials Science Forum Vols. 595-598

Abstract: Most of equipments used in glass industry are superalloys containing up to 30 wt% chromium. The ability of these alloys to resist against silicate melt corrosion is directly linked to the formation of a chromia (Cr2O3) layer at the alloy/melt interface which can be protective under particular conditions. These conditions have been previously identified and are the temperature, melt composition and redox conditions. The aim of the present study is to establish the relationship between the protective (or non protective) behaviour of the chromia layer with the solubility of chromia in silicate melts under given conditions. The combination between results from the electrochemical study of pure chromium corrosion and total Cr solubility in Na2O-xSiO2 melts clearly indicates that both parameters are linked: the lower the total Cr solubility, the better the Cr is corrosion resistant.

Abstract: The heavy refractory metals and alloys Molybdenum (Mo), Molybdenum – Silicon – Boron (Mo–Si-B; “MoSiBor”), Tungsten (W), Tungsten – Copper (W-Cu), Tungsten – Nickel – Iron (W-Ni-Fe; “Densimet D 176 and 185”) and Tungsten – Nickel – Molybdenum - Iron (W-Ni- Mo-Fe; “Densimet D2M”) were pack-treated at 1100°C with Silicon - powder to form siliconized zones and/or intermetallic phases which are intended to be more oxidation resistant than the plain base materials. These materials (especially the W-based ones) are used at ambient conditions as counterweights, radiation shields etc. because of their high density as well as at high temperatures (600 – 900°C) as metal forming tools, electrodes etc. because of their refractory metal content. In both areas of conditions oxidation of the plain materials occurs and leads to lower functionality or destruction. A suitable oxidation test has been defined to check the presumably enhanced oxidation resistance of the pack-treated materials: an isothermal high temperature oxidation test at 700 and 900°C for one week. At these conditions all untreated materials would have been more or less strongly oxidized. Improved oxidation resistance could be found for the materials with pack-cementation treatment except for sintered Tungsten (92% dense), sharp etched D 185 and D 176 at 900°C and Tungsten – Copper at both temperatures. More stable and dense superficial oxides were formed which led to decreased oxidation rates and could help to increase functional stability and the lifetime of the components. Different pack-treatments e.g. with chromium or silicon plus chromium could improve the behaviour of the materials which failed within this work.

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.

Abstract: Metal dusting behaviour of alloy800H was investigated in a laboratory-pressurized reactor where CO-H2-CO2-H2O gas mixtures flowed continuously, simulating the gas environments in actual syngas production plant. Four test conditions, having a variety of gas pressures and H2O contents, were conducted for a long rod specimen that was imposed thermal gradient of 777°C to 454°C in a longitudinal direction. After an exposure of 100h at high pressure, metal dusting has initiated with a bell-shape distribution on the test specimens at temperatures between 600°C and 700°C. Gas pressure has accelerated metal dusting even if the gas atmospheres had the same gas composition. On the contrary, metal dusting has been alleviated with increasing the H2O content in the test gas. Oxide scale characterisations were analyzed by using Raman spectroscopy and XPS at the test specimen surfaces exposed at different temperature. The ratio of a protective Cr2O3 scale to spinel oxides such as FeCr2O4 and MnCr2O4 has increased with increasing the exposure temperature, with reducing the gas pressure, and with lowering the H2O content. These behaviours have been interpreted from thermodynamic indices: carbon activity ac and the oxide scale integrity related to oxygen potential Po2 as a function of temperature.

Abstract: Oxidation testing of four gamma-prime (γ') strengthened alloys used extensively in aero and/or land-based gas turbine engines was performed. The studied alloys, which included R-41 alloy, Waspaloy alloy, 263 alloy and the new HAYNES® 282® alloy were selected because they belong to a family of alloys distinguished by their possession of sufficient fabricability to be produced in sheet or plate form. The test program included both static and dynamic oxidation testing. The static oxidation tests were performed in flowing air at temperatures of 871, 927 and 982°C (1600, 1700, and 1800°F). The dynamic (burner rig) oxidation tests were performed at 871°C (1600°F). The results of these test programs will be presented.

Abstract: In the present study, a thermowell made of Incoloy 800 (33Ni-21Cr-46Fe) was used as a part to cover and therefore to protect a thermocouple inserted in a tube used in petrochemical process. It was then subjected to the atmosphere in the tube containing moving carbon particles at 900°C. One of the critical failure types observed was the combination of both solid particle erosion and oxidation. Thermowell degradation, which accounted for the corrosion-affected-erosion of materials, was significantly observed in the area where impingement at the head surface area of thermowell. The physicochemical characterization revealed that the thermowell consisted of two layers. The inner one is the incoloy 800 base metal, whereas the outer layer was clad with a cobalt base superalloy. Kinetic of oxidation of both materials in each layer, which was performed in air at 1173K presented the different parabolic rate. Metal oxide and surface morphology of thermowell, which was investigated in the simulated condition, were then examined by XRD, OM and SEM. A finite element software was furthermore applied to simulate the flow direction and the velocity of the particles. It was found that the maximum speed of particles (180 m s-1) was at the surface where the impingement angle of particle is 90°. Comparing to the failure of themowell in service, it was concluded that the failure of thermowell depends primarily on the impingement angle of particle. Design of the shape of thermowell, as well as the cladding with the higher erosion-corrosion resistance materials, were recommended to be carried out to improve the erosion-corrosion resistance of this part.

Abstract: The formation of carbon filaments which occurs at carbon activities ac > 1 in a range of temperatures 450-700°C is a major problem in many chemical, petrochemical and refinery processes where hydrocarbons or other strongly carburizing atmospheres are involved. An excessive carbon deposition causes deterioration of the furnace alloys, such as an important migration of carbon into the alloys. In order to better control and limit this deterioration, this work has been performed to on one side get a more accurate understanding of the mechanisms of formation of catalytic coke and on the other side to find remedies as the injection of selected additives in the feed. Thermogravimetric analyses (TGA) were performed on iron samples in simulated conditions of isobutane dehydrogenation. X ray diffraction (XRD) and scanning electron microscopy examinations were used to identify the different steps during the formation of the catalytic coke. The selection of appropriate remedies to reduce the catalytic coke deposition, requires accurate understanding on both mechanisms of the catalytic particles formation and of the growth of the graphite filaments. We have studied the first steps of the catalytic coke formation on high purity iron that has been previously reduced or oxidised. The comparison of the catalytic coke deposition kinetics indicates that the mass gain is much faster on a pre oxidised state than on a reduced one. In refinery and petrochemical processes, several methods can be selected in order to limit the deposition phenomena of catalytic coke: selection of an appropriate metallurgy, protection of the surfaces by application of coatings, injection of additives with the feed. Steric inhibitors (that block the adsorption sites and slow down the germination and diffusion steps) such as sulfur additives are currently industrially used but special care has to be taken in order to prevent consequential secondary effects such as, for catalytic refinery process, the deactivation of catalysts. Based on TGA experiments, the accurate amount of inhibitor to be injected has been selected regarding the oxidising state of the iron surface.

Abstract: Gas turbines are critical components in the combined cycle power systems being developed to generate electricity from solid fuels, such as coal and biomass. The use of such fuels to produce fuel gases introduces the potential for significant corrosive and erosive damage to gas turbine blades and vanes. Single crystal superalloys have been developed for use with clean fuels but are now being deployed in industrial gas turbines. The performance of these materials, with coatings, has to be determined before they can be used with confidence in dirtier fuel environments. This paper reports results from a series of laboratory tests carried out using the ‘deposit replenishment’ technique to investigate the sensitivity of candidate materials to exposure conditions anticipated to cause type I hot corrosion in such gas turbines. The materials investigated have included the single crystal nickel-based superalloys CMSX-4 and SC2-B, both bare and with Pt-Al coatings. The exposure conditions within the laboratory tests have covered ranges of SOx (50 and 500 volume parts per million, vpm) and HCl (0 and 500 vpm) in air, as well as 4/1 (Na/K)2SO4 deposits, with deposition fluxes of 1.5, 5 and 15 5g/cm2/h, for periods of up to 500 hours at 900°C. Data on the performance of materials has been obtained using dimensional metrology: pre-exposure contact measurements and post-exposure measurements of features on polished cross-sections. These measurement methods allow distributions of damage data to be determined for use in the development of materials performance modelling. In addition, the types of damage observed have been characterised using standard optical and SEM/EDX techniques. The damage rates of the single crystal materials without coatings are too high for them to be used with confidence in gas turbines fired with gases derived from ‘dirty fuels’. Under the more severe combinations of gas composition, deposition flux and metal temperature, the corrosion rates of these materials with Pt-Al coatings are also excessive. The data produced from these tests has allowed the sensitivity of hot corrosion damage to changes in the exposure environment to be determined for the single crystal alloys and coating systems examined.

Abstract: The oxidation behaviour of binary Fe-Cr alloys containing 10 and 20 mass % Cr, respectively, was studied in Ar-20%O2, Ar-7%H2O and in Ar-4%H2-7%H2O at temperatures between 800 and 1050°C. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar-O2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the surface oxide scale, the present results reveal that the presence of water vapor also affects the transport processes in the alloy. The enhanced internal oxidation, which is likely the result of water vapor increasing the solubility and/or the diffusivity of oxygen in the alloy, explains the frequently observed effect that Fe(Ni)Cr alloys with intermediate Cr contents (e.g. 10-20%, depending on temperature) exhibit protective oxidation in dry gases but breakaway type oxidation in steam. The temperature dependence of the change from protective to non-protective behaviour in Ar-H2O differs quantitatively, but not qualitatively from that in Ar-O2.

Abstract: Two FeCrAlRE alloys, a commercial, 0C404, and a model alloy in the form of thin foils, with different Mn, Nb, Mo and Ti concentrations were subjected to cyclic oxidation in lab air at 1100°C. The oxidized samples were studied by gravimetry, Grazing-Incidence X-ray Diffraction (GI-XRD), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-ray (EDX) analysis. The two FeCrAl alloys exhibit different oxidation kinetics; however, both alloys have the same weight gain after 500 hours exposure. During the early stages the scale consists mainly of α-Al2O3 together with some oxide particles containing Mn, Al, Fe and Cr formed on the alloys. After 500 hours the 0C404 scale locally also consists of larger polycrystalline regions of Mn-Cr-Al spinel. In addition, Si-rich oxide, chromia and Al-Cr oxide could be observed at the metal/oxide interface.