Internal Alumina Research Articles

Determination of the Maximum Surface Chromia Thickness for the Nondestructive Identification of Internal Alumina Scales on a Heat-resistant Alloy Using Cathodoluminescence

Determination of the Maximum Surface Chromia Thickness for the Nondestructive Identification of Internal Alumina Scales on a Heat-resistant Alloy Using Cathodoluminescence

Nondestructive, Rapid Identification of Aluminum Nitride and Internal Alumina Scales on a Heat-Resistant Alloy Using Cathodoluminescence

Nondestructive, Rapid Identification of Aluminum Nitride and Internal Alumina Scales on a Heat-Resistant Alloy Using Cathodoluminescence

The corrosion behavior of Co-Cr-Al alloys exposed to mixed oxygen-sulfur atmospheres at 900 °C

Three Co-8Cr-xAl alloys (x = 0, 3, 5 wt.%) were exposed to three mixtures (Gas 1: H2-CO2, Gases 2 and 3: H2-CO2-H2S) at 900 °C for 20 h. Gas 1 produced external chromia scales (plus internal alumina for Co-8Cr-3/5Al). Gas 2, with p(S2) below stability of Co sulfides, favored a transition from external chromia scales, observed in Gas 1, to exclusive external alumina scales for both Co-8Cr-3/5Al alloys. Conversely, Gas 3 was very aggressive for both Co-8Cr and Co-8Cr-3Al, forming a liquid Co-S phase, while with Co-8Cr-5Al it still formed external Al2O3 scales. Al additions improved the alloys resistance to the sulfur attack.

The corrosion behavior of Ni-Cr-Al alloys exposed to mixed oxidizing-carburizing atmospheres at 800 °C

Ni-10/15Cr-7Al and Ni-5Cr-10Al (at.%) were exposed at 800 °C to three C/O gas mixtures: two (Gases 1,2) could oxidize both chromium and aluminum, while the third gas (Gas 3) only aluminum. Ni-5Cr-10Al formed always alumina scales. Reaction of Ni-10/15Cr-7Al with Gases 1 and 2 formed external chromia plus internal alumina, while Gas 3 produced external alumina plus internal chromium carbides. The external alumina growth on Ni-10/15Cr-7Al in Gas 3 is attributed to a reduced critical aluminum content for internal-to-external oxidation with respect to Gases 1 and 2. Oxidation maps are used to interpret the results in Gases 1 and 2.

Corrosion Behavior of Fe-Base Austenitic Alloys with Various Al Additions in a Carburizing–Oxidizing Mixture at 900 °C

The corrosion behavior of Fe–19Ni–13/21Cr–xAl (x = 0, 2, 6 at%) alloys in (vol%) 15CH4–70H2–15CO2 atmosphere was investigated at 900 °C. The presence of oxygen significantly improves the resistance to carbon attack. Continuous oxide scales formed on all alloys. A few carbides appeared in Fe–19Ni–21Cr–xAl (x = 0, 2 at%). The addition of 6 at% Al promoted the formation of external Al2O3 which could work with external Cr2O3 to resist carbon attack, while 2 at% Al only resulted in the appearance of internal alumina.

Oxidation of Ni-Cr-Co-Al-Mo-Ti-Re-Ta-W-Ru Single Crystals at 1000 <sup>o</sup>C in Air

Three kinds of Ni-based single crystals with the compositions of 63.8Ni-7.5Cr-5.1Co-4.8Al-1.9Mo- 0.9Ti-3Re-11.8Ta-1.2W, 61.4Ni-7.4Cr-5Co-4.8Al-1.8Mo-0.9Ti-3.1Re-11.6Ta-4W, and 60.9Ni-7.5Cr-5Co-4.8Al- 2Mo-1Ti-2.9Re-10.9Ta-1.2W-3.8Ru, in wt%, were cast in a Bridgman furnace. In the cast alloys, Cr, Co, Re, Mo, W, and Ru became microsegregated in dendrites consisting of γ-Ni, while Ni, Ta, and Al microsegregated in interdendrites consisting of eutectic γ/γ . The cast alloys were oxidized at 1000 oC up to 275 h in air to study the effect of alloying elements on high-temperature oxidation. The oxide scales consisted primarily of CrTaO4, with some NiCr2O4, NiO, and α-Al2O3. The oxidation resistance was dependent on the formation and continuity of the α-Al2O3 scale. Ta and W were beneficial, while Ru was harmful in improving the oxidation resistance. The selective oxidation of Al in dendrites led to the formation of thin, uniform α-Al2O3 scales, i.e., uniform oxidation. The competitive oxidation of active elements such as Al, Ti, and Ta in interdendrites led to the formation of porous, crack-susceptible oxide nodules, i.e., nodular oxidation. Less active elements such as Ru, Re, Ni, Co, Mo, W, and Cr tended to enrich in the vicinity of the oxide nodules. The oxidation progressed through the outward diffusion of cations and the inward diffusion of oxygen. This inward diffusion formed internal alumina islands, beneath the oxide scale.

Isothermal oxidation of Inconel 625 superalloy at 800 and 1000 °C: Microstructure and oxide layer characterization

Superalloys are widely used in applications at high temperatures and severe environments. Inconel 625 is one of the most applied superalloys due to its high oxidation resistance and mechanical properties, however studies in air lab and temperatures not so extreme as 800 °C are still incipient, mainly regarding microstructure characterization and phases formed during aging at high temperatures. The aim of this study was investigating the Inconel 625 isothermal oxidation behavior at 800 and 1000 °C for 120 and 240 h in air lab. Thermodynamic simulation was used to guide microstructural characterization performed by SEM/EDS and XRD techniques. As-received microstructure presented titanium-niobium (carbo)nitrides and precipitates at grain boundaries. After oxidation, the oxide layer was identified as predominantly chromia in all conditions, providing high oxidation resistance, with some manganese oxidation, suggesting spinel MnCr2O4 formation at 1000 °C, besides ABO4 oxide. Internal alumina oxidation was also observed. Delta phase (Nb,Mo)Ni3 was noticed at grain boundaries (800 °C) and at metal/oxide interface for both temperatures as result of chromium depletion. Some spallation zones were observed at 1000 °C, but this fact did not decrease the good oxidation resistance, as observed at 800 °C. Finally, photoelectrochemistry (PEC) result demonstrated a p-type semiconductor character for chromia scale after 120 h at 800 °C, that was associated to the porous oxide layer and delta phase formation.

Influence of oxidation temperature on the microstructure evolution of NiCoCrAlYTa powders

In this paper, the microstructure evolution in NiCoCrAlYTa alloy powders at two different oxidation temperatures (900 and 1100 °C) was investigated. The results showed that higher oxidation temperature gave higher oxidation rate. Increasing temperature accelerated the diffusion of alloying elements outwards to the powder surface and oxide growth rate. Alumina was the main oxide when the Al content in the powder was high enough. With Al depletion, β phase was depleted by phase transformation of β-γ′-γ at 900 °C and β-γ at 1100 °C. When the Al content in the powder was not enough to maintain the growth of alumina layer, spinel phases were highly formed above the alumina layer and internal alumina begun to largely form. Y-rich and Ta-rich phases were detected beneath the oxide layers at long oxidation time. The diffusion behavior of alloying elements in the different oxidation stages was discussed.

Stability of External α-Al2O3 Scales on Alloy 602 CA at 1100–1200 °C

An external ultrathin α-Al2O3 scale grown on the Ni-base alloy 602 CA during air oxidation at 800 °C was characterized by means of high-resolution TEM/EDX and electron diffraction. Alloy samples pre-oxidized at 800 °C were subsequently exposed at 1100, 1150 and 1200 °C for up to 100 h. Whereas the external alumina remained stable at 1100 °C, with the increasing exposure temperature, the pre-grown alumina scale tended to break down resulting in an external chromia scale accompanied by internal alumina precipitation. The transition from external to internal Al oxidation was investigated using SEM/EDX/EBSD. The critical Al depletion at the scale-alloy interface during the post-exposure at 1100–1200 °C was modeled using the CALPHAD-based thermodynamic-kinetic approach.

Supercritical Water Gasification of Biomass in a Ceramic Reactor: Long-Time Batch Experiments

Supercritical water gasification (SCWG) is an emerging technology for the valorization of (wet) biomass into a valuable fuel gas composed of hydrogen and/or methane. The harsh temperature and pressure conditions involved in SCWG (T > 375 °C, p > 22 MPa) are definitely a challenge for the manufacturing of the reactors. Metal surfaces are indeed subject to corrosion under hydrothermal conditions, and expensive special alloys are needed to overcome such drawbacks. A ceramic reactor could be a potential solution to this issue. Finding a suitable material is, however, complex because the catalytic effect of the material can influence the gas yield and composition. In this work, a research reactor featuring an internal alumina inlay was utilized to conduct long-time (16 h) batch tests with real biomasses and model compounds. The same experiments were also conducted in batch reactors made of stainless steel and Inconel 625. The results show that the three devices have similar performance patterns in terms of gas production, although in the ceramic reactor higher yields of C2+ hydrocarbons were obtained. The SEM observation of the reacted alumina surface revealed a good resistance of such material to supercritical conditions, even though some intergranular corrosion was observed.

External α-Al2O3 scale on Ni-base alloy 602 CA. – Part I: Formation and long-term stability

A multi-step oxidation treatment was developed for the NiCrAl-base alloy 602 CA to promote the growth of protective α-Al2O3 scales during exposure at 1100–1200°C in air. Above 1000°C, the alloy forms external chromia scales accompanied by internal alumina precipitation. On the other hand, external alumina grows on alloy 602 CA surface at 800–900°C. The alloy specimens were pre-oxidized at 800°C to form the external α-Al2O3 scale and further exposed at 1100 and 1200°C. The resulting oxide scales were characterized with SEM/EDX, EBSD, and TEM. The pre-grown alumina scale demonstrated good adherence, long-term stability and healing capability.

Investigation of Oxide Bifilms in Investment Cast Superalloy IN100: Part II. Characterization

Oxide bifilms are a proposed casting inclusion reported to have been observed in vacuum investment cast polycrystalline Ni-based superalloys. Ongoing research seeks to determine if current superalloy casting practices can result in the formation of oxide bifilms, and subsequently if it is possible to observe and characterize this phenomenon. The effect of casting atmosphere, turbulence, filtering, hot isostatic pressing, and heat treatment has been investigated to identify the critical parameters that have been reported to result in bifilm formation in Ni-based superalloy IN100. Scanning Auger microscopy (SAM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) were utilized to characterize samples from each casting condition. In situ ultrahigh vacuum Auger fractography did not indicate the presence of bifilms on the fracture behavior of IN100 in any processing condition. SAM analysis identified a sulfur-enriched monolayer on the surface of dendritic casting porosity, and identified heterogeneous Ti oxycarbide inclusions in air cast IN100. SEM analysis also indicated the presence of Ti oxycarbide inclusions in air cast IN100, and determined that these inclusion structures consist of fine blocky external M(Ti, Mo)C carbide enveloping an internal core of alumina. HR-TEM analysis indicated that none of the oxycarbide inclusion interfaces exist as discontinuous unbound interfaces, and that the internal alumina core is an ultra-fine polycrystalline structure.

Scale Formation of Alloy 602 CA During Isothermal Oxidation at 800–1100 °C in Different Types of Water Vapor Containing Atmospheres

The oxidation behavior of the nickel base Alloy 602 CA in atmospheres, relevant to gas separation units in fossil fired power plants, was investigated in the temperature range 800 to 1100 °C. Isothermal oxidation kinetics were determined in Ar–7 %H2O, Ar–4 %H2–7 %H2O, Ar–20 %O2–7 %H2O and, for comparative purposes, in Ar–20 %O2. The alloy formed an external alumina scale during oxidation at 800 °C, regardless of the atmosphere. Increasing the temperature results in gradual replacement of the alumina scale by an external chromia layer and internal alumina precipitates. The chromia growth rate is affected by the gas composition, due to titanium incorporation in the scale and outwardly protruding metallic nodules. The external alumina scale formation was strongly affected by surface cold work during specimen grinding. This effect, which promoted external alumina scale formation, was retained during exposure at 800 °C but was rapidly lost at higher temperatures resulting in external chromia scale formation and internal oxidation of aluminum.

Modelling compositional changes in nickel base alloy 602 CA during high temperature oxidation

The high chromium nickel base alloy 602 CA is used as construction material for high temperature components in aggressive environments such as gas burners or heat exchangers. The high temperature oxidation behaviour of this alloy is characterised by the formation of an external Cr2O3 scale and internal precipitation of alumina. The main goal of the present investigation was to elucidate the effect of aluminum on the subscale chromium depletion phenomena. The chromium concentration profiles were found to be flat and sometimes even increasing towards the oxide-alloy interface. The experimental observations were interpreted in terms of alloy thermodynamics, i.e. effect of oxidation induced compositional changes on chemical potentials of the alloy constituents. The concentration profiles in alloy 602 CA observed during high temperature exposure were modelled using the CALPHAD approach for thermodynamic and kinetic calculations. Thermodynamic calculations revealed that the Al depletion in the alloy subsurface due to internal alumina precipitation leads to enhanced Cr diffusion towards the alloy surface.

The Initial Oxide Scale Development on a Model FeNiCrAl Alloy at 900 °C in Dry and Humid Atmosphere: A Detailed Investigation

The investigated alumina forming FeNiCrAl model alloy shows protective oxidation behavior in dry and humid environment at 900 A degrees C. Hence, this type of alloy may replace conventional chromia forming austenitic alloys in aggressive oxidizing/reducing environments. A detailed investigation of the oxide scale development reveals a complex initial scale development. Firstly, at alloy grain boundaries, a thin Al rich oxide forms which is replaced by transient alumina platelets in dry and equiaxed alpha-Al2O3 crystallites in humid atmosphere. The scale at alloy grain centers develops via a layered scale of external chromia:Fe/Ni metal inclusions:internal alumina to a layered external spinel:internal alumina scale in dry atmosphere. In humid condition an additional oxide feature appears on the center of large alloy grains i.e. thick oxide protrusions. Despite the initially different phase compositions a continuous protective alpha-Al2O3 scale forms both atmospheres.

Atomistic Modeling of Corrosion Resistance: A First Principles Study of O2 Reduction on the Al(111) Surface Covered with a Thin Hydroxylated Alumina Film

In the early stage of corrosion of Al or Al alloys (i.e., during the initiation of localized corrosion), an oxide film is generally present on the surface. This work investigates the possibility for a cathodic reaction to occur on these oxide films. We discuss realistic models of supported oxide films on Al(111) in order to disentangle the factors determining the reactivity towards O2. Three components of the complex film formed on Al(111) can be identified: an ultrathin under‐stoichiometric AlxOy interface layer, an intermediate Al2O3 phase with γ‐alumina structure, and an hydroxylated AlOOH surface termination with boehmite structure. The electron transfer to O2 molecules depends on the workfunction, Φe, of the metal/oxide interface and on the thickness of the inner Al2O3 phase. The electron transfer takes place both from the metal‐oxide interface and the oxide surface to the adsorbed O2 molecule. Very important is the role of the hydroxyl groups at the surface: they eliminate the Al surface states and stabilize the surface; they allow the reduced O2 − species to capture protons and transform into hydrogen peroxide in a non‐activated process. H2O2 is further reduced to two water molecules, in a series of two‐electron mechanisms. These reactions take place only when the internal alumina phase is ultrathin (here 0.2 nm). As soon as an Al2O3 inner layer develops (film thickness of about 1 nm), the film becomes unreactive and passivates the Al(111) surface. The results help to shed light on the complex reactions responsible for metal corrosion.

The Halogen Effect for Ni-Base Superalloys – A Thermodynamic Study

A new method for the oxidation protection of Ni-base superalloys with relatively low Al-content is proposed. By using the halogen effect the Al activity on the surface can be increased. Thus, the formation of a pure protective alumina scale becomes possible. The alloys IN738 and IN939 are considered in the present paper. Thermodynamic calculations for fluorine and chlorine predict the existence of the halogen effect for both alloys at temperatures between 900°C and 1200°C. The results also predict a change of the oxidation mechanism from internal alumina formation to external oxidation.

In situ developed laminates with large microstructural differences between layers of similar composition

Fabrication of alumina-aluminium titanate laminates that combine high strength external layers with flaw tolerant internal layers is limited by the difficulty of the co-sintering of layers with large differences in the green state. This work describes a new method to obtain alumina-aluminium titanate layered materials constituted by layers with large differences in terms of grain size starting from green bodies with similar microstructures. The approach is based on the effect of small amounts of titania (TiO2) as agents for alumina grain growth enhancement. Starting from a fine grained green body that combined alumina layers with composite layers made of mixtures of alumina and titania, a two step sintering schedule led to a layered structure with external layers of small grain sized alumina combined with internal alumina layers with large grain size due to the diffusion of titanium. The large grain sized alumina layers conferred flaw tolerant behaviour to the laminate due to crack branching and bridging.

Anodized Aluminum Oxide Films as Nanostructured Template for the Grow of Carbon Nanotubes

AbstractA summary of our recent work related to the growth of carbon nanotubes (CNTs) inside the pores of anodized alumina oxide (AAO) films is presented. The surface topography of the AAO film was studied by atomic force microscopy, while the internal pore structure of the AOO templates was concluded from the shape of the CNTs obtained. The effect of the different experimental approaches and of the anodization voltage value on the features of the pores and CNTs is also discussed, as well as the catalytic effect in the acetylene pyrolysis of the Co particles and internal alumina walls. The CNTs wall structure observed by HRTEM and its implication on some physico-chemical properties of the CNTs are also presented.

Microstructural hot corrosion behaviour of austenitic Fe‐29.7Mn‐8.7Al‐1.04C alloy in fused Na2SO4 and Na2SO4‐NaCl mixture

A short‐time hot corrosion test was performed on the austenitic Fe‐29.7Mn‐8.7Al‐1.04C alloy in sodium sulphate at 900°C. The corrosion scales formed on the alloy investigated were studied by scanning electron microscopy and X‐ray mapping techniques. The hot‐corrosion scales morphology of the austenitic Fe‐29.7Mn‐8.7Al‐1.04C alloy were characterized by the formation of Al2S3 and α‐MnS sulphides at and beneath the internal alumina scale. No fluxing of the scales was observed. The effect of the addition of sodium chloride to the fused sodium sulphate on the hot corrosion scales morphology was also investigated.