Aluminide Layer Research Articles

Degradation of Aluminide Layers During Cyclic Oxidation of Ferritic 430 Stainless Steel

In order to increase the performance of the preferred Ferritic 430 SS for manufacturing automobile and motor-cycle exhaust systems. The aluminizing coating on the surface of bare steel was applied by hot-dipping method in a molten pure aluminum. The high temperature oxidation of the aluminized steel was cyclically studied at 900 °C and 1000 °C in static air. The degradation of intermetallic layers during cyclic oxidation were analyzed by means of X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The crack perpendicular to the specimen surface rapidly propagated through the FeAl and Fe3Al layers due to a thermal expansion mismatch upon cooling to room temperature. The accumulation of voids generated crack at the interface between the aluminide layer and the steel substrate. Oxygen is allowed to penetrate into the aluminide layer crack, rapidly forming alumina oxide and closing the crack. Some of the aluminide layers peeled off due to this rapid growth. Thus, the protective Al2O3 layer degraded and later, the substrate was oxidized subsequently to form iron-rich oxide (Fe2O3) at 1000 °C.

The Pack-Cementation Process of Iron-Aluminide Coating on China Low Activation Martensitic and 316L Austenitic Stainless Steel

In a fusion blanket design, ceramic coating such as Al2O3, Er2O3, Y2O3, TiC, TiN and TiC/TiN etc., has been considered as a tritium permeation barrier (TPB) on structural materials (e.g. RAFMs, 316L) by many countries in the past 20 years. The Al2O3 film prepared by in-situ oxidation of the iron aluminide layer is considered one of the most attractive because of the slow-growing steady protective oxide scale and its excellent self-healing ability. In order to obtain a transition aluminide layer with a certain aluminum content and thickness on two kinds of substrates such as the Reduced Activation Ferritic/Martensitic (RAFM) and 316L stainless steel, wide research efforts have been made on the effect of different pack chemistry, temperature and time on the properties and thickness of the aluminizing layers. The results indicated that a dense and uniform coating with a thickness about 20μm was formed on CLAM (a Chinese RAFM steel) and 316L substrates for the pack material with low Al content (about 32wt.%). This aluminide coating had a surface aluminum content about 40-50at.% and was mainly consisted of ductile FeAl phase. For the pack material with high Al content (about 50wt.%), the thickness and the surface aluminum content of the aluminizing coating had great increases and there were mainly brittle Fe2Al5 phase. Especially some cracks were observed across this coating on CLAM substrate due to the mismatch in coefficient of thermal expansion (CTE) between the coating and substrate.

Prepartion of Al-Mn Alloy Coatings on Low Carbon Steel by Hot-Dip-Aluminizing

Low carbon steel were coated by hot-dipping into a molten bath containing Al-2 wt%Mn, Al-9wt%Mn, Al-13wt%Mn, respectively. The phase composition, morphology and the adhesion of the aluminide layer were characterized by XRD, SEM, EDAX, OM and scratch tester. The results show that the coating layers is mainly composed of Al, FeAl3, Fe2Al5 and MnAl6 phase. The coatings consists of two-layers structure, i.e., topcoat Al-Mn alloy layer and tongue-like intermetallic compound, and the coating layer is about 800 μm thick. The adhesion of the Al-Mn alloy coatings were characterized by Lc value and were found to be about ~30N. The adhesion gradually decrease with the increase of the Mn contents in alloy coatings.

Effect of Titanium-Aluminum Ratio on the Thermal Explosion Processing of TiAl-TiB0.6 Layered Composites

Bi- and trilayer composites of titanium aluminides (Ti–Al) and substoichiometric titanium boride (TiB0.6) have been fabricated from green compacts placed under pressure. Ti–Al blends with ratios of 3:1, 1:1, or 1:3 have been prepared by ball milling. Subsequent to the preparation of the precursor blends, plate-shaped green compacts of Ti–Al and Ti-B0.6 were pressed at room temperature. Green compacts were stacked in a layered geometry and were heated rapidly under a static pressure of about 10 bar. After the temperature reached 1300–1400°C, the Ti–Al and TiB0.6 phases were synthesized by the conversion of the reactants into the product phases (also known as “thermal explosion” processing) and simultaneously consolidated and joined under a pressure of 80–100 bar. Scanning electron microscopy (SEM) examination showed that the Ti–Al layer was highly heterogeneous, and the extent of heterogeneity depended strongly on the respective elemental ratio of Ti to Al. Furthermore, the conversion from reactants to products was found to be determined by the overall heat available during the thermal explosion processing step. Unlike the Ti–Al, the TiB0.6 layer was more uniform; however, the combination of heat and pressure was found to be insufficient to fully densify the aluminide layer. These and other features of the layered intermetallic structures will be discussed.

Formation of Ni Aluminide Coating Containing Reactive Elements by Simultaneous Electrodeposition and Cyclic Oxidation Resistance

A Ni aluminide layer containing Zr or Hf was formed on a Ni specimen by the simultaneous electrodeposition of Al and Zr or Al and Hf using a molten-salt bath. When the simultaneous electrodeposition of Al and Zr was carried out using molten NaCl-KCl containing 3.5 mol%AlF3 and 0.05 mol%ZrF4, the electrodeposited layers were formed in the order of Ni2Al3, NiAl3 and Al from the Ni substrate side. The ZrAl3 particles were uniformly formed in the surface region of the NiAl3 and Al layers. On the other hand, when the simultaneous electrodeposition of Al and Hf was carried out using molten NaCl-KCl containing 3.5 mol%AlF3 and 0.05 mol%HfF4, the electrodeposited layer consisted of Ni2Al3 as the inner layer and NiAl3 as the outer layer were formed with HfAl3 particles uniformly formed in the surface region of the NiAl3 layer. For the sample treated with the simultaneous electrodeposition of Al and Zr, no significant change in the mass gain was observed during the cyclic-oxidation test at 1423 K, suggesting that the sample had a high cyclic-oxidation resistance. Similarly, the sample treated by the simultaneous electrodeposition of Al and Hf had a high cyclic-oxidation resistance. An adhesive scale, having localized inward penetrations consisting of Al2O3 containing ZrO2 or HfO2, was formed on the samples having the high cyclic-oxidation resistance.

The mechanism of oxide whisker growth and hot corrosion of hot-dipped Al–Si coated 430 stainless steels in air–NaCl (g) atmosphere

The mechanisms of oxide whisker growth and hot corrosion of 430 stainless steel (430SS) and aluminide 430 stainless steel hot-dipped in a Al–10 wt.%Si molten bath (430HDAS) were studied at 750 and 850 °C in air mixed with 500 and 990 vppm NaCl (g). The results showed that the loose Cr 2O 3 scale which formed on the 430SS could not prevent the corrosion of 430SS in a 500 vppm NaCl (g) atmosphere, resulting in the formation of Fe 2O 3 scale. Fe 2O 3 whiskers grew at the grain boundary of the Fe 2O 3 scale. However, no Fe 2O 3 whiskers formed on the Fe 2O 3 scale while 430SS was exposed in a 900 vppm NaCl (g) atmosphere. During the initial high-temperature corrosion of 430HDAS in a 500 vppm NaCl (g) atmosphere, a dense Al 2O 3 scale formed on the surface of the specimens. Also, Al 2O 3 whiskers grew on the Al 2O 3 scale. As exposure time increased, cyclic chlorination/oxidation degraded the protective aluminide layer and caused the formation of Fe 2O 3 scale and Fe 2O 3 whiskers. The morphology of Fe 2O 3 whiskers formed at 750 °C is more slender than those formed at 850 °C. The formation and growth of both Fe 2O 3 and Al 2O 3 whiskers may be attributed to the chloridation of both the steel substrate and aluminide layer, accelerating the diffusion rate of metallic ions in the oxide scales.

Isothermal and thermal cycling oxidation of hot-dip aluminide coating on flake/spheroidal graphite cast iron

Two types of cast iron, flake graphite and spheroidal graphite cast iron, with ferrite matrix and similar composition, were aluminized by hot-dipping. As-coated aluminide layer consists of the outer Al topcoat, inner Fe–Al intermetallic layer and dispersed graphite. Isothermal and thermal cycling oxidation tests of aluminized specimens have been conducted. Cast irons with aluminide coating exhibit higher oxidation resistance than without the coating. However, different graphite structure results in diverse quality of aluminide coatings. Aluminide coating on flake graphite cast iron exhibits less oxidation resistance and adhesion to the substrate.

Effect of silicon on the formation of intermetallic phases in aluminide coating on mild steel

Mild steel was coated by hot-dipping into molten baths containing pure aluminum, Al–0.5Si, Al–2.5Si, Al–5Si and Al–10Si (wt.%) at 700°C for 180s. Silicon’s effect on the formation of the intermetallic phase in the aluminide layer was investigated by using a combination of scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). The micrograph observation showed that the aluminide layer resulting from hot-dipping in pure aluminum possesses the thickest intermetallic layer and a rough interface between intermetallic layer and steel substrate. As the silicon content in the molten bath increased, the thickness of the intermetallic layer decreased substantially and the interface between the intermetallic layer and the steel substrate became flat. On the other hand, EDS and EBSD observation revealed the aluminide layer resulting from hot-dipping in Al–2.5Si not only possessed FeAl3 and Fe2Al5, but also formed cubic τ5(C)-Al7(Fe,M)2Si (M=Mn, Cr or Cu) above the FeAl3 and scattered τ1-(Al,Si)5Fe3 in the Fe2Al5. However, as the content of silicon in the molten aluminum bath increased, τ5(C)-Al7(Fe,M)2Si began to be replaced by hexagonal τ5(H)-Al7Fe2Si and τ6-Al4FeSi.

Microstructure and Erosion-Corrosion Behavior of Hot-Dipping Al-Mn Alloy Coatings on Low Carbon Steel

Low carbon steel was coated by hot-dipping into a molten bath containing Al-2 wt.%Mn. The phase composition, morphology and the erosion-corrosion behavior of the aluminide layer were characterized by XRD, OM, SEM and erosion-corrosion tester, respectively. The results show that the coatings are mainly composed of Al, FeAl3, Fe2Al5 and MnAl6 phase. The coatings consist of two-layer structure, i.e., toplayer Al-Mn alloy layer and tongue-like intermetallic compound. The thickness of the coating layer is about 800 μm and all the coating layers show good adhesion to the steel substrate. Compared with the pure Al coatings, the Al-Mn alloy coatings exhibit lower wear rate irrespective of the rotation speed. The hot-dipped Al-Mn coatings possess considerable erosion-corrosion resistance.

ALUMINIZING OIL CASING STEEL N80 BY A LOW-TEMPERATURE PACK PROCESSING MODIFIED WITH ZINC ADDITION

Different aluminide coatings were prepared on oil casing steel N80 at a relatively lower temperature of 530°C for 2 h by pack powder modified with different content of zinc (Zn). The cross-sectional microstructure, element distribution and properties of as-aluminized oil casing steel N80 were investigated by SEM, EDS, micro-hardness test and electrochemical corrosion measurement. Results show that aluminide coating with around 50 μm in thickness can be successfully achieved by a low-temperature pack aluminizing processing with the addition of Zn . Zn in the pack powder can enhance the uniformity and continuity of the coating layer, while it has little effect on the thickness of as-packed coating with the increasing content of Zn from 38.8 wt.% to 84.4 wt.%. As the content of Zn is over 58.8 wt.%, two layer coating consisting of pure Zn layer and Fe–Al aluminide layer can be formed on oil casing steel N80 substrate. Furthermore, oil casing steel N80 with aluminizing coating shows a higher microhardness than that of original one except in the depth range of pure Zn layer, but the microhardness of oil casing steel substrate does not decrease after aluminizing which can be inferred that low-temperature aluminizing processing reported here will not bring any damages on the mechanical properties of oil casing steel N80 . Additionally, a lower self-corrosion current density of oil casing steel N80 with aluminizing coating also indicates that low-temperature aluminizing processing is helpful to the corrosion resistance of oil casing steel N80 .

Microstructural evolution of intermetallic layer in hot-dipped aluminide mild steel with silicon addition

Mild steel was coated by hot-dipping into molten pure aluminum, Al–0.5 Si, Al–2.5 Si, Al–5 Si and Al–10 Si (wt.%) baths at 700°C for 180seconds. The microstructure and phase constitution of the aluminide layers were characterized by means of optical microscope, scanning electron microscope with energy dispersive X-ray spectroscopy, X-ray diffraction and electron backscatter diffraction. Also, the thicknesses of the intermetallic layers and the metal losses of the steel substrate were measured to investigate the interaction between mild steel and aluminum baths. The results revealed that the additions of silicon to the aluminum baths caused Al7Fe2Si and Al2Fe3Si3 phases to form above the FeAl3 layer and in the Fe2Al5 layer, respectively. As the silicon content in the aluminum bath increased, the thickness of the intermetallic layer decreased, and the intermetallic layer/steel substrate interface transformed from an irregular morphology into a flat morphology. The decrease of the thickness of the intermetallic layer was principally attributed to the detachment of the Al7Fe2Si layer from the intermetallic layer into the aluminum bath. The flattened intermetallic layer/mild steel substrate interface was due to the formation of Al2Fe3Si3 precipitates in the Fe2Al5 layer by the serration-like steel substrate reacting with the Fe2Al5 layer containing solid-solute silicon.

Formation and Oxidation Behavior of Diffusion-Barrier-Coating System on γ-TiAl

The results of the work presented here refer to a high temperature oxidation of TiAl alloys coated with a protective layer of nickel aluminide. The nickel aluminide layer was formed by the method of pack aluminization of nickel coating which is electrochemically positioned onto the alloys surface. Prior to nickel plating, TiAl alloys underwent the process of aluminization leading to the formation on their surface of a layer enriched with Al. It has been established that during the aluminization the electrochemical nickel layer transforms into a protective layer of nickel aluminide Ni2Al3. The alloys Ti48Al2Cr2Nb and Ti48Al7Nb covered with the coating were exposed to oxidation in the air atmosphere at a temperature of 800oC. The layer of NiAl is covered with the Al2O3 scales during the oxidation. Inter-metallic phases containing Ni, Al, Ti, Nb are formed in the coating and in the near-surface alloy layer.

Improvement of oxidation resistance of TiAl6V4 alloy by siliconizing from liquid phase using melts with high silicon content

This work deals with a study of properties of layers prepared by a simple, cheap, and efficient method of siliconizing from liquid phase. They were prepared using the melts AlSi20 and AlSi28 (wt.%); their high silicon content provides for almost a twofold rate of the layers formation. Main component of the layers formed by siliconizing is τ 2 − phase [Ti(Al x Si 1− x ), where x = 0.15–0.3]. This phase is also formed in aluminium melts containing 5 and 10 wt.% of silicon, as documented in [1–4]. The growth rate of silicide–aluminide layers is a parabolic function; the rate constant for melts AlSi20 and AlSi28 expressed in m 2/s was determined as: k AlSi 20 = 22.344 exp − 227,412.8 R T and k AlSi 28 = 7,936,635 exp − 329,741 RT . This equation is valid under experimental conditions. The protective layers with a high silicon content are particularly suitable for high-temperature applications as they contain thermodynamically stable phases, such as TiSi 2, SiO 2 and Al 2O 3 creating an efficient barrier against diffusion of oxygen and nitrogen. The protective effect of these layers was studied by cyclic oxidation at temperatures 1123 and 1223 K. These layers rich in silicide–aluminide phase have been proved to provide, in spite of their inhomogeneity, for excellent protection of the titanium alloy against high-temperature oxidation.

Aluminizing Mechanism and Corrosion Resistance of Pipeline Steel X80 by Combined Pack Cementation Process under Low Temperature

In order to improve the corrosion resistance of pipeline steel X80 and maintain its good mechanical properties simultaneously, a low-temperature pack aluminizing process was carried out at 723 K on pipeline steel X80 after a surface mechanical attrition treatment (SMAT). The phase composition, microstructure and element distribution of the as-aluminized pipeline steel X80 were characterized by XRD, SEM and EDS, respectively.The results show that the as-received aluminide layer consists of Fe2Al5, which exhibits a good cohesion with SMATed pipeline steel X80 substrate with the thickness of around 90 μm. The element concentrations of Al and Fe atoms show a gradual change in the range of aluminide layer. After corrosion test processed in 3.5% NaCl solution, there is no obvious corrosion crack or uphills piled up by corrosion products on the surface of as-aluminized SMATed pipeline steel, which can conclude that pack aluminizing assisting by SMAT at low-temperature is an effective way for protecting pipeline steel X80 against corrosion.

Thermal Stability of Microstructure of Aluminide Layer Deposited by CVD Method on CMSX 4 Nickel Base Superalloy

In the paper the aluminide layer was deposited by CVD method on the CMSX 4 single crystal nickel base alloy. The aluminizing process was carried out at the 1050 °C during 8 h. The chemical vapor deposition process was performed by means of AlCl3 aluminum chloride. The effects of aluminizing were verified in microscopic examination (microstructure and depth layer) and chemical composition on the surface and cross-section of aluminide layer. The oxidation resistance in the air atmosphere at the 1100 °C during 1000 h was a criterion of efficiency of CVD process. The hardness distribution on the cross-section aluminide layer before and after oxidation test was investigated. The parabolic mass change was observed during oxidation. Under oxidation test during 120 h on the grain size of NiAl phase was observed the phase transformation β NiAl →Ni3Al. The increase of oxidation time causes decreasing of substrate hardness and stabilization of Topologically Closed Packed phases.

Functional Properties of Aluminide Layer Deposited on Inconel 713 LC Ni-based Superalloy in the CVD Process

Functional Properties of Aluminide Layer Deposited on Inconel 713 LC Ni-based Superalloy in the CVD Process

Study of microstructure and phase evolution of hot-dipped aluminide mild steel during high-temperature diffusion using electron backscatter diffraction

Mild steel was coated by hot-dipping into a molten aluminum bath. The microstructure and phase evolution in the aluminide layer during diffusion at 750°C in static air were analyzed by electron backscatter diffraction (EBSD). The results showed that the aluminide layer of the as-coated specimen consisted of an outer aluminum topcoat, minor FeAl3 and major Fe2Al5, respectively. Also, Fe2Al5 possessed a tongue-like morphology, which caused corresponding serration-like morphology in the steel substrate. A portion of the peaks of serration-like substrate were isolated, after short exposure at 750°C, and accompanied by the formation of voids, which continued to appear with further exposure at 750°C. As the aluminum topcoat was consumed, FeAl3 phase disappeared and left an aluminide layer of Fe2Al5 phase. After 60min of exposure, FeAl2 and FeAl phases formed at the interface between Fe2Al5 and the steel substrate. With increasing exposure time, the voids condensed and the serration-like morphology disappeared, while FeAl2 and FeAl phases kept growing. After prolonged exposure, the aluminide layer was composed of FeAl2 and FeAl and possessed a flat interface between FeAl and steel substrate.

EBSD study of crystallographic identification of Fe–Al–Si intermetallic phases in Al–Si coating on Cr–Mo steel

5Cr–0.5Mo steel was coated by hot-dipping in a molten Al–10wt.% Si bath at 700°C for 10, 60, 120 or 180s. The identification of the phases in the Fe–Al–Si intermetallic phases formed in the aluminide layers during hot-dipping was carried out by using a combination of scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The EDS results show a τ5(H)-Al7Fe2Si phase, which exhibited 2 distinct morphologies, small particles widely dispersed and a continuous layer. Also revealed by EDS were τ6-Al4FeSi and τ4-Al3FeSi2 phases, which showed plate-shaped morphology, in an Al–Si topcoat. However, the XRD results show the intermetallic phases in the aluminide layer were composed of outer cubic τ5(C)-Al7(Fe,Cr)2Si and inner hexagonal τ5(H)-Al7Fe2Si. EBSPs and mapping functions in EBSD helped to clarify the confused phase identifications yielded by EDS and XRD. In this way, the small intermetallic particles and the continuous intermetallic layer were identified as cubic τ5(C)-Al7(Fe,Cr)2Si and hexagonal τ5(H)-Al7Fe2Si, respectively, and the plate-shaped intermetallic phase was identified as monoclinic τ6-Al4FeSi and tetragonal τ4-Al3FeSi2 with the same metallographic morphology. EBSD proved to be a very effective technique for local phase identification of aluminide layers with complicated multiphase morphologies.

The dependence of high temperature resistance of aluminized steel exposed to water-vapour oxidation

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 H 2 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 pO 2-steam than in a high pO 2-steam.

Microstructure and high temperature oxidation behavior of hot-dip aluminized coating on high silicon ductile iron

High silicon ductile iron was coated by hot-dipping into an Al molten bath. The oxidation behavior of the aluminized alloy and the bare substrate was studied in air at 750 °C. The results showed that the coating layers consisted of three layers, in the sequence of Al, Fe–Al intermetallic and Si pile-up layers from the external topcoat to the substrate. The intermetallic layer was composed of outer FeAl 3 and inner Fe 2Al 5. The outer rod-shaped FeAl 3 dispersed in the aluminum topcoat, while the inner tongue-like Fe 2Al 5 formed in the metallic layer becoming the major phase in the aluminide coating layer. Those three layers of aluminum, Fe 2Al 5 and silicon pile-up layer exhibited hardness of HV 50, HV 1100 and HV 450, respectively. In this study, when the as-coated specimens were examined, Fe–Al–Si compounds could not be found. But the silicon pile-up at the interface between the substrate and the Fe–Al intermetallic layer could be seen in all the as-coated specimens. The graphite nodules were noticed in the substrate. The presence of graphite nodules in the substrate might be markers of hot-dipping. After hot-dipping in Al all the specimens contained graphite nodules in the aluminide layer. The oxidized graphite nodules resulted in cracks propagating in aluminide coating. Even though graphite nodules meant the existence of crack in the aluminide coating, the high temperature oxidation experiments indicated that the aluminide coating could prevent the oxidation of substrate effectively even at 750 °C.