Aluminide Diffusion Coatings Research Articles

Characterization of Iron Aluminide Diffusion Coatings Obtained after Friction Surfacing

Iron aluminides are considered as candidate materials for high temperature applications for their excellent high temperature corrosion and oxidation resistance. In the present work, iron-aluminide coatings were developed by friction surfacing (6351 aluminum alloy deposited on a low-carbon steel substrate) followed by a diffusion heat treatment. The initial coatings were found to be geometrically homogenous and adhered well to the steel substrate. The heat treatment process was carried out at 550 °C for 48, 72 and 96 h and the resulting coatings were characterized in terms of microstructure, chemical composition, hardness distribution and phase composition. After heat treatment, the coating/substrate interface morphology was modified and presented patterns typical of Fe-Al intermetallic formation, as well as a substantial increase in hardness (>900 HV) relative to the initial as-deposited condition. With the diffusion treatment, initially Fe2Al5 was found to develop in the coatings, which was converted into FeAl2 after longer exposures.

The formation of Al-Si aluminide coatings by pack cementation method on TNM-B1 intermetallic alloy

The TiAl intermetallics are the promising material for aerospace application. According to its insufficient oxidation resistance above 900oC the using of protective coatings is necessary. The diffusion aluminide coatings based on TiAl2 or TiAl3 phases permits to formation of alumina scale on the surface of TiAl alloys. The pack cementation with Si doping is one of the most popular method of this type of coatings production. In present article the influence of Si content in the pack, process time and temperature during pack cementation process were investigated. The thickness of obtained coating was in range 20-50 μm. When Si content was higher the formation of titanium silicides was observed using almost all analysed values of process parameters. The results showed that using of 24 wt. % Si containing pack and process parameters: 4h/950oC enables to obtain the coating characterized by optimal thickness and structure. The porosity and cracks in coatings according to TiAl phases brittleness was observed.

Oxidation Behavior of Overlay NiCoCrAlY and Diffusion Aluminide Coatings Deposited on a Directionally Solidified Nickel-Based Superalloy: A Comparative Study

Oxidation Behavior of Overlay NiCoCrAlY and Diffusion Aluminide Coatings Deposited on a Directionally Solidified Nickel-Based Superalloy: A Comparative Study

Aluminide Diffusion Coatings on IN 718 by Pack Cementation.

This paper addressed the issues of both direct and indirect synthesis of Ni aluminides by pack cementation (pure Ni and IN 718 superalloy). On the Al-Ni diffusion twosome under pressure, at temperatures below and above the Al melting temperature, the appearance and evolution of diffusion porosity because of the Kirkendall–Frenkel effect manifestation was highlighted. It has been confirmed that, as the temperature rises above the Al melting temperature, the porosity decreases. Nickel-based superalloys, and in particular IN 718, significantly increase their performance by increasing the aluminides proportion in the top diffusion coating. This is made possible by changing the value of the Al and Ni weight percentage ratio in this area (noted as Al/Ni). In the case of the diffusion twosome between IN 718 and pack aluminizing mixtures, having in their composition as active components Al powder, Ferroaluminum (FeAl40) or mixtures of Al and Fe powders, at processing temperatures above the Al melting temperature, by modifying the active component of the mixture, substantial changes in the Al/Ni values were observed, as well as in the maximum %Al in the diffusion coating and of its thickness. It was found that, when switching from Al to FeAl40 or powder mixture (Al + Fe), the Al/Ni value changes between 3.43 and 1.01, from initial subunit values. The experiments confirmed that the highest %Al in the top aluminized diffusion coating, for IN 718, was obtained if the powder mixture contained 66.34 wt.% Al.

Synthesis of self-healing NiAl-Al2O3 composite coatings by electrochemical way

Nickel aluminide diffusion coatings act like an aluminium reservoir to form protective alumina scales at high temperatures. However, interdiffusion phenomena play a critical role in the metallurgical integrity of these coatings. This work explores a new type of self-healing coating for high temperature oxidation applications. Micro-reservoirs consisting of an aluminium-rich intermetallic core (Al3Ni2) and an aluminium oxide shell are embedded in a nickel aluminide matrix (NiAl). This new type of coating has been synthesized by an electrochemical chemical route to trap the micro-reservoirs in an electrodeposited nickel matrix followed by aluminization via a slurry route. Under high temperature oxidation conditions, the micro-reservoirs allow to form a diffusion barrier and to release Al into the coating matrix. These two simultaneous phenomena make it possible to maintain a sufficient amount of aluminium and ensure the unique formation of α-alumina at the surface of the coatings, hence to possibly increase the lifespan of the aluminium diffusion coatings.

Diffusion aluminide coating modified via electroless nickel plating for Ni-based superalloy protection

Diffusion aluminide coatings are widely employed to increase oxidation resistance of superalloy components. This work presents an innovative modified diffusion coating on a Ni-based superalloy obtained by the deposition of an electroless nickel layer (ENL) before the pack cementation aluminizing process. ENL was also employed as a vehicle to embed α-Al2O3 nanoparticles within the aluminide coating, to promote a faster formation of a stable protective oxide. Plain and modified coatings were compared by means of XRD, SEM and EBSD. Differences in oxidation kinetics were evaluated with isothermal oxidation tests at 1050 °C: modified coatings present higher protective capabilities, characterized by a lower parabolic oxidation constant than standard aluminides. SEM and EDS analysis after 1000 h of test demonstrated a continuous and well-adherent thermally grown oxide (TGO) scale that was still protective, in contrast to the degradation evidenced in plain coatings. Nanoparticles mobility was not promoted by the diffusion treatments, thus preventing any beneficial effect on TGO growth and high temperature oxidation resistance. The proposed procedure based on deposition of an external electroless nickel layer, therefore, demonstrates a remarkable increase of protective performances of the aluminide coatings.

High temperature oxidation behavior of aluminide coatings applied on HP-MA heat resistant steel using a gas-phase aluminizing process

Aluminide diffusion coatings were applied on a high-performance micro-alloyed (HP-MA) heat resistance steel, using a single step gas-phase aluminizing process. Powder mixtures with different compositions were employed for the coating process to achieve low-Al and high-Al coatings. This paper investigates the phase composition, growth mechanism, as well as high temperature oxidation behavior of aluminide-coated HP-MA steels. X-ray diffraction (XRD), scanning electron microscope (SEM), and field emission scanning electron microscopy (FESEM) were used to investigate characteristics of coated specimens. To evaluate the oxidation behavior, specimens were heated to 1100 °C for 100 h in the air atmosphere. Weight measurements were conducted at regular ten-hour time intervals. Furthermore, the parabolic rate constant (kp) values were obtained from the (ΔW/A) vs. oxidation time graphs. Moreover, stresses generated in oxide scale, formed on the aluminide coatings, were theoretically calculated following the first cycle of oxidation. Results showed that the stresses created in the oxide scale exceed the compressive strength of alumina, resulting in its spallation. After 100 h of cyclic oxidation test, the thickness of the oxide scale for the HP-MA steel, low-Al, and high-Al aluminide coated specimens was 22, 14, and 6 μm, respectively. The oxidation behavior of all specimens followed the parabolic rate law. The value of kp was the lowest at all oxidation time intervals for the coatings with higher Al content. This was due to the presence of more Al and Cr elements in the high-Al aluminide coating, improving the ability to form an outer dense alumina layer.

The influence of Al addition and aluminizing process on oxidation performance of arc melted CoCrFeNi alloy

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%), NH4Cl (20%) and Al2O3 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 Al2O3 layer resulted in lower mass gain during the oxidation tests while mixed oxides and Cr2O3 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.

Evaluating the efficacy of aluminide coatings to improve oxidation resistance of high performance engine valve alloys

Increasing peak cylinder pressures and operating temperatures of turbo-charged internal combustion engines (ICEs) engines present new challenges for existing materials employed in engine exhaust valves. Oxidation induced degradation of current Ni based alloys for these components will be a life-limiting mechanism while strict cost margins in the automotive transportation industry further limit the choice of suitable candidate materials. Metallic diffusion aluminide coatings can provide a cost-effective way to improve the oxidation resistance of underlying materials. In this study, the high temperature oxidation behavior of diffusion nickel aluminide coatings on two Ni based alloys (commercially available alloy 31V and a developmental alloy) during cyclic oxidation behavior in air+10% H2O at 900 °C was investigated. A complete depletion of the beneficial Al-rich β-(NiFe)Al phase was observed in the coating on alloy 31V while a significant fraction of this phase was retained in the coating on the second alloy. A coupled thermodynamic-kinetic model showed that the disappearance of the Al-rich phase in the coating on 31V was mainly due to the combined effect of steeper chemical potential gradients of Al and Ti between the coating and the substrate and their faster diffusivities compared to the coated developmental alloy. A higher content of Fe was shown to support the retention of β-(NiFe)Al phase in the coating on the developmental alloy while the presence of Ti in the alloy was shown to be detrimental for long-term coating performance.

Diffusion Aluminide Coatings for Hot Corrosion and Oxidation Protection of Nickel-Based Superalloys: Effect of Fluoride-Based Activator Salts

The influence of two different fluoride-based activator salts (NH4F and AlF3) was studied for diffusion aluminide coatings obtained via pack cementation on a Ni-based superalloy (René 108DS). The resistance to oxidation and hot corrosion was assessed as a function of the concentration of activator salts used during the synthesis process by means of pack cementation. Two different concentrations were selected for activator salts (respecting the equimolarity of fluoride in the synthesis) and the obtained diffusion coatings were compared in terms of morphology, thickness and composition, as well as in terms of microstructural evolution after high temperature exposure. Isothermal oxidation tests were conducted at 1050 °C in air for 100 h in a tubular furnace. The oxidation kinetics were evaluated by measuring the weight variation with exposure time. The microstructural evolution induced by the high temperature exposure was investigated by SEM microscopy, EDS analysis and X-ray diffraction. Results showed that the coatings obtained with AlF3 activator salt are thicker than those obtained using NH4F as a consequence of different growth mechanism during pack-cementation. Despite this evidence, it was found that the NH4F coatings show a better oxidation resistance, both in terms of total mass gain and of quality of the microstructure of the thermally grown oxide. On the other hand, coatings produced with high concentration of AlF3 exhibited a better resistance in hot corrosion conditions, showing negligible mass variations after 200 h of high temperature exposure to aggressive NaCl and Na2SO4 salts.

Steam Oxidation of Aluminide-Coated and Uncoated TP347HFG Stainless Steel under Atmospheric and Ultra-Supercritical Steam Conditions at 700 °C

The efficiency of ultra-supercritical (USC) steam power plants is limited by the materials properties, in particular, the steam oxidation resistance of the currently used steels at temperatures higher than 600 °C. Under these conditions, steam oxidation results in the development of thick oxide scales which spall and can accumulate in tube bends leading to blockage, overheating and premature creep rupture, as well as erosion of downstream components such as steam valves and turbine blades. Most published work related to oxidation testing is carried out at atmospheric pressure, with significantly less testing of austenitic steels in supercritical steam, and rarely including protective coatings. Indeed, the effect of high-pressure steam in the oxidation process is not quite understood at present. This paper covers a comparison of the behaviour of TP347HFG stainless steel at 700 °C under atmospheric pressure and 25 MPa, with and without slurry-applied diffusion aluminide coatings. The results show a very protective behaviour of the aluminide coatings, which develop a very thin Al-rich protective oxide, and no significant difference between the two environments. In contrast, the uncoated steel exhibited a different behaviour. Indeed, under atmospheric pressure after 3000 h, very thin scales, rich in Cr and not surpassing 5 to 10 µm in thickness, covered the samples along with some much thicker Fe-rich oxide nodules (up to 150 µm). However, under 25 MPa, a thick multilayer scale with a non-homogeneous thickness oscillating between 10 to 120 µm was present. A microstructural investigation was undertaken on the oxidised uncoated and coated substrates. The results suggest that pressure increases the oxidation rate of the chromia former steels but that the oxidation mechanism remains the same. A mechanism is proposed, including early detachment of the outer growing scales under supercritical pressure.

Growth Kinetics of TiAl3 Diffusion Coating by Pack Cementation on Beta 21-S

The growth kinetics of protective diffusion aluminide coatings were investigated on Ti based alloy Beta-21S. The coating was prepared by halide activated pack cementation using CrCl3 as transport agent and pure aluminum (high activity) as masteralloy. The coating was composed of only TiAl3 layer whose growth was controlled by solid state diffusion following a parabolic law. The morphology of the coating was quite similar with interdiffusion products present in a bulk semi-infinite diffusion couple as showed by previous literature studies. The transport phenomena are significantly affected by the alloying elements present in the alloys and dissolved in the coating layer. The activation energy estimated at 108 kJ mol−1 determined for temperatures in the range 660-760 °C, indicates a bulk diffusion process with a possible grain boundary contribution and/or kinetic limitations associated to the mobility of the gaseous species during the aluminization.

Development of aluminide diffusion coatings on ODS ferritic-martensitic steel for corrosion resistance in high temperature super critical-carbon dioxide environment

Aluminide diffusion coatings were applied on an oxide dispersion strengthened ferritic-martensitic (ODS-FM) steel as surface modification methods. Either thin layer of Al or NiAl was deposited on the ODS-FM steels, which were subjected to inter-diffusion heat treatment (IDHT) to produce Al-IDHTed or NiAl-IDHTed specimens, respectively. After corrosion test in high temperature super-critical carbon dioxide (S-CO2), both Al-IDHTed and NiAl-IDHTed specimens showed significantly improved corrosion resistance with lower weight gains and effectively supressed carburization in the matrix. The improvement in corrosion resistance was attributed to the presence of continuous alumina layer on top of surface layer. For Al-IDHTed specimen, extensive diffusion of Al from surface layer and inter-diffusion zone to substrate was observed after the S-CO2 corrosion test. However, for NiAl-IDHTed specimen, an Al-rich surface layer was well-maintained after S-CO2 corrosion as Ni acted as a diffusion barrier.

Formation Mechanism of Aluminide Diffusion Coatings on Ti and Ti-6Al-4V Alloy at the Early Stages of Deposition by Pack Cementation.

The diffusion coatings were deposited on commercially pure Ti and Ti-6Al-4V alloy at up to 1000 °C for up to 10 h using the pack cementation method. The pack powders consisted of 4 wt% Al (Al reservoir) and 4 wt% NH4Cl (activator) which were balanced with Al2O3 (inert filler). The growth kinetics of coatings were gravimetrically measured by a high precision balance. The aluminised specimens were characterised by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). At the early stages of deposition, a TiO2 (rutile) scale, other than aluminide coating, was developed on both materials at <900 °C. As the experimental temperature arose above 900 °C, the rutile layer became unstable and reduced to the low oxidation state of Ti oxides. When the temperature increased to 1000 °C, the TiO2 scale dissociated almost completely and the aluminide coating began to develop. After a triple-layered coating was generated, the coating growth was governed by the outward migration of Ti species from the substrates and obeyed the parabolic law. The coating formed consisted of an outer layer of Al3Ti, a mid-layer of Al2Ti and an inner layer of AlTi. The outer layer of Al3Ti dominated the thickness of the aluminide coating.

Effect of diffusion coatings on the high temperature properties of nickel-chromium-superalloys

The halide-activated pack cementation method was utilized to deposit aluminide or silicide coatings on Inconel 617 and Hastelloy X superalloys. Aluminide and silicide diffusion coatings were formed at 850[Formula: see text]C for 2 h in nitrogen atmosphere, using a pack mixture containing pure aluminum (Al) or silicon (Si) and aluminum oxide (Al2O3) powders with activators of NH4Cl and AlF3. Aluminide-coated alloys showed homogeneous and uniform microstructures. Al diffused into the alloy inwards and aluminide diffusion coatings of [Formula: see text]17 [Formula: see text]m thick were formed inside the alloy. It was shown that the Al coatings played a key role in blocking off the excessive corrosion products at a high temperature for the alloys. The enhanced thermal stability and improved wear resistance were achieved in the aluminide coatings. In contrast to the aluminide coating, the silicide coating played a negative role, unable to provide the protective layer. The microstructural evolution and thermal stability of the aluminide- and silicide-coated alloys have been elucidated.

Corrosion behavior of aluminide diffusion coatings in low temperature molten carbonate electrolysis environments

Interest on low‐temperature molten carbonate electrolysis processes (T &lt; 600 °C) for advanced energy conversion and storage applications appears to be progressively growing in literature. However, the corrosion of metallic materials in molten carbonate salts in equilibrium with the acidic CO2‐rich electrolysis gas atmospheres has been scarcely studied. In this work, the corrosion stability of a low‐activity FeAl aluminide diffusion coated 316 L stainless steel has been tested in a ternary carbonate eutectic mixture Li2CO3‐Na2CO3‐K2CO3 by 48 h‐immersion tests at 500 °C under CO2 atmosphere. Before the corrosion testing, the coated steel samples were pre‐oxidized in air at 750 °C in order to generate a protective thin alumina film on the aluminide coating surface. The salt immersion results indicate that FeAl aluminide coatings are not stable in acid molten carbonate environments due to rapid transformation of the pre‐existing alumina film into a non‐protective mixed Fe‐Al oxide crystalline layer. Corrosion degradation has been found to consist of both general and localized forms of attack. Factors affecting FeAl aluminide coating corrosion have been analyzed and discussed in detail.

Diffusion stable aluminide coatings: Behaviour under steam and oxy-fuel fire-side conditions at 650 °C and accelerated diffusion tests at 900 °C

Two diffusion aluminide coatings obtained at 1050 °C were produced to protect ferritic steels both from steam oxidation and fire-side corrosion at 650 °C. One of them was a simple aluminide coating, whereas the other one was enriched with Cr prior to the aluminizing step. As opposed to aluminide coatings obtained at 700 °C, the new “as deposited” coatings do not exhibit through-thickness cracks and the Al content at the surface is lower. The Cr enriched coating has twice as much Cr at the surface than the aluminide coating produced under the same conditions but without Cr enrichment. Cr at the surface may increase the lifetime of this coating as it is known that it decreases the critical amount of Al required to maintain a protective alumina scale. The two aluminide coatings are very stable under atmospheric steam, as well as under an oxy-combustion atmosphere at 650 °C. They show no significant degradation by diffusion, nor evidence of substrate attack despite the development of some cracks, likely formed during the tests cooling cycles. None of these cracks reached the substrate. This behaviour contrasts that of the coating obtained at 700 °C, which exhibits fast diffusion during exposure at 650 °C. Accelerated diffusion experiments were also carried out at 900 °C under steam, comparing the Cr enriched and unenriched aluminide coatings in order to establish if the higher Cr content present in the enriched coating allows maintenance of the protective alumina scale for longer exposures. However, Cr from the substrate diffused outwards equalizing the Cr surface content of both coatings.

Study on the mechanisms of formation of aluminized diffusion coatings on a Ni-base superalloy using different pack aluminization procedures

The mechanisms of formation and microstructure of pack aluminized diffusion coatings depend on a wide range of parameters. These impact coatings differently depending on the processing mode selected (out-of-pack or in-pack) and are still the subject of debate. The need to systematize data on the influence of processing mode on the mechanisms of the formation of aluminized coatings was the motivation for this study, which reviews the influence of processing mode based on an analysis of experimental data for aluminized coatings processed in-pack and out-of-pack on a Ni alloy with Al at unit activity at 800 °C [low temperature with high Al activity (LTHA)] and 1100 °C [high temperature with high Al activity (HTHA)]. The results show that processing mode determined the amount of Al deposited at the surface and that the formation of a larger halide-depleted zone during out-of-pack diffusion accounted for the reduced deposition of Al. Processing temperature and Al concentration at the surface of the alloy determined the mobility of the elements in the coating and, consequently, the coating thickness and dominant diffusion mechanisms. Aluminization with the specimen immersed in the pack resulted in a preferential inward diffusion of Al for both processing temperatures. In contrast, coatings processed by out-of-pack diffusion showed a dependence on processing temperature. When a HTHA process was used, the coatings were thinner and formed in a two-stage diffusion process, while a LTHA process failed to form aluminide diffusion coatings.

Integrity of Co-Cr-C coated P92 steel for power plant pipework applications

The main limiting factor for P92 steel, in power plant at high temperature, is the increased oxidation damage on the inside surface which causes enhanced damage of components. Industry have attempted to address this problem by applying oxidation resistant coatings to the inside surface of the P92 pipework to prevent damage. Aluminide diffusion coatings have been a particular focus for research to date, however they have been found to have a number of detrimental effects on the creep properties and coating-substrate integrity. This paper introduces a Co-Cr-C type coating, composed of Cr3C2 particles electro-deposited within a cobalt matrix. On exposure to high temperature oxidation conditions the coating is shown to form a cobalt and chromium rich oxide which is slow growing, adherent and ideal for oxidation resistance. When applied to P92 substrate and exposed at service relevant temperatures the coated system retains its integrity and appears suitable for long term service. The coated P92 system is also shown to retain its integrity during high temperature creep testing and coating application does not have a negative effect on the mechanical properties of P92. Overall the Co-Cr-C coating has a number of superior properties compared to previously investigated coatings.

Monitoring the Degradation Process of Inconel 600 and its Aluminide Coatings under Molten Sulfate Film with Thermal Cycles by Electrochemical Measurements

With a specially designed electrochemical cell, the changes in impedance behavior for Inconel 600 and aluminide diffusion coatings under molten sulfate film with thermal cycles (from <TEX>$800^{\circ}C$</TEX> to <TEX>$350^{\circ}C$</TEX>) were monitored with electrochemical impedance measurements. It was found that corrosion resistance for both materials increased with lower temperatures. At the same time, the state of molten salt was also monitored successfully by measuring the changes in impedance at high frequency, which generally represents the resistance of molten salt itself. After two thermal cycles, both Inconel 600 and aluminide diffusion coatings showed excellent corrosion resistance. The results from SEM observation and EDS analysis correlated well with the results obtained by electrochemical impedance measurements. It is concluded that electrochemical impedance is very useful for monitoring the corrosion resistance of materials under molten salt film conditions even with thermal cycles.