Codeposited chromium and silicon diffusion coatings for Fe-Base alloys via pack cementation

The simultaneous deposition of Cr and Si into plain carbon, low-alloy, and austenitic steels using a halide-activated pack-cementation process is described. Equilibrium partial pressures of gaseous species have been calculated using the STEPSOL computer program to aid in designing specific processes for codepositing the desired ratios of Cr and Si into a given alloy. The calculations indicate that NaCl-activated packs are chromizing, while NaF-activated packs deposit more Si with less Cr. The use of a dual activator (e.g., NaF+NaCl) allows for the deposition of both Cr and Si in the desired amounts. Single-phase ferritic coatings (150–250 microns thick) with a surface concentration of 20–35 wt.% Cr and 2–4% Si have been grown on AISI 1018, Fe-2.25 Cr-1.0Mo-0.15C, and Fe-0.5 Cr-0.5 Mo-0.2C steels using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF+NaCl activator, and a silica filler. Two-phase coatings (approximately 75 microns thick) containing 20–25 wt.% Cr and 2.0–2.4% Si have been obtained on 304 stainless steel using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF activator, and an alumina filler. The same pack chemistry allowed the diffusion of Cr and Si into the austenitic Incoloy 800 alloy without a phase change. A coated Fe-2.25 Cr-1.0 Mo-0.15 C coupon with a surface concentration of Fe-34 wt.% Cr-3Si was cyclically oxidized in air at 700°C for over four months and 47 cycles. The weight gain was very low (<0.2 mg/cm²) with no scale spalling detected. Coated coupons of AISI 1018 steel, and Fe-0.5 Cr-0.5 Mo-0.2C steel have shown excellent oxidation-sulfidation resistance in reducing, sulfur-containing atmospheres at temperatures from 400 to 700°C and in erosion and erosion-oxidation testing in air at 650 and 850°C.     

An investigation of pack-aluminide coating on steel

Aluminide coatings are known to protect steels from oxidation and corrosion in hydrocarbon and sulfur-bearing atmospheres. Pack cementation is ideally suited for forming these coatings on small intricate components, wherein a diffused layer is formed which is well bonded to the substrate. Even though pack aluminide coated steels are being commercially used, there has not been any systematic investigation of the factors that control the coating formation. The present investigation has been carried out to define the boundary conditions under which diffusion in the solid phase determine the coating kinetics. The effect of pack activity and temperature on the structure and kinetics of aluminde layer formation on EN-3 steel has been investigated. The coating characteristics were evaluated by metallography, EPMA, X-ray diffraction, and scanning electron microscope (SEM). Oxidation resistance of the coated samples were compared to that of 304 stainless steel after heating in air at 900°C for 72 h. The surface aluminum composition was found to be about 20% by weight which remained constant with time in the temperature range of 750°C–900°C. Weight gains and layer thicknesses obeyed parabolic relationship with time at all temperatures. Under these conditions, the system constitutes a vapor-solid diffusion couple. Interdiffusion coefficient values in the Fe-Al system have been determined, and the activation energy has been calculated to be 57 Kcals/mole, which agrees well with the literature values.     

The Effect of Water Vapor on the Oxidation Behavior of CVD Iron-Aluminide Coatings

The oxidation behavior of iron-aluminide coatings, Fe₃Al or (Fe,Ni)₃Al, produced by chemical-vapor deposition (CVD) was studied in the temperature range of 700–800°C in air + 10 vol.% H₂O. A typical ferritic steel, Fe–9Cr–1Mo, and an austenitic stainless steel, 304L, were coated. For both substrates, the as-deposited coating consisted of a thin (<5μm), Al-rich outer layer above a thicker (30–50 μm), lower-Al-content inner layer. In addition to coated and uncoated Fe–9Cr–1Mo and 304L, cast Fe–Al model alloys with similar Al contents (13–20 at.%) to the CVD coatings were included in the oxidation exposures for comparison. The specimens were cycled to 1000 1 hr cycles at 700°C and 500 1 hr cycles at 800°C, respectively. The CVD coating specimens showed excellent performance in the water-vapor environment at both temperatures, while the uncoated alloys were severely attacked. These results suggest that an aluminide coating can substantially improve resistance to water-vapor attack under these conditions.     

Effects of Chromium on the Oxidation Performance of β-FeAlCr Coatings

-FeAl coatings containing various Cr contents of 6.5–45 wt.%were produced with a closed-field, unbalanced magnetron sputter (CFUMS)deposition technique. Cyclic oxidation tests at 1100°C in air for100 1-hr cycles and isothermal exposures at 1000°C in pure O₂ for100 hr were carried out with the coatings and an as-cast FeAlspecimen. All of the coatings showed good scale-spallation resistanceduring cyclic oxidation and the coating with 6.5 wt.% Cr exhibited thelowest oxidation rates in both cyclic and isothermal oxidationexposures. After oxidation, fine-grain ridge-type oxide scales formed onthe coatings, while the oxide scale formed on the cast FeAl showed alarge quantity of -Al₂O₃ blades and large interfacial voids on thebase–alloy surface. The transformation from to -Al₂O₃was accelerated due to the presence of Cr in the coatings. The fasttransformation considerably reduced oxidation rates, suppressed fastoutward Al diffusion for the growth of a -Al₂O₃ scale, and preventedthe formation of interfacial voids that played a major role in causing thescale spallation.     

Oxidation Behavior of Cast Ni3Al Alloys and Microcrystalline Ni3Al+5% Cr Coatings with and without Y Doping

Ni₃Al+5% Cr and Ni₃Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al₂O₃ formed on the coatings as the main oxidation products, with the formation of - and -Al₂O₃ scales at 900 and 1200°C, respectively. The spallation resistance of the Al₂O₃ scales formed on the coatings was superior to the oxide scales formed on cast Ni₃Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. I: Oxidation Kinetics and Morphology at 800°C

The oxidation of nickel-matrix/aluminum-particle composite coatings was studied using thermogravimetric (TG) analysis in air at 800°C for up to 100 hr. Long-term oxidation behavior was investigated with furnace exposures up to 2000 hr. The coatings were applied to nickel substrates by the composite electrodeposition technique and vacuum heat treated for 3-hr at 825°C prior to oxidation testing. The heat-treated coatings contained a two-phase (Ni)+(Ni₃Al) microstructure and the overall coating composition was approximately 7 wt.% Al. Also examined were uncoated nickel substrates and bulk Ni–Al alloys containing 6.2, 9.0, and 14 wt.% Al. For all samples, mass-gain kinetics were obtained from thermogravimetric (TG) experiments and furnace exposures and the composition and morphology of the oxidation products were examined using optical microscopy, scanning-electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). An outer NiO layer and an inner -Al₂O₃ layer formed on the composite-coating surface. The addition of a small amount of Si (about 1–2 at.%) was found to have little effect on Ni–Al composite-coating oxidation behavior. The Ni–Al coatings behave similarly to bulk + (Ni₃Al) or single-phase (Ni₃Al). In addition, at lower temperatures, such as 800°C, the coatings benefit from a small grain size that enhances Al diffusion to the surface to form the protective alumina layer. Based on oxidation kinetics and morphology, a critical Al content of about 6 wt.% was found, below which internal oxidation and higher oxidation mass gains were observed.     

Simultaneous chromizing — Aluminizing coating of low-alloy steels by a halide-activated,pack-cementation process

The simultaneous chromizing — aluminizing of low-alloy steels has achieved Kanthal-like surface compositions of 16–21Cr and 5–8 wt.% Al by the use of cementation packs with a Cr-Al masteralloy and an NH₄Cl activator salt. An initial preferential deposition of Al into the alloy induces the phase transformation from austenite to ferrite at the 1150°C process temperature. The low solubility of carbon in ferrite results in the rejection of solute C into the austenitic core, thereby preventing the formation of an external Cr-carbide layer, which would otherwise block aluminizing and chromizing. The deposition and rapid diffusion of Cr and Al into the external bcc ferrite layer follows. Parabolic, cyclic-oxidation kinetics for alumina growth on the coated steels in air were observed over a wide range of relatively low temperatures (637–923°C).     

Simultaneous chromizing-aluminizing coating of austenitic stainless steels

Chromium and aluminum were simultaneously co-deposited by diffusion into austenitic stainless steel substrates, by a single-step, pack-cementation process. The mechanism for the formation of diffusion-coated products on 304 and 316 stainless steels and on Incoloy 800 is discussed. The morphologies of the phases formed at the surface, i.e., an external beta layer and an underlying multiphase interdiffusion zone, are presented. The formation of the brittle, , outer layer was minimized by variations in the pack composition and activator. The coated 304 and 316 steels exhibited excellent scaling resistance upon oxidation in air at 1000°C.     

Oxidation-Resistant Aluminide Coatings on γ-TiAl

The long-term application of TiAl alloys based on the -phase at temperatures above 750–800°C requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, these alloys, containing large amounts of titanium, suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti–50 at.% Al alloy to TiAl₃, the most promising, oxidation-resistant phase in the Ti–Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800–1000°C in air for up to 300 hr. The aluminide coating greatly improves the oxidation resistance of -TiAl, forming a protective alumina scale. The rapid aluminum interdiffusion between the TiAl₃ coating and the -TiAl substrate determined the effective life of the coating. In addition, the oxidation behavior of the TiAl₂ phase formed by interdiffusion of the coating system was studied by oxidation of cross sections.     

Effect of Cr,Co, and Ti additions on the high-temperature oxidation behavior of Ni3Al

The oxidation behavior of Ni₃Al+2.90 wt.% Cr, Ni₃Al+3.35 wt% Co, and Ni₃Al+2.99 wt.% Ti alloys was studied in 1 atm of air at 1000, 1100, and 1200°C. Isothermal tests revealed parabolic kinetics for all three alloys at all temperatures. Cyclic oxidation for 28 two-hour cycles produced little spallation at 1000°C, but caused partial spallation at 1100°C. Especially, at 1200°C severe spallation in all three alloys was observed. Although additions of Cr, Co, or Ti to Ni₃Al alloys slightly increased the isothermal-oxidation resistance, the additions tended to decrease the cyclic-oxidation resistance. The major difference in the oxidation of the three alloys compared with the oxidation of pure Ni₃Al alloys was the existence of small -Al₂O₃ particles in the middle of the -Al₂O₃ scale and the formation of irregularly shaped Kirkendall voids at the alloy-scale interface.     

High-Temperature Oxidation of an Aluminized NiCr Alloy formed by a Magnetron-Sputtered Al Diffusion Coating

Aluminium diffusion coatings were obtained on Ni–20Cr substrate by sputtering an aluminium film, followed by a two stage diffusion treatment in an argon inert gas atmosphere (first stage at 600°C, second at 900 or 1100°C). Aluminides obtained at 900°C and 1100°C are close to those obtained by pack cementation process with high aluminium activity. These diffusion coatings are able to develop alumina scales during isothermal oxidation at high temperatures, whereas the untreated substrate had a chromia-forming behaviour. The weight gain recorded at 1100°C on coated sample is then smaller than the one of uncoated NiCr at 950°C. Presence of chromium was detected in the diffusion coating and Cr-rich precipitates were observed at the diffusion coating/substrate interface. After oxidation at 900°C and 1100°C, only α-Al₂O₃ was revealed by XRD. An intermediate scale with a “whiskered” morphology could however be observed after 48 hr oxidation at 900°C. After 100 hr of oxidation at 1100°C, the Ni_xAl_y diffusion phases were no longer detectable and the upper part of the oxide scale spalled away during cooling. Large cavities appeared at the initial location of the diffusion coating/substrate interface.     

Degradation behaviour of ZrO2-Ni/Al gradient coatings in molten Zn

In order to improve the corrosion resistance of metallic materials in molten zinc, ZrO₂-Ni/Al gradient coatings were sprayed on the surface of the Fe-0.35-0.44 wt.% C steel. The corrosion behaviour and corrosion mechanism of the ZrO₂-Ni/Al gradient coatings in molten zinc were studied. The ZrO₂-Ni/Al gradient coatings on the surface of steels prolonged the lifetime of samples and changed the corrosion behaviour of the samples in molten zinc. The lifetime of the ZrO₂-Ni/Al gradient coatings immersed in molten zinc at 620 °C is 28 days, which is 4 times as long as that of the general ZrO₂ coatings. The ZrO₂-Ni/Al gradient coatings were corroded in molten zinc at 620 °C, which was caused by zinc atom diffusing along the crystal boundary and pores of the ZrO₂-Ni/Al gradient coatings, and reacting with Ni/Al particle in the ZrO₂-Ni/Al gradient coatings. The corrosion mechanism of the coatings in molten zinc at 620 °C was crystal boundary corrosion, pitting corrosion and reaction corrosion.     

Mechanical behaviors and energy absorption properties of Y/Cr and Ce/Cr coated open-cell nickel-based alloy foams

In this study,Y-and Ce-modified Cr coatings applied by pack cementation method were prepared on the surface of open-cell nickel-based alloy foam.The morphologies and microstructures of Y-and Ce-modified Cr coatings with various Y and Ce contents were investigated in detail.Then,the effects of Y and Ce addition on the mechanical properties of open-cell nickel-based alloy foams were analyzed and compared.Simultaneously,the energy absorption capacity and energy absorption efficiency of the Y-and Ce-modified Cr coated alloy foams were discussed and compared at the room and high temperatures.The results show that Cr coatings containing minor amounts of rare earth element(Y and Ce) are well adhered to the nickel-based foam struts.Especially,the microstructure of the 2 wt% Ce-modified Cr coating is denser and uniform.In addition,the compressive strength and plateau stress of Y-and Ce-modified Cr coated alloy foams firstly increase and then decrease by increasing the Y and Ce contents at room and high temperatures.The energy absorption capacity of Y/Cr and Ce/Cr coated alloy foams increases linearly with the strains increasing.The Ce/Cr coated alloy foams can absorb more energy than Y/Cr coated alloy foams in the plateau and densification regions at room temperature.Compared to those at room temperature,the Y-and Ce-modified Cr coated alloy foams show higher energy absorption efficiency when deformation within 10%-30% at high temperature.     

Thermally-induced formation of hexagonal AlN in AlCrN hard coatings on sapphire: Orientation relationships and residual stresses

Cubic AlCrN coatings were epitaxially grown onto Al₂O₃(00.1) substrates by reactive magnetron sputtering at 500°C from Al/Cr targets with an atomic ratio of 70/30. The coatings were vacuum annealed at 1000°C for 2 hours in order to induce formation of wurtzite-type AlN. The as-deposited and annealed coatings were characterized using X-ray diffraction techniques. Pole figure measurements revealed orientation relationships of the cubic AlCrN phase with respect to the substrate. Residual stress characterization indicated compressive stresses of -1246 MPa in the as-deposited cubic AlCrN phase. After annealing, the residual stresses in the hexagonal wurtzite-type Al(Cr)N and the Al-depleted cubic Cr(Al)N phase are -132 and 346 MPa, respectively. The stress changes can be interpreted as a consequence of point defect recovery at temperatures above deposition temperature and Al(Cr)N formation in the annealed coating.     

Oxidation Behavior of ODS Fe–Cr Alloys

Four experimental oxide dispersion strengthened (ODS)Fe-(13–14 at. %)Cr ferritic alloys were exposed for up to 10,000 hr at 700–1100 °C in air and in air with 10vol.% water vapor. Their performance has been compared to other commercial ODS and stainless steel alloys. At 700–800°C, the reaction rates in air were very low for all of the ODS Fe–Cr alloys compared to stainless steels. At 900°C, a Y₂O₃ dispersion showed a distinct benefit in improving oxidation resistance compared to an Al₂O₃ dispersion or no addition in the stainless steels. However, for the Fe-13 %Cr alloy, breakaway oxidation occurred after 7,000 hr at 900°C in air. Exposures in 10 % water vapor at 800 and 900°C and in air at 1000 and 1100°C showed increased attack for this class of alloys. Because of the relatively low Cr reservoirs in these alloys, their maximum operating temperature in air will be below 900°C.     

Decarburization of Fe-Cr-C steels during high-temperature oxidation

Isothermal oxidation treatments were carried out on Fe-Cr-C steels. The steels containing 0.08, 0.15, 0.17, 0.88, 1.51, and 12.77wt.% Cr and 0.10, 0.49, 1.19, 0.18, 1.05, and 1.63 wt.% C were oxidized in ambient air at temperatures of 900, 1000, and 1200°C. Steels containing 13.22, 12.90, 12.52, and 12.77wt.% Cr and 0.15, 0.30, 0.50, and 1.63 wt.% C were heated (1100°C/3hr) in a flowing atmosphere of O₂-N₂-He in a SETARAM thermobalance. Evidence of decarburization of the steels is given by metallographic observations, by direct measurements of carbon diffusivities from the decarburization profiles in the oxidized samples, and by the results of kinetics measurements.³ Carbon diffusion coefficients were measured by the standard sectioning method in the samples oxidized in air. A. generalized equation for carbon diffusivity in Fe-Cr-C alloys is developed in terms of N_Cr[wt.%], N_C[wt.%], and T[K].     

High-Temperature Oxidation of Ti-48Al-2Nb-2Cr and Ti-25Al-10Nb-3V-1Mo

The short-term oxidation behavior of a-TiAl alloy (Ti-48Al-2Nb-2Cr) was compared andcontrasted to that of an₂-Ti₃Al base(Ti-25Al-19Nb-3V 1Mo) alloy. Oxidation ofTi-25Al-10Nb-3V-1Mo was found to occur at a moderate rate at 800°C, in aN₂ + 20% O₂ environment. A largeincrease in the oxidation rate occurred above thistemperature. This large weight increase was attributedto a breakdown in the protective oxide scale on the surface of the₂ intermetallic alloy, therebypermitting rapid diffusion of oxygen and nitrogen to thesurface of the intermetallic. The oxidation rate of thisalloy at 1200°C was not significantly higher thanthe oxidation rate at 1000°C. In contrast, theoxidation rate of Ti-48Al-2Nb-2Cr remained low up to1200°C. At this temperature, a significant increasein oxidation was observed and was attributed to acceleratedoxygen diffusion through the ₂ phaseand increased solubility of oxygen in the gamma phase ofthe intermetallic microstructure. This weight increaseoccurred despite the fact that at 1200°C, theintegrity of the oxide layer formed on the surface ofthis alloy was maintained. The results of this studyillustrate the need for developing protectiveenvironmental coatings tailored to the individualintermetallic alloy.     

Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam

Several Fe-Cr-Ni austenitic stainless steels (Cr wt.%: 13–25, Ni wt.%: 15) were oxidized in steam for 1000 hr at 500–900°C. The oxide scales were examined and categorized with respect to the chromium concentration and the grain size of the base metal. Experiments showed three conditions for the critical bulk Cr concentration and the oxidation temperature at which the oxidation behavior changed drastically. Metallographic examination showed that two of these three conditions resulted from the internal-external transition of Cr₂O₃ either on the metal surface or along the grain boundaries of the base metal. Attempts were made to interpret these conditions from the available oxidation theories. Atkinson's treatment was employed with some modification to incorporate the grain-boundary diffusion of Cr in the base metal. The calculation basically explained the internal-external transition for the oxidation of these steels.     

Microstructure development during pack aluminization of nickel and nickel–chromium wires

Pack aluminization – a chemical vapor deposition process widely used to form protective coatings on Ni-based superalloy components – was used to form shells of Ni₂Al₃, NiAl and/or γ′-Ni₃Al on the surface of γ-Ni wires with diameters of 127 μm. The growth kinetics of these Al-rich intermetallic shells are studied as a function of aluminization time and pack activity at 1000 °C. Similar kinetics but additional phases (Cr/Ni two-phase shell, Cr silicide particles and Al-rich particles distributed in Ni₂Al₃) are found in the shells of pack-aluminized Ni–20 wt.% Cr wires with similar diameters. Fully homogenous Ni–Al and Ni–Cr–Al wires are achieved by interdiffusion at 1200 °C between the deposited Al-rich intermetallic shells and the Ni-rich core of both types of wires. Upon subsequent aging at 900 °C, wires with γ/γ′ structure and high hardness indicative of precipitation strengthening are obtained.     

Inhibited aluminization of an ODS FeCr alloy

Aluminide coatings are of interest for fusion energy applications both for compatibility with liquid Pb–Li and to form an alumina layer that acts as a tritium permeation barrier. Oxide dispersion strengthened (ODS) ferritic steels are a structural material candidate for commercial reactor concepts expected to operate above 600 °C. Aluminizing was conducted in a laboratory scale chemical vapor deposition reactor using accepted conditions for coating Fe- and Ni-base alloys. However, the measured mass gains on the current batch of ODS Fe–14Cr were extremely low compared to other conventional and ODS alloys. After aluminizing at two different Al activities at 900 °C and at 1100 °C, characterization showed that the ODS Fe–14Cr specimens formed a dense, primarily AlN layer that prevented Al uptake. This alloy batch contained a higher (> 5000 ppma) N content than the other alloys coated and this is the most likely reason for the inhibited aluminization. Other factors such as the high O content, small (~ 140 nm) grain size and Y–Ti oxide nano-clusters in ODS Fe–14Cr also could have contributed to the observed behavior. Examples of typical aluminide coatings formed on conventional and ODS Fe- and Ni-base alloys are shown for comparison.