Early high temperature oxidation behaviour of Ti(C,N)-based cermets in air

Isothermal oxidation behaviour of two Ti(C,N)-based cermets (TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32-x)Ni-xCr, x = 0 and 6.4 wt%) was investigated in air at 800-1100 °C up to 2 h. Mass gains exhibited neither linear nor parabolic law during isothermal oxidation. The oxide scales formed at 800-1100 °C were multi-layered, consisting of NiO outerlayer, NiTiO₃ interlayer and TiO₂-based innerlayer. The internal oxidation zones formed at 1000-1100 °C consisted of Ti-based, Ni-based and Mo-based complex oxides. Cermet with 6.4 wt% Cr exhibited superior oxidation resistance, due to the presences of Cr_0.17Mo_0.83O₂ in TiO₂-based innerlayer of the oxide scale and Cr-rich Ti-based complex oxide in the internal oxidation zone.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

High temperature oxidation characteristics of Nb–10W–XCr alloys

The effect of Cr content on the microstructure and cyclic oxidation behavior of Nb–10W–XCr alloys with four different compositions has been investigated. Experiments were conducted in air at 900 °C and 1300 °C; the oxidation kinetics have been evaluated in terms of weight change per unit area with respect to exposure time. Alloy's microstructure consists of Nb solid solution phase regions surrounded by a network of NbCr₂ Laves phase. A trend of improvement in oxidation resistance with increase of the intermetallic phase is observed at 1300 °C and oxidation kinetics follow a parabolic behavior. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates than alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄, and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content. Results delineate the influence of microstructure and composition on the oxidation mechanisms of these alloys that represent a promising base for high-temperature intermetallic alloy development.     

The role of microstructure in the high temperature oxidation mechanism of Cr3C2-NiCr composite coatings

Composites of Cr₃C₂-NiCr provide superior oxidation resistance to WC-Co composites, which has seen them applied extensively to components subjected to combined high temperature erosion and oxidation. This work characterises the variation in oxidation mechanism of thermally sprayed Cr₃C₂-NiCr composites at 700 °C and 850 °C as a function of heat treatment. Carbide dissolution during spraying increased the Ni alloy Cr concentration, minimising the formation of Ni oxides during oxidation. Compressive growth stresses resulted in ballooning of the oxide over the carbide grains. Carbide nucleation with heat treatment reduced the Ni alloy Cr concentration. The oxidation mechanism of the composite coating changed from being Cr based to that observed for NiCr alloys.     

High-temperature oxidation behavior of Nb-Si-Cr alloys with Hf additions

The oxidation behavior of two alloys from the Nb-Si-Cr system containing hafnium has been investigated under isothermal and cyclic conditions. Nb-20Si-20Cr-(5,10)Hf alloys (composition in atomic percent) were exposed to air for 24 and 168 h over a range of temperatures from 700 °C to 1400 °C. A gravimetric method was used to determine the oxidation kinetics; weight gain per unit area as a function of temperature or time. Computed isothermal sections of the quaternary Nb-Si-Cr-Hf phase diagrams were used for alloy selection. XRD, SEM and EDS were used to characterize the phases present in the oxidation products and the alloys. Oxidation experiments revealed extremely good oxidation resistance at 700 °C and 800 °C and above 1200 °C under isothermal conditions for both alloys. Partial pesting was observed when the samples were exposed to 800 °C. Complete oxide formation was observed above 1000 °C for 5Hf and above 900 °C for 10Hf up to 1200 °C. Beneficial effects have been observed with the addition of 10Hf to the alloy compared to 5Hf at 700 °C, 1200 °C and 1300 °C resulting in a reduction of weight gain per unit area.     

High temperature oxidation of Fe-Al and Fe-Cr-Al alloys: The role of Cr as a chemically active element

Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the α-Al₂O₃ film which is formed on the surface provided temperature is above 900 °C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales.     

Oxidation performance of Mo3Si with Al additions

The Mo₃Si alloys with different aluminum contents were fabricated by the arc-melting and drop-casting technique, heat treated and then exposed to air at 700, 800, 900 and 1000 °C in order to assess their oxidation behavior. Line scan studies led to the assumption that the oxide scale thermally grown at 1000 °C was composed of SiO₂ which was located closer to the alloy matrix and Al₂O₃ around the outer surface of the oxidized sample, while the Mo oxide volatilized at this oxidation temperature. The results also showed that the unalloyed sample (Mo₃Si) underwent a pest reaction in a short time of exposure, while the sample with 16 at.% Al exhibited the best oxidation behavior, which could be attributed to the formation of SiO₂ and Al₂O₃ in the oxide scale.     

Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

Corrosion of a chromia-forming nickel base alloy, Haynes 230^®, has been investigated under impure helium containing a few Pa of CO and H₂O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H₂O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H₂O is the major oxidant of chromia-forming nickel base alloys in impure helium.     

Cyclic oxidation behaviour of Ti3Al-based alloy with Ni-Cr protective layer

The influence of a thin 80Ni-20Cr (at.%) protective coating on the cyclic oxidation of a Ti-24Al-11Nb (at.%) alloy based on Ti₃Al at 600 and 900 °C in air was investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The results of the oxidation tests showed that deposited Ni-Cr layer provides an improved oxidation resistance due to the formation of protective oxide scale which barriers the outward Ti diffusion into the scale. In some extent surface formation of the nitride layer also prevents diffusion of alloying elements from the matrix. Although oxidation at 900 °C is faster than that at 600 °C, a remarkable reduction in mass gain of the alloy with protective coating was observed. The thickness of oxide scale on the coated samples is approximately two times less than that formed on the uncoated samples treated under the same exposure conditions (120 h).     

Metal dusting behaviour of Cr-Ni steels and Ni-base alloys in a simulated syngas mixture

Long-term laboratory exposure tests for various Cr and Ni content steels and Ni-base alloys were conducted at 650 °C in a 60vol.%CO-26%H₂-11.5%CO₂-2.5%H₂O gas mixture simulating syngas environments. Upon isothermal heating, alloys with 15% and 20% Cr had many pits on the surface after a brief exposure, while no pit was found on alloys containing of 60% Ni and more than 23% Cr exposed for up to 5000 h. The thermal cycling accelerated pit initiation drastically, resulting that all test specimens except 30%Cr-60%Ni alloy suffered from metal dusting. From a measurement of pit depths, Ni proved to be an effective alloying element to retard the pit growth: growth rate for 75% Ni alloy has achieved double-digit decrease compared to that for 20% Ni. Microscopic observations has revealed that platelet graphite aligned perpendicular at the boundary of gas/metal of pits. The length of the platelet graphite for high Ni alloys was appreciably longer than that for low Ni steels. This can be interpreted from the difference of super saturation of carbon.     

Oxidation of single crystal PWA 1483 at 950 °C in flowing air

The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10⁻⁶ mg²/(cm⁴ × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO₂ and NiTiO₃ but mostly Cr in Cr₂O₃ and (Ni/Co)Cr₂O₄ besides NiTaO₄. This outer scale is connected to a discontinuous layer of Al₂O₃ and an area of γ′-depletion within the base material.     

Unusual oxidation behaviour of Zr50Cu50 alloy at high temperatures

The oxidation of Zr₅₀Cu₅₀ alloy at 500-700 °C is characterized by preferential oxidation of zirconium, while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₁₄Cu₅₁ phase. The strong reaction at 800 and 850 °C resulted in the total corrosion of the specimens in 21 and 15 h, respectively. The oxidation at elevated temperatures showed an anomalous decrease of the oxygen consumption rate in the temperature range 930-1000 °C, corresponding to the preferentially oriented crystallites of ZrO₂ in the oxide scale at 900 and 1000 °C. The oxide layer consists of ZrO₂ and CuO in the whole temperature interval of the oxidation. The reaction kinetics obeys a parabolic rate law. An activation energy of 92.0 ± 0.3 kJ/mol has been estimated.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

The oxidation behavior of Fe-20Cr alloy foils in a synthetic exhaust-gas atmosphere

Oxidation tests of rare-earth-modified and Ti-modified Fe–20Cr alloy foils, which are under consideration for catalytic converter supports, were performed in a synthetic exhaust-gas atmosphere (N₂+H₂O+CO₂) between 900°C and 650°C. Between 900°C and 750°C, the rare earths had no effect on oxide growth rates while Ti increased growth rates. Oxide growth rates for the rareearth alloys at 800°C and 750°C are much lower than those found in the literature for oxidation of Fe–Cr alloys or pure Cr in O₂-rich atmospheres. The slow growth rates for the rare-earth alloys agree with literature data for oxidation of stainless steels containing >20% Cr in wet atmospheres and are caused by growth of an oxide scale only one grain thick. At temperatures 700°C, Fe–20Cr alloys grow massive Fe oxides; however, this can be suppressed by adding rare earths or Ti. To ensure good oxide adherence, free sulfur must be eliminated in the alloy by tying it up with a reactive-element addition. Both Ti and the rare earths can be used to tie up S, but the rare earths are more effective. For converter applications, the optimum alloy composition may contain rare earths for good oxide adherence and a small amount of Ti to suppress growth of Fe-rich oxides.     

KCl-induced high temperature corrosion of the austenitic Fe-Cr-Ni alloys 304L and Sanicro 28 at 600 °C

The influence of KCl(s) on the high temperature oxidation of the austenitic alloys 304L and Sanicro 28 at 600 °C in O₂ + H₂O environment is reported. 0.10 mg/cm² KCl(s) was added before exposure. The samples are investigated by grazing angle XRD, SEM/EDX, and AES. In the absence of KCl, both alloys show protective behaviour in dry O₂. In O₂ + H₂O environment, alloy 304L suffers local breakaway corrosion while Sanicro 28 still shows protective behaviour. The oxidation of both alloys is strongly accelerated by KCl. KCl reacts with chromium in the normally protective corundum-type oxide, forming K₂CrO₄. This depletes the scale in chromia and leads to the formation of a non-protective, iron-rich scale. The significance of KCl-induced corrosion in real applications is discussed and the oxidation behaviour of the two steels is compared.     

Effect of lamellar microstructure on oxidation kinetics of Fe3Al sintered by hot isostatic pressing

The present paper focuses on the investigation of the relationship between microstructure of Fe₃Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe₃Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe₃Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al₂O₃ stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al₂O₃. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe₃Al samples are supposed to be the main factors that determine observed peculiarities after Fe₃Al oxidation at 900 °C and 1000 °C.     

The air oxidation of two-phase Cu-Cr alloys at 700–900°C

The oxidation in air of three two phase Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt. % was studied at 700–900°C. The alloys corroded nearly parabolically, except at 900°C, when the corrosion rates decreased with time more rapidly than predicted by the parabolic rate law. The corrosion rate decreased for higher Cr contents in the alloy under constant temperature and generally increased with temperature for the same alloy composition. The scales were complex and consisted in most cases of an outermost copper oxide layer free from chromium and an inner layer composed of a matrix of copper oxide or of the double oxide Cu₂Cr₂O₄, often containing particles of chromium metal surrounded by chromia and then by the double oxide. Metallic copper was also frequently mixed with chromia. Cr-rich regions tended to form continuous chromia layers at the base of the scale, especially at the highest temperature. No chromium depletion was observed in the alloy.     

Oxidation of a quaternary three-phase Cu-20Ni-20Cr-5Fe alloy at 700-900 °C in 1 atm of pure O2

The oxidation of a quaternary Cu-Ni-Cr-Fe alloy containing approximately 20 at.% Ni, 20 at.% Cr and 5 at.% Fe, balance Cu (Cu-20Ni-20Cr-5Fe), was studied at 700-900 °C in 1 atm of pure oxygen. The alloy is composed of a mixture of three phases, where the lightest α phase with the largest Cu content forms the matrix, while the other two, much richer in Cr, form a dispersion of isolated particles. At variance with the ternary three-phase Cu-20Ni-20Cr alloy examined previously, which was unable to form protective chromia scales over the alloy surface even after an extended period of oxidation, the present alloy formed complex external scales containing mixtures of the oxides of the various components plus a deep internal region containing a mixture of alloy and oxide phases. With time, a very irregular and thin but essentially continuous chromia layer formed at the bottom of the mixed internal oxidation region, producing a gradual decrease of the oxidation rate. Thus, the addition of 5 at.% Fe to Cu-20Ni-20Cr alloy is able to decrease the critical Cr content required to form the most stable oxide and promotes the formation of a continuous chromia scale under a lower Cr content in spite of the simultaneous presence of three different phases.     

Continuous and cyclic oxidation of T91 ferritic steel under steam

The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700 °C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr)₃O₄. The intermediate layer was porous magnetite (Fe₃O₄) followed by a compact thinner layer of hematite (Fe₂O₃). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time.