Al2O3 Oxide Scale Research Articles

Aluminide Coatings by Means of Slurry Application: A Low Cost, Versatile and Simple Technology

The present study focused on demonstrating the versatility of the slurry deposition technique to produce aluminide coatings to protect components from high-temperature corrosion in a broad temperature range, from 400 to 1400 °C. This is a simpler and low-cost coating technology used as an alternative to CVD and pack cementation, which also allows the coating of complex geometries and offers improved and simple repairability for a lot of industrial applications, along with avoiding the use of non-hazardous components. Slurry aluminide coatings from a proprietary water-based-Cr6+ free slurry were produced onto four different substrates: A516 carbon steel, 310H AC austenitic steel, Ti6246 Ti-based alloy and TZM, a Mo-based alloy. The resulting coatings were thoroughly characterised by FESEM and XRD, mainly so that the identification of microstructures and appropriate phases was reported for each coating. The importance of surface preparation and heat treatment as key parameters for the coating final microstructures was also evidenced, and how those parameters can be optimised to obtain stable intermetallic phases rich in Al to sustain the formation of a protective Al2O3 oxide scale. These coating systems have applications in diverse industrial environments in which high-temperature corrosion limits the lifetime of the components.

High-temperature oxidation behaviors of Co-free Cr30Fe30Ni30Al5Ti5 dual-phase multi-component alloys with multi-scale nanoprecipitates

In the context of the growing research interest in multi-component alloys (MAs) and their exceptional performance under extreme environments, the high-temperature oxidation resistance and applications of MAs have attracted significant attention in the field of metallic materials. While the cost-effective and mechanical properties of Co-free MAs are of great importance, their oxidation resistance remains insufficiently understood. In this work, we designed multiple heterogeneous structures within a cast dual-phase Cr30Fe30Ni30Al5Ti5 MA by tailoring the Al and Ti ratio, which consists of body-centered-cubic (BCC) grains reinforced by multi-scale nanoprecipitates (i.e., L21, B2, and η phase) and an L12-strengthened face-centered cubic (FCC) skeleton. Isothermal oxidation experiments at 800 °C, 1000 °C, and 1200 °C with varying exposure durations were conducted. The oxidation kinetics at 800 °C and 1000 °C followed a parabolic law, while both low weight increment and oxidation rate confirm remarkable oxidation resistance. At 800 °C, the oxides mainly consist of Cr2O3 and Al2O3, while are dominated by (TiO2 + Cr2O3) and the mixed oxides of Al2O3, TiO2, and Ti2O above 1000 °C. Importantly, the inability to form a continuous Al2O3 oxide scale at higher temperatures led to a deterioration in oxidation resistance. These findings offer valuable insights into underlying mechanisms contributing to oxidation resistance for Co-free MAs.

Oxidation Behavior of Atmospheric Plasma Sprayed NiAl Coating at High Pressure Oxygen Atmosphere

The high-temperature and high-pressure oxygen-rich combustion gases often cause the high temperature gas conduit in rocket engine to suffer severe oxidation, resulting in a reduced service life. The present study deposited a dense NiAl coating on GH4202 superalloy using atmospheric plasma spraying and investigated its oxidation behavior in a high temperature and high-pressure oxygen atmosphere. The results showed that the NiAl coating presented excellent bonding with the superalloy substrate throughout tests. The uncoated GH4202 substrate was severely oxidized during high pressure oxygen atmosphere. It was found that only a continuous very thin Al2O3 oxide scale evolved on the NiAl coating surface and no oxidation was observed at the interface between the NiAl coating and GH4202 substrate. Moreover, no significant inner oxidation occurred to NiAl coating. Therefore, the present APS NiAl coating can provide effective protection of superalloy from oxidation at high pressure oxygen combustion gas atmosphere.

Microstructural evolution and high-temperature oxidation resistance of NiCoCrAlYSiHf coatings after surface thermal modification with millisecond laser

NiCoCrAlYSiHf coatings prepared by arc ion plating was thermally modified via millisecond laser technique. The microstructural evolution and oxidation resistance of the coatings before and after laser thermal modification were compared. Laser thermal modification significantly reduces the surface roughness of the coating, modifies the microstructure (introduction of dislocations, nano-particle precipitation), regulates the elemental distribution (Y-Al oxides), and forms ultrafine-grained particles on the surface. The experimental results show that these evolutions provide conditions for the improvement of the oxidation resistance of the coatings. In particular, Y-Al oxides can play an effective pinning role during long-term oxidation. And the ultrafine-grained particles formed on the surface promote the formation of Al2O3 oxide scale. Therefore, laser thermal modification can be an effective processing method to extend the service life of NiCoCrAlYSiHf coatings.

Modeling Oxidation of AlCoCrFeNi High-Entropy Alloy Using Stochastic Cellular Automata.

Together with the thermodynamics and kinetics, the complex microstructure of high-entropy alloys (HEAs) exerts a significant influence on the associated oxidation mechanisms in these concentrated solid solutions. To describe the surface oxidation in AlCoCrFeNi HEA, we employed a stochastic cellular automata model that replicates the mesoscale structures that form. The model benefits from diffusion coefficients of the principal elements through the native oxides predicted by using molecular simulations. Through our examination of the oxidation behavior as a function of the alloy composition, we corroborated that the oxide scale growth is a function of the complex chemistry and resultant microstructures. The effect of heat treatment on these alloys is also simulated by using reconstructed experimental micrographs. When they are in a single-crystal structure, no segregation is noted for α-Al2O3 and Cr2O3, which are the primary scale-forming oxides. However, a coexistent separation between Al2O3 and Cr2O3 oxide scales with the Al-Ni- and Cr-Fe-rich regions is predicted when phase-separated microstructures are incorporated into the model.

Insights into the surface pretreatment on the oxidation behavior of anodized TiAl alloy

The influence of surface pretreatment on the oxidation performance of the anodized TiAl alloy was investigated. Before anodization in NH4F containing ethylene glycol, TiAl alloy was sand blasted or ground by SiC paper. Results showed that the humps on the rough surface serve as crystallographic templates for the nucleation of the isostructural α-Al2O3 at the initial stage of oxidation. The derived Al2O3 act as the crystal nuclei for the formation of continuous Al2O3 oxide scale during thermal exposure, therefore providing good oxidation resistance. The surface roughness acts a key role to improve the oxidation resistance of the anodized TiAl alloy.

High temperature oxidation resistance of arc ion plating NiCoCrAlY coating modified via laser shock peening

In this study, laser shock peening (LSP) was used to treat NiCoCrAlY coating prepared via arc ion plating (AIP), which is to improve the high temperature oxidation resistance of NiCoCrAlY coating. The oxidation condition is continuous oxidation at 1100 ℃. The results showed that after 150 h oxidation, the samples without LSP treatment had oxide film damage and large-area internal oxidation, and the coating nearly failed. The samples treated by LSP had intact oxide film and strong antioxidant properties. This is attributed to the fact that LSP helps to eliminate large particle clusters on the sample surface, which provides a prerequisite for the uniform growth of thermally grown oxide (TGO). Furthermore, LSP introduces high density dislocations, high density stacking faults and twins, which refines the grains of the coating and provides more diffusion channels for the selective oxidation of Al elements. Due to above reasons, LSP treated sample can form a continuous and uniform Al2O3 oxide scale faster at the initial stage of oxidation, which hinders the diffusion reaction of other elements and keeps TGO in a stable growth stage for a long time. It is proved that LSP is a potential technology for improving the oxidation resistance of NiCoCrAlY coating.

The hot corrosion behaviors and mechanisms of hot-dipped Ti-Al-Si coatings on TiBw/Ti6Al4V composite against NaCl and Na2SO4

The Ti-Al-Si coatings were prepared on TiBw/Ti6Al4V composites by a two-step method comprising hot dip and heat treatment processes to enhance the hot corrosion resistance. Results show that Ti-Al-3Si (wt%) and Ti-Al-20Si (wt%) coatings were mainly composed of the Ti(Al,Si)3 and Ti3AlSi5 phase, respectively. Both of the coatings can protect the substrate against the deposited NaCl, 25 wt%NaCl+ 75 wt%Na2SO4, and Na2SO4 salts. In particular, the coatings exhibited superior hot corrosion resistance in the deposited Na2SO4 salts, which is attributed to the formation of dense and continuous Al2O3 oxide scales and the slow dissolution process of sulfate during the hot corrosion process. Moreover, the Ti-Al-Si coatings exhibited serious external oxidation when hot corroded in the deposited 25 wt% NaCl+ 75 wt%Na2SO4 and Na2SO4 salts, in which more corrosion products and pores were formed due to the autocatalytic reaction processes. Since the deposited NaCl salt would rapidly dissolve the oxide scale and destroy its compactness and continuity, which presented a more serious hot corrosion effect.

Effect of Water Vapor on the Microstructure of Al2O3 on the Free-Standing MCrAlY Alloy at 1100 °C

The oxidation resistance of the MCrAlY binding coat is due to the formation of protective Al2O3 oxide scale at high temperature. The oxidation behavior of NiCrAlYHf alloy in 1100 °C air and air-water vapor atmosphere was studied. The effect of water vapor on the microstructure and distribution of reactive elements was discussed. The results showed that the oxide scale in air has a double layer structure composed of columnar and equiaxed crystals, while the oxide scale in water vapor contains fine alumina grains, which provides more channels for the diffusion of reactive elements. In addition, The Cr element in the oxide scale is mainly concentrated in the outer equiaxed crystal zone, and the Hf oxide is mainly concentrated in the columnar crystal boundary. In air-water vapor atmosphere, the Cr element is uniformly distributed in the oxide scale.

Corrosion behavior of Al-containing MAX-phase coatings exposed to oxygen containing molten Pb at 600 °C

Four types of MAX-phase coating samples (Cr2AlC, V2AlC, Ti2AlC, and Ti3AlC2), synthesized via physical vapor deposition and post-annealing of elemental multilayer films, have been investigated toward their potential corrosion protection in 10-6 wt% oxygen-containing molten Pb for 3200 h at 600 °C. Three MAX-phase coatings (Cr2AlC, Ti2AlC, and Ti3AlC2) are corrosion resistant against the molten Pb, via formation of a protective oxide layer. Specifically, various oxide scales formed on each sample: an Al2O3 scale, containing a low amount of Cr2O3 was formed in case of the Cr2AlC coating; a mixed Al2O3 and TiO2 oxide scale was observed for both Ti2AlC and Ti3AlC2 coatings. A more complex mixed oxide layer consisting of VO2, V4O7, and Pb3(VO4)2 phases is formed on the V2AlC coating, while the MAX-phase coating was almost entirely consumed. According to their corrosion performance against oxygen containing molten Pb, the order of corrosion resistance is: Cr2AlC > Ti2AlC > Ti3AlC2 > V2AlC.

Cyclic Oxidation Properties of the Nanocrystalline AlCrFeCoNi High-Entropy Alloy Coatings Applied by the Atmospheric Plasma Spraying Technique

Microcrystalline and nanocrystalline AlCrFeCoNi high-entropy alloy (HEA) coatings were applied on Inconel 718 superalloy using the atmospheric plasma spraying (APS) process. The high-temperature oxidation behavior of the microcrystalline and nanocrystalline AlCrFeCoNi HEA-coated superalloy was examined at 1100 °C under the air atmosphere for 50 cycles under cyclic heating and cooling (1 h for each cycle). The oxidation kinetics of both nanocrystalline- and microcrystalline-coated superalloys were accordingly analyzed by weight change measurements. We noted that the uncoated and coated samples followed the parabolic rate law of the oxidation. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDS), elemental mapping and X-ray photoelectron spectroscopy (XPS) were used to analyze the oxidized coated and uncoated samples. In the HEA-coated superalloy, Fe, Ni, Co and Al were oxidized in the inter-splat region, whereas the splats, which consisted mainly of Ni and Cr, remained unoxidized. Due to the formation of compact and adhesive thin NiO, CoO oxides and spinels together with the Al2O3 oxide scale on the surface of the coating during oxidation, the developed nanocrystalline HEA coating showed better oxidation resistance compared with the microcrystalline HEA coating.

Breaking the continuity of the Al2O3 oxide scale by additions of Cr in Co-Al-W-based superalloys

The paper presents the effect of small amounts of Cr alloying on the oxidation behavior of a Co-9Al-9W-2Si alloy. A small amount of Cr alloying inhibits the formation of a continuous Al2O3 scale in the oxide layers. The exact mechanism is explained by studying the transient oxidation stages. Cr addition can increase the diffusion rate of oxygen in the oxidation scale. Al is consumed very fast by forming Co(Al,Cr)2O4. This impedes the formation of Al2O3. As a result, the few formed Al2O3 particles do not merge to a continuous scale in the oxide layer.

High temperature corrosion behavior of Ni and Co base HVOF coatings exposed to NaCl-KCl salt mixture

The high-temperature corrosion behavior of NiMoCrW and CoNiCrAlY base coatings produced by high velocity oxy-fuel technique (HVOF) has been analyzed in dry air atmosphere and in presence of NaCl-KCl salt mixture at 650 °C for 360 h. Their behavior was compared with that of NiCr coating used as reference. Their corrosion resistance in presence of NaCl-KCl was measured by dimensional metrology analysis and the corroded samples were characterized by XRD and SEM techniques. The coatings exhibited excellent oxidation resistance in dry air atmosphere due to the formation of Cr2O3 and Cr-rich spinels, as well as the formation of Al2O3 in CoNiCrAlY coating. Their corrosion resistance in presence of NaCl-KCl was significantly lower due to chlorine-induced active oxidation and due to the formation of chromates, identified by XRD as K2CrO4. However, both coatings presented higher corrosion resistance in presence of the alkali salts compared to that of NiCr coating. The presence of Mo in the NiMoCrW coating improved the high-temperature corrosion resistance in presence of the alkali salts and the formation of a stable Al2O3 oxide scale on CoNiCrAlY coating reduced the penetration of chlorides and the degradation of the protective oxide scale by chromates formation.

Effect of water vapor on high-temperature oxidation of NiAl alloy

The high-temperature oxidation of NiAl is studied with dry oxygen and water vapor. The oxidation in H2O results in a thicker Al2O3 oxide scale than that in O2. The oxide scale formed initially is a single layer of γ-Al2O3 that subsequently transforms into a α-Al2O3/γ-Al2O3 bilayer structure, in which the inner α-Al2O3 layer formed in H2O has a higher porosity than that in O2. Further density functional theory calculations show that H protons derived from H2O molecules penetrate into the oxide lattice and boost the formation of lattice vacancies in both α-Al2O3 and γ-Al2O3, thus enhancing the oxide scale growth.

High-temperature oxidation behavior of modified 4Al alumina-forming austenitic steel: Effect of cold rolling

The oxidation behavior and mechanism of as-received and 30 % cold-rolled alumina-forming austenitic (AFA) steel were investigated in dry air at 700 °C. The results show that the mass gain per unit area curves of as-received and 30 % cold-rolled steels subject to near-parabolic law before 100 h oxidation time. Two samples both show higher high-temperature oxidation resistance due to the formation of dense Al2O3 oxide scale. Gradual spallation of outer scale results in the formation of continuous and dense alumina scale. Dislocations can act as short-circuit diffusion channel for the diffusion of Al from alloy matrix to surface, and also provide nucleation sites for B2-NiAl phase, which ensure the continuous formation of Al2O3 scale.

Preparation, characterization and oxidation behavior of CeO2-gradient NiCrAlY coatings applied by HVOF thermal spraying process

Conventional and CeO2-gradient NiCrAlY coatings (0.5–2.0 wt% of nano-CeO2 from the inner layer to layers adjacent to the outer surface) were produced by the high-velocity oxy-fuel spraying (HVOF) process. Microstructural characteristics, phases analysis and high-temperature oxidation behavior of the functionally graded (FG) NiCrAlY/nano-CeO2 coatings were investigated and compared with those monolayer (ML) NiCrAlY coatings with without CeO2 nanoparticles. The microstructure of the ML- and FG-coatings, as well as processed powders, were characterized using a transmission electron microscope (TEM), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), microhardness testing and Raman spectroscopy. On account of the calculation of oxidation behavior of the coating, the ML- and FG-coatings were exposed to laboratory's air at 1000 °C up to 100 h (short-term exposure) and 5000 h (long-term exposure) and moreover, the weight gain versus oxidation time and oxide growth rate were subsequently examined. Obtained results indicated that FG-NiCrAlY/nano-CeO2 coating had a better oxidation resistance as well as a lower oxide growth rate compared to the rest of the ML-coatings. In addition, increasing oxidation resistance of FG-NiCrAlY/nano-CeO2 coating can be attributed by controlling Al and O diffusion due to the formation of relatively thin, dense, and sticky Al2O3 oxide scale. Besides, all cases of the oxidized ML- and FG-coatings had a parabolic rate behavior owing to their diffusion-controlled oxidation under short- and long-term oxidation tests.

Aluminide and silicide diffusion coatings by pack cementation for Nb-Ti-Al alloy

Halide activated pack cementation technique was successfully used to elaborate protective coatings, based on silicon- or aluminum-rich intermetallic compounds, for a Nb-Ti-Al alloy. Thermodynamic computations showed that growth mechanism of coating involves different chemical reactions. The substrate shows a poor behavior in thermal oxidation with the formation of non-protective oxides and high solubility of oxygen in its microstructure. The effectiveness of the deposited MAl3 and MSi2 coatings to protect the substrate was demonstrated. Reliability and high oxidation resistance of these diffusion coatings are promoted by their good adherence to the substrate, capability to accommodate thermo-mechanical stress and ability to form adherent and compact Al2O3 and SiO2 oxide scales. These oxide scales grow during the oxidation process at 900 and 1000 °C according to very low kinetics as demonstrated by the curves of mass change versus time. The mass changes of the coated substrate with aluminide and silicide were <1.5 mg·cm−2 after 650 h of exposure at 1000 °C.

Oxidation kinetics and mechanisms of aluminum modified ultra-pure low chromium ferritic stainless steels

The oxidation behavior of ultra-pure low chromium ferritic stainless steels with aluminum contents of 0, 0.16, and 1.51 wt.% has been investigated at 800 and 1000 °C in air for up to 100 h. Aluminum promotes the appearance of parabolic nature of oxidation, and the parabolic rate constant significantly decreases to about 1.1 × 10−7 mg2 mm−4 h−1 at 1000 °C and 1.4 × 10−3 μg2 mm−4 h−1 at 800 °C for the steel with 1.51 wt.% Al. The TiO2 + Al2O3 oxide scale at 1000 °C and Al2O3 + Cr2O3 oxide scale at 800 °C significantly improve oxidation resistance.

Microstructure and super oxidation resistance of the network structured Ti-Al-Si coating

To improve oxidation resistance of the titanium matrix composites, the network structured Ti-Al-Si coating was successfully prepared on TiBw/Ti6Al4V composites with network microstructure utilizing hot dip siliconizing and subsequent heat treatment. The cycle oxidation tests showed that the weight gains of the Ti-Al-Si coating were only 8.98%, 5.99% and 3.65% of the TiBw/Ti6Al4V composites at 973K, 1073K and 1173K for 100 h. This result indicates super oxidation resistance of the network structured Ti-Al-Si of TiBw/Ti6Al4V composites, which can be ascribed to the three reasons: (1) the continuous Al2O3 oxide scale was formed in the outermost layer, (2) there were no cracks existed in the oxidized coating due to the unique network structure of the coating, (3) the Ti5Si4 layer in the interface could effectively retard the interfacial reaction between the Ti(Al,Si)3 coating and composite substrate.

Cycle oxidation behavior and anti-oxidation mechanism of hot-dipped aluminum coating on TiBw/Ti6Al4V composites with network microstructure

Controlled and compacted TiAl3 coating was successfully fabricated on the network structured TiBw/Ti6Al4V composites by hot-dipping aluminum and subsequent interdiffusion treatment. The network structure of the composites was inherited to the TiAl3 coating, which effectively reduces the thermal stress and avoids the cracks appeared in the coating. Moreover, TiB reinforcements could pin the TiAl3 coating which can effectively improve the bonding strength between the coating and composite substrate. The cycle oxidation behavior of the network structured coating on 873 K, 973 K and 1073 K for 100 h were investigated. The results showed the coating can remarkably improve the high temperature oxidation resistance of the TiBw/Ti6Al4V composites. The network structure was also inherited to the Al2O3 oxide scale, which effectively decreases the tendency of cracking even spalling about the oxide scale. Certainly, no crack was observed in the coating after long-term oxidation due to the division effect of network structured coating and pinning effect of TiB reinforcements. Interfacial reaction between the coating and the composite substrate occurred and a bilayer structure of TiAl/TiAl2 formed next to the substrate after oxidation at 973 K and 1073 K. The anti-oxidation mechanism of the network structured coating was also discussed.