Protective Alumina Layer Research Articles

Effect of Al content on the microstructure, mechanical property and lead-bismuth eutectic corrosion resistance of refractory AlxTiNbZrMoV coatings produced by magnetron sputtering

In present study, refractory AlxTiNbZrMoV (x=0.2, 0.5, 0.9 and 1.2 in molar ratio) high-entropy alloy coatings were prepared by magnetron sputtering technology. The microstructure, mechanical property and lead-bismuth eutectic corrosion resistance of the AlxRHEA coatings were investigated. With the increase of Al content, the coatings maintained the amorphous structure, and no intermediate phase was generated. The hardness and elastic modulus of the AlxRHEA coatings were higher than that of Al-free coating. After the LBE corrosion, formation of surface Fe-oxide layer occurred in the four AlxRHEA coatings. The Al0.5RHEA and Al0.9RHEA coatings exhibited improved corrosion resistance. The results indicate that for HEA, the formation of protective alumina layer can be on the basis of Al enrichment on the coating surface to continue to improve the oxidation resistance.

Study on the corrosion behavior of laser surface remelted and laser cladding of ferritic/martensitic steels after exposure to lead-bismuth eutectic at 700 °C

This study explores two vital structural materials, T91 (Fe-9Cr) and SIMP (Fe-11Cr) steels, in the context of lead-cooled fast reactors and accelerator-driven sub-critical systems (ADS). Lead-bismuth eutectic (LBE) functions as a key coolant and spallation target material due to its impressive thermal conductivity, neutron yield, and chemical properties. Unfortunately, materials in contact with LBE are prone to severe corrosion at elevated temperatures (T>500 °C), compromising their integrity. To bolster corrosion resistance, we utilized laser remelting and laser cladding to apply FeCrAl/TiN coatings on the steel surfaces. Our study scrutinizes the corrosion behavior of steel in LBE saturated with oxygen at 700 °C and investigates the underlying causes. Following 240 h of exposure to corrosion, T91 and SIMP steels subjected to laser remelting displayed substantial oxide scale formation. Lead-bismuth atoms infiltrated the outer oxide layer (Fe3O4), diminishing adhesion between the inner and outer oxide layers, leading to the detachment of the outer oxide layer. The inner oxide layers, composed of Fe-Cr spinel, were approximately 123 μm thick for T91 steel and 77 μm for SIMP steel, underscoring SIMP steel's superior corrosion resistance. For T91 steel treated with laser cladding FeCrAl/TiN coating, a characteristic duplex oxide layer with a total thickness of around 83 μm was formed, with noticeable deposition of Pb-Bi atoms at the interface between the outer and inner oxide layers. Conversely, only a protective alumina layer safeguarded SIMP steel from LBE corrosion. This outcome emphasizes the efficacy of laser cladding FeCrAl/TiN coating in providing superior protection for SIMP steel over T91 steel. Our research significantly contributes to the development of anti-corrosion coatings for high-temperature LBE environments.

Growth of AlN coating on Al-6061 alloy surface

The objective of the study was to grow uniform nitride coatings over Al-6061 alloys to resist fluorine diffusion. This paper discusses a process of carrying out surface nitridation of an Al-6061 alloy substrate. Through this method, a dense aluminum nitride coating can be grown on Al-6061 alloy with complete area coverage of the samples having complex geometries. The method uses nitrogen gas alone as the nitrogen carrier, and not ammonia derivatives used in conventional processes. Magnesium powder was also used in the process to reduce the protective alumina layer on the alloy and to facilitate the aluminum nitride growth. The process led to uniform coating coverage over the flat sample surfaces as well as over the surfaces of drilled holes in the sample. The coatings were stable when subjected to several 400 °C thermal cycles.

Cyclic oxidation of slurry silicide-aluminide coatings formed on Ti-6Al-4V alloy

ABSTRACT A proprietary slurry cementation method was used to produce silicide-aluminide coatings on alloy Ti-6Al-4V. The cementation processes were carried out at 900–1100 °C for 2–6 h. The coatings exhibited a stratified configuration characterised by a phase arrangement that demonstrates favourable attributes pertaining to mechanical and corrosion resistance capabilities, namely, the presence of aluminium-enriched phases at the coating surface and titanium-enriched phases proximity to the substrate. The cyclic oxidation behaviour at 800 °C for 30 twenty-two-hour cycles was investigated. The characterisation of the structure of coatings was carried out through the use of scanning microscopy, X-ray microanalysis, and X-ray diffraction techniques. The coatings have excellent resistance to cyclic oxidation, resulting from the gradient structure and the establishment of a protective alumina layer on the surface. The coatings were characterised by good resistance to thermal shocks. The microcracks formed during thermal shocks were filled with an alumina, which renders the coatings self-healing.

The effect of Al and Ti on the microstructure, mechanical properties and oxidation resistance of γ′-Ni3(Al, Ti) strengthened austenitic stainless steels

Both Al and Ti are the key elements for the formation of γ′-Ni3(Al, Ti) phase and protective oxide scale in the γ′-Ni3(Al, Ti) precipitation strengthened austenitic stainless (AS) steels. To obtain good mechanical properties, especially at elevated temperatures, high volume fraction of γ′ phase is required, which is strongly dependent on the total amounts of Al and Ti. For a given concentration of (Al + Ti), changing Al or Ti addition also has significant effects on the microstructure, mechanical properties and oxidation resistance of alloys. In this study, three alloys are designed by moderately adjusting Al and Ti addition while keeping (Al + Ti) concentration constant (8.7 at.%). It is found that increasing Ti and decreasing Al addition lead to fine Laves precipitates and close spacing between them, which in turn refine grain size and enhance strength at temperature lower than 750 °C. But at about 800 °C, the strength of there alloys is almost the same. Increasing Al and decreasing Ti addition have no apparent influence on the γ′ phase but promote the formation of B2–NiAl phase, resulting in reduced ductility at elevated temperature. Besides, high Al addition facilitates the formation of continuous protective oxide layer (mainly alumina) and is beneficial for oxidation resistance at 750 °C for 500 h. As Ti replaces Al, the integrity of protective alumina layer is destroyed and the discontinuous Al-rich oxides are interlaced with Cr-rich and Ti-rich oxides.

Immobilisation mechanism for nuclear waste containing aluminium by supersulfated cement containing phosphogypsum

Supersulfated cement (SSC), which features a relatively low pH value, is increasingly considered as an effective alternative to Portland cement-based material for solidifying nuclear waste. Phosphogypsum (PG), an industrial waste, can be used to prepare SSC, but the impurities may affect the efficiency of SSC for nuclear waste immobilisation. Therefore, PG was pretreated by calcination and lime neutralization. The mechanism of supersulfated cement containing phosphogypsum for immobilizing nuclear waste was investigated in this study. Results show that the corrosion of aluminium rods by the PG-based SSC is much lower than that of Portland cement, of which calcined PG-based SSC has the least corrosion on aluminium rods. At the w/c ratio of 0.4, the pH value of PG-based SSC is lower than that of Portland cement. For calcined PG-based SSC with different w/c ratios, a high w/c ratio reduced the corrosion of aluminium rods slightly. The immobilisation mechanism of the SSCs on nuclear waste is as follows: when the aluminum-containing nuclear waste comes to contact with the cement paste, the protective layer of alumina is slowly dissolved, and aluminum reacts with OH− to form hydrogen and Al(OH)3. The formation of hydrogen bubbles may force cement away from the aluminum surface and allow ettringite and Al(OH)3 to form in water-filled pores around the metal, which relatively slows the rate of the corrosion reaction. Corrosion will end until the surface of the aluminum rod is covered with corrosion products or the free water in the cement is exhausted.

Effect of aluminium and alumina ratio of aluminide coating on 304 stainless steel by slurry aluminizing

Aluminide coating formed protective layer of alumina that protect from high oxidation. However, the composition of coating that determined the coating’s properties should be intensively considered. The aim of this study is to develop an aluminide coating on 304 stainless steel using low temperature aluminizing. The aluminide coating was developed using slurry aluminizing with and without alumina addition. The coating then was annealed at 650 °C for 2, 4, 6 and 8 h. The coatings were characterized for morphology, coating thickness, coating composition, phases and hardness using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray Diffraction (XRD) and Vickers microhardness. For slurry composition containing only Al and 7Al:3Al2O3, the aluminide coating successfully developed on the surface. However, for 5Al:5Al2O3 and 3Al:7Al2O3 slurry compositions, the coating produced was not properly adhered onto the substrate. Most of the surface of the substrate remained uncoated. A single layer of FeAl3 was formed at the additive layer and a thin layer of FeAl was formed at the interdiffusion zone. The addition of alumina has no effect on the type of phase formed in the coating. When alumina was added to the slurry, the coating thickness was reduced. Vickers microhardness measurements revealed that the additive layer, interdifussion zone (IDZ), and substrate have hardness in the range of 760–920 HV, 580–680 HV, and 200–230 HV, respectively.

Acid Dissolution Behavior of Ferritic FeCrAl Tubes Candidates for Nuclear Fuel Cladding

The international materials community is engaged in finding safer alternatives to zirconium alloys for the cladding of fuel in light water reactors. One solution is to replace the zirconium cladding using ferritic iron-chromium-aluminum (FeCrAl) alloys, which offer extraordinary resistance to high-temperature reaction with air or steam due to the formation of a protective alumina layer on the external surface. It is important to characterize the behavior of FeCrAl not only during accident conditions but in the entire fuel cycle, which may include reprocessing of the used fuel after it is removed from the power reactors. The reprocessing may involve the dissolution of the fuel rods in mineral acids. Little or nothing is known on the dissolution of FeCrAl alloys in common mineral acids, therefore the objective of this research was to study the dissolution of typical cladding tubing having two compositions of FeCrAl (APMT and C26M) in three acids (H2SO4, HNO3, and HCl) as a function of the temperature using both standard ASTM immersion tests as well as electrochemical tests. The dissolution behavior of the FeCrAl alloys is compared to the dissolution capability of other traditional nuclear materials such as austenitic stainless steels (304SS and 316SS) and austenitic nickel alloys (Alloy 600 and Hastelloy C-276). Results show that both C26M and APMT have a higher dissolution capability in the studied mineral acids, which will be beneficial for reprocessing procedures.

Effects of tungsten alloying and fluorination on the oxidation behavior of intermetallic titanium aluminides for aerospace applications

Current limitations to a wider use of intermetallic TiAl alloys in aircraft and automotive engines arise from an insufficient oxidation resistance at temperatures above approximately 800 °C. In this paper, the high temperature oxidation behavior of three engineering γ-TiAl-based alloys at 900 °C in air is reported. The performance of the TNM alloy (Ti-43.5Al–4Nb–1Mo-0.1B), the 4822 alloy (Ti–48Al–2Cr–2Nb), and the Nb-free IRIS alloy (Ti–48Al–2W-0.08B) is compared (all chemical compositions are given in at.%). During testing in air non-protective mixed oxide scales developed on all untreated samples, but with different compositions and thicknesses. These different oxide layers are characterized and their formation mechanisms are discussed. The presence of W in the IRIS alloy leads to a better oxidation behavior compared to untreated TNM and 4822. This behavior was changed in the direction of a protective alumina layer formation via the so-called “fluorine effect”. The above-mentioned alloys were treated with fluorine via a liquid phase process by evenly spraying a fluorine containing polymer on all faces of the specimens. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated specimens. Due to the fluorination all treated test coupons exhibited slow oxidation kinetics. The results of isothermal as well as thermocyclic exposure tests are presented and discussed in the view of the chemical composition and processing conditioned microstructure of the three investigated γ-TiAl-based alloys.

Mechanism for breakaway oxidation of the Ti2AlC MAX phase

The good oxidation resistance of MAX phases up to temperatures around 1200 °C can be compromised for long exposure due to the breakaway of the protective alumina layer. Herein, we unveil a mechanism of breakaway oxidation of the Ti2AlC MAX phase, identifying the main trigger and the solutions to avoid it. It is caused by excessive rumpling of the oxide scale on surfaces with arithmetical mean roughness (Ra) > 3 µm and constitutes a key factor in subsequent consumption of Ti2AlC. First, the oxide scale experienced rumpling due to significant radial stresses generated at the Ti2AlC/oxide interface. Second, scale blistering resulted from substantial buckling due to the evolution of in-plane stresses and lateral lengthening. Third, blister collapse and exposure of the underlying Al-depleted Ti2AlC surface led to rapid ingress of oxygen and oxide/substrate interface recession. The self-healing ability of Ti2AlC has been restrained and breakaway oxidation kinetics following a linear trend have been initiated. Similarly, breakaway oxidation was observed on micro-damaged surfaces. A mixed oxide layer with high porosity mainly composed of rutile titanium dioxide (TiO2) promptly formed on these surfaces, gradually consuming the base Ti2AlC material.

Structured catalysts for the conversion of liquefied petroleum gas to hydrogen-rich gas and for anode off-gas afterburning

A number of mixed oxide Ce0.75Zr0.25O2 supports were prepared, tested for reproducibility, and characterized by physicochemical methods. The most reproducible preparation method was adapted for depositing the mixed oxide on a FeCrAlloy mesh substrate coated by a protective alumina layer. Based on the obtained structured Ce0.75Zr0.25O2/θ-Al2O3/FeCrAl support, the Rh/Ce0.75Zr0.25O2/Al2O3/FeCrAl and Pt/Ce0.75Zr0.25O2/Al2O3/FeCrAl catalysts were prepared and tested in the reactions of partial oxidation of LPG and deep oxidation of anode gases, respectively. Rh/Ce0.75Zr0.25O2/Al2O3/FeCrAl provided complete conversion of LPG into synthesis gas of a composition close to the equilibrium one. Pt/Ce0.75Zr0.25O2/Al2O3/FeCrAl provided complete conversion of all components of the anode gases at GHSV = 20,000–40,000 h−1; however, at higher GHSV values, methane conversion decreased. The studies on the effect of methane content on deep oxidation of anode off gases showed that methane conversion began to decrease at a 1.5-fold excess of methane and dropped to 50% at a 10-fold excess of methane.

Oxidation Behavior of Ta–W–Ti–Al Multialloys Prepared by Spark Plasma Sintering

In this study, Ta–W–Ti–Al multialloys were prepared by spark plasma sintering (SPS) as a new powder metallurgy technology based on discharge activation and thermoplastic deformation process. The oxidation behavior of the as-obtained sintered alloys were investigated under 1000 and 1200°C in air atmosphere. Microstructures and phase compositions of alloys and oxides were analyzed using various analytical methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and backscatter electron microscopy (BSE). The sintered samples possessed relative densities reaching up to 98% with achievable densification. The densities of all specimens with high Ti contents exceeded 100%, suggesting the great impact of Ti addition on liquid-phase precipitation. Intermetallic compounds were formed in Ta–W–Al alloys, and samples with high Ti contents displayed grain boundary phases rich in titanium and oxygen. The addition of Ti and Al effectively improved the oxidation resistance of the resulting alloys. A protective alumina layer was formed after oxidation at 1000°C that made oxidation kinetics of the alloys obeying pseudo-parabolic laws. At the oxidation temperature of 1200°C, Al2O3 reacted with Ta2O5 to form the AlTaO4 alloy with reduced protective effect. The mass gain in the alloys after oxidation at 1200°C for 4 h was about 7 times higher compared to processing at 1000°C.

On oxide formation on a single crystalline Ni-based superalloy at 900 °C in SO2 containing atmosphere: The effect of surface treatment

The oxidation behavior of the single crystalline SCA425+ (17Cr-10Al-3Ta-Ni bal.) after two different surface finish procedures (grinding and polishing) was investigated after cyclic exposures at 900 °C in SO2 containing atmosphere up to 260 h. The microstructure and chemistry of the produced oxide scales were studied using high-resolution methods, SEM/EDX, TEM, and STEM. The investigation revealed that grinding provides more effective diffusion paths, so that the material is able to form a protective continuous alumina layer, while an extensive internal oxidation takes place on the polished sample. In addition, a numerical model, based on the cellular automata approach, was developed and used to consider the effect of dislocation pipe diffusion in combination with diffusion processes and oxide formation. Good agreement between the model and experimental results was obtained.

Hot corrosion behavior of thermal barrier coating on Cr2AlC and CoNiCrAlY substrates at 950°C in presence of Na2SO4+V2O5 molten salts

In this study, Cr2AlC MAX phase and CoNiCrAlY substrates were prepared by spark plasma sintering (SPS) method and Yttria-stabilized zirconia (YSZ) ceramic coating was deposited on the substrates by atmospheric plasma spray (APS) technique. The hot corrosion behavior of these samples was examined at 950°C in a cyclic mode in the presence of Na2SO4+V2O5 molten salts. After each cycle, samples were evaluated by energy dispersive spectroscopy (EDS) equipped scanning electron microscopy (SEM) and x-ray diffraction (XRD) methods. According to the results, Cr2AlC/YSZ sample has better hot corrosion resistance compared to the CoNiCrAlY/YSZ system. Formation of rod shape YVO4 phase and also phase transformation during the hot corrosion are the main factors in the destruction of the YSZ top coat. According to the EDS results, the thermally grown oxide (TGO) layer in Cr2AlC/YSZ is alumina and in CoNiCrAlY/YSZ system TGO is consisted of alumina and spinel oxides. Also, it was found that the spinel oxide in CoNiCrAlY/YSZ system is porous and does not have appropriate mechanical properties compared to the dense and protective alumina layer and can be a suitable area for the interfacial cracking.

Machine learning assisted design of \u03b3\u2032-strengthened Co-base superalloys with multi-performance optimization

Designing a material with multiple desired properties is a great challenge, especially in a complex material system. Here, we propose a material design strategy to simultaneously optimize multiple targeted properties of multi-component Co-base superalloys via machine learning. The microstructural stability, γ′ solvus temperature, γ′ volume fraction, density, processing window, freezing range, and oxidation resistance were simultaneously optimized. A series of novel Co-base superalloys were successfully selected and experimentally synthesized from >210,000 candidates. The best performer, Co-36Ni-12Al-2Ti-4Ta-1W-2Cr, possesses the highest γ′ solvus temperature of 1266.5 °C without the precipitation of any deleterious phases, a γ′ volume fraction of 74.5% after aging for 1000 h at 1000 °C, a density of 8.68 g cm−3 and good high-temperature oxidation resistance at 1000 °C due to the formation of a protective alumina layer. Our approach paves a new way to rapidly design multi-component materials with desired multi-performance functionality.

Oxidation behavior of Ti45Al8.5Nb alloy anodized in NH4F containing solution

Ti45Al8.5Nb was anodized in ethylene glycol and NH4F containing solution to improve the oxidation resistance. Isothermal oxidation test shows that the anodized specimens can provide excellent oxidation resistance and spallation resistance under 1000 °C. During the high temperature oxidation process, a compact and continuous alumina layer would generate on the anodized Ti45Al8.5Nb alloy. This protective alumina layer can efficiently decrease the inward diffusion of oxygen and outward diffusion of the substrate element. The improved oxidation resistance is assigned to the halogen effect caused by the fluorine component generated during the anodization process.

Oxidation behaviour of the MAX-phases Ti2AlC and (Ti, Nb)2AlC at elevated temperatures with and without fluorine treatment

MAX-phase materials have shown great potential for different technical applications due to their mechanical properties. If the main group element is aluminium their excellent oxidation resistance also makes them attractive for several high temperature applications. As an example the thermodynamically stable MAX-phase Ti2AlC forms a thin, protective alumina layer in oxidising atmospheres at elevated temperatures. This alumina layer is formed due to the high Al activity within the material and prevents further attack by the environment. However, high temperature oxidation tests at 900 °C in air of “technical” Ti2AlC which is not pure single-phase Ti2AlC led to the formation of a non-continuous alumina scale which is intersected by a mixed TiO2/Al2O3 scale. Furthermore, internal oxidation was observed. This “technical” material consists of two phases namely Ti2AlC plus γ-TiAl due to the manufacturing route. Such γ-TiAl-grains are preferentially oxidised. This type of internal attack can be suppressed by a preceding fluorine treatment.

First report on cold-sprayed AlCoCrFeNi high-entropy alloy and its isothermal oxidation

Abstract

Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials

Molten chloride salts are promising advanced high-temperature (400–800 °C) thermal energy storage (TES) and heat transfer fluid (HTF) materials in next generation concentrated solar power (CSP) plants for higher energy conversion efficiencies. However, severe corrosion of structural materials in contact with molten chloride salts is one of the most critical challenges limiting their applications at elevated temperatures. In this work, two corrosion mitigation strategies are investigated to alleviate the hot corrosion of structural materials in molten chloride salts: (1) adding corrosion inhibitor and (2) using a Fe-Cr-Al alloy with a protective alumina layer on the surface after pre-oxidation. Three commercial high temperature Fe-Cr-Ni alloys (SS 310, Incoloy® 800 H and Hastelloy® C-276) were exposed to molten MgCl2-NaCl-KCl (60–20–20 mol%) mixed salts with 1 wt% Mg as corrosion inhibitor, for 500 h at 700 °C under inert atmosphere. By addition of the Mg inhibitor, the corrosion rates of the studied alloys were found to be significantly reduced, more precisely by ~ 83% for SS 310, ~ 70% for In 800 H and ~ 94% for Ha C-276 compared with the exposure tests without Mg addition. The corrosion mitigation mechanism of Fe-Cr-Ni based alloys in molten chloride salts by adding Mg is discussed based on corrosion thermodynamics. To assess the second mitigation strategy two pre-oxidized alumina forming Fe-Cr-Al alloys were exposed to the same molten chloride salts without Mg corrosion inhibitor under the same conditions. It is observed that the adherent alumina scales can effectively inhibit the dissolution of Cr and Fe and the bulk penetration of corrosive impurities. Overall, both strategies offer enormous potential for enhancing the expected lifetime of commercial alloys in molten chloride salts.

Material-Versatile Ultrabroadband Light Absorber with Self-Aggregated Multiscale Funnel Structures.

Broadband light absorbers are essential components for a variety of applications, including energy harvesting and optoelectronic devices. Thus, the development of a versatile absorbing structure that is applicable in various operating environments is required. In this study, a material-versatile ultrabroadband absorber consisting of metal-coated self-aggregated Al2O3 nanowire bundles with multiscale funnel structures is fabricated. A high absorptance of ∼0.9 over the AM 1.5G spectrum (300-2500 nm) is realized for absorbers with a range of metal coatings, including Al, W, and titanium nitride (TiN). We demonstrate that the plasmonic nanofocusing and index-matching effects of the funnel structure result in strong ultrabroadband absorption for the various metal coatings, even though the coating materials have different optical properties. As an example of applicability in an operating environment, in the evaluation of the thermal-oxidation resistance, the Al-coated solar absorber exhibits superior performance to those coated with refractory materials such as W and TiN because of the protective alumina layer formed on the Al surface.