FeAl Coatings Research Articles

Microstructure, corrosive wear and electrochemical properties of Al2O3 reinforced FeAl coatings by laser cladding

PurposeThis study aims to investigate the influences of Al2O3 mass fraction on the corrosive wear and electrochemical behaviors of FeAl–xAl2O3 coatings.Design/methodology/approachFeAl–xAl2O3 coatings were prepared on S355 steel by laser cladding to improve its corrosive wear and electrochemical properties.FindingsThe average coefficients of friction and wear rates of FeAl–xAl2O3 coatings are decreased with the Al2O3 mass fraction, and the Al2O3 plays a positive role in the corrosion wear resistance. Moreover, the charge transfer resistance of FeAl–xAl2O3 coatings is increased with the Al2O3 mass fraction, showing the FeAl–15%Al2O3 coating has the best corrosion resistance. The findings show the corrosion resistance of FeAl–15%Al2O3 coating is the highest among the three kinds of coatings.Originality/valueAl2O3 was first added into FeAl coatings to further improve its corrosive wear and electrochemical properties by laser cladding.

Effect of the incorporating of refractory NbC precipitates in intermetallic iron-aluminide coatings on corrosion and high-temperature oxidation behavior

This study aims to investigate the effects of growing niobium carbide (NbC) particles into intermetallic iron-aluminide (FeAl) coatings on ductile cast iron (SGI) by thermo-reactive diffusion technique (TRD). The study compares the corrosion and oxidation behavior of the FeAl-NbC coatings with SGI, FeAl, and NbC coatings. Corrosion tests were conducted through polarization tests in a 3.5 wt% NaCl solution, while oxidation tests were performed at 900 °C for 4, 16, and 64 h. Before and after corrosion and oxidation tests, the coatings were examined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The findings show that FeAl coatings with NbC particles had lower graphite nodules, porosity, and surface roughness values compared to FeAl coatings. FeAl-NbC composite coatings provided better corrosion and oxidation resistance compared to untreated SGI, FeAl, and NbC coatings. The results of the comparative analysis of FeAl-NbC, FeAl, NbC, and untreated SGI specimens indicate that corrosion resistance in a 3.5 wt% NaCl solution and oxidation resistance at 900 °C followed the order FeAl-NbC > FeAl > NbC > SGI.

Microstructure evolution of γ-Al2O3/FeAl tritium permeation barrier coatings under 6.4 MeV Fe3+ ion irradiation

γ-Al2O3/FeAl coatings for tritium permeation barrier (TPB) application in fusion reactors were fabricated via electro-chemical deposition and selective oxidation. The TPB coatings consisted of a γ-Al2O3 scale (200 nm), an upper interlayer of FeAl (20 μm) and a lower interlayer of Fe3Al (10 μm). Their potential in-reactor performance was investigated based on 6.4 MeV Fe3+ ion irradiations at 400 °C, up to ion fluences of 1.0 × 1019 ions·m−2 and 1.0 × 1020 ions·m−2, at depth regions entailing the oxide scale and the top interlayer. Combined grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM) analysis confirmed the overall phase stability and grain structure stability in γ-Al2O3/FeAl coatings after irradiation. Cavities dominated the damage microstructure for both γ-Al2O3 and FeAl phases, but were not observed at the sub-layer interfaces. The underlying mechanisms for defect production were discussed. Finally, a brief comparison between heavy-ion and fission neutron damage effects in Al2O3-based TPB coatings is made, aiming to bring new insights on the coating integrity and performance reliability during fusion reactor operation.

Strategy to Generate Oxide-Free Molten Metal Droplets for Fully Dense Corrosion-Resistant Coatings by Atmospheric Plasma Spraying

Thermal spray techniologies are very efficient to deposit metal coatings, which have been applied to different industrial fields for protection of metals from wear and corrosion. However, severe oxidation during inflight of spray particles introduces large amount of oxide inclusions in the coating which limits lamellar bonding formation and thus full utilization of coating material performance potential. In this project, the spray powders containing the deoxidizers such as carbon and boron are designed to develop in-situ deoxidizing effect to generate oxide-free molten metal droplets by air plasma spraying (APS) in ambient atmosphere for depositing dense coatings with sufficiently bonded splats. The thermodynamic and kinetic conditions for continuous deoxidization during whole in-flight molten droplets are presented. The experiments were conducted for NiCr, NiCrCu, CuNi coatings with boron as deoxidizer and for NiAl and FeAl coatings with dispersed diamond as deoxidizer. Results show that through powder design and spray condition optimization different spray particles can be heated to temperatures from 2100°C to 2500°C. It was found that the oxide contents in all coatings decreases with increasing spray distance, which indicates that deoxidizing effect of deoxidizer is maintained during whole spray particle in-flight. The examination shows that all APS coatings present dense microstructure. The electrochemical test reveals that the corrosion only occurs to coating surface and no trace of corrosive solution penetration into coating. As a result, the corrosion-resistant metal coatings can be realized by APS in ambient atmosphere through developing in-situ deoxidizing effect and subsequently oxide-free molten droplets with using deoxidizer-containing spray powders.

Improvement of Microstructure and Sliding Wear Property of Cold-Sprayed FeAl Intermetallic Compound Coating by Annealing Treatment

Nanograin Fe(Al) solid solution alloy coating was firstly produced through cold-spraying technology using mechanically alloyed powder. Then, the above-mentioned coating was annealed at different temperatures to explore its influence on the phase constitution, microstructure, microhardness and dry sliding wear property of the coatings. Results exhibited that an FeAl phase appeared in the coatings after 500 °C treatment and remained stable with increasing annealing temperature. The annealing temperature had a considerable effect on the microstructure, microhardness and wear behavior of the FeAl coating. The existing laminated structure in the as-sprayed coating gradually faded away with increasing temperature and finally obtained a dense coating microstructure with no particle interface when annealed above 950 °C. Nanograin began to evidently grow at temperatures over 800 °C. The microhardness of the FeAl coating stayed at 400 Hv0.1 at temperatures below 800 °C, then it quickly dropped to 300 Hv0.1 at 950 °C and remained nearly unchangeable up to 1100 °C. The dry sliding wear mechanism of the FeAl coating annealed at low temperatures below 700 °C was mainly delamination of the oxide layer and showed a high wear rate within the order of magnitude range of 10−4, whereas FeAl coatings annealed at high temperatures above 950 °C were worn by microploughing and little oxidation and exhibited very low wear rates within the order of magnitude range of 10−6.

Effect of the technological parameters of plasma-arc spraying of flux-cored wire on the structure and properties of intermetallide coatings based on Fe3Al

Existing techniques for applying intermetallide layers are characterized by low productivity, difficulties associated with the maintenance and operation of technological equipment, as well as significant costs for the purchase of materials for spraying. Therefore, modern science shows considerable interest in the development of new, highly effective technologies to form intermetallide coatings on the surface of articles. Such promising techniques include the technology of plasma-arc spraying (PAS) of flux-cored wires. This technique has a number of significant advantages, namely high performance, relative simplicity, as well as the affordability of equipment and materials for coating. This paper reports a study into the structure and properties of coatings obtained by flux-cored wire PAS, in which the steel sheath and aluminum powder filler interact when heated with the exothermic effect of Fe3Al synthesis. The influence of technological parameters of PAS process on the structure and properties of Fe-Al coatings was investigated by means of mathematical planning of the experiment. It was found that in all samples the main phase is an intermetallide of the Fe3Al type. Tests for gas-abrasive wear resistance at room temperature showed that the wear resistance of coatings exceeds the stability of steel S235 by an average of 2 times. As a result of studying the electrochemical properties in a 3-% aqueous solution of NaCl and in a 0.5-% solution of H2SO4, the score of corrosion resistance for these media was determined, which was, respectively, 4 and 5 (coatings belong to the group of "resistant"). In this regard, the practical use of coatings based on the Fe3Al intermetallide is recommended for protection against oxidation, corrosion, and gas-abrasive wear of components and assemblies in the heat power industry (heat exchanger pipes, catalytic converters, steam turbine blades, shut-off valves, etc.)

Electrochemical corrosion and solid particle erosion response of Y2O3 dispersed FeAl coatings deposited by detonation spray

The electrochemical corrosion and erosion wear behaviors of detonation sprayed ODS FeAl powder coatings on T91 substrate were investigated. Electrochemical corrosion resistance of the deposited coatings was evaluated in three different aqueous media, namely 3.5 wt% (0.61 M) NaCl, 2 N (1 M) H2SO4 and 3.5 wt% (0.25 M) Na2SO4 and are compared with T91 substrate and detonation sprayed Cr3C2–25NiCr coatings (on T91 substrate). The ODS FeAl coatings shown improved corrosion resistance in 3.5 wt% Na2SO4 than T91 substrate. However, the Cr3C2–25NiCr coatings have shown consistently better corrosion resistance than T91 substrate and ODS FeAl coatings in all the aqueous media. The corrosion mechanisms investigated imply that the higher corrosion rate of FeAl coatings as compared to the Cr3C2–25NiCr coatings is well compensated by the uniform corrosion in the case of former than the localized and intense corrosion in the case of later coatings and substrate material. In contrast, the solid particle erosion wear studies conducted at two different temperatures namely, room temperature (25 °C) and 400 °C have revealed that the ODS FeAl coatings exhibited 4 to 5 times improved wear resistance than that of Cr3C2–25NiCr coatings at room temperature and 1.5 to 2.5 times at a higher temperature (400 °C). Also, the ODS FeAl coatings have performed marginally better than the T91 substrate at both the test temperatures. The results obtained together with the observations made demonstrate a leap step towards the development of novel thermal spray grade feedstock materials as an alternative to industrially well-known Cr3C2–25NiCr coatings and open up a new avenue of techno-economic benefits that can be further explored.

Development of FeAl Coatings by a Combined Aluminizing Process to Improve the Dry Sliding Wear Resistance of HH309 Alloy

Development of FeAl Coatings by a Combined Aluminizing Process to Improve the Dry Sliding Wear Resistance of HH309 Alloy

Tribological wear of Fe-Al coatings applied by gas detonation spraying

Comparative tests of gas detonation (GDS) coatings were carried out in order to investigate the influence of spraying parameters on abrasive wear under dry friction conditions. The tests were carried out using the pin-on-disc (PoD) method at room temperature. The microstructure of the coatings was analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM / EDS) methods. The results showed that with specific GDS process parameters, the main phases in both coatings were FeAl and Fe3Al involving thin oxide films Al2O3. The tribological tests proved that the coatings sprayed with the shorter barrel of the GDS gun showed higher wear resistance. The coefficient of friction was slightly lower in the case of coatings sprayed with the longer barrel of the GDS gun. During dry friction, oxide layers form on the surface, which act as a solid lubricant. The load applied to the samples during the tests causes shear stresses, thus increasing the wear of the coatings. During friction, the surface of the coatings is subjected to alternating tensile and compressive stresses, which lead to delamination and is the main wear mechanism of the coatings.

Corrosion behaviour of iron and nickel aluminide coatings under the synergistic effect of NaCl and water vapour

Fe-Al and Ni-Al coatings were prepared by aluminizing ferritic-martensitic steels without and with pre-electroplating a Ni film, respectively. The corrosion behaviour of two aluminide coatings in marine environment, where high temperature oxidation and corrosion occur under the synergistic effect of solid NaCl and water vapour, were investigated. The results indicated that Ni-Al coatings showed superior corrosion resistance to Fe-Al coatings, the improved performance is mainly due to the beneficial influence of the Ni-plating layer on inhibiting self-sustainable oxychlorination, tungsten outward diffusion and interdiffusion behaviour. The detrimental effects of Fe on breakaway oxidation of Fe-Al coatings are discussed.

Electric arc spraying of intermetalic Fe–Al coatings using different solid and powder wires

Electric arc spraying of intermetalic Fe–Al coatings using different solid and powder wires

Mechanically induced grain refinement, recovery and recrystallization of cold-sprayed iron aluminide coatings

FeAl coatings can be processed by cold spraying conditions only under the use of well-tuned parameter sets that guarantee shock-loading conditions, which provide sufficient ductility in iron aluminide intermetallic compounds and are capable to ensure sufficient heat creation and thus thermally softened interfaces for bonding. Original microstructures, present in the powder feedstock particles, locally change under the respectively applied loads and temperatures. Given the challenge due to the anomaly softening of such ordered alloys, the present study describes and discusses the differences of coating microstructures in terms of the effect of spraying parameters such as particle size, gas pressure, spraying distance and substrate heating. Electron backscatter diffraction analysis provides elucidation on the grain refinement mechanisms and internal particle strain upon particle impact. The microstructural characterization is complemented by the misorientation analysis within single grains, which is correlated with the spraying parameters and the respective thermal energy input. The misorientation analysis allows for analyzing grain refinement, internal grain bending and for elucidating associated stress relief mechanisms. The results show that the cold spray deposition of FeAl intermetallics can be explained by mechanically induced adiabatic shear instabilities as well as different recovery and recrystallization mechanisms causing a local stress relief.

Al2O3/FeAl interfacial behaviors by yttrium doping in high temperature oxidation

FeAl coatings with yttrium (Y) doping were prepared at 600 °C on pure iron. The anti-flaking properties of Al2O3-oxide films at 1000 °C oxidation was improved significantly by Y doping in FeAl coatings. The spallation was 0.47 mg/cm2 for 200 h of oxidation and only ~1/6 of that without Y. The first principle calculation results demonstrate that the interfacial bond strength were enhanced with Y doping, that is because Y substituted primarily in the Al1 and Al2 position at the Al2O3/FeAl interfacial zone as the most negative interfacial cohesive energy of −13.94 eV and −12.72 eV. Remarkably, it is found that the maximum tensile strength of Y–FeAl/Al2O3 reaches to 20.48 GPa on uniaxial tensile strain, which is 1.33 times than that of FeAl/Al2O3 interface (15.36 GPa). Meanwhile, compared with a pure interface, Y doping would change ductile fracture on the Al2O3/FeAl interface as the strain was enhanced significantly by 49.12%. The bond length of the Al1–Y showed little change under external stress, which resulted in an increase in bonding strength of the Al2O3/FeAl interface and the inhibition of the generation and propagation of micro-cracks. The formation of a YAlO3 layer induced a release of thermal stress at the Al2O3/FeAl interface. Due to the improvement of interfacial strength, the enhancement of interfacial toughness and the formation of a YAlO3 stress-release layer between the Al2O3 film and FeAl coating, therefore, Y doping can significantly improve the anti-flaking performance of the Al2O3-oxide films in high temperature oxidation.

Investigation of alumina film formed over aluminized RAFM steel by plasma assisted heat treatment

Reduced Activation Ferritic Martensitic (RAFM) steel is currently under intense consideration as a structural material for the blanket applications in fusion reactor. The concept of blanket module utilizes both solid i.e. Li2TiO3 and liquid breeder material, i.e. Eutectic Pb-17Li operating at 320–480 °C. The critical issues like liquid metal corrosion of RAFM steel, tritium permeation into RAFM steel and magneto hydrodynamic drag generation due to flowing Pb–17Li is already reported. αAl2O3 + FeAl coatings have been found promising to mitigate these challenges & reported with a substrate of P91 steels. However, coatings with hot dipping process on RAFM are scarcely reported as RAFM steel is in under development. Hence, an experimental investigation is done to examine the effects of heat treatments to form FeAl with a top layer of Al2O3. In-RAFM steel (9Cr-1.4W-0.06Ta) samples were hot dipped in a molten bath of 93%Al & 7%Si at 730 °C. These hot dipped samples were subjected to various heat treatments in 3 different routes. In the 1st route, normalizing heat treatment has been carried out at 980 °C for 30 min followed by thermal tempering in a muffle furnace at 760 °C for 90 min. 2nd route consists of plasma tempering along with normalizing in which O2 was used at low pressure (5 mbar) at 760 °C/90 min with pulsed DC at −520 V. Whereas in the third route, to form α-Al2O3, plasma assisted heat treatments were conducted directly without normalizing up to 24 h. The transformations of these phases have been analyzed through X-ray diffraction. Moreover, to confirm Al2O3 coatings, its thickness as well as the case depth of diffused FeAl, cross-sections of coated samples were investigated by mapping through SEM equipped with energy dispersive x-rays. Among these heat treatments, α-Al2O3 + FeAl formation with a thickness of ∼60–70 μm was observed in route 2. Route 1 resulted in to the formation of θ-Al2O3 + FeAl while route 3 indicated less diffusion of Al in to the substrate with defects.

Study on neutronics of VVER-1200 with accident tolerant fuel cladding

Protective coatings deposited on traditional Zr-based fuel cladding materials potentially enhance accidental tolerance of Light Water Reactors due to reduced interaction between water and Zr alloys during accidental conditions. Enhancement of cladding performance and reduction of negative effects, such as heat generation, hydrogen production, oxidation, and embrittlement can be achieved by deposition of various coatings that are widely studied around the world. Even though thermal, mechanical, and chemical responses during accidental conditions are the main concern for these materials, the actual application of coated materials will be linked to standard operating conditions including economic consequences and transient response. Reference VVER-1200 fuel system with five coatings (Cr, CrN, ZrSi2, Cr2AlC, Fe(Cr)(Al)) is investigated from the neutron-physical point of view using the calculation package SCALE 6.2.2. In addition to the multiplication factor, specific fuel safety criteria are also examined for both the fresh fuel assemblies and fuel assemblies with burn-up up-to 70 MWd/tHM. All studied coatings entail certain negative effects in comparison with the reference case. The closest behavior to the reference VVER-1200 fuel assembly is found for ZrSi2 coating followed by Cr2AlC and Fe(Cr)(Al) coatings. Both Cr and CrN materials exhibit significant differences compared to the reference case.

Functional and Fractal Properties of Fe-Al Coatings After Gas DetonationSpraying (GDS)

Functional and Fractal Properties of Fe-Al Coatings After Gas DetonationSpraying (GDS)

Sliding Wear Behavior of Fe-Al Coatings at High Temperatures

The medium and high temperature tribological behavior of different iron aluminide thermal spray coatings was investigated. Several powders synthesized through different routes (ball milling, self-decomposition, and self-propagating high-temperature sintering (SHS)) were evaluated. High heterogeneity of conventional High Velocity Oxygen Fuel (HVOF) coatings plays a vital role in their sliding performance, but as long as their integrity is preserved under high temperature oxidizing conditions, the wear rates are found to be acceptable, as it occurs in the case of ball milled Fe-40Al (at.%) powder. The friction phenomenon and wear mechanisms were analyzed in detail through the wear track morphology, contact surface, and friction coefficients. The occurrence of brittle phases in the sprayed coatings, which are also present when tested at high temperatures, appeared to be crucial in accelerating the coating failure.

TEM/STEM study of Zircaloy-2 with protective FeAl(Cr) layers under simulated BWR environment and high-temperature steam exposure

FeAl(Cr) thin-film depositions on Zircaloy-2 were studied using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with respect to oxidation behavior under simulated boiling water reactor (BWR) conditions and high-temperature steam. Columnar grains of FeAl with Cr in solid solution were formed on Zircaloy-2 coupons using magnetron sputtering. NiFe2O4 precipitates on the surface of the FeAl(Cr) coatings were observed after the sample was exposed to the simulated BWR environment. High-temperature steam exposure resulted in grain growth and consumption of the FeAl(Cr) layer, but no delamination at the interface. Outward Al diffusion from the FeAl(Cr) layer occurred during high-temperature steam exposure (700 °C for 3.6 h) to form a 100-nm-thick alumina oxide layer, which was effective in mitigating oxidation of the Zircaloy-2 coupons. Zr intermetallic precipitates formed near the FeAl(Cr) layer due to the inward diffusion of Fe and Al. The counterflow of vacancies in response to the Al and Fe diffusion led to porosity within the FeAl(Cr) layer.

The FeAl coatings deposited by Laser Engineered Net Shaping

The FeAl coatings deposited by Laser Engineered Net Shaping

Artificial neural network-based prediction technique for coating thickness in Fe-Al coatings fabricated by mechanical milling

ABSTRACTThe objective of this study was to evaluate the effect of milling time, milling speed and particle size of initial powders on the coating thickness of Fe-Al intermetallic coating by using artificial neural network (ANN). Coating morphology and cross-section microstructures were evaluated using a scanning electron microscope (SEM). It was found that an increase in the milling time provided an increase in the coating-layer thickness due to the cold welding process between particles and the steel substrate. The microstructure of the coating surface was refined by ball impacts in the milling process. As a result of this study, the ANN was found to be successful for predicting the coating thickness of Fe-Al intermetallic coatings. The correlation between the predicted values and the experimental data of the feed-forward back-propagation ANN was quite adequate. The mean absolute percentage error (MAPE) for the predicted values didn’t exceed 7.46%. The ANN model can be used for predicting the coating thickness of Fe-Al intermetallic coating produced for different milling time, milling speed and particle size.