External Alumina Research Articles

Effect of thermal cycling on protective properties of alumina scale grown on thin Haynes 214 foil

The isothermal and cyclic oxidation of a thin alloy 214 foil (130μm thick) has been studied in air at 1200°C for up to 720h. Haynes 214 forms a duplex oxide scale consisting of pure, columnar alumina overlaid by mixed Ni(Al,Cr)2O4 spinel with minor inclusions of (Al,Cr)2O3. A transition from external alumina to chromia scaling (abbreviated as TACS) occurs at the oxide-alloy interface after the Al reservoir of the foil is exhausted due to oxidation. The tensile cracking of the oxide scale upon thermal cycling is shown to promote the early transition from protective to accelerated breakaway oxidation.

The Effect of Environmental Sulfur on the Establishment and Structural Stability of Alumina Scales

Single-phase γ-Ni of composition (in at.%) Ni–6.3Al–5.4Cr is borderline between forming Al2O3 internally or externally. Oxidation of this alloy in air and O2 + 1 %SO2 was carried out at 1,000 °C for 20 h. In air, the alloy oxidized in a mixed mode, with regions forming a non-protective product of internal Al2O3/NiAl2O4 and external NiO. When oxidized in O2 + 1 %SO2, the alloy formed a continuous Al2O3 scale. Thus, a small amount of sulfur in the atmosphere promoted the transition from internal to external Al2O3-scale formation. In a parallel study, single-phase γ′-Ni3Al of composition (in at.%) Ni–5Cr–20Al–3Pt–0.1Hf–0.05Y was oxidized at 900 °C for 20 h in air, O2 + 0.1 %SO2 and in air with an Na2SO4 deposit. For all conditions, external alumina scales were established. Metastable Ө-Al2O3 formed when oxidation took place in air alone, whereas the stable α-Al2O3 formed during oxidation in O2 + 0.1 %SO2 and in air with an Na2SO4 deposit. Thus, sulfur from the salt deposit or gas atmosphere promoted the Ө-Al2O3 → α-Al2O3 transformation.

Evaluation of Alumina-Forming Austenitic Foil for Advanced Recuperators

A corrosion- and creep-resistant austenitic stainless steel has been developed for advanced recuperator applications. By optimizing the Al and Cr contents, the alloy is fully austenitic for creep strength while allowing the formation of a chemically stable external alumina scale at temperatures up to 900°C. An alumina scale eliminates long-term problems with the formation of volatile Cr oxy-hydroxides in the presence of water vapor in exhaust gas. As a first step in producing foil for primary surface recuperators, three commercially cast heats have been rolled to ∼100 μm thick foil in the laboratory to evaluate performance in creep and oxidation testing. Results from initial creep testing are presented at 675°C and 750°C, showing excellent creep strength compared with other candidate foil materials. Laboratory exposures in humid air at 650–800°C have shown acceptable oxidation resistance. A similar oxidation behavior was observed for sheet specimens of these alloys exposed in a modified 65 kW microturbine for 2871 h. One composition that showed superior creep and oxidation resistance has been selected for the preparation of a commercial batch of foil.

Influence of Aluminum and Rhenium on the Isothermal Oxidation Behavior of CoNiCrAlY Alloys

The oxidation behavior of a Co32Ni21Cr8Al0.6Y (wt%) alloy with and without the addition of 3.5 wt% rhenium, 2 wt% aluminum or a combination of the two was investigated at 1000 °C. Results showed that increasing the Al content from 8 to 10 wt% led to an increase of the alloy β-phase, but did not affect the oxidation behavior. Re addition induced (Cr,Re,Y)-rich phase to precipitate in the alloy, accelerated the θ- to α-alumina transformation, reduced the oxidation rate and enhanced the rate of alloy Al diffusion. Adding both Al and Re further improved the oxidation behavior by promoting the development of the external alumina scale and suppressing the formation of Ni, Co containing spinel. This alloy also showed the largest reduction of oxidation rate and emerged to be the most beneficial. A continuous Cr–Re rich layer was observed at the oxide/alloy interface of the Re, Al containing alloy after longer oxidation times, but this layer is not expected to affect the continued growth of the alumina scale.

Effect of Nb on oxidation behavior of high Nb containing TiAl alloys

The isothermal oxidation behavior of Ti-45Al-8Nb and Ti-52Al-8Nb alloys at 900 °C in air was investigated. The early oxidation behaviors were studied by using XPS and AES. And the microstructure and the composition of the oxidation scale were studied by using XRD and SEM. The results show that the oxidation behavior of TiAl alloy is significantly improved by Nb addition. Nb substitutes for Ti in TiO 2 as a cation with valence 5, and thus to suppress TiO 2 growth. The (Ti,Nb)O 2-rich layer is a dense and chemically uniform which is more protective than the TiO 2 layer. Nb addition also lowers the critical Al content to form an external alumina. Nb 2Al phase is formed in the metallic matrix at the oxide–metal interface on the high Nb containing TiAl alloys.

A new concept of oxidation protection of Ni-base alloys by using the halogen effect

The Ni-base alloys with Al-contents of less than 10 wt% are widely used in high temperature technology due to their beneficial mechanical properties. However, their oxidation resistance may be insufficient at temperatures above 1000°C. Oxidation of these Ni-base alloys does not form a pure continuous Al2O3 protective scale on the surface, but rather a complex layer structure. This structure is characterized by inward growing oxides showing a discontinuous alumina scale. A new method for the formation of a dense protective alumina scale on the surface is now presented. The method is based on the halogen effect, which was successfully applied for TiAl-alloys. Thermodynamic calculations show the preferred formation of gaseous Al-halogenides within a certain region of fluorine partial pressures. The fluorine treatment is performed by ion implantation. The implantation parameters are defined by using Monte Carlo simulations. Following these results fluorine implantations of the Ni-base alloy IN 738 are performed to meet the required fluorine content near the surface. After oxidation at 1050°C a thin continuous external alumina scale is formed on the surface, whereas the untreated alloy shows a mixed oxide scale with significant inward growing oxides. The results offer a new and innovative way to protect the Ni-base alloys against oxidation.

The halogen effect for Ni-base alloys – A new method for increasing the oxidation protection at high temperatures

A new method for oxidation protection of Ni-base alloys is presented. The method is based on the halogen effect. Thermodynamic calculations show the preferred formation of gaseous Al-halogenides within a certain region of fluorine partial pressures. Ion implantation has been chosen for fluorine treatment. The implantation parameters are defined by using the T-DYN simulation software. Based on these results fluorine implantations of the Ni-base alloy IN 738 are performed to meet the required fluorine content near the surface. The fluorine depth profiles are analyzed by using the PIGE-technique. After oxidation at 1050°C a dense protecting external alumina scale is formed on the surface.

A new concept of oxidation protection of Ni-base alloys by using the halogen effect

The Ni-base alloys with Al-contents of less than 10 wt% are widely used in high temperature technology due to their beneficial mechanical properties. However, their oxidation resistance may be insufficient at temperatures above 1000°C. Oxidation of these Ni-base alloys does not form a pure continuous Al2O3 protective scale on the surface, but rather a complex layer structure. This structure is characterized by inward growing oxides showing a discontinuous alumina scale. A new method for the formation of a dense protective alumina scale on the surface is now presented. The method is based on the halogen effect, which was successfully applied for TiAl-alloys. Thermodynamic calculations show the preferred formation of gaseous Al-halogenides within a certain region of fluorine partial pressures. The fluorine treatment is performed by ion implantation. The implantation parameters are defined by using Monte Carlo simulations. Following these results fluorine implantations of the Ni-base alloy IN 738 are performed to meet the required fluorine content near the surface. After oxidation at 1050°C a thin continuous external alumina scale is formed on the surface, whereas the untreated alloy shows a mixed oxide scale with significant inward growing oxides. The results offer a new and innovative way to protect the Ni-base alloys against oxidation.

Effect of Zn Additions on the Oxidation of Cu–2 at.% Al and Cu–4 at.% Al Alloys in 1 atm O2 at 800 °C

Four ternary Cu-Zn-Al alloys containing 5 or 10 at.% Zn and 2 or 4 at.% Al plus an alloy containing 2 at.% Al and 15 at.% Zn have been oxidized at 800 A degrees C in 1 atm O(2), and their behavior has been compared with that of the corresponding binary Cu-Zn and Cu-Al alloys. For the alloy containing 4 at.% Al, which is already able to form external alumina scales, the addition of Zn is only effective in reducing the mass gain during the fast, initial-oxidation stage. Conversely, the addition of 15 at.% Zn to Cu-2Al is able to prevent the formation of external scales containing mixtures of the Cu and Al oxides, resulting in the formation of external alumina scales after an initial stage of faster rate, producing a limited third-element effect. Finally, the addition of Al to both Cu-5Zn and Cu-10Zn is able to prevent the internal oxidation of Zn, producing a kind of reversed third-element effect. Possible mechanisms for these effects are examined on the basis of general treatments concerning the scaling behavior of ternary alloys.

Sandwich materials formed by thick alumina tapes and thin-layered alumina–aluminium titanate structures shaped by EPD

The objective of this work was to analyse the potential of the alumina (Al 2O 3)–titania (TiO 2) system to fabricate laminates with very thin deflecting layers. Alumina–aluminium titanate structures formed by thick self-supported alumina tapes (400 μm) joined together by layered alumina–aluminium titanate structures shaped by EPD have been developed and their mechanical response has been evaluated as a function of the thickness of the layered structures. For this purpose the stability of alumina and alumina–titania suspensions in ethanol was optimized and the EPD parameters to control the growth of layers on non-conductive substrates were analysed. Two different sandwich structures formed by external alumina tapes joined by multilayer systems built by EPD were fabricated. Differences between the multilayer systems were based on the thickness of the layers. Large portions of the central parts of the laminates with thicker layers remained un-fractured while multiple delaminations were observed in the central parts of the specimens with thinner layers that presented wood-like fracture. The fracture modes were reflected in the load-displacement curves as higher loads were needed to fracture the more resistant thicker ligaments whereas larger displacements were admitted by the specimens with thinner layers due to multiple delaminations.

The Effect of Si Additions on the High-Temperature Oxidation of a Ternary Ni–10Cr–4Al Alloy in 1 ATM O2 AT 900–1000 °C

The oxidation of four Ni–10Cr–ySi–4Al alloys has been studied in 1 atm O2 at 900 and 1000 °C to examine the effects of various Si additions on the behavior of the ternary alloy Ni–10Cr–4Al, which during an initial stage formed external NiO scales associated with an internal oxidation of Cr + Al, later replaced by the growth of a chromia layer at the base of the scale plus an internal oxidation of Al. The addition of 2 at.% Si was able to prevent the oxidation of nickel already from the start of the test, but was insufficient to form external alumina scales at 1000 °C, while at 900 °C alumina formed only over a fraction of the alloy surface. At 1000 °C the addition of 4 at.% Si produced external chromia scales plus a region of internal oxidation of Al and Si, a scaling mode which formed over a fraction of the alloy surface in combination with alumina scales also by oxidation at 900 °C. Conversely, the presence of about 6 at.% Si produced external alumina scales over the whole sample surface at 900 °C, but only over about 60 % of the alloy surface at 1000 °C. The changes in the oxidation modes of the ternary Ni–10Cr–4Al alloy produced by Si additions have been interpreted by extending to these quaternary alloys the mechanism of the third-element effect based on the attainment of the critical volume fraction of internal oxides needed for the transition to the external oxidation of the most-reactive-alloy component, already proposed for ternary alloys.

Accelerated Corrosion of Fe–xCr–10Al Alloys Containing 0–20 at.% Cr Induced by Sulfur and Chlorine in a Reducing Atmosphere at 600 °C

The corrosion behavior of five Fe–xCr–Al alloys with a constant Al content of 10 at.% and Cr contents ranging from 0 at.% to 20 at.% was examined at 600 °C in a H2–HCl–H2S–CO2 gas mixture providing 3.7 × 10−22 atm O2, 2.4 × 10−14 atm Cl2 and 3.9 × 10−9 atm S2. All the alloys formed duplex scales containing an outermost layer of iron oxide plus an inner layer composed of mixtures of the oxides of all the alloy components. Besides, a region of internal attack of Al or Al + Cr, whose depth decreased with increasing Cr content, formed in all the alloys. The simultaneous presence of chlorine and sulfur in the gas mixture significantly accelerated the corrosion of all the alloys with respect to their oxidation in a simpler H2–CO2 mixture providing the same oxygen pressure, by forming thick and cracked scales. The effect was particularly large for the high-Cr alloys due to their inability to form external protective alumina scales in the present gas mixture.

Compatibility Issues for a High Temperature Dual Coolant Blanket

One proposed U.S. test blanket module (TBM) for ITER uses ferritic-martensitic alloys with both eutectic Pb-Li and He coolants at ∼475°C. In order for this blanket concept to operate at higher temperatures (∼750°C) for a DEMO-type reactor, several Pb-Li compatibility issues need to be addressed. A SiC/SiC composite flow channel insert is proposed to reduce the steel dissolution rate (and the magnetohydrodynamic pressure drop). Prior capsule testing examined dense, high-purity SiC in Pb-Li at 800°-1200°C and found detectable levels of Si in the Pb-Li after 2,000h at 1100°C and 1,000h at 1200°C. Current capsule experiments are examining several different SiC/SiC composite materials at 1000°C. Another issue involves Pb-Li transport between the first wall and heat exchanger. Aluminide coatings on type 316 stainless steel and Al-containing alloys capable of forming an external alumina scale have been studied in capsule experiments at 700° and 800°C for 1,000h. Model aluminide coatings made by chemical vapor deposition reduced the dissolution rate for 316SS at 800°C by a factor of 50.

Effects of minor alloy additions and oxidation temperature on protective alumina scale formation in creep-resistant austenitic stainless steels

Niobium was found to be beneficial for alumina scale formation in newly developed creep-resistant austenitic stainless steels. Additions of both Ti and V degraded alumina scale formation, whereas alloys with individual additions of Ti or V were able to form protective alumina scales. The ability to form external alumina scales was lost in the lower Al- and Nb-containing alloys between 800 and 900 °C. Compositions with the potential to form alumina at 900 °C were identified.

Aluminium depletion in NiCrAlY bond coatings by hot corrosion as a function of projection system

Three different projection system are used to prepare NiCrAlY bond coats over metallic substrates: atmospheric plasma spray (APS), high velocity oxyfuel (HVOF) and high frequency pulse detonation (HFPD). These coatings were tested in hot corrosion experiments with sprayed Na 2SO 4 at 1000 °C for 20 and 100 h experiments in air. Coatings surface composition after thermal treatment was characterised by XRD and SEM. Cross section of coatings were analysed by SEM-EDX. A relationship between microstructural characteristics of initial coatings and final performance in hot corrosion was found in terms of porosity percentage: plasma sprayed coatings present higher percentage of porosity compared to HVOF and HFPD projection systems for the same composition and Al is heavily consumed in interparticle oxidation. This Al depletion in turn involves intrinsic chemical failure and surface layer is comprised by a porous spinel of mixed oxides. On the other hand, high energy projection systems produce dense coatings allowing the Al migration to external alumina layer, particularly in the case of HVOF coating.

Hot corrosion of a novel electrodeposited Ni–6Cr–7Al nanocomposite under molten (0.9Na, 0.1K) 2SO 4 at 900 °C

A novel Ni-6Cr-7Al nanocomposite (in wt.%) was fabricated by co-electrodeposition of Ni with Cr and Al nanoparticles from a nickel sulfate bath. The nanocomposite consisted of nanocrystalline Ni matrix and dispersed nanoparticles of Cr and Al. The nanocomposite, compared with conventionally arc-melted Ni-6Cr-7A1 alloy, exhibited a dramatically increased hot corrosion resistance under molten (0.9Na, 0.1 K)(2)SO4 in air at 900 degrees C, because the unique structure of the nanocomposite favored the rapid formation of external alumina scale. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effect of yttrium addition on the isothermal oxidation behavior of sputtered K38 nanocrystalline coating at 1273 K in air

The influence of various amounts of Y addition on the isothermal oxidation behavior of sputtered Ni-based superalloy K38 nanocrystalline coating at 1273 K was investigated. The result indicated that the addition of 0.05 and 0.1 wt.% Y in nanocrystalline coating enhanced the selective oxidation of Al, reduced scale growth rate and retarded the phase transformation from metastable to stable alumina. In contrast, 0.5 wt.% Y addition significantly increased the oxidation rate of the coating, the incorporation of Ti in the external alumina scale accelerated the phase transformation to stable alumina.

High temperature water vapour corrosion of rare earth disilicates (Y,Yb,Lu) 2Si 2O 7 in the presence of Al(OH) 3 impurities

Potential environmental barrier silicates based on rare earth disilicates containing Y, Yb and Lu have been investigated for their corrosive behavior in a gas stream containing water at 1500 °C. No currently used test method is unambiguous: silica or silica-forming tubing cause high internal P Si ( OH ) 4 , which should artificially slow down corrosion rates and alumina tubing cause alumina contamination via P Al ( OH ) 3 . We used the latter and report on the details of the interaction. In Y 2Si 2O 7 and a number of solid solutions with a Lu or Yb content up to 50% this contamination resulted in the formation of a melt. Depending on further impurities, particularly Ca, melt formation is accompanied by oxyapatite or monosilicate crystallisation. On cooling rare earth garnets crystallize from the melt. The melt ± oxyapatite/monosilicate formation does not create a protective effect. The corrosion kinetics is linear; the rates are slower than those of pure silica, but only at a level reflecting reduced silica activity due to dilution by a factor of 2–5. Porosity causes fast initial mass losses. The formation of secondary phases inside the material induces crack formation. A disilicate layer in those systems is unlikely to become an effective environmental barrier for non-oxide systems. For Yb and Lu silicates there are indications for the formation of a rare earth garnet layer during the corrosion process at high temperature, which has protective power. This causes a logarithmic law for the corrosion kinetics: for extended times the mass loss drops asymptotically. Under the assumption that the garnet formation also removes the sink for external alumina, the protection for Si-removal may even become perfect. The total mass change is a balance between Al-input and silica loss, which makes it currently impossible to formulate a quantitative equation for the time dependence of the process.

Restructuring of polygalacturonate on alumina upon hydration—Effect on phosphate sorption kinetics

Hydration of organic coatings in soils is expected to affect the sorption of oxyanions onto hydrous Fe and Al oxides. We hypothesized that the hydration of polygalacturonate (PGA) coatings on alumina (Al 2O 3) increases their permeability for phosphate. Pure and PGA-coated alumina were equilibrated in deionized water for 2 and 170 h at pH 5 and 20 °C before studying (i) their porosity with N 2 gas adsorption and 1H NMR relaxometry, (ii) structural changes of PGA-coatings with differential scanning calorimetry (DSC), and (iii) the kinetics of phosphate sorption and PGA desorption in batch experiments. Scanning electron micrographs revealed that PGA molecules formed three-dimensional networks with pores ranging in size from <10 to several hundred nanometers. Our NMR results showed that the water content of intraparticle alumina pores decreased upon PGA sorption, indicating a displacement of pore water by PGA. The amount of water in interparticle alumina pores increased strongly after PGA addition, however, and was attributed to water in pores of PGA and/or in pores at the PGA-alumina interface. The flexibility of PGA molecules and the fraction of a PGA gel phase increased within one week of hydration, implying restructuring of PGA. Hydration of PGA coatings increased the amount of phosphate defined as instantaneously sorbed by 84%, showing that restructuring of PGA enhanced the accessibility of phosphate to external alumina surfaces. Despite the fact that the efficacy of phosphate to displace PGA was higher after 170 h than after 2 h, a higher phosphate surface loading was required after 170 h to set off PGA desorption. Our findings imply that the number of PGA chain segments directly attached to the alumina surface decreased with time. We conclude that hydration/dehydration of polymeric surface coatings affects the sorption kinetics of oxyanions, and may thus control the sorption and transport of solutes in soils.

Study of the oxide scale integrity on Ni‐base alloy CMSX‐4 during isothermal and thermal‐cycling exposure

AbstractThe excellent isothermal high‐temperature corrosion resistance of the single crystalline Ni‐base superalloy CMSX‐4, which is due to external alumina formation, is substantially reduced under thermal‐cycling conditions. Particularly, tapered edges exhibit a high cracking and spallation rate leading to premature onset of breakaway oxidation in combination with strong internal corrosion attack. To characterize these effects, specimens with different geometries were prepared, i.e., rectangular‐shaped and wedge‐shaped specimens. Measurements under isothermal and thermal‐cycling conditions at a temperature of 1100°C in air were carried out using a self‐designed thermobalance. By using wedge‐shaped specimens the conditions for spallation, Al depletion and eventually, breakaway oxidation in combination with internal corrosion (Al2O3, TiN and AlN) could be correlated with the change in thickness of the specimen and the duration of the high‐temperature dwell time. As compared to rectangular‐shaped specimens, wedge geometries revealed a much higher susceptibility to oxide spallation and consequently, a higher rate of aluminium depletion. An evaluation of the surface fraction affected by oxide spallation and an analysis of the spalled oxidation products were carried out in order to determine the role of dwell time duration as well as the effect of the number of thermal cycles.