A surface study of the oxidation of type 304L stainless steel at 600 K in air

The oxidation of type 304L stainless steel at 600 K in air was studied using a number of surface-analytical techniques, including Auger electron spectroscopy (AES), scanning electron microscopy with energy-dispersive analysis of X-rays (SEM-EDAX), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). Spectral analysis showed that a duplex oxide was formed, the outer layer of which formed rapidly and was essentially iron (III) oxide. Beneath this was a mixed iron-chromium oxide. SIMS sputter-profile curves showed region of relatively low iron concentration in the oxide film at the metal-oxide interface. This resulted from the rapid diffusion of iron within the oxide film. The oxide grain boundaries were examined using SEMEDAX. Higher chromium and silicon levels were detected in these regions compared with the corresponding grain centers. AES indicated the presence of silicon as SiO₂.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature III: Modelling and Simulation of Void-induced Duplex Oxide Growth

9Cr–1Mo steel forms in CO₂ at 550?°C a duplex oxide layer containing an outer magnetite scale and an inner Fe–Cr rich spinel scale. The inner spinel oxide layer is formed according to a void-induced oxidation mechanism. The kinetics of the total oxide growth is simulated from the proposed oxidation model. It is found that the rate limiting step of the total oxide growth is iron diffusion through high diffusion paths such as oxide grain boundaries in the inner Fe–Cr rich spinel oxide layer. In the proposed oxidation model, a network of nanometric high diffusion paths through the oxide layer allows the very fast supply of CO₂ inside pores formed at the oxide/metal interface. Its existence is demonstrated to be physically realistic and allows explaining several observed physical features evolving in the oxide layer with time.     

Effects of aluminum on the oxidation of 25Cr-35Ni cast steels

The oxidation kinetics and morphological development during reaction of two cast austenitic steels at 1000°C in pure dry oxygen at 20 kPa are reported. Both steels contained approximately 25 wt.% Cr and 35 wt.% Ni and, in addition, one steel contained 3.3 wt. %. Both steels oxidized to form external scales consisting mainly of Cr₂O₃ with a thin outer layer of manganese rich spinel. Scale growth kinetics were parabolic, and somewhat faster rates were observed for the aluminum bearing steel. In both steels, deep internal oxidation occurred at the site of primary (interdendritic) carbides. The kinetics of this process were parabolic, and rate control was attributed to oxygen diffusion along the interface between internal oxide and matrix metal. In the aluminum-free steel, interdendritic carbides were converted to chromium rich oxide, but when aluminum was present, a sheath of aluminum rich oxide formed around the carbides. In this latter case, the rate of interdendritic penetration was somewhat slower. The aluminum bearing steel also formed large numbers of rod-shaped Al₂O₃ precipitates within the austenitic dendrites. Deepening of the Al₂O₃ precipitate zone also proceeded according to parabolic kinetics at a rate consistent with rate control by diffusion of oxygen along the oxide-alloy interfaces.     

The Effects of Molybdenum Addition on High Temperature Oxidation Behavior at 1,000?°C of Type 444 Ferritic Stainless Steel

The influence of molybdenum addition on the oxidation behavior of ferritic stainless steel (FSS) was studied at 1,000?°C up to 100?h in air under isothermal conditions. The results were compared with molybdenum-free FSS in order to explain the role of molybdenum on the oxidation of Type 444 FSS. It is shown that molybdenum plays a similar protective role as the one observed with silicon by promoting the precipitation of Laves phase. Moreover, the Fe₂(Nb, Mo) Laves phases with a small amount of silicon are found nearby the oxide–metal interface and deep in the metallic matrix along the grain boundaries. These highly protective Laves phases hinder the external diffusion of iron, chromium, and manganese cations from the matrix and prevent the internal diffusion of oxygen which leads to the lower oxidation rate and the better scale adherence.     

Substrate Depletion Analysis and Modeling of the High Temperature Oxidation of Binary Alloys

To predict concentration changes in binary alloy due to preferential oxidation during high temperature oxidation, a model based on the flux balance of the oxidized element at the moving oxide/alloy interface has been developed. These changes are driven by the alloy oxidation rate k _c . The model is numerically solved considering different initial alloy concentration profiles and a possible diffusion enhancement effect close to the alloy surface. After validating the model by comparison with Wagner’s analytical solution for Ni_30wt%–Pt alloy oxidised at 850 °C, we show that the effects of an initial alloy depletion due to the presence of a passive oxide layer are cancelled after very short oxidation times. We also show that a diffusion acceleration due to work-hardening in the vicinity of the alloy surface induces an inflexion point in the depletion profile.     

Modelling of the oxide scale formation on Fe-Cr steel during exposure in liquid lead-bismuth eutectic in the 450–600 °C temperature range

Previous studies showed that the oxidation of T91 (Fe-9Cr martensitic steel) in liquid Pb-Bi eutectic leads to the formation of a duplex oxide layer containing an inner Fe_2.3Cr_0.7O₄ spinel layer and an upper magnetite layer. The magnetite layer is easily removed by the Pb-Bi flow when the oxygen concentration is low and the flow velocity is high. This phenomenon is not currently understood. The magnetite layer growth rate is limited by the iron diffusion in the oxide layer lattice. The Fe-Cr spinel layer grows in the nanometric cavities formed at the Fe-Cr spinel /T91 interface by the outwards diffusion of iron. Due to this mechanism the growth rate of the Fe-Cr spinel layer is linked to that of magnetite. A modelling of this mechanism is presented. The modelling is in agreement with the experimental data in the case of a high oxygen concentration. However, the calculated oxide scale thicknesses are systematically lower than the experimental values in the case of a low oxygen concentration when the iron diffusion only occurs via interstitials in the oxide scale. Consequently, the estimation of the iron diffusion coefficient, when diffusion occurs via interstitials, is not reliable. To have a better estimation of this iron diffusion coefficient in the Fe-Cr spinel, when diffusion occurs via interstitials, a fit is done using experimental data coming from the European DEMETRA project. Although this evaluation is only based on a fit on the experimental data, it permits to estimate the oxide layer growth kinetics in case of the formation of the described duplex oxide layer, for each oxygen concentration (leading to a vacancy and/or an interstitial diffusion), each temperature between 450 and 620 °C and each hydrodynamic flow. This model shows that the hydrodynamic flow affects the corrosion rate only by the removal of the upper magnetite layer leading to an increase of the oxygen concentration at the spinel/magnetite interface. The oxidation mechanism is thus neither changed by the Pb-Bi flow nor by the oxygen concentration. However, the oxygen concentration modifies the iron diffusion process in the oxide lattice.     

The development of silicon-containing oxides during the oxidation of iron-chromium-base alloys

The influence of silicon on the oxidation of Fe-14% Cr and Fe-28% Cr has been studied at high temperature, with particular emphasis on the development and nature of the healing SiO₂ layer. In general, silicon is a less effective addition than aluminium to these alloys in improving oxidation resistance because SiO₂ grows at a lower rate than α-Al₂O₃. Hence, silicon is a less successful oxygen secondary getter and development of a complete healing layer of SiO₂ is less rapid than that of α-Al₂O₃ on a corresponding aluminium-containing alloy. Nonetheless, the addition of only 1% Si to Fe-28% Cr causes a marked reduction in the overall oxidation rate, particularly by facilitating development of the Cr₂O₃ scale. Precipitates of SiO₂ form at the alloy/scale interface. These grow inwards and laterally until they eventually link up to establish a continuous healing layer at the interface after several hundred hours exposure at 1000°C. Similar features are observed for Fe-14% Cr-3% Si but the healing SiO₂ layer develops after a much shorter time for Fe-14% Cr-10% Si, due to the high silicon availability. In every case, the healing layer has been shown to be amorphous SiO₂. Although this phase is very protective during isothermal oxidation, it is a site of weakness during cooling and scale spallation is very extensive from specimens where the SiO₂ is continuous, with failure occurring cohesively within that layer. Ion implantation of silicon into Fe-14% Cr and Fe-28% Cr gives a reduced oxidation rate due to facilitation of a more rapid establishment of a Cr₂O₃ scale. Similar implantation of yttrium into the ternary alloys assists in development of the silicon-containing oxide layer, possibly associated with an influence on the nucleation of the oxide precipitates in the early stages of exposure.     

Isotope exchange studies of oxidation mechanisms in nickel-base superalloys using FIB-SIMS techniques

The thermo-mechanical stability of the oxide layer that grows between the metallic bond coat and the ceramic top coat on superalloy turbine blades determines the lifetime of the system. Understanding the mechanisms of oxidation of the bond coat that is applied to the superalloy is key to improving the performance of the thermal barrier coatings. FIB-SIMS (Focused Ion Beam-Secondary Ion Mass Spectrometry) techniques in conjunction with tracer diffusion experiments represent a powerful tool in characterizing this oxide layer. This paper presents the results of oxidation studies on single crystal nickel-base superalloys/bond coat systems. In these studies, a two-stage oxidation experiment is used where ¹⁸O₂ serves as a tracer element during the second stage oxidation. The aluminium oxide grown in ¹⁶O₂ during the first stage oxidation represents an initial layer of oxide. Mass spectra collected by FIB-SIMS reveal the counter mass transportation of inward oxygen diffusion and outward diffusion of aluminium. New oxide formation during the second stage oxidation under an ¹⁸O₂ enriched environment is observed at both the gas/oxide interface as well as the oxide/superalloy interface. FIB-SIMS scanning enables high-resolution isotope maps, in particular ¹⁸O⁻, to be captured. These confirm the existence of new oxide forming at the oxide/superalloy interface with clear indications of short circuit diffusion paths through the existing oxide. These data allow the diffusion mechanisms for different superalloy/bond coat systems to be identified and contrasted, allowing the role of alloying additions to be elucidated.     

Mechanism of Iron Oxide Scale Reduction in 5%H<Subscript>2</Subscript>–N<Subscript>2</Subscript> Gas at 650–900 °C

The reduction behaviour of the oxide scale on hot-rolled, low-carbon steel strip in 5%H₂–N₂ gas at 650–900 °C was studied. In general, the reduction rate of the oxide scale at the centre location was more rapid than that at the near-edge location. In both cases, the reduction rates at 650 °C were extremely low and the rates increased with increased temperature, reaching their maxima at 850 °C. Arrhenius plot of the rate constant derived from the early parabolic stage revealed that the reduction mechanism at 650–750 °C differed from that at 750–850 °C, with the former being oxygen diffusion in α-Fe and the latter most likely iron diffusion in wustite. In all cases, a thin iron layer formed on the scale surface within a very short time and then the thickness of this layer remained essentially unchanged, while the scale layer was gradually reduced via outward migration of the inner wustite–steel interface, as a result of inward iron diffusion through the wustite layer to that interface. More rapid oxygen diffusion through the thin surface iron layer than the oxygen supply rate through interface reaction was believed to result in a lower oxygen potential at the outer iron–wustite interface, thus providing a driving force for iron to diffuse through the wustite layer. The inner wustite–iron interface became undulating initially; then with the rapid advance of some protruding sections, some parts of the wustite layer were reduced through first, and finally the remaining wustite islands were reduced to complete the reduction process. Porosities were generated when wustite islands were reduced due to localized volume shrinkage. Higher oxygen concentrations in the scales of the near-edge samples were believed to be responsible for their slower reduction rates than those of the centre location samples.     

Chromium-depleted zones and the oxidation process in stainless steels

It is suggested that, during the oxidation of stainless steels, matter is conserved at the oxide-metal interface by the creation of a dynamic balance between the chromium diffusion fluxes in the alloy and in the oxide. It is shown that the rate of oxidation is insensitive to alloy composition so that a necessary consequence is that the rate-controlling process is always diffusion through the oxide. In addition, the interfacial concentration of chromium remains invariant with time at a value higher than that in thermodynamic equilibrium with the oxide. Some of the predictions made with regard to the depth and kinetics of growth of chromium-depleted zones within the alloy have been checked experimentally inoxidation tests in CO₂ at 1123° K on a 20Cr-25Ni stainless steel containing a dispersion of TiN particles. It is concluded that the matter-conservation hypothesis is valid for this material.     

The Initial Stages in the Oxidation of TiAl

The oxidation behaviour of titanium aluminides containing 36 wt.-% Al (Ti36Al) and 35 wt.-% Al plus 5 wt.-% Nb (Ti35Al5Nb) has been investigated by electron microscopic methods with emphasis on transmission electron microscopy (TEM). The oxidation experiments were carried out at 800 to 1000°C in laboratory air for 0.5 h to 4 h. In addition thermogravimetric measurements were made. It has been shown that the shortterm oxidation of TiAl can be divided into two stages. In stage I the preferred formation of aluminium oxide leads to an aluminium depletion of the metal subsurface zone and the subsequent formation of titanium nitrides which enhances the oxidation rate. After consumption of the depletion layer a repeated cycle of aluminium oxide formation, subsequent local depletion of the metal subsurface zone in Al and consumption of the Ti-rich metal phase by nitride formation is observed leading to linear oxidation behaviour (stage II). In the niobium containing alloy the dissolution of alumina in titania is decreased and thus the formation of aluminium oxide at the metal/oxide interface is favoured. By electron diffraction it has been found that the aluminium oxide formed at the metal/oxide interface most probably is an aluminium oxynitride Al₂₇O₃₉N. The aluminium depleted metal phase has been analyzed to consist of α₂-Ti₃Al and a new cubic phase with a composition between of α₂-Ti₃Al and γ-TiAl.     

Effects of the Presence of Water Vapour on the Oxidation Behaviour of Low Carbon–Low Silicon Steel in 1 %O2–N2 at 1,173 and 1,273 K

The effect of water vapour on the oxidation behaviour of a low carbon and low silicon steel in 1 %O₂–N₂ at 1,173 K (2.5 and 17.2 %H₂O) and 1,273 K (17.2 %H₂O) was examined. In most cases, the oxidation kinetics were linear initially and turned to parabolic after 15 and 30 min, respectively for 1,173 and 1,273 K. However, for oxidation in 1 %O₂–N₂ at 1,273 K for longer than 60 min, the apparent parabolic rate decreased with increased oxidation time. The presence of water vapour resulted in increased oxidation rates, both in the linear and parabolic stages. Improved scale–steel interface adherence was believed to be responsible for the increased oxidation rates. Fast growth of certain oxide grains formed in 1 %O₂–N₂ at 1,273 K and bridging of the fronts of these fast growing grains to form a continuous layer resulted in the formation of a line of porosities inside the scale, thus reducing the total cross-sectional area for outward iron diffusion, and hence reducing the apparent oxidation rate. The presence of 17.2 %H₂O in 1 %O₂–N₂ prevented formation of porosities in the scale.     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

Influence of water vapour on high temperature oxidation of steels used in petroleum refinery heaters

The oxidation behaviours of three different steels used in the construction of petroleum refinery heaters were investigated by thermogravimetric analysis (TGA) technique. C‐5, P‐11 and P‐22 steel samples were tested in two different environments: air and CO₂ + 2H₂O + 7.52N₂, a gas composition which simulates the combustion products of natural gas, at 450 and 500 °C. P‐22 steel had the best oxidation resistance at both temperatures in air. In CO₂ + 2H₂O + 7.52N₂ environment, the oxidations of all the steels were accelerated and C‐5 exhibited better oxidation resistance than P‐22 and P‐11. Analyses of oxidation products by optical microscopy, SEM‐EDX and XRD were carried out to correlate TGA results to oxide composition and morphology. The lower oxidation rate of P‐22 in air was explained with reference to the formation of a protective Cr‐containing oxide layer between the steel and the iron oxide scale. The higher oxidation rates of chromium containing steels in CO₂ + 2H₂O + 7.52N₂ environment were attributed to the depletion of protective Cr‐containing oxide scale, which was deduced from the lower Cr content of this layer than that formed in air oxidation, as a result of probably faster oxidation of Cr even inside the steel. Therefore, the oxidation mechanisms of Fe? Cr alloys with intermediate Cr contents at higher temperatures could also be valid for steels with low chromium contents such as P‐22 (2.25%) even at 450 and 500 °C.     

Effect of Element Diffusion Through Metallic Networks During Oxidation of Type 321 Stainless Steel

A detailed study was conducted on localized oxidation on Type 321 stainless steel (321ss) using synchrotron x-ray nanobeam analysis along with Raman microscopy. The results showed the presence of metallic nanonetworks in the oxide scales, which plays an important role in the continued oxidation of the alloy at 750 °C. A mechanism is proposed to explain the rapid oxidation of 321ss in complex gaseous environments at elevated temperature. Neutral metal atoms could diffuse outward, and carbon atoms could diffuse inward through the metallic nanonetworks in oxide layers. Alternately, diffusion tunnels can dramatically affect the phase composition of the oxide scales. Since the diffusion rate of neutral metal and carbon atoms through the metallic nanonetworks can be much faster than the diffusion of cations through Cr₂O₃, the metallic nanonetwork provides a path through the protective Cr₂O₃ layer for the rapid outward diffusion of metallic chromium and iron atoms to the nonprotective spinel layer. This diffusion process affects the solid-state reaction near the alloy-oxide boundary, and a dense Cr₂O₃ protective layer does not form. The classic stable structure of the oxide scales, with a dense Cr₂O₃ layer at the bottom, is damaged by the rapid diffusion through the tunnel at the reaction front, resulting in locally accelerated oxidation. This process can subsequently lead to “breakaway” oxidation and catastrophic failure of the alloy.     

Oxidation Behavior of Ni–Cr–Fe-Based Alloys: Effect of Alloy Microstructure and Silicon Content

The oxidation behavior of Ni–Cr–Fe-based alloys in a low oxygen partial pressure atmosphere (H₂–H₂O) was investigated in terms of the effect of alloy microstructure and their silicon content. It was found that the formation and growth kinetics of the oxide scale are rather sensitive to the alloy microstructure and their corresponding Si contents. Oxide ridges were found to form in areas with eutectic structure, while a thin and homogeneous oxide scale formed on austenite matrix. The thicknesses of the oxide ridges and the oxide layer on the austenite matrix were dependent of their corresponding Si contents. The austenite/carbide phase boundaries in eutectic structure can offer fast diffusion paths for metal outward diffusion, which leads to the formation of ridge-like oxide features. The continuous SiO₂ sub-layer formed at the oxide scale/metal interface on the austenitic matrix acted as an effective diffusion barrier to metal outward diffusion, resulting in rather thin and uniform oxide scales.     

A study of the Cr-depleted surface layers formed on four Cr-Ni steels during oxidation in steam at 600° C and 800° C

Three commercial steels with 18% Cr-11% Ni, 19% Cr-25% Ni, and 19% Cr-35% Ni and a laboratory-made 20% Cr-35% Ni steel have been oxidized in steam at 600°C and 800°C. Two types of oxides are distinguished: a thin layer rich in Cr₂O₃ and patches of thick oxide of Fe, Ni, Cr spinels. The Cr concentration profile in the steels under the first type has been determined by microprobe analysis of ground and pickled or annealed specimens oxidized for various times. The variation of the Cr concentration in the alloy at the alloy-oxide interface with surface treatment, time, and temperature is discussed. It is concluded that one reason for the more extensive nodular growth on pickled or annealed specimens is their greater Cr depletion after short time oxidation. The interdiffusion coefficients in the Cr-depleted zones have been determined on ground and annealed specimens oxidized at 800°C. The acceleration of the diffusion rate due to grinding is clearly demonstrated. A fair agreement with diffusion data from diffusion couples is found.     

Breakaway oxidation of Fe10%Cr and Fe20%Cr at temperatures up to 600°C

In the oxidation of Fe10%Cr and Fe20%Cr in air at 600°C, nucleation is followed by a period when growth rates are extremely low, and this ends in a transition to a fast linear rate. Microstructural and micro-analytical studies of the thin protective oxide (2–50nm) indicate that it is a duplex layer of Fe₂O₃ and M₃O₄ of rather variable thickness and composition. The heterogeneity of the oxide layer results from differences in diffusion rates in both the oxide and alloy. The transition to higher oxidation rates arises because of a build-up of compressive growth stresses which disrupts the protective layer. Protection cannot be re-established because during failure the alloy close to the alloy-oxide interface is deformed, causing growth of a non-coherent chromium-containing oxide.     

Effect of Nb Addition on Oxidation Mechanisms of High Cr Ferritic Steel in Ar–H2–H2O

High chromium ferritic steels are being used as construction materials for interconnects in solid oxide electrolysis cells (SOEC). Addition of niobium in the range of a few tenths of a percent is suitable for increasing the high-temperature creep strength of this type of ferritic steel. In the present work, the high-temperature isothermal oxidation behavior of a niobium containing ferritic steel at 800 °C was investigated in Ar–4%H₂–4%H₂O gas simulating the service environment in an SOEC (cathode side) and compared with that of a Nb-free counterpart alloy. Gravimetric data were correlated with the results from microstructural analyses using, among others, scanning and transmission electron microscopy as well as glow discharge optical emission spectroscopy. Atom probe tomography was used for obtaining atomic-scale insight into the segregation processes in external oxides and their interfaces. The oxidation rate was substantially higher for the Nb-containing than for the Nb-free alloy. Both alloys formed double-layered oxide scales consisting of inner chromia and outer MnCr₂O₄ spinel. Additionally, a thin layer of rutile-type Nb(Ti,Cr)O₂ oxide of 200–300 nm thickness was observed at the scale–alloy interface in the Nb-containing steel. Nb addition to the alloy led to its segregation at chromia grain boundaries which affected the diffusion of Cr and other solute species such as Ti, Mn and Si.

     

Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900�C1000?��C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 °C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates ??-Ni₃Nb and/or (Ni, Mo)₆C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of ??-Ni₃Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The ??-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo)₆C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.