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The aluminide coating layer was formed on wire arc additive manufactured (WAAM) Inconel 625 (IN625) Ni-based superalloy by a pack-aluminizing process at 700 °C for 3 h. The aluminide coatings were evaluated utilizing X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and nanoindentation techniques. In addition, the oxidation performances of the aluminide coatings were compared with as-built WAAM IN625 samples based on their exposure in the open-air environment for 5, 25, and 50 h at 1000 °C. The aluminizing process provided a dense aluminide coating with a thickness of 35 µm, continuous throughout the surface. The coating layer consists of mainly NiAl, Ni 2 Al 3 , Cr 2 Al, and MoAl 5 phases and exhibited a nano-hardness of 12.85 ± 0.43 GPa. Moreover, the applied heat treatment also improved the surface hardness and elasticity modules of WAAM Inconel 625. The stability of aluminide phases (NiAl, Ni 2 Al 3) at temperatures exceeding 1000 ℃ and the formation of stable Al 2 O 3 oxide islands on the surface provided 6.63 times, 2.70 times, and 2.65 times better oxidation resistance in the aluminized samples than the as-built WAAM IN625 in the 5 h, 25 h and 50 h oxidation periods at 1000 ℃, respectively. In contrast, the increase in the oxidation time changed the oxidation mechanism of as-built WAAM IN625 from Cr 2 O 3 to Cr 2 O 3 and spinel phases such as NiCr 2 O 4, NiMoO 4, and NiO. On the other hand, the increase in the oxidation time in aluminized samples caused Kirkendall voids formation and their degradation. Therefore, it was concluded that the mechanical properties could be improved, and the oxidation resistance of these alloys could be improved with the aluminizing heat treatment. [ABSTRACT FROM AUTHOR]

Electrical contacts are used in electromechanical horns to make and break the circuit. Electrical contacts are generally made of tungsten refractory material due to its high melting point, high hardness, and hence good abrasion and wear resistance among all refractory metals. However, the oxidation resistance of tungsten is poor, and thus tungsten contacts used in electrical appliances undergo severe high-temperature oxidation during making and breaking electric circuit. The tungsten oxide (WO3) layer, being not very conductive with respect to thickness, can cause electrical connectivity loss and subsequently cause horn failure. In other circumstances, if oxides grow thicker, they can peel off easily due to the frequent impact during make-and-break operations, leading to a high wear rate. Thus, to improve the life of horn, aluminizing and chromizing coating treatments on tungsten contact have been developed throughes halide activated pack cementation process. The behaviour of the developed coatings under isothermal oxidation at 1000 °C for different time intervals from 2 to 100 h was studied. To evaluate the rate of oxidation, the weight gain versus time of bare and coated tungsten contacts was analysed. Macroscopic morphological analysis indicates isothermal oxidation at 1000 °C resulted into severe oxidation of bare W within 10 h and aluminized W within 25 h. On the other hand, chromized W showed negligible oxidation even after 100 h at 1000 °C. The mass gain of chromized tungsten contacts after 100 h of isothermal oxidation was 2.2 mg/cm2, which was drastically lower than the aluminized (276.9 mg/cm2) and bare (270.02 mg/cm2) tungsten contacts. The parabolic oxidation rate constant (kp in units g2 cm−4 s−1) for chromized tungsten contact was obtained as 4.7 × 10−11, which is quite low compared to aluminized and bare tungsten of 6.3 × 10−7 and 1.0 × 10−6, respectively. This suggests that the chromizing coating effectively improved the oxidation resistance of tungsten contact. [ABSTRACT FROM AUTHOR]

The study aims to investigate the effect of Al2O3 and Al additions to Nickel-base superalloys as a coating layer on oxidation resistance, and structural behavior of nickel superalloys such as IN 738 LC. Nickel-base superalloys are popular as base materials for hot components in industrial gas turbines such as blades due to their superior mechanical performance and high-temperature oxidation resistance, but the combustion gases’ existence generates hot oxidation at high temperatures for long durations of time, resulting in corrosion of turbine blades which lead to massive economic losses. Turbine blades used in Iraqi electrical gas power stations require costly maintenance using traditional processes regularly. These blades are made of nickel superalloys such as IN 738 LC(Inconel 738). Few scientists investigated the impact of Al2O3 or Al additions to Nickel-base superalloys as coating layer by using the slurry coating method on oxidation resistance to enhance the Nickel-base superalloy’s oxidation resistance. In this study, IN 738 LC is coated with two different coating percentages, the first being (10 Al+90 Al2O3) and the second being (40 Al+60 Al2O3). Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) were performed on all samples before and after oxidation. According to the results, SEM images of the surface revealed that the layer of the surface has a relatively moderated porosity value and that some of the coating layers contain micro-cracks. The best surface roughness of specimens coated with 60 % alumina+40 % aluminum was 5.752 nm. Whereas, the surface roughness of specimens coated with 90 % alumina+10 % aluminum was 6.367 nm. Results reveal that alloys with both Al2O3 and Al additions have reported a positive synergistic effect of the Al2O3 and Al additions on oxidation resistance. Moreover, the NiCrAl2O3 thermal coating has good oxidation resistance and the effective temperature of anti-oxidation is raised to 1100 °C in turn reducing the maintenance period of turbine blades [ABSTRACT FROM AUTHOR]

Platinum-modified-βNiAl is a bond coat (BC) material for thermal barrier coatings (TBCs) applications applied on aeroengine hardware to reduce their surface temperatures. However, it is desirable to minimize its production and material costs by the low-cost alternatives of similar performance. It has been acknowledged that the small concentration of the reactive elements (REs), such as Zr, Hf, and Y, could tremendously enhance the oxide adhesion even in some cases better than Pt modified counterparts. The aim of this study was to design and fabricate the Zr-modified-βNiAl bond coat on CMSX-4 superalloy using an aluminizing method. Moreover, the study focused on the development of a systematic understanding of underlying mechanisms behind the beneficial effects of REs. Initially, three sets of BCs were prepared: Zr-free βNiAl (undoped), Al and Zr co-deposited in a single-step process (1SP), and Zr and Al, which were individually deposited in two processing steps (2SP): zirconizing and aluminizing. Such three sets of BCs helped to understand the processing, as well as Zr and Al effects on scale adhesion. In particular, 1SP/2SP BCs showed uniformity of Zr in the form of precipitates and networks that caused hardness enhancement. All BCs were isothermally oxidized at 1150oC for 100 hours wherein 2SP revealed the best spallation resistance, microstructural stability, and its Zr-oxide pegs were extended to substrates. In addition to the Zr effect, BC Al content was found to affect the oxide adhesion equally. Under identical Zr contents (of 1SP and 2SP = 1at %), the higher Al showed the better spallation resistance while lower Al caused the inverse effect of Zr owing to its reactive nature that was termed as over doping. Moreover, it was established that over-doping either local or into entire BC, accelerated the Al depletion that destabilized the βNiAl into γ'-Ni3Al phase. An extensive discussion is presented in the light of the observed results. [ABSTRACT FROM AUTHOR]

A high-temperature antioxidant layer on Ti-6Al-4V alloy was prepared by a two-step method of REO-boriding followed by aluminizing (REOs are rare-earth oxides, which are Nd 2 O 3 , Gd 2 O 3 , and Tm 2 O 3). The microstructure and oxidation behavior of the layer were investigated at temperatures of 700 °C, 800 °C, and 900 °C in air. The findings unveiled that the layer significantly enhances the antioxidant capacity of Ti-6Al-4V alloy. After cyclic oxidation at 900 °C for 100 h, the average weight gain of the layer is only one fortieth of that of the matrix. TiAl 3 in the layer can react with O 2 to form a dense continuous Al 2 O 3 layer, which effectively inhibits the diffusion of O 2 to the matrix. During the cyclic oxidation, the thicker TiAl 2 , TiAl, and Ti 3 Al diffusion layers are formed sequentially between TiAl 3 layer and matrix, and TiAl 2 layer can also provide the good oxidation resistance. • REO-borided and aluminized coatings were prepared by a simple two-step process. • The coatings have excellent high-temperature oxidation resistance. • Oxygen reacts with Al 3 Ti to form a dense continuous Al 2 O 3 layer to resist oxidation. [ABSTRACT FROM AUTHOR]

ASTM P23 steel (Fe-2.25Cr-1.6W-0.1Mo in wt.%) was hot-dip aluminized and oxidized at 800∘C and 1000∘C for 20 h in air in order to determine the effect of aluminizing on the microstructure, hardness, and oxidation resistance of P23 steel. Aluminizing effectively increased the oxidation resistance of P23 steel by forming protective α -Al2O3 scales. During oxidation, outward diffusion of substrate elements and inward transport of Al and oxygen occurred simultaneously. The oxidation and interdiffusion formed voids in the coating, lowered the microhardness, and transformed the original (Al-rich topcoat)/(Al 1 3 Fe4 layer) to either (thin α -Al2O3 scale)/(Al5Fe2 layer)/(AlFe layer)/(AlFe3 layer)/(α -Fe(Al) layer) at 800∘C or (thick α -Al2O3 scale)/(AlFe3 layer)/(α -Fe(Al) layer) at 1000∘C. At 1000∘C, Fe2O3 was also formed in addition to α -Al2O3 scale, due to the enhanced outward diffusion of Fe, thus suppressing the formation of cracks in the coating. [ABSTRACT FROM AUTHOR]

Abstract Refractory high-entropy alloys (RHEAs) emerge as promising candidate materials for ultrahigh-temperature applications. One critical issue to solve for RHEAs is their balanced oxidation resistance and mechanical properties, mainly room-temperature ductility for the latter. Recently, it was found that existing ductile RHEAs are subject to catastrophic accelerated oxidation, also known as pesting. In this work, both alloying and surface coating, are applied to enhance the oxidation resistance of ductile RHEAs, with the focus on surface coating using the pack cementation method and more specifically, aluminizing. The oxidation resistance of two RHEAs, Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr, one recently identified ductile RHEA which pests in the temperature range of 600–1000 °C, and Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5 , the newly designed ductile RHEA which does not pest but embrittles after oxidation, are studied after aluminizing at 900 °C using three different pack components. Aluminizing, if using the appropriate pack cementation parameters, can avoid pesting in Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr and alleviate the oxidation induced embrittlement in Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5 , and holds the promise for further improving the RHEAs as potential ultrahigh-temperature materials. Highlights • Pack cementation using different pack compositions were experimented on two RHEAs. • Proper aluminizing can avoid pesting in the Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr RHEA. • Proper aluminizing can improve the embrittlement in Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5. • Aluminizing is promising to further improve RHEAs for high-temperature applications. [ABSTRACT FROM AUTHOR]

The T92 steel plate was hot-dip aluminized, and oxidized in order to characterize the high-temperature oxidation behavior of hot-dip aluminized T92 steel. The coating consisted of Al-rich topcoat with scattered Al3Fe grains, Al3Fe-rich upper alloy layer with scattered (Al, Al5Fe2, AlFe)-grains, and Al5Fe2-rich lower alloy layer with scattered (Al5Fe2, AlFe)-grains. Oxidation at 800 °C for 20 h formed (α-Al2O3 scale)/(AlFe layer)/(AlFe3 layer)/(α-Fe(Al) layer), while oxidation at 900 °C for 20 h formed (α-Al2O3 scale plus some Fe2O3)/(AlFe layer)/(AlFe3 layer)/(α-Fe(Al) layer) from the surface. During oxidation, outward migration of all substrate elements, inward diffusion of oxygen, and back and forth diffusion of Al occurred according to concentration gradients. Also, diffusion transformed and broadened AlFe and AlFe3 layers dissolved with some oxygen and substrate alloying elements. Hot-dip aluminizing improved the high-temperature oxidation resistance of T92 steel through preferential oxidation of Al at the surface. [ABSTRACT FROM AUTHOR]

This study investigated the effect of powder-pack aluminizing treatment on the high-temperature oxidation of ER307 stainless steel components fabricated by wire arc additive manufacturing (WAAM) during isothermal oxidation at 1000 °C for 5 h, 25 h, and 50 h. Scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD), X-Ray fluorescence (XRF), nanoindentation testing, and oxidation testing were used to characterize the aluminized and non-aluminized samples produced by WAAM. The results showed that the powder-pack aluminizing increased the surface nano-hardness up to 13.95 GPa and the modulus of elasticity up to 159 GPa, as well as improving the microstructure of WAAM ER307 stainless steel. Indeed, aluminide coatings remained stable up to temperatures exceeding 1000 °C, and the growth of hematite, the main oxide phase, was inhibited by a preferential alumina growth (Al 2 O 3), resulting in an improvement in oxidation resistance in the range of 46–70 %. In addition, owing to the advantages of low-temperature aluminizing, the microstructure, mechanical properties, and oxidation resistance of these alloys have been improved without causing sigma phase formations, which constitute a significant problem in high-temperature heat treatment of stainless steels. • Aluminizing minimized residual stresses and elemental segregation of WAAM stainless. • Aluminizing improved microstructure, mechanical properties, and high-temperature oxidation resistance of WAAM ER307. • The aluminide coatings improved the oxidation resistance of WAAM ER307 by 46% to 70% at 1000 °C. • The increase in the oxidation time has an effect on the crystal lattice structure and the fragile oxide phases. • The growth of hematite, the main oxide phase, was inhibited by a preferential alumina growth (Al 2 O 3). [ABSTRACT FROM AUTHOR]

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. In the present work the microstructural features and oxidation behavior of an aluminized Co-base MCrAlY-coating on a Ni-based superalloy have been investigated in the temperature range 925-1075 °C. Microstructural studies of the oxidized coatings by SEM/EBSD were complemented with numerical thermodynamic calculations using the software package ThermoCalc. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY. During high-temperature air exposure alumina based surface scales were formed but the oxidation induced Al depletion of the aluminized coating did not result in formation of the γ’-(Ni3Al) phase. Rather, the subscale formation of Co/Cr-rich phases was observed and a direct transformation of β- into γ-Ni phase after longer times. It is expected that these subscale microstructural changes thus affect the alumina formation and growth as well as the critical aluminum depletion in a different manner as in the case of corresponding β-NiAl coatings, although a direct comparison between various coating systems was not possible on the basis of the present results. [ABSTRACT FROM AUTHOR]

Recently γ-TiAl intermetallics have been successfully applied on low pressure turbine blades of modern jet engines mostly due to their low density, high specific strength and creep resistance, all of which make them excellent alternatives for Ni-based superalloys. However, due to insufficient high temperature oxidation resistance above 800 °C there is the necessity for development of protective coatings that will allow the formation of a thin and slow growing α-alumina oxide scale. The paper presents the results of investigations concerning the behavior and influence of silicon on the substructure of aluminized ɣ-TiAl alloy in the as-deposited state as well as after long-term isothermal high temperature oxidation. The Si-modified aluminide coating was produced by pack cementation method and oxidized isothermally at 950 °C for 3000 h. The detailed microstructural examination was performed using analytical SEM and STEM techniques along with Focused Ion Beam (FIB) technique which was utilized for the preparation of a sample from the metal-scale interface after long-term oxidation test. The addition of silicon to the aluminide coating resulted in the formation of nanometric Ti 5 Si 3 precipitates that were found to bind both Ti and Nb from the alloy. It was demonstrated that the investigated ɣ-TiAl with Si-modified aluminide coating is capable of forming a continuous and uniform α-alumina oxide scale at 950 °C that remains adherent for 3000 h. The oxide scale was found to obey the growth mechanism typical for Ni-based superalloys and consisted of outer equiaxed and inner columnar grains. [ABSTRACT FROM AUTHOR]

In this work a pack cementation of germanium-doped aluminum and silicon coatings on low alloy steel type-T21 has been applied. This gives significant improvement in the oxidation. Steel-T21 was coated with germanium-doped aluminizing-siliconizing. Diffusion coating was carried out at 1050oC for 6 h under an Ar atmosphere by simultaneous germanium-doped aluminizing-siliconzing process. Cyclic oxidation tests were conducted on the coated steel-T21 alloy in the temperature range oxide 300-900oC in air for 102 h at 3 h cycle. The results showed that the oxidation kinetics for coated system in air can be represented by parabolic curve .Oxide phases that formed on coated system are SiO2 and Cr2O3. A neural network model of oxidation kinetics has been proposed to model the oxidation kinetic. The neural model shows good agreement with the experimental data.

The processes occurring during the formation of combined aluminide coatings on nickel alloys using gas and plasma vacuum technology are considered. It is shown that the external surface of the coating has the structure of a secondary β-NiAl solid solution after the application of a diffusion coating from an Al-Si-Y alloy directly on the surface of alloys or on samples preliminary aluminized or chromoaluminized at 1000°C. The results of heat-resistance tests at temperatures of 1000 and 1050°C are presented. [ABSTRACT FROM AUTHOR]

Conventional and gradient CoNiCrAlYSi coatings were produced using high-velocity oxy-fuel (HVOF) and an additional step of diffusional over aluminizing (pack cementation) techniques on the Inconel-738 substrate. Hot corrosion and cyclic oxidation performance of the conventional and the gradient coatings were investigated by exposing samples to a molten salt of NaSO-20 % wt. NaVO at 880 °C and 1-h exposure at 1100 °C in air. Corrosion and cyclic oxidation rates were determined by measuring the weight gain at regular time intervals. X-ray diffraction, field emission scanning electron microscopy, and electron probe microanalysis techniques were used to characterize the coatings and the thermally grown oxide. Increase in the amount of β aluminum-rich phase and the formation of high-chromium layer beneath the outer aluminum-rich layer and the low surface roughness of gradient coating increased the hot corrosion and oxidation resistance by a factor of 1.7. In addition, the gradient coating showed better rehealing of the thermally formed alumina scale due to its possession of more β phase as a Al reservoir. [ABSTRACT FROM AUTHOR]

The isothermal oxidation resistance of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy alloy is analyzed and the microstructural evolution of the oxide layer is studied. The limited aluminum, about 3.6 at %, leads to the non-continuous alumina. The present alloy is insufficient for severe circumstances only due to chromium oxide that is 10 μm after 1173 K for 360 h. Thus, the aluminized high-entropy alloys (HEAs) are further prepared by the industrial packing cementation process at 1273 K and 1323 K. The aluminizing coating is 50 μm at 1273 K after 5 h. The coating growth is controlled by the diffusion of aluminum. The interdiffusion zone reveals two regions that are the Ti-, Co-, Ni-rich area and the Fe-, Cr-rich area. The oxidation resistance of aluminizing HEA improves outstandingly, and sustains at 1173 K and 1273 K for 441 h without any spallation. The alumina at the surface and the stable interface contribute to the performance of this Al0.2Co1.5CrFeNi1.5Ti0.3 alloy. [ABSTRACT FROM AUTHOR]

Aluminizing of nickel pre-plated AISI 316 is prepared by a high-activity pack at 1050 °C. The effect of initial nickel layer with different thicknesses on microstructure and oxidation behavior of coating is investigated. After aluminization, the surface microstructure of stainless steel mainly consists of β-(Fe, Ni)Al as matrix with β-FeAl + α -(Fe, Cr) precipitates and an inter-diffusion layer with γ phase. Aluminized coating on specimens with 10 and 20 µm primary thicknesses of the nickel layer includes three layers. The outer zone is made up of β-NiAl thin layer and a β-(Fe, Ni)Al layer. As the nickel layer increases to 50 and 100 µm, aluminide layers consist of outer and inner zone with β-NiAl and Ni3Al phase, respectively. Oxidation tests at 950 °C show that the oxidation resistance of aluminide coatings improves and oxidation kinetics follows a sub-parabolic rate law by increase in thickness of initial nickel layer. [ABSTRACT FROM AUTHOR]

Composite palladium membranes based on porous stainless steel (PSS) substrate are idea hydrogen separators and purifiers for hydrogen energy systems, and the surface modification of the PSS is of key importance. In this work, the macroporous PSS tubes were aluminized through pack cementation at 850 °C in argon, followed by an oxidation with air at 600 °C. Palladium membranes were prepared by electroless plating. Their permeation performances were tested, and the hydrogen permeation kinetics was discussed. The substrate materials and the palladium membranes were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD). An Al 2 O 3 -enriched surface layer with small pore size was created through aluminizing and oxidation treatments, which greatly improves the membrane integrity. The intermetallic diffusion between the palladium membranes and the PSS substrate material was not observed after a heat-treatment at 500 °C under hydrogen for 200 h. However, the aluminizing and oxidation treatments still need to be further optimized in order to improve the membrane permeability and selectivity, and particularly, the high diffusion resistance of the substrate materials greatly limited the hydrogen flux. [ABSTRACT FROM AUTHOR]

Highlights: [•] Slurry aluminized and uncoated 9–12wt.% Cr steels exposed to KCl deposit in various atmospheres. [•] Corrosion rates and mechanisms at 560 and 610°C are studied and compared. [•] The materials are degraded by active oxidation process. [•] The coating does not provide the steels with protection against corrosion beneath KCl deposit. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the formation of ceramic coatings by a combined processing of low-temperature pack aluminizing and oxidation treatment on the surface of X80 pipeline steel substrates in order to improve the corrosion resistance ability of X80 pipeline steel. First, Fe-Al coating consisting of FeAl3 and Fe2Al5 was prepared by a low-temperature pack aluminizing at 803 K which was fulfilled by adding zinc in the pack powder. Pre-treatment of X80 pipeline steel was carried out through surface mechanical attrition treatment (SMAT). Further oxidation treatment of as-aluminized sample was carried out in the CVD reactor at 833 K under oxygen containing atmosphere. After 1 h duration in these conditions, ceramic coating consisting of α-Al2O3 was formed by in situ oxidation reaction of Fe-Al coating. Those coatings have been characterized by different techniques including X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscope (EDS), respectively. Ceramic coating shows a dense and uniform microstructure, and exhibits good coherences with X80 pipeline steel substrates. By electrochemical corrosion test, the self-corrosion current density of X80 pipeline steel with as-obtained ceramics coating in 3.5% NaCl solution shows an obvious decrease. The formation of α-Al2O3 ceramic coating is considered as the main reason for the corrosion resistance improvement of X80 pipeline steel. [ABSTRACT FROM AUTHOR]

Pack cementation is an attractive method for the formation of aluminum coatings on metal substrates. When applied to porous stainless steel substrates, the aluminum vapour species can diffuse into the pores and coat the interior surfaces. In this study, aluminum coatings were grown on commercial porous UNS 430 stainless steel and freeze cast UNS 440 stainless steel substrates by the pack cementation method. The effects of pack mixture composition, temperature, time, and the morphology of the substrates were characterized by scanning electron microscopy. It was shown that the degree of coverage of the aluminum coatings increased with porosity of the substrate. In addition, increased reaction time and temperature of the pack led to a more homogeneous distribution of aluminum across the substrate. [ABSTRACT FROM PUBLISHER]

Abstract: The oxidation response of TZM alloys has been investigated at the exposure of dynamic plasma flame with a variation of exposure time. In order to examine the effect of pack cementation coatings, aluminum coatings on TZM alloys have been carried out with a powder mixture of Al, NH4Cl and Al2O3 (25:5:70wt.%), resulting that molybdenum aluminides (~200μm) were synthesized on the surface of TZM. When the TZM alloys were exposed in the dynamic plasma flame, the temperature of the specimen was increased up to 1550°C. While a significant weight loss was observed for the uncoated TZM alloy due to a volatile MoO3 phase formation, the Al coated TZM alloys did not exhibit any weight loss during the exposure of the flame. It appears that the excellent oxidation resistance is attributed to the Al2O3 formation during the flame tests. The oxidation stability of the coated and uncoated TZM alloys has been discussed in terms of weight changes and structural evolution. [Copyright &y& Elsevier]

The vacuum plasma sprayed CoNiCrAlY coatings are modified by filtered cathodic vacuum arc deposition aluminizing. The microstructure and oxidation resistance of the coatings are investigated. The parabolic law is obeyed for the aluminized coatings after oxidation at 1100 °C for 100 h. Its parabolic kinetic constant is 0.080 mg/cm h, which is lower than that of as-sprayed coatings. The continuous and dense AlO scale is formed earlier due to the increase of Al concentration, and the spinels hardly exist. The oxidation resistance is improved obviously after filtered cathodic vacuum arc deposition aluminizing. [ABSTRACT FROM AUTHOR]

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450 μm each, the titanized coating consisted of two layers with a thickness of 100 μm each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200 μm each. The XRD investigation of the coatings showed that the aluminized coating consisted of AlO, AlCr, FeAl, and FeAl phases; the titanized layers contained TiO, NiTi, FeNi, and FeTiO phases; and the two-step coating contained AlNi, TiAl, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050 °C for 100 h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones. [ABSTRACT FROM AUTHOR]

Abstract: Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950°C for 6h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti3Al or Ti2Al phase, and the outer layer was TiAl3 and Al2O3 phase. The cyclic oxidation treatment at 1000°C for 51h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl3, θ-Al2O3 and compact α-Al2O3 at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium. [Copyright &y& Elsevier]

Abstract: NiCoCrAlY coatings were prepared on superalloy K417G using arc ion plating (AIP), then some specimens coated with NiCoCrAlY were aluminized using a pack cementation method. The high temperature oxidation and hot corrosion behavior of the ordinary and the aluminized NiCoCrAlY coatings were studied. The results indicated that α-Al2O3 scales grew on both coatings during oxidation at 1000°C and 1100°C in air, and the aluminized NiCoCrAlY coating always showed lower oxidation rates and lower degradation rates. The hot corrosion tests were carried out by applying salt films of 25wt.% K2SO4 +75wt.% Na2SO4 mixture on the specimens and exposing in air at 900°C. The aluminized NiCoCrAlY coating showed a longer lifetime by forming a continuous Al2O3 scale. But, cracks and spallation of the oxide scales occurred on the ordinary NiCoCrAlY coating after 80h. [Copyright &y& Elsevier]

Abstract: Hot-dip aluminized AISI 1005 steel was isothermally oxidized at temperatures varying from 700 to 800°C in air, mixed steam and pure steam at atmospheric pressure. The high-temperature steam resistance of aluminized steel depended on the oxidation temperature. It was shown that in atmospheres that contain water-vapour, the alumina formed on the surface coating had less protection than in a dry air atmosphere. The presence of water-vapour enhanced Fe-ion transport processes in the alumina, most likely via the incorporation of hydrogen. The rapid growth of sporadic iron oxide nodules on the coating surface and at the interface was accelerated by the rapid outward diffusion of Fe-ions; high-pressure H2 from water-vapour dissociation; and crack formation in the aluminide layer. Thus the oxidation rate was increased, resulting in a substantial weight gain with temperature and time. In addition, the aluminum exhibited a greater tendency to become internally oxidized in a low pO2-steam than in a high pO2-steam. [ABSTRACT FROM AUTHOR]

Abstract: Protective coatings, deposited mainly by thermal spraying and diffusion techniques, are considered a solution to extend the lifetime of many components in the energy production sector, such as heat exchangers. In this paper, some results are presented for uncoated, aluminized and chromized-aluminized 9Cr–1Mo steel, subjected to air and to impacts by 200μm silica particles at angles of 30° and 90° and speeds of 7.0–9.2ms−1 at 550–700°C, in a laboratory fluidized-bed rig, to determine whether or not aluminized and chromized-aluminized diffusion coatings could protect the steel under such conditions. Erosion-oxidation damage was characterized by measurement of the mean thickness changes using a micrometer and examination of worn surfaces by scanning electron microscopy. Under most conditions, the coatings provided some protection to the substrate: under 30° impacts, up to 650°C, and under 90° impacts, at 700°C, both coatings were effective, whereas under 90° impacts, up to 650°C, only the chromized-aluminized coating gave significant protection. However, for 30° at 700°C, the oxide scale on the substrate was protective and the coatings were not needed. Explanations for these observations are presented in this paper, in terms of interactions between the erosion and oxidation processes for the materials. [Copyright &y& Elsevier]

Abstract: Aluminizing is often used to improve steel''s resistances to corrosion, oxidation and wear. This article reports our recent attempts to further improve aluminized carbon steel through surface nanocrystallization for higher resistances to corrosion and corrosive wear. The surface nanocrystallization was achieved using a process combining sandblasting and recovery heat treatment. The entire surface modification process includes dipping carbon steel specimens into a molten Al pool to form an Al coat, subsequent diffusion treatment at elevated temperature to form an aluminized layer, sandblasting to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. The grain size of the nanocrystallized aluminized surface layer was in the range of 20–100nm. Electrochemical properties, electron work function (EWF), and corrosive wear of the nanocrystalline alloyed surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface of carbon steel exhibited improved resistances to corrosion, wear and corrosive wear. The passive film developed on the nanocrystallized aluminized surface was also evaluated in terms of its mechanical properties and adherence to the substrate. [Copyright &y& Elsevier]

High entropy alloys are new promising alloys for high-temperature applications. Sluggish diffusion, which is one of their various important properties, contributes to their stability and oxidation resistance. Some Al-containing alloys and/or aluminized alloys have superior resistance due to the alumina layer forming on their surfaces during high-temperature oxidation. In this study, to see the effect of the Al-content and aluminizing process on a HEA, FCC structured -CoCrFeNi and BCC + B2 structured -CoCrFeNiAl alloys were produced by arc melting technique. The produced CoCrFeNi was aluminized at 700 °C for 4 h with the mixture of Al (40%), NH 4 Cl (20%) and Al 2 O 3 40%. The surface of aluminized CoCrFeNi alloy was composed of Al-rich phases like diffusion aluminide coatings. Samples were exposed to isothermal oxidation tests at 950 °C for 5, 25, 50 h. SEM, EDS and XRD analysis results show that both Al addition and aluminizing process improved the oxidation performance of CoCrFeNi alloy. The formation of the Al 2 O 3 layer resulted in lower mass gain during the oxidation tests while mixed oxides and Cr 2 O 3 formations on the surface of CoCrFeNi led to higher mass gain. In addition, no dramatic difference was observed between the oxidation performance of CoCrFeNiAl and aluminized CoCrFeNi. • The formation of porous structured mixed oxides leads to the spallation of the oxide scales in CoCrFeNi alloy. • Aluminizing process enhanced the oxidation behavior of CoCrFeNi alloy. • Al 2 O 3 formation in CoCrFeNiAl and aluminized CoCrFeNi after the oxidation tests increased the oxidation resistance. [ABSTRACT FROM AUTHOR]

Aluminized coatings were produced on iron by means of two different processes: electron beam deposition under UHV of Al on iron samples previously covered with 57Fe films, and hot-dipping of iron samples in molten aluminium. Aluminized samples were submitted to thermal treatments in order to promote interdiffusion at the Fe–Al interface and favour the formation of Fe–Al intermetallic compounds of composition suitable to protect the underlying iron from oxidation. Phase composition, structure and morphology of both as deposited and thermally treated coatings were characterized by means of X-ray diffraction, Mössbauer spectroscopy and metallographic techniques. Significant differences among the effects of the Fe–Al interdiffusion occurring for Al layers produced with the two processes are pointed out and discussed. [ABSTRACT FROM AUTHOR]

Abstract: “Simple” Pt-enriched γ + γ′ coatings (16–19 at.% Al) were fabricated on René 142 and single-crystal N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment in vacuum at 1175 °C. By introducing a secondary short-term aluminizing step via pack cementation with NaCl activator, “enriched” γ + γ′ coatings were achieved with an increased Al content (∼22 at.% Al). The changes in composition profiles between the simple and “enriched” γ + γ′ coatings were discussed. Incorporation of reactive elements, such as Hf, into the γ + γ′ coating during the aluminizing process also was explored. The cyclic oxidation performance of the “enriched” γ + γ′ coatings was evaluated at 1100°C. [Copyright &y& Elsevier]

Abstract: A simple and inexpensive method to improve oxidation resistance of titanium alloys, liquid-phase aluminizing and siliconizing by an Al–Si alloy, has been proposed. The surface reaction at 1013K for 20min on a Ti60 alloy using Al–10wt.%Si melt as reaction medium resulted in a modified layer with a thickness of about 50μm, composed of TiAl3 and TiSi2. The coating significantly improved the isothermal oxidation resistance of the alloy at 1073K. An approximately continuous alumina-rich scale was formed at the outermost surface after oxidation. SiO2 was also detectable in the oxide scale. It was inferred that during oxidation, the TiSi2 in the coating was oxidized to SiO2. In the meantime, the TiSi2 was reduced to a lower silicide, Ti5Si4. The formation of the alumina-rich scale (containing silica) at the outermost surface accounted for the improvement in the oxidation resistance. [Copyright &y& Elsevier]

Abstract: A nanocrystalline Ni–30Cr–8Al–0.5Y coating was deposited on a Ni-base superalloy by magnetron sputtering. Post-aluminizing was performed on the sputtered coating to improve the corresponding high-temperature properties. The isothermal oxidation behavior at 1000∼1100 °C and hot-corrosion behavior in the presence of 75 wt.% Na2SO4 +25 wt.% K2SO4/NaCl film at 900 °C of the sputtered coating with and without aluminizing have been investigated using TGA, SEM/EDS, XRD and EPMA, respectively. The results indicated that the sputtered NiCrAlY coating possessed excellent oxidation resistance at 1000 °C due to the presence of extensive amount of chromium and good amount of aluminum. Because of the excessive consumption of Al in the coating, it lost protection at 1100 °C. The mass gain of the aluminized NiCrAlY coating was a little higher than that of the sputtered coating due to the formation of the rapid growth θ-Al2O3 phase at 1000 °C. At 1100 °C the oxidation resistance of the latter was better than that of the former. The sputtered coating provided a limited protection at the transient hot-corrosion stage in the molten salt film and degenerated completely with time. On the contrary, the aluminized NiCrAlY coating showed much better hot-corrosion resistance in the presence of 75 wt.% Na2SO4 +K2SO4/NaCl film as a result of the formation of a continuous and protective Al2O3 scale. [Copyright &y& Elsevier]

Abstract: High-temperature diffusion of a hot-dip aluminized titanium is conducted to study microstructure changes and oxidation behavior of the aluminized titanium. After aluminizing, the titanium substrate is covered by a black layer in which tiny block-shaped TiAl3 particles are scattered in aluminum matrix. Based on the diffusion experiment results, the thickness of the aluminum diffusion layer at 800°C increases with diffusion time. However, the aluminum diffusion layer at 900°C grows and reaches its maximum thickness in 6h, and then the thickness of the aluminum diffusion layer is reduced with prolonged diffusion time. An inversion of the diffusion layer thickness versus time appears for the aluminized titanium treated at 1000°C, and the thickness of the diffusion layer keeps declining with diffusion time. The phases present in the outer and middle sublayers are titanium-rich TiAl3 and equilibrium TiAl3, respectively. However, the phase in inner sublayer changes from titanium-rich TiAl3 to TiAl2 and TiAl as diffusion temperature and time increase. Through energy-dispersive X-ray and X-ray diffraction analysis, the oxides formed in the oxidation process are Al2O3 and Al2TiO5. Although the oxide scale formed on the surface of the aluminized titanium has an insufficient stability and integrity, the thermal oxidation resistance of the aluminized titanium is still improved by over 5 times compared with that of the pure titanium. [Copyright &y& Elsevier]

Abstract: A simple method is described for enriching the surface aluminum concentration of carbon steel. The process of weld overlaying steel substrate with pure aluminum wire results in an aluminum concentration of ∼15–17at.% (8–9wt.%) for depths of ∼2mm. The aluminum-enriched surface shows an improved oxidation and sulfidation resistance. The paper gives details of the process along with microstructural analysis and oxidation resistance improvement results. [Copyright &y& Elsevier]

The deposition of Al on 1010 steel from mixtures of trimethyl (TMA) and triethyl (TEA) aluminum alkyls in argon and hydrogen was studied. Unlike earlier work with SiH/H no aluminum diffusion onto the steel was observed even when the samples were heated to 800°C. Coatings obtained with TEA at temperatures between 300-400°C resulted in overlays of elemental Al or Al oxide after exposure to ambient conditions. No deposition was obtained at temperatures higher than 400°C in a hot-wall reactor due to rapid decomposi tion of TEA on the walls of the reactor. Deposition with TMA at temperatures between 400-800°C resulted in overlays of Al carbide. For deposition at tem peratures higher than 500°C, a cold-wall reactor was used. The Al overlay coatings resulting from TMA were dense, homogeneous, and adhered well to the steel. Both kinds of coatings provided oxidation protection to the steel in air at 800°C. The performance of the coatings produced with TMA was superior to the coatings produced with TEA and similar to the performance of an Alonized steel. [ABSTRACT FROM AUTHOR]

• A low temperature process to prepare Al 2 O 3 /Fe-Al gradient hydrogen permeation barrier is proposed. • The mechanism for the different growth behavior of Al 2 O 3 in the polished and unpolished samples is also discussed. • The deuterium permeation reduction factor value of the Al2O3/Fe-Al coated 316 L in this work are ∼10,000 at 400 °C. Tritium permeation barrier (TPB) coating technology is one of the most effective methods to solve the problem of permeation and leakage of tritium through the structural materials to the coolant, environment, and other functional components in tritium breeder blanket modules (TBMs). Alumina coatings prepared by in-situ oxidation of aluminized steel shows a high permeation reduced factor (PRF) and self-healing ability. However, to eliminate the Al 5 Fe 2 phase in the aluminizing process, the stainless-steel substrate is often heat treated under high temperature conditions for a long time which leads to the degeneration of steel. In this work, a gradient alumina TPB coating was prepared on 316 L stainless steel by aluminizing process at low temperature. The influence of Al 5 Fe 2 on the formation of Al 2 O 3 is studied. The mechanism for the different growth behavior of Al 2 O 3 in the polished and unpolished samples is also discussed. Compared to the polished samples, the existence of porous Al 5 Fe 2 layer in unpolished sample changes the diffusion behavior of Al and leads to the formation of an un-continuous and rough Al 2 O 3 layer. [ABSTRACT FROM AUTHOR]

• Reactive air aluminizing improved the oxidation resistance of H230 superalloy. • A three-zone microstructure observed in the diffusion coating on H230 superalloy. • Alumina growth due to phase transformation of β-NiAl to γ'-Ni 3 Al during Oxidation. The effect of reactive air aluminizing (RAA) on the high temperature oxidation behavior of the H230 superalloy is studied at 850 °C for 600 h. Mass gain values are measured and microstructure is studied by XRD, SEM, and EDX. Post oxidation mass gain for the aluminized H230 is reduced compared to wrought H230. The RAA process lead to formation of a three-zone diffusion coating with dominant β-NiAl in the two outer zones and increased γ'- Ni 3 Al after high temperature oxidation, which is attributed to the alumina scale growth during the high temperature oxidation. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs), or multi-principal-element refractory alloys, have been intensively studied in recent years, due to their attractive potential for ultrahigh-temperature structural applications. One formidable challenge facing the materials development for RHEAs though, is to simultaneously achieve good oxidation resistance and good mechanical properties, which unfortunately often impose contradictory microstructural requirements. So far, there exist no ductile RHEAs that possess reasonable oxidation resistance, and the formation of protective oxide scales such as alumina in them is never seen. Here we report, for the first time, a strategy to form protective alumina scale in ductile RHEAs upon high temperature exposure, using a tailor-designed two-step pack aluminizing process. Very importantly, the oxidation resistance of ductile RHEAs improves tremendously, as evidenced from harsh cyclic oxidation tests, thus providing a huge impetus to push forward the further development of RHEAs, towards ultrahigh-temperature applications. • a two-step pack aluminizing process forms protective alumina scale in ductile RHEAs. • a TiAl 3 /TiAl/Ti 2 AlNb and substrate multi-layered gradient structure is achieved. • Crack-free TiAl 3 coating forms on the top surface of Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5 • The oxidation resistance of ductile RHEAs improves tremendously after aluminizing. [ABSTRACT FROM AUTHOR]

The skutterudite CoSb 3 is a well-known thermoelectric material widely used to produce electricity from heat. This material is commonly employed under vacuum but in a near future it is expected to be used at high temperatures under oxidative atmospheres, e.g. in air. The lifetime of the material may be affected by the oxidative environment, considerably limiting the use of the thermoelectric equipment. The present research describes the degradation mechanisms of CoSb 3 from oxidation experiments carried out under a flow of synthetic air at 800 K. The skutterudite material is progressively oxidized after 15 h, 50 h, 100 h and 1000 h of treatment, producing three oxides on the CoSb 3 surface (CoSb 2 O 4 /CoO·Sb 2 O 3 , Sb 2 O 4 and CoSb 2 O 6). These three oxides have different growth kinetics and they are produced in various amounts as a function of the oxidation time. Next, as a solution for an appropriate oxidation protection, this work explores the chemical vapor deposition (CVD) process named pack cementation to synthesize a protective coating on CoSb 3. This process produces a surface layer made of aluminum antimonide (AlSb) and cobalt aluminide (Al 9 Co 2). The oxidation experiments carried out on the coated CoSb 3 highlight the protective properties of this innovative surface layer. The coating is a protective barrier against oxygen that keeps the CoSb 3 substrate unaffected by the flow of air at 800 K for 1000 h. Consequently, pack cementation is an efficient process to synthesize a protective surface layer that makes CoSb 3 usable under oxidative environments. Unlabelled Image • The oxidation behavior of CoSb 3 in air at 800 K is explored for 1000 h. • The progressive degradation of CoSb 3 produces a surface oxide layer made of several phases. • A surface layer is synthesized by aluminum pack cementation to protect CoSb 3. • The surface layer is made of AlSb and Al 9 Co 2 and protects CoSb 3 from oxidation. [ABSTRACT FROM AUTHOR]

The CoSb 3 skutterudite compounds are thermoelectric materials used to produce electricity from heat. Unfortunately, these materials quickly oxidize in air at elevated temperature. To solve this problem, this work explores the possibility to produce a protective aluminide surface layer to limit the interactions with oxygen. To synthesize the layer, this research work experiments the pack cementation process. A good understanding of the process requires the knowledge of the thermodynamic phase equilibria of the involved elements. However, this ternary system has never been considered in literature. Therefore, the first part of the research concerns the study of the isothermal section at 600 °C of the Al–Co–Sb ternary system to improve the knowledge of the phases that can be formed during the pack cementation process. Next, the aluminide surface layer is synthesized, then characterized by coupling scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis. The results reveal that the obtained coating is made of two crystalline phases, AlSb is the major one and Al 9 Co 2 is the minor one. At last, the oxidation experiments point out the protective properties of the surface layer against oxygen penetration, keeping the substrate safe for 1000 h under a flow of synthetic air at 527 °C. Consequently, the aluminized CoSb 3 is a promising thermoelectric material useable in oxidative environments. • The isothermal section at 600 °C of the Al–Co–Sb ternary system is explored. • An aluminide layer is synthesized by pack cementation on the CoSb 3 thermoelectric material. • The obtained layer is made of two phases, AlSb the major one and Al 9 Co 2 the minor one. • The aluminide layer protects CoSb 3 from oxidation in air at 527 °C for 1000 h. [ABSTRACT FROM AUTHOR]

This work presents the mechanisms of formation on pure nickel of full thermal barrier coating systems (aluminide coating, thermally grown oxide and thermal barrier top coating) from micro-sized Al particles dispersed in a slurry annealed in different atmospheres (Ar, synthetic air, water vapour and mixtures thereof). The simultaneous formation of nickel aluminides and of a thermal barrier made of sintered hollow alumina spheres involved self-propagating high-temperature synthesis in all conditions. However, the microstructures and adherence of the top coatings changed markedly with either a diffusion step (700°C-2 h) or a complete heat treatment (700°C-2 h + 1100°C-2 h) depending on the atmosphere. Whereas fast consumption of Al occurred in Ar to form the nickel aluminides, synthetic air and water vapour fostered the peripheral oxidation of Al micro-sized particles that impeded the release of Al and its diffusion towards the substrate. This resulted in heterogeneous diffusion layers but thicker top coatings with better sintering and thicker alumina shells. • Hollow alumina spheres coatings were obtained from Al microspheres slurries using different oxidizing atmospheres. • Under inert gas atmosphere (Ar), the oxide shell of the microspheres was too thin and result in collapsed coatings. • Using oxidizing atmospheres (synthetic air or Ar + H 2 O vap), the coatings showed poor adherence due to excessive oxidation. • However, a good compromise between adherence and oxide shell thickness was achieved using hybrid atmosphere heat treatment. [ABSTRACT FROM AUTHOR]