Corrosion Resistance and Microstructure of High-Alumina Refractories, Based on the Rotary Slag Test

The corrosion resistance and microstructure of four highalumina and two chromia-containing high-alumina refractories were evaluated. Analysis of the refractories after exposure in the rotary slag test showed that the chromia-containing products had better slag resistance than the chromia-free products. Three factors contributed to this conclusion, including formation of a Cr-spinel layer at the slag/refractory interface, formation of an impermeable mullite layer just below the interface, and improved matrix bonding from chromia-alumina solid-solution formation.     

Densification characteristics of chromia/alumina castables by particle size distribution

The quality of the refractories applied on integrated gasification combined cycle should be a key factor that affects both the reliability and the economics of gasifier operation. To enhance the workability of chromia/alumina castables, three types of ultrafine alumina powder were added to improve the workability. Densification behavior of such castables in the presence of ultrafine alumina was assessed through the measurement of parameters like flow value, viscosity, bulk density, apparent porosity, and microstructure evaluation by an SEM study. It's proved that the specific surface area and particle size distribution of ultrafine powders in matrix parts greatly influence the densification behavior of these castables.     

Oxidation limited life times of chromia forming ferritic steels

Long term, cyclic oxidation studies of three high – Cr, ferritic steels were carried out at 800°C and 900°C in air. It was found that with decreasing sample thickness the life time of the mentioned alloys decreases due to breakaway phenomena. This effect is caused by faster exhaustion of the chromium reservoir from the bulk alloy in case of thinner components. The observed life time limits can be predicted with reasonable accuracy by a theoretical model, using oxide growth rate parameters, initial alloy Cr content and critical Cr content required for protective chromia scale formation. In the calculation it has, however, to be taken into account that the oxidation rates of the steels increase with decreasing specimen thickness.     

Effect of component thickness on lifetime and oxidation rate of chromia forming ferritic steels in low and high pO2 environments

Abstract

Long term oxidation tests were carried out with a high-Cr ferritic steel at 800°C and 900°C in simulated cathode and anode gas of a solid oxide fuel cell (air and an Ar/H₂/H₂O mixture respectively). It was found that with decreasing sample thickness the life time of the steel decreases due to breakaway phenomena. This effect is caused by faster exhaustion of the chromium reservoir from the bulk alloy in the case of thinner components. During air exposure the oxidation rates increase with decreasing specimen thickness and this has to be taken into account in the calculation of the Cr-reservoir exhaustion. This thickness dependence is not found during the exposures in simulated anode gas. Hence, especially for thin walled components, the oxidation rates in anode gas are substantially smaller and thus the life times are longer than during air exposure. The differences in oxidation behaviour in the two environments are discussed on the basis of scale formation mechanisms involving microcrack formation in the surface oxide scale and depletion of major and minor alloying additions in the bulk alloy.     

Metal dusting resistance of high chromium alloys

Samples of 5 high Cr‐alloys were discontinuously exposed for 10,000 hours under severe metal dusting conditions, i. e. in flowing 49%CO‐49%H₂‐2%H₂O at 650°C. After each of the 11 exposure periods the mass change was determined and any coke removed and weighed. Metallographic cross sections were prepared after about 4,000 h and 10,000 h. The high Cr‐alloys: 1. PM 2000 (Fe‐19%Cr‐5.5%Al‐0.5%Ti‐0.5%Y₂O₃), 2. Cr‐44%Fe‐5%Al‐0.4%Ti‐0.5%Y₂O₃, 3. Cr‐50%Ni, 4. Cr‐5%Fe‐1%Y₂O₃ and 5. porous chromium showed no or only minute metal dusting attack. Compared to the attack on reference samples of Alloy 601 (Ni‐23%Cr‐14%Fe‐1.4%Al), the metal dusting symptoms were negligible on the 5 high Cr‐alloys, minor coking and pitting and no internal carburization was observed. Because of the high Cr‐content, carbon solution and ingress should be minute, and in addition are inhibited by the formation of a chromia scale, as confirmed for four of the Cr‐rich alloys, and formation of an alumina scale on PM 2000. These alloys could be used for parts exposed to severe metal dusting conditions, and in fact, 50Cr‐50Ni has been applied successfully under such conditions.     

Selective Oxidation and Sub-surface Phase Transformations in Chromium-bearing Austenitic Steels

A series of Fe–15Cr–(2–3)Mo–(0.7–2.5)C (compositions in weight percent) steels was oxidised at 850°C and P_{O_2} = 5.8 × 10^–20 atm, where iron oxide is unstable. All grew external Cr₂O₃ scales according to parabolic kinetics. Depletion of chromium from alloy subsurface regions led to dissolution of chromium-rich carbides if the original alloy carbon level was less than 1.2%. Simultaneous decarburisation caused a transformation of the original austenitic or austenoferritic structure into single-phase ferrite, stabilised by the molybdenum. Diffusion analysis of the concentration profiles within this transformed zone led to satisfactory agreement with the known diffusion coefficient for chromium in ferrite. At high carbon levels, decarburisation was slow, resulting in low chromium concentrations at the internal alloy–carbide interfaces. In these cases, the carbide dissolution did not proceed and chromia scaling rates were slowed.     

Microstructure,adhesion, and tribological properties of conventional plasma-sprayed coatings on steel substrate

A variety of metallic and oxide coatings were deposited under various conditions on 1020 mild steel substrate by conventional plasma spraying. The coating thickness, microhardness, cohesion and adhesion failure loads, friction coefficient, and abrasive wear resistance were evaluated. The coatings were classified as follows, in order of decreasing microhardness and wear resistance: alumina, chromia, 316 stainless steel, Ni-5% Al, elemental aluminum and aluminum-polyester. Wear resistance increased with increasing microhardness and decreasing friction coefficient. The microhardness and wear resistance of high-velocity oxy-fuel (HVOF) diamond jet (DJ)-sprayed aluminum were found to be superior to those of plasma-sprayed aluminum. Plasma or flame-sprayed metallic coatings adhered well to the substrate. The cohesion, adhesion, microhardness, and wear resistance of alumina coatings exceeded those of equally thick chromia coatings.     

Sulfur in chromia

The solubility of sulfur in chromia has been studied in H₂-H₂O-H₂S tagged with³⁵S at 973 and 1173 K at low oxygen and sulfur partial pressures typical for coal gasification-systems. For monocrystalline samples, it has been shown that sulfur-containing species are only present adsorbed on the surface of the specimens and can be removed by ultrasonic cleaning in acetone. The surface coverage after 4 weeks of exposure to H₂-H₂O-H₂S was between 2.8 and 19.3% of a monolayer. In polycrystalline chromia, sulfur was located only in pores and cracks of the sample. In dense, compact areas of the specimens the solubility of sulfur was below the detection limit of autoradiography, which was estimated to be better than 0.17 ppm.     

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.