Search

Results are presented for research on the problem of increasing the stability and durability of a diffusion aluminized coating on carbon steel 45 when held in an oxidizing atmosphere at 1100°C. A method is proposed for aluminizing from a three-layer impregnation mixture in which resorption of the aluminized layer is excluded and good oxidation resistance of the protected steel is provided. Experimental tests of an aluminized specimen made of steel 45 at a temperature of 1100°C for 200 hours showed that under the specified conditions there is 10% better oxidation resistance than for steel 12X18H10T and 1140% better than for unaluminized steel 45. [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]

A new slurry coating design incorporating a first layer of Cr microparticles at the substrate surface and a surmounting second layer of Al microparticles to enrich the substrate with Al was investigated. The microstructure of the coating was tailored by adjusting the initial thicknesses of Cr and Al layers. The intermediate layer of Cr microparticles acted as a barrier to molten Al by forming Al-Cr intermetallic compounds thereby suppressing the highly exothermic reactions usually observed between molten Al and nickel-based materials. This also significantly decreased the inward diffusion rate of Al towards the substrate at 700 °C. Annealing at 1100 °C then promoted the outward diffusion of nickel atoms from the substrate and their combination with Al from the synthesized Al-Cr intermetallic phases compounds resulting in the direct formation of β-NiAl with expelled Cr. The mechanisms of formation of the diffusion layers and the influence of Cr on the aluminizing progress are discussed. • The Cr-free Al slurry composition results in high-activity aluminide coatings. • The Cr interlayer reacts with molten Al by forming AlxCry intermetallics. • The formation of AlxCry intermetallics decreases the Al activity. • The Cr/Al ratio of the coating system has a tremendous influence on the mechanisms of formation and on the final coating microstructure. • The appropriate slurry coating design can lead to the formation of low-activity Cr-modified aluminide coatings. [ABSTRACT FROM AUTHOR]

Abstract The mechanisms of formation of nickel aluminide coatings from alternate deposition of Al and Cr microparticles and subsequent heat treatment in Ar were studied on pure nickel. For this purpose, tailored amounts of Al and Cr water-based slurries were successively deposited on pure nickel following two different architectures (Al/Cr and Cr/Al double-layer systems). Regardless of the coating architecture, the addition of Cr microparticles was found to decrease the thermodynamic activity of Al upon aluminizing through the formation of Al x Cr y phases. This considerably limited the inward diffusion of Al towards the substrate at low temperature (e.g. 650 °C). Whereas the formation of δ-Ni 2 Al 3 was still observed with the Al/Cr double-layer system, its formation was completely suppressed with the Cr/Al one. By adjusting the composition of the deposited layers, i.e. the thickness of both Cr and Al layers, it was possible to directly form the β-NiAl phase with a further annealing at high temperature (e.g. 1000 °C for 3 h). This fostered the outward diffusion of nickel and the dissolution of synthesized Al x Cr y phases. Undissolved Cr-rich phases were also observed in the diffusion layers, which confirmed the outward growth of the coating typical of low-activity aluminizing. Highlights • Double-layer deposits (Al/Cr and Cr/Al) with tailored amounts of Al and Cr successfully elaborated by slurry on pure Ni. • The Ni substrate-Al/Cr configuration results in high-activity aluminide coatings. • The Ni substrate-Cr/Al configuration results in low-activity aluminide coatings. • The mechanisms of formation of such coatings involve the appearance of intermediate Al x Cr y intermetallics. [ABSTRACT FROM AUTHOR]

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]

Hot-dipping aluminizing of 45 steel was carried out in molten Al baths containing 0.0, 1.0, 3.0, and 5.0 wt.% Ni at 710°C for 10, 120, 300, and 600 s. The coatings were analyzed by x-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The coating hot-dipped in the Al-5Ni bath consisted of an outer Al-Ni topcoat (α-Al, θ-Al3Fe, Al3Ni, τ1-Al9FeNi), minor τ1-Al9FeNi, minor θ-Al3Fe, and major η-Al5Fe2layers, respectively, while no τ1-Al9FeNi layer was identified in the coating hot-dipped in the Al-1Ni and Al-3Ni bath. Diffusion path model was introduced to explain this phenomenon. Ni as an alloying element added into Al bath decreased the growth rate of η-Al5Fe2layer. The average thickness of η-Al5Fe2layer followed the parabolic law in hot-dipping in the Al-5Ni bath. Also, η-Al5Fe2had the largest growth rate among the intermetallic layers. [ABSTRACT FROM AUTHOR]

Coatings improve the high temperature corrosion resistance of materials by enriching the subsurface zone with elements that form protective oxide scales, which increase the material lifetime. Interesting coatings are aluminum (Al) diffusion coatings formed in a pack cementation process e.g., on austenitic steels. The coating procedure may lead to the risk of crack formation within the coating because of the detrimental combination of high brittleness of the Al-rich intermetallic phases that form, the coating thickness, and the mismatch of the coefficients of thermal expansion (CTE) between the material and the coating. By means of a new coating design, Al diffusion coatings consisting of less brittle intermetallic phases with a CTE closer to that of the substrate can be applied with a controlled coating thickness. For this purpose, the required coating manufacturing parameters such as process temperature, process time, and powder composition are predicted by the presented coating design model. [ABSTRACT FROM AUTHOR]

Abstract: 12% Cr heating resistant steels with single (martensite) and dual (martensite and ferrite) phases were prepared to investigate not only the effect of the phase difference in substrate on the alloy formation but also the growth kinetic of the alloy layer during aluminizing and diffusion treating. Scanning electron microscopic observation and X-ray diffraction analysis revealed that the alloy layer was identified as Fe4Al13 ( FeAl3) during aluminizing and annealing treatment and the thickness of the layer in both specimens increased as the temperature and time increased. For comparison of the parabolic coefficient and activation energy of two specimens, the specimen with the single phase presented a higher coefficient and lower activation energy than that with dual phases. This may be due to structural differences relating to the ease in which the Al atom migrates into the substrate and the fact that martensite contains a larger fraction of defects. Additionally, the results for the annealing treatments for diffusion after aluminizing clarified that both specimens showed lower activation energies than during aluminizing treatment because the formation of voids and their aggregation during annealing treatment after aluminizing contributed to lower activation energies for diffusion. [Copyright &y& Elsevier]

Despite the large industrial use of pack aluminizing processes for the protection of parts (e.g. cooling channels) of gas turbine blades, systematic studies relating the formation mechanisms and the chemical composition of pack aluminized layers to important process parameters, including the nature of the base alloy and the temperature, are scarce. In this study, 4 different alloys (pure Ni, Ni–20Cr, Inconel 738 and directionally solidified CM247LC) were subjected to a pack aluminizing process at three different temperatures (950°C, 1000°C, 1040°C), using a pack mix containing a fluorine-based activator; the results were compared to those obtained with a chemical vapor deposition (CVD) aluminizing process performed at 1040°C with the same fluorine-based gaseous precursor. The microstructural characterization, performed by SEM+quantitative EDX analysis, XRD and nanoindentation testing, shows that, during the heating stage of the pack aluminizing process, Al is transported to the sample surface at temperatures too low to allow significant simultaneous diffusion of Ni; therefore, a δ-Ni2Al3 outer layer is formed by inward Al diffusion below the alloy surface, and its growth then continues during the isothermal stage as well. As a result, the chosen isothermal treatment temperature does not affect growth mechanisms, although it modifies the overall thickness of the aluminized layer. δ-Ni2Al3 is converted to β-NiAl after a subsequent vacuum heat treatment at 1120°C. In a CVD process, where gaseous precursor are introduced only after attaining the isothermal treatment stage, Al and Ni diffuse simultaneously from the very beginning of the aluminizing process and β-NiAl is directly developed. Less mobile species (heavy atoms, such as W) in the alloy composition hinder all diffusion phenomena, both during pack aluminizing and during subsequent vacuum heat treatment: after aluminizing, precipitates are developed within the δ-Ni2Al3 outer layer and, after vacuum heat treatment, the resulting β-NiAl layer exhibits a compositional gradient. [ABSTRACT FROM AUTHOR]

Conventional and gradient CoNiCrAlYSi coatings were produced by using low vacuum plasma spray and an additional step of diffusional over aluminizing (pack cementation) techniques on an Inconel-738 substrate. Hot corrosion of these coatings was investigated using NaSO-20wt%NaVO molten salt at 880 °C for 800 h. Hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20 h. X-ray diffraction, field emission scanning electron microscopy and electron probe micro analysis techniques were used to determine the nature of phases, investigation of the thermally grown oxide, examination of the surface attack and determination of the elemental distribution. The gradient coating showed better performance by re-healing alumina scale due to its possession of more β phase as Al reservoir. Results indicated that pack cementation process caused an increase in amount of aluminum-rich β phase and better hot corrosion properties of gradient coatings owing to the Al enrichment in the outer layer and rapid formation of protective oxide on the surface. [ABSTRACT FROM AUTHOR]

Abstract: Coating of aluminide on a TiAl-based alloy (49.1at.% Al) was carried out by thermal spraying pure aluminum and subsequent diffusion treatment at 1100°C. The growth of a Ti–Al intermetallic layer in the coating layers during diffusion treatment for 120–1800s as well as the oxidation resistance of the aluminized TiAl-based alloy was investigated. The outermost surface layer was comprised of Al-rich intermetallic TiAl3 and contained pores. In contrast, an Al concentration gradient layer consisting of Ti2Al5, TiAl2, and Al-rich TiAl containing 55at.% Al was formed between the outermost layer and the substrate. The thickness of the outermost layer decreased with increasing diffusion time, while the thickness of the intermediate layer grew to approximately 30μm. In addition, the coating/substrate interface changed from a wavy to a linear form with the growth of the intermediate layer. The aluminized coating, at all diffusion times, showed good oxidation resistance in cyclic oxidation tests at 900°C in air. [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]

Abstract: Foil aluminizing of steel, wherein Al diffuses to the base steel material by diffusion treatment after hot pressing of the aluminum foil at a lower temperature than the melting point, was performed in this study. Alloy layers formed by diffusion at temperatures ranging from 700 to 1000 °C were investigated, and their features were compared with those of hot-dip aluminized steel. In hot-dip aluminizing, an intermediate Fe2Al5 layer was formed between the aluminum layer and the base steel during aluminum coating before the diffusion treatment. In contrast, the coating layer of the foil-aluminized steel specimen after diffusion bonding of the aluminum foil consisted only of the Al layer. An Fe2Al5 layer, an FeAl layer and an Al diffused layer were formed in both the aluminized specimens subsequent to the diffusion treatment. The numbers of voids formed in the Fe2Al5 layer and at the FeAl/Al diffused layer interface of the foil-aluminized specimens are smaller than those for hot-dip aluminizing. Moreover, the FeAl and Al diffused layers are formed with a greater thickness in the foil-aluminized steel under identical diffusion conditions. The Al concentration in hot-dip aluminized steel decreased in stages from the surface to the base steel, whereas, in the foil-aluminized steel, it decreased gradually. The Fe2Al5/base steel interface in the foil-aluminized steel was thus indistinct, and cross-sectional hardness also decreased gradually. [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: Hot dip aluminizing of carbon steels with different carbon concentration ranging 0.2–1.1wt.% was carried out in a molten Al–9.08wt.% Si–0.98wt.% Fe alloy at 660°C. The steel specimens lost weight as a result of dissolution into the melt, and an intermetallic layer was formed on the surface of them. The specimens showed varied dissolution rates depending on carbon concentration. The specimen with the highest carbon content exhibited the slowest dissolution rate. The thickness of the intermetallic layer increased with dipping time following a parabolic relationship. The growth rate of the layer decreased with increase of the carbon content. A diffusion mechanism to control the dissolution of the carbon steel into the molten aluminum alloy was suggested, and the effect of carbon content on the dissolution of the steel substrate into the melt was discussed in connection with the proposed diffusion mechanism and microstructural observations. [Copyright &y& Elsevier]

Kinetics of formation of iron aluminide diffusion coatings by chemical vapor deposition (CVD) on pure iron was studied in the temperature range 1173–1373 K. AlCl3 vapor, produced by the sublimation of anhydrous aluminum chloride, was used for transporting aluminum from an aluminum source (pure aluminum) in the CVD reactor to the substrate to produce the coating. The reactor pressure was maintained at 1.33±0.133 kPa. The first step in developing a theoretical model for the coating process is to establish the kinetics of the process. This was accomplished by monitoring the weight gain and thickness as a function of time and temperature. The growth followed a parabolic rate law with the rate constant (by weight gain) steadily increasing with substrate temperature from 0.0114 mg2 cm-4 s-1at 1198 K to 0.0836 mg2 cm-4 s-1 at 1348 K. The rate constants were 4.138×10-6 and 6.554×10-6 mm2 s-1 (by thickness) at substrate temperatures 1273 and 1323 K, respectively. The activation energy for the coating process was 211±8 kJ mol-1. [Copyright &y& Elsevier]

Formation of aluminide diffusion coatings on iron, nickel and cobalt by pack cementation and chemical vapor deposition (CVD) involves a series of similar steps, the slowest among them are the transport of aluminum bearing species from the vapor phase to the substrate by gas-phase diffusion and the solid-phase diffusion of aluminum into the substrate to form the aluminide phases. The former increases the surface concentration of aluminum in the coating while the latter decreases it. Modeling of the process is based on the observation that the surface composition of the coating tends to reach a steady state value in a short time after the commencement of the process, at which stage the rates of the above two processes are equal. However, in the case of iron aluminide coatings produced by low pressure chemical vapor deposition (1.33±0.13 kPa), the rate of transport of aluminum to the substrate is much faster than the solid phase diffusion of aluminum into the iron substrate since the diffusion coefficients of the vapor species are inversely proportional to pressure and the diffusion layer thickness is reduced considerably at low pressures. Under this condition, the vapor transport is no longer a rate-determining step and the composition of the aluminide coating is decided by the kinetics of the solid phase diffusion. This model could explain the kinetics of the process, surface composition and concentration profile of the coating. [Copyright &y& Elsevier]

The process of hot dipping pure aluminum on a stainless steel (1Cr18Ni9) followed by oxidation was studied to form a surface oxide layer. The thickness of the top aluminum on the steel substrate increases with increasing aluminizing time, while the thickness of the aluminum layer in the steel decreases as the increase in dipping temperature. Lower temperature and longer time favor a thicker layer of the aluminum on the substrate. The thickness of the intermetallic layer in the steel substrate increases with dipping temperature and time. However, the higher aluminizing temperature does not appear to have a significant effect on the thickness of the intermetallic layer. The oxidation treatment of the aluminized steel at 800 °C results the formation of a top oxide layer on the steel surface, composed of α-alumina, Al4Cr and Al17Cr9. The aluminizing and oxidation treatment of the stainless steel creates about 120 μm thickness of top oxide layer which has an extremely sound adherency to the steel substrate and a greatly improved properties of thermal shock withstanding, high temperature oxidation resistance and anti-liquid aluminum corrosion. [Copyright &y& Elsevier]

A new powder liquid coating method is proposed for the aluminization of Fe. Mixed powder slurries of Al + Ti or Al + Al2O3 are pasted onto Fe specimens, and the specimens are then dried and heated in a vacuum. Unlike hot dipping or powder pack cementation, this technique can be used to aluminize specimens selectively without the need for special equipment or halides. The amount of Al adhering to the substrate is determined by the Al–Ti reaction or coalescence of molten Al in Al2O3 powder during heat treatment. The Al concentration profile of the modified layer can be controlled by adjusting the powder mixing ratio or heat treatment conditions. The properties of the modified layer are analyzed using a new formulation, where the diffusion equation is treated numerically with consideration of the concentration dependence of the interdiffusion coefficient. The calculated profiles are stable and in good agreement with the experimental data. [Copyright &y& Elsevier]

The process of hot dipping pure aluminum on a steel substrate followed by oxidation was studied to form a surface layer of aluminum oxide resistant to the corrosion of aluminum melt. The thickness of the pure aluminum layer on the steel substrate is reduced with the increase in temperature and time in initial aluminizing, and the thickness of the aluminum layer does not increase with time at given temperature when identical temperature and complete wetting occur between liquid aluminum and the substrate surface. The thickness of the Fe–Al intermetallic layer on the steel base is increased with increasing bath temperature and time. Based on the experimental data and the mathematics model developed by the study, a maximum exists in the thickness of the Fe–Al intermetallic at certain dipping temperature. X-ray diffraction (XRD) and energy dispersive X-ray (EDX) analysis reveals that the top portion of the steel substrate is composed of a thin layer of α-Al2O3, followed by a thinner layer of FeAl3, and then a much thicker one of Fe2Al5 on the steel base side. In addition, there is a carbon enrichment zone in diffusion front. The aluminum oxide surface formed on the steel substrate is in perfect condition after corrosion test in liquid aluminum at 750 °C for 240 h, showing extremely good resistance to aluminum melt corrosion. [Copyright &y& Elsevier]

In the present study aluminum diffusion coatings were prepared on alloy 800H within a few minutes in air using several surface heating methods: induction heating, a natural gas burner, and a heating mat. All such methods are highly relevant for on-site applications, because they are commonly known from post welding heat-treatments. The very short (3 or 5 min) dwell times at temperatures between 700 and 1000 °C were sufficient to produce continuous diffusion coatings with all heat treatment methods. With the heating mat more of the Al-poor (Fe, Cr, Ni)Al phase is formed during diffusion treatment at 1000 °C due to the slower heating and cooling rates in comparison to the other techniques. • Established surface heat treatments are employed to manufacture diffusion coatings within a few minutes. In air. • Oxidation of Al-particles and diffusion temperature influence the coating thickness for the different heating methods. [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]