Influence of Oxygen Dissociation on the Oxidation of Iron

Kinetics of the oxidation of iron in fluxes of atomic and molecular oxygen have been studied over a wide temperature range from 573 to 1273 K. The influence of oxygen pre-dissociation on the oxidation rate was found negligible at 573–1073 K. At temperatures above 1073 K, when only FeO is formed, the rate of iron oxidation in atomic oxygen is substantially higher than that in molecular oxygen. Decarburization occurs during the first stage of oxidation of iron containing carbon. The rate of carbon elimination in atomic oxygen exceeds that in molecular oxygen due to the higher chemical potential of atomic gas.     

Microstructure and mechanical properties of a directionally solidified and aged intermetallic Ni–Al–Cr–Ti alloy with β-γ′-γ-α structure

Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′(L1₂)-particles (Ni₃Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates.     

Corrosion Behavior of 9Cr-1Mo Steel in Sulfur Dioxide Environment

Corrosion behavior of annealed 9Cr-1Mo steel was studied in SO₂ environment at 1173 K, at flow rates from 8.33 × 10^?7 to 33.33 × 10^?7 m³/s, and parabolic rate law was followed. The rate constants were found to be independent of flow rate, within the range of flow rate investigated. Corrosion at temperatures from 973 to 1173 K, at a constant flow rate of 16.66 × 10^?7 m³/s, at 1 atmospheric pressure, for 6 h also exhibited parabolic law, however, the rate constants were observed to increase significantly with rise in temperature. The outer layer of the scale formed at 973 K was essentially of iron oxide, with small amount of chromium oxide whereas the inner layer was predominantly of chromium sulphide and chromium oxide. The scale formed at 1173 K was multilayered, in contrast to double layered formed at 973 K and 1073 K. The outer thick layer of the scale formed at 1173 K, consisted of iron oxide followed by thin substrate of chromium sulphide, iron sulphide/iron oxide, and chromium sulphide/chromium oxide toward the substrate. A model is proposed for the process of corrosion of 9Cr-1Mo steel in SO₂ environment, based on the present investigation.     

Mechanical properties of an iron oxide formed by corrosion in reinforced concrete structures

The paper deals with elastic properties of iron oxides formed in reinforced concrete structures. Due to the difficulty to perform mechanical tests on the real oxides presented in the form of (multiple) laminated stratums, the elastic modulus of iron oxides remains unknown. Young’s moduli of porous compact “synthetic oxides” in powder form, obtained in laboratory conditions, were measured from both acoustic measurements and oedometric tests. The elastic modulus of the compact polycrystalline iron oxide is deduced with respect to two models, a micromechanical one and a Hertz’ theory. The full method is validated on a well-known material, the alumina.     

Mechanical Properties and Microstructure of VPS and HVOF CoNiCrAlY Coatings

In this study, high velocity oxy-fuel (HVOF) and vacuum plasma spraying (VPS) coatings were sprayed using a Praxair (CO-210-24) CoNiCrAlY powder. Free-standing coatings underwent vacuum annealing at different temperatures for times of up to 840 h. Feedstock powder, and as-sprayed and annealed coatings, were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The hardness and Young’s modulus of the as-sprayed and the annealed HVOF and VPS coatings were measured, including the determination of Young’s moduli of the individual phases via nanoindentation and measurements of Young’s moduli of coatings at temperatures up to 500 °C. The Eshelby inclusion model was employed to investigate the effect of microstructure on the coatings’ mechanical properties. The sensitivity of the mechanical properties to microstructural details was confirmed. Young’s modulus was constant up to ~200 °C, and then decreased with increasing measurement temperature. The annealing process increased Young’s modulus because of a combination of decreased porosity and β volume fraction. Oxide stringers in the HVOF coating maintained its higher hardness than the VPS coating, even after annealing.     

Corrosion behaviour of nanocomposite TiSiN coatings on steel substrates

Nanocomposite TiSiN coatings were deposited on tool steels. Detailed mechanisms that govern the corrosion of these coated steels were revealed, following immersion tests in a 70% nitric acid solution. Pitting originated preferentially from coating defect sites and expanded with increasing immersion time. Both Young’s modulus and hardness measured by nanoindentation decreased as the corrosion damage intensified. A thin oxide layer formed from the thermal annealing of the as-deposited samples at 900 °C was found to be effective against corrosive attack. In addition, compressive residual stress was noted to suppress the propagation of corrosion-induced cracks. The role of residual stress in controlling the corrosion resistance of these ceramic-coated steels is clarified by finite element analysis.     

Dynamic elastic modulus and vibration damping behavior of porous silicon carbide ceramics at elevated temperatures

The piezoelectric ultrasonic composite oscillator technique (PUCOT) has been used to measure the Young’s modulus, E, the mechanical damping, Q ⁻¹, and the strain amplitude, ε, of a sintered silicon carbide containing pores (Hexoloy-SP). The silicon carbide material used in this study had at least 14 vol% porosity. Young’s modulus was found to have a linear temperature dependence from room temperature to 740 °C. The damping was near 10⁻⁴ and was independent of strain amplitude above room temperature.     

Sulfidation behavior of an aluminum-manganese steel

An alloy steel containing 4.5 weight percent (w/o) manganese, 8.8 w/o aluminum, and 0.36 w/o carbon was sulfidized at temperatures of 973, 1073, and 1173 K in flowing H₂/H₂S gas mixtures corresponding to sulfur partial pressures in the range 10^–8-10^–4 atm. Slow parabolic weight uptake kinetics were observed at T=973 K whenP _{s 2}10^-7 atm and at T =1073 K when PS ₂=10^-7 atm. Under these conditions, a thin external scale rich in -MnS was formed. At higher values of PS ₂ at these temperatures, and at all values of PS ₂ [(Fe, Mn)S plus (Fe, Mn)Al₂S₄] porous layer, which grew by inward sulfur transport, and an outer region of FeS, which grew by outward diffusion of iron.     

Sulfidation properties of Fe-6.1 at.% Mo alloy at 973–1273 K

Sulfidation of an Fe-6.1 at% Mo alloy was investigated in H₂S-H₂ atmospheres, 10^?4 ? P_s2 ? 10²Pa, at 973-1273 K. The reaction kinetics are parabolic except at 1273 K as liquid sulfide formation leads to catastrophic corrosion. This solid-liquid transformation between Fe₂Mo₂S₄ and Mo₂S₃ occurs at 1214 ± 9 K. At 1073 K and P_s2 = 10^?4Pa, growth of a duplex Mo₂S₃/FeMo₂S₄ scale offers high resistance to sulfidation. At 973, 1073 and 1173 K, 10^?2 ? P_s2 ? 10²Pa, parabolic sulfidation kinetics of the same magnitude as for pure iron describe growth of a duplex scale composed of an inner (FeMo₂S₄ + Mo₂S₃) layer and at an outer FeS layer. Marker measurements indicated that growth of the inner two-phase layer was supported by inward migration of sulfur and that growth of the outer FeS layer resulted from outward migration of iron.     

Effect of scale constitution on the carburization of heat resistant steels

Several austenitic heat-resistant steels were exposed to alternating periods of carburization at 1273 K [a _c= 1,po₂<10^–28 atm] and oxidation at 973°K [a _c O,po₂ = 0.2 atm]. In all cases the depth of internal carbide precipitation increased with cumulative carburization time. It was found that the carburization rates of high nickel content alloys were unaffected by intermittent oxidation cycles, whereas the low nickel, high iron content alloys experienced a reduction in carburization rate subsequent to oxidation treatment. The latter group of alloys formed external scales of chromium-rich M₇C₃ which were shown by sulfur tracing experiments to be gas permeable. It was concluded, therefore, that oxidation of these materials led to blockage of cracks and holes in the scales, thereby decreasing the surface carbon activity and hence the carburization rate. High nickel, low iron alloys formed external scales of chromium-rich M₇C₃ covered by Cr₃C₂. These scales were shown to have very low gas permeabilities. It was concluded that the carbon activity at the surface of these alloys was controlled by scale-alloy equilibration, and was therefore not affected by brief periods of oxidation. The pattern of carbide scale formation is qualitatively consistent with the thermodynamics of the Fe-Cr-C system.     

Mechanical Properties of LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript> and Thermal Cycling Behaviors of Plasma-Sprayed LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript>/YSZ Thermal Barrier Coatings

Recently, extensive efforts have been made to develop new thermal barrier coating (TBC) materials which can operate at temperatures above 1523 K over a long term. In this article, LaTi₂Al₉O₁₉ (LTA) was synthesized by solid-state reaction at 1773 K, and the mechanical properties of the LTA bulk were evaluated. The microhardness is about 14 GPa, comparable to that of YSZ bulk, whereas the Young’s modulus is about 44 GPa, lower than the value of YSZ. However, the fracture toughness of 0.8-1 MPa m^1/2 is much lower than that of bulk YSZ. A double-ceramic-layer LTA/YSZ TBC structure was proposed and the TBC sprayed by plasma spraying. Thermal cycling tests of the TBC specimens were performed at 1373 K with a dwell time of 10 min. The LTA remained good stability with ZrO₂ and Al₂O₃. However, the single layer LTA TBC was cracked at the LTA/bond coat interface after about 300 cycles, due to its poor thermal shock resistance, while the YSZ TBC yielded a lifetime of about 1000 cycles. The LTA/YSZ TBC remained intact even after 3000 cycles, exhibiting a promising potential as new TBC materials.     

Note on the Temperature Stability of Wüstite in Surface Oxide Films on iron

Abstract

Davies et al. have shown that, when iron is oxidised at temperatures above 700°, the oxide film present on flat specimens contains mainly wüstite (FeO), in fact the three oxides FeO, Fe₂O₄ and Fe₂O₄ are all present, approximately in the ratio 95 : 4 : 1. Below 570°, FeO is not stable, as shown by the iron–oxygen equilibrium diagram₂ and Collongues & Chaudron₃ have shown that in vacuo wüstite decomposes to give iron and magnetite at temperatures below 570°. Wüstite formed as an oxide layer on iron oxidised at temperatures above 570° should also become unstable on cooling to room temperature. The following results show how the degree of wüstite breakdown can be easily estimated.     

A determination of the chromium concentration for ‘healing’ layer formation during the oxidation of chromium-depleted 20cr-25ni-nb stainless steel

High temperature oxidation tests have been performed at 973–1123 K in a CO₂ based gas on specimens of 20Cr-25Ni-Nb stainless steel in which the surface chromium concentration was reduced by prior vacuum annealing at 1273 K. It is shown that the rapid initial oxidation rate decreases as a result of the formation of a chromium-rich ‘healing’ layer at the oxide-metal interface. For oxidation temperatures ≥ 1073 K, a chromia-type layer readily forms whereas lower oxidation temperatures favour the formation of a chromium-rich spinel after considerably longer oxidation times. Use of electron probe micro-analysis to determine the pre-existing chromium depletion profile and measurements of the depth into the steel at which this ‘healing’ layer forms enable an assessment to be made of the chromium concentration necessary to form this layer. This concentration increased from 15.2 w/o at 1123 K to ≥ 18.5 w/o at 973 K.     

Effect of tin on high-temperature sulphidation of Fe-Cr alloys in H2/H2S atmospheres

The high temperature sulphidation behaviour of Fe-46Cr-xSn (x = 0; 0.2; 0.5; 1; 2) alloys has been studied at temperatures of 1073, 1173 and 1273 K in H₂/H₂S mixtures with different sulphur vapour partial pressures of 10⁻¹, 10⁻³ and 10⁻⁵ Pa. Thermogravimetric studies in combination with scanning electron microscope (SEM), with energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques, have displayed a significant influence of the sulphur partial pressure on the composition and growth rate of the sulphide scale. The results have shown that addition of tin increases the sulphidation rate of Fe-46Cr alloys but not considerably (except at temperatures of 1073 and 1173 K combined with sulphur partial pressure of 10⁻⁵ Pa). The metallic core of the studied samples was enriched in tin and iron, moreover tin was found in the internal layer close to the metallic core as metallic Fe_xSn_y inclusions with tin concentrations of up to 12 at.%.     

Influence of sulfur on the nature and morphology of carbides formed on the surfaces of Fe-Ni-Cr alloys at high temperatures

The influence of sulfur upon surface carbide formation on a range of Fe-Ni-Cr alloys has been investigated at a temperature of 1273 K. The addition of small amounts of H₂S, up to 100 ppm (range of = 2.2×10^–12 to 5.5×10 su–11 bar), to a H₂-CH₄ carburizing atmosphere (a_c=0.8) was found to significantly modify the behavior from that normally observed in the sulfur-free environment. The carburization of these alloys in the H₂-CH₄ atmosphere led to the formation of globular particles of M₇C₃ on the surface of the alloy, but the addition of H₂S affected the type, morphology, and distribution of the surface carbides. Initially, the lower carbon containing M₂₃C₆ was formed, which transformed to M₇C₃ at a rate determined by the concentration of sulfur in the environment. The morphology of the M₇C₃ was modified by the presence of sulfur, and the carbide exhibited a preferred crystallographic orientation in the [001] direction. Particles of manganese sulfide were formed on the commercial alloys at > 2.3 × 10^–11 bar at 1273 K, and these served as nucleation sites for carbides sothat, in contrast to the behavior in sulfur-free conditions, complete surfacecarbide layers were formed.     

Hydrogen effects on stainless steel passive film fracture studied by nanoindentation

Hydrogen effects on the fracture stress of passive film formed on 316L stainless steel were studied by nanoindentation. Hydrogen accumulated primarily on the sample surface and in the passive film. Displacement excursion related to film fracture appeared in load-controlled load–displacement curves when electrochemically polished and anodically passivated samples were indented. The excursion disappeared after the passivation film was removed electrochemically or by nanowear. The critical load, excursion depth and length decreased with higher hydrogen concentration, along with Young’s modulus and fracture stress of the passive film.     

Rate of FeO Reduction from a CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag by Carbon-Saturated Iron

Reaction rates have been measured for the reduction of FeO from a lime-silica-alumina slag over carbon-saturated iron in graphite crucibles. The initial FeO concentrations were under 5 pet in a base slag composition of 46 to 47 pet CaO, 38 to 39 pet SiO₂, 15 to 16 pet A1₂O₈.The reaction rate was found to be proportional to about the second power of the analyzed iron content of the slag. Two rate constants were obtained for simultaneous reactions at the slag-metal and slag-graphite interfaces. Values for these constants at 1430°C (2606°F) were k₁ = 0.00058 g FeO per (min) (sq cm) (pet FeO)² at the metal and k₂ = 0.00012 (same dimensions) at the graphite. The temperature coefficient was not measured independently for the two reactions, but the net effect of a 140°C rise in temperature was small.Iron droplets rose to the surface of the slag with gas bubbles and collected in a ring adjacent to the crucible wall in amount equal to or greater than the weight of iron calculated from FeO reduction, in those runs with both slag and metal. When only slag was present in the crucible, the iron beads were more evenly distributed over the entire crucible wall and were smaller in size and total amount.It is not possible to deduce a reaction mechanism to interpret the observations on the basis of these experiments alone, but several alternatives have been discussed in terms of intermediate reaction products, dissociation of FeO in the slag, diffusion, nucleation of gas or metal, and surface phenomena leading to reaction through iron films on rising bubbles of CO in the slag.The rate of reduction of FeO in the absence of sulphur is qualitatively consistent with the part that this reaction is believed to play in the desulphurization of iron by slags under similar experimental conditions.     

Internal oxidation of Ag-in alloys: Stress relief and the influence of imposed strain

The kinetics of internal oxidation of silver-indium alloys containing 3.5, 5.9, and 9.8 at.% In in air at temperatures 773 to 973 K were established by TGA with no load applied to the specimens. Silver nodules free of oxide particles were observed to form at the surface during internal oxidation. The volume of these silver nodules was comparable to the total volume increase caused by internal oxidation. The alloys were also creep tested during oxidation in air at creep rates varying from 10^–7 to 5×10^–5 s^–1 at 773, 873, and 973 K. The parabolic rate constants k_p for the internal oxidation of the solute were determined from the measured widths of the internal oxidation zones. A small increase in k_p was observed with increased strain rate. The large volume change associated with internal oxide formation resulted in a stress gradient between the stress-free surface and the internal oxidation front which is under a high compressive stress. Stress relief occurred by transport of silver to the surface. A Nabarro-Herring creep type mechanism based on lattice diffusion of Ag cannot account for the high rate of silver transport to the surface. Pipe-diffusion controlled creep is proposed as the mechanism of stress accommodation by silver diffusion.     

High-temperature sulfidation of Fe-Mn-Cr alloys

Alloys of composition (in weight percent) Fe-10Mn-10Cr, Fe-10Mn-25Cr, and Fe-25Mn-10Cr were reacted at temperatures of 973 and 1073 K with flowing hydrogen-hydrogen sulfide mixtures corresponding to equilibrium sulfur partial pressures of 10^?3 and 8 Pa. Sulfide-scale-growth kinetics and morphologies were compared with those found on pure iron and on the binary alloys Fe-25Cr and Fe-25Mn. All alloys reacted according to parabolic kinetics after an initial period of slow approach to this steady state. Of the materials examined, the binary Fe-25Mn showed the slowest sulfidation rates, except at 973 K and a sulfur pressure of 8 Pa, where Fe-10Mn-25Cr had the best performance. Ternary alloys provided improved performance only when a scale layer of Cr₃S₄ was formed, an event dependent on temperature and sulfur activity. Multilayered scales were always formed on the ternary alloys, and the role of these layers in controlling sulfidation rates is discussed.     

Effect of Thermal Exposure on Mechanical Properties of a Plasma-Sprayed Nanostructured Thermal Barrier Coating

A nanostructured thermal barrier coating (TBC) was deposited by air plasma spraying. The effect of microstructural evolution on nano-hardness and Young’s modulus has been investigated by nanoindentation technique after exposure at 1200 °C in air for different times. The results showed that the sintering process of nanostructured TBC at 1200 °C was divided into two stages. TBC completely kept the nanostructure with the grain size <100 nm at the first stage of 10 h thermal exposure. The nanostructure was lost gradually at the second stage from 10 to 200 h thermal exposure. During the first stage, nano-hardness and Young’s modulus increased rapidly for TBC densification, and Weibull bimodal distribution of both Young’s modulus and nano-hardness disappeared as grain grew and most microcracks were healed. The structure of TBC did not change basically, and nano-hardness and Young’s modulus increased slightly at the second stage.