The Oxidation of cobalt-tantalum base alloys containing carbon

The oxidation of cobalt-tantalum carbon alloys, containing 10 and 15 wt.% Ta and carbon in the range 0–1 wt%, was carried out in oxygen and air at atmospheric pressure at 900, 1000 and 1100°C. The alloys oxidised according to the parabolic rate law with activation energy of about 38 Kcal/mole. In general, the addition of tantalum decreases the oxidation rates, in comparison with cobalt and with the same mass of chromium added to cobalt. Again, the presence of carbon in the Co-Ta alloys decreases its oxidation rates in comparison with carbon-free alloys. The scales formed on Co-Ta and Co-Ta-C alloys consist mainly of an outer layer of cobalt oxide, CoO, and an inner porous layer of mixture of oxides: cobalt oxide; CoO, tantalum oxide; Ta₂O₅, and solid solution of these two oxides; CoTaO₄ at all temperatures in the range of 900°-1100°C. The binary Co ?10% Ta and Co ?15% Ta show an internal oxidation along the internal phase, increasing of alloy tantalum content increases the density of the internal phase. The presence of carbon in the ternary Co-Ta-C alloys has little effect and there is no apparent preferential penetration along the tantalum carbide network. In contrast to carbide present in Co-Cr-C alloys, where these carbides were preferentially attacked, the outer scale was disrupted, due to the formation of carbon gaseous oxides.     

Corrosion behaviour of the new gas turbine alloy 2100 GT in hot gases and combustion products

Not only excellent high temperature mechanical properties are needed to establish a new gas turbine alloy, but also a very good oxidation behaviour, together with good resistance to so‐called “hot corrosion”. This paper describes experimental studies on the corrosion behaviour in hot gases and combustion products of a new Ni‐Cr‐Ta alloy 2100 GT in comparison to the commercially established alloys 230, C‐263 and 617. Alloy 2100 GT is a newly developed cobalt, tungsten and molybdenum free Ni‐base superalloy of Krupp VDM. It contains as major alloying elements 25 wt.‐% chromium, 8 wt.‐% tantalum, 2.4–3 wt.‐% aluminium and 0.2–0.3 wt.‐% carbon. High temperature strength is achieved by the addition of tantalum, resulting in significantly increased solid solution strengthening, carbide hardening due to the formation of primary precipitated tantalum carbides, and γ′‐precipitation hardening by aluminium and tantalum. The isothermal oxidation tests showed that the parabolic rate constant of alloy 2100 GT is similar to that of alumina‐forming alloys. This is achieved by the remarkably high aluminium content for a wrought alloy. Additions of yttrium improve the spalling resistance under thermal cycling by the formation of very thin and tightly adherent oxide layers. No deleterious effect caused by the addition of tantalum could be found. In the cyclic oxidation tests performed at temperatures between 700°C and 1200°C alloy 2100 GT showed the lowest mass change of all the alloys investigated. Na₂SO₄ has been found to be a dominant component of alkali salt deposits on gas turbine components at elevated temperatures. Combustion gases contain SO₂ because of the impure nature of the fuel. To investigate the hot corrosion behaviour of alloy 2100 GT, tests were performed with salt deposits containing 0.1 mol Na₂SO₄ and a test gas comprising air and 0.1% SO₂. Test temperatures were 600°C, 700°C, 850°C and 950°C. Alloy 2100 GT exhibited the best performance at all test temperatures. It was the only alloy which did not suffer any fluxing of the oxide layer and only slight internal sulphidation was observed.     

The sulphidation properties of titanium-, manganese- and niobium-bearing Fe---25Cr alloys in H2---H2S mixtures at 800°C

An investigation of high-temperature sulphidation properties of 4 wt% Ti-, 9 wt% Mn- and 8 wt% Nb-bearing Fe---25Cr alloys has been carried out in H₂-H₂S mixtures of sulphur partial pressure in the range of 10⁻³ < pS₂ < I Pa at 800°C. On the whole, the sulphidation kinetics of all alloys obeyed the parabolic law after the initial period of reaction. In some cases, fluctuations in the weight gain-time curves arose due to cracking of sulphide scales. Compared with the weight gain of Fe---25Cr alloy, the additions of alloying elements improved the sulphidation resistance. The effect of 8 wt% Nb was relatively major, but the effects of 4 wt% Ti and 9 wt% Mn were minimal. The addition of these elements did not change the surface morphologies or improve the structure of sulphide scales. Combining the sulphidation kinetics and the analysis of sulphide scale structure, the sulphidation mechanisms of these Fe---25Cr-base alloys have been proposed and the effects of titanium, manganese and niobium have been discussed.     

Sulphidation of Cu and Cu-Ni alloys in H2S/argon mixture

The sulphidation of cold-worked Cu and annealed Cu and Cu-Ni alloys containing 10 and 50wt.%Ni, has been studied in dry H₂S/argon (1 : 20) mixture at 500°C for the cold-worked, and in the range 320–500°C for the annealed samples. The reaction kinetics were linear in all tests. The rate of sulphidation decreased with increasing amount of cold work. The scale on annealed sample showed considerable blister formation and grain growth which were appreciably less on samples with increasing cold work. There was no blister formation on alloys. From combined techniques of X-ray powder and diffractometry, the sulphide scale on Cu was found to be mainly Cu₂S, while that on the alloys a thick scale of Ni₃S₂ was associated with a thin layer of Cu₂S. Of the two alloys, Cu-10%Ni sulphidized faster than Cu-50%Ni at all temperatures. The activation energy of sulphidation of annealed Cu was about 74 kJ/mole compared with 40 kJ/mole for the alloy. The dissociation of the adsorbed gas at the gas/scale interface is considered to be the rate-controlling process for the linear kinetics.     

Effect of chromium content on the sulphidation behaviour of CoCr alloys in H2/H2S mixtures (Ps2 = 15 torr)

Pure cobalt and CoCr binary alloys containing up to 25wt% Cr, all sulphidize rapidly according to a parabolic rate law at 800 and 1000°C in H₂/H₂S atmospheres of an effective sulphur partial pressure of 15 torr (H₂-65%H₂S at 1000°C and H₂-85%H₂S at 800°C). There is a maximum in the parabolic rate constant at 10wt%Cr at 1000°C, and at 5wt%Cr at 800°C; the latter, however, is not so pronounced.At 800°C, the scales on the alloys consist of an inner layer of cobalt—chromium sulphide solid solution and an outer layer of CoS_1+x containing no chromium; this latter sulphide decomposes on cooling from the reaction temperature to Co₉S₈ and Co₃O₄. The scales on the 1 and 5wt%Cr alloys also contain an intermediate layer of Co₉S₈. The scales on the Co-17 and 25wt%Cr alloys at 1000°C are similar to those at 800°C. However, on the more dilute alloys, a molten cobalt—chromium sulphide solution forms, which may be covered with a thin crust of solid CoS_{1 + x}.The rate-controlling mechanism appears to be the diffusion of chromium and cobalt ions through the cobalt—chromium sulphide solid solution, where this forms, the outer scale layer having little influence on the overall sulphidation rate.     

The effect of phosphorus addition on the corrosion behavior of ARC-MELTED Ni10TaP alloys in 12 M HCl

The corrosion resistance of arc-melted Ni10TaP alloys containing 0, 10 and 20 at% phosphorus in 12 M HCl solution at 30 °C was investigated. The alloys containing 0 and 10 at% phosphorus suffer severe corrosion. The addition of 20 at% phosphorus to crystalline Ni10Ta alloy results in a three-orders-of-magnitude decrease in the corrosion rate. The open circuit potentials of the Ni10Ta alloys containing 0 and 10 at% phosphorus stay almost constant in the active region of nickel, while the open circuit potential of the Ni10Ta20P alloy increases almost linearly in the initial 2 h. The Ni10Ta alloy consists of intermetallic Ni₈Ta and immersion in 12 M HCl results in faceting dissolution. Ni10Ta10P alloy is composed of major Ni₈Ta and Ni₃P phases and minor Ni₂Ta and Ni₂P phases. Immersion of Ni10Ta10P alloy leads to preferential dissolution of the Ni₈Ta phase and to continuous thickening of the corrosion product film consisting mostly of tantalum as cations. Ni 10Ta20P alloy consists of Ni₂Ta, Ni₃P, Ni₂P and NiP phases. Immersion of Ni10Ta20P alloy gives rise to initial increase in elemental phosphorus on the surface as a result of selective dissolution of nickel and selective oxidation of tantalum. The formation of elemental phosphorus with a high cathodic activity is responsible for the initial ennoblement of the open circuit potential and for the formation of the passive film in which tantalum is highly concentrated. The higher corrosion resistance of Ni10Ta20P alloy than Ni10Ta10P alloy is attributable to the formation of the Ni₂Ta phase with a higher tantalum content than the Ni₈Ta phase which is the readily corroded major intermetallic phase in the Ni10Ta10P alloy.     

The corrosion of a Co–15 wt% Ce alloy in mixed oxidizing-sulfidizing gases at 600–800°C

The corrosion behavior of a Co–Ce alloy containing approximately 15 wt% Ce has been studied at 600–800°C in several H₂–H₂S–CO₂ mixtures, providing sulfur pressures of 10⁻⁸ atm at 600, 700 and 800°C and of 10⁻⁷ atm at 800°C, and oxygen pressures of 10⁻²⁴ atm at 600°C and 10⁻²⁰ atm at 700 and 800°C. At 600°C, the alloy corrodes more slowly than pure cobalt but more rapidly than pure cerium while, at 700°C, it corrodes at about the same rate as pure cerium, but much faster than pure cobalt. At 800°C under 10⁻⁸ atm S₂, i.e. a value below the stability of the cobalt sulfides, the alloy corrodes rather slowly but, under 10⁻⁷ atm S₂, the rate is very high, although slightly lower than that of pure cobalt. The scaling kinetics are generally intermediate between linear and parabolic but are sometimes irregular. The corrosion of this alloy produces multilayered scales, containing an outermost layer of almost pure cobalt sulfide, an intermediate complex layer composed of a mixture of compounds of the two metals and, finally, an innermost region of internal attack of cerium by both oxygen and sulfur. Cerium is not able to diffuse outwards and remains in the alloy consumption region. In the intermediate region cobalt sulfide forms a continuous network which allows the growth of the external CoS_y layer, although at rates that are reduced with respect to those of pure cobalt. Thus, a cerium content of 15 wt% is not sufficient to prevent or even to significantly reduce the sulfidation of the base metal. These results, as well as the details of the microstructure of the scales, are interpreted by taking into account the limited solubility of cerium in the base metal and the presence of an intermetallic compound, rich in cerium, in the alloy.     

On the mechanism of high-temperature sulphur corrosion of binary alloys

Taking into account the phase composition and morphology of the scales three groups of alloys may be distinguished. The first of them contains alloys forming monophase scales which constitute solid solutions of sulphides of both alloy components (AgCu type). The mte of corrosion of these alloys is a monotonous function of their composition, and the mechanism of scale formation depends on the geometrical configuration of the reaction system. The second group includes alloys the components of which form sulphides of limited mutual solubility or sulphides having spinel structures (FeCr type); the scale may then be a mono- or double phase one and the exact structure is a function of the concentration of the alloying element. The metals belonging to the third group form mutually immiscible sulphides (CuZn type). The scales form double layers, the external layer being made up by sulphides of the base metal, while the internal layer is a heterophase mixture of the sulphides of both alloy components. The kinetics and the mechanism of the corrosion of these alloys is largely independent from the atmosphere (elemental sulphur or H₂? H₂S mixture).     

Optimization of electrolyte concentration for surface modification of tantalum using plasma electrolytic nitridation

Plasma Electrolytic Nitriding (PEN) is a cathodic atmospheric plasma process which has shown a promising deposition of metal coatings that exhibits a significant adhesion to the substrate as well as high deposition rates. The structure of tantalum alloy, microhardness and corrosion resistance behavior after cathodic plasma electrolytic nitriding (PEN) in electrolyte containing urea and distilled water were investigated. An Optical microscope (OM), X-ray diffractometer and scanning electron microscopy (SEM) were used to characterize the phase composition of the modified layer and its surface morphology. The corrosion resistance properties of nitrided tantalum alloy are investigated. It was shown that various electrolytes provided metallic tantalum (Ta), TaN_0.43, TaN_0.1, Ta₄N, Ta₄N₅ and TaN phases and nitrogen solid solution in tantalum. The cathodic PEN with 78 wt% urea and 21.6 wt% distilled water had a microhardness of 1198.18 VHN, which was selected as the best sample in term of electrolyte composition.     

The Sulfidation of a Co-15 wt.% Ce Alloy in H2-H2S Mixtures at 600-800°C

The sulfidation behavior of a Co-Ce alloycontaining approximately 15 wt.% Ce has been studied at600-800°C in H₂-H₂S mixturesproviding a sulfur pressure of 10^-8 atm, butalso of 10^-7 atm at 800°C. At 600 and 700°C the alloy corrodes moreslowly than pure cobalt, but more rapidly than purecerium. At 800°C under 10^-8 atmS₂, which is below the stability of thecobalt sulfides, the alloy corrodes quite slowly, but under 10^-7 atmS₂ it corrodes very rapidly and practicallyat the same rate as pure cobalt. The sulfidationkinetics are generally irregular, except for a few casesof nearly parabolic behavior. The sulfidation of the alloy produces duplexscales, containing an outermost layer of practicallypure cobalt sulfide and an inner complex layer where thetwo components are simultaneously present. Cerium is not able to diffuse out of thealloy-consumption region, where it forms a ceriumsulfide mixed with cobalt sulfide and an innermostregion where cerium sulfide is mixed with cobalt metal.The cobalt sulfide forms a continuous network which allowsthe growth of the external CoS_y layer, eventhough at rates reduced with respect to pure cobalt.Thus, a cerium content of 15 wt.% is not sufficient toprevent the sulfidation of the base metal. Theseresults as well as the details of the microstructure ofthe scales grown on the alloy are interpreted by takinginto account the limited solubility of cerium in the base metal and the presence in the alloy ofan intermetallic compound rich in cerium.     

Factors influencing the performance of a Cr-Ni-Fe alloy exposed to sulfidising/oxidising/carburising environments at 800°C

Studies concerned with the corrosive degradation of engineering alloys exposed to multi-reactant gaseous environments containing sulphur, oxygen and carbon-bearing species have intensified during the past decade. In particular, the attack experienced by materials exposed in coal conversion plants has become one of the major areas of concern. These process atmospheres are generally characterised by low oxygen activities and high sulphur and carbon activities and sulphidation can therefore pose significant problems in the design and operation of such plant. This paper presents the results of studies carried out on a laboratory cast austenitic 25 Cr-35 Ni-Fe “model” alloy exposed to a range of H₂-CO-H₂O gas mixtures containing H₂S additions such that the oxygen and carbon activities remain constant (pO₂ = 10^?21 bar, a_c = 0.3) whilst the sulphur activity is varied systematically within the range, pS₂ = 10^?9 to 10^?7 bar at 800°C. Tests have also been carried out in an equivalent sulphur-free atmosphere for reference purposes. The influence of surface condition upon corrosion behaviour has received some attention by comparing the results obtained on ground samples with those obtained on work-free electropolished specimens. The kinetics and mechanisms by which this alloy corrodes have been established for exposure periods ranging from 5 min to 5000 h. These studies provide a clear understanding of the factors which can lead to catastrophic sulphidation attack in these types of environment and they also establish the critical dependence of long-term corrosion resistance upon the sulphur activity of the atmosphere.     

Effect of presulfidation on the oxidation behavior of Co-based alloys. Part I. Presulfidation at sulfur partial pressures above the dissociation pressure of cobalt sulfide

The effects of presulfidation in H₂-H₂S atmospheres of sulfur activity sufficient to form cobalt and chromium sulfides on the oxidation rates of Co-Cr binary alloys containing 0–25 wt.% Cr and Co-25 wt.% Cr alloys containing 0–2 wt.% C have been investigated. Presulfidation increases the oxidation rate, but the effect is not very dramatic. Carbon additions to the Co-25 wt.% Cr alloy progressively increase the oxidation rate, but not to as great an extent as a simple model based on the reduction of the chromium activity in the alloy. Sulfur released from the preformed sulfides by oxidation diffuses into the alloy precipitating fresh sulfides, there appears to be no outward diffusion of sulfur through the oxide scale. These internal sulfides have a liquid-like morphology in cobalt-base alloys when the oxidation is carried out at 1000°C, as compared to 800°C in corresponding nickel-base alloys. When the sulfide layer produced during the presulfidation is thin, so that oxidation destroys the continuous sulfide layer, the subsequent scale morphologies after oxidation exhibit many features similar to samples subjected to hot corrosion in environments containing sodium sulfate.     

The oxidation of Co-20% Cr base alloys containing Nb or Ta

Alloys based on Co-20% Cr containing approximately 4, 7 or 10wt% Nb or Ta were oxidized in oxygen and air at 900, 1000 and 100°C for times up to 350 h. In general, the addition of Nb accelerated the oxidation rate, although this effect was small at the lowest temperature. Futhermore, little evidence could be found for the development of a protective Cr₂O₂ layer. In contrast, the addition of Ta proved beneficial at all temperatures, promoting the development of protective oxide scales. A critical difference appeared to be the ability of the Ta-containing alloys to form a compound oxide, CrTaO₄, whereas no similar phase could be detected in the scales on the Nb-containing alloys. The Ta-rich oxides formed a layer adjacent to the metal, while a Cr-rich layer was formed outside it. It is possible that the Ta reduced the oxygen activity at the surface of the alloy, preventing the formation of cobalt-containing oxides which might otherwise disrupt the protective scale. Both elements have restricted solubility in Co-20% Cr, forming intermetallic compounds which oxidize internally. In the case of the Nb-containing alloys, a process occurs, during oxidation, which produces a change in the intermetallic deep in to the alloy, though there is no similar change in the Ta-containing alloys. This process has not yet been defined.     

The effectiveness of aluminizing in protecting iron-base alloys in sulphidizing gases at high temperature

Following earlier research which has shown that Al₂O₃ scales are more effective than Cr₂O₃ scales in protecting iron-nickel-base alloys against sulphur-containing gases, several commercial steels, 310 stainless, 314 stainless, and 321 stainless, and an experimental ferritic steel, FeCrAlHf, have been pack aluminized to develop aluminide coatings for applications in mixed-gas environments of high sulphur and low oxygen potential. Results are presented for long-term exposures to H₂/1.6% H₂O/1.1% H₂S at H₂S at 750°C and 1000°C under thermal-cycling conditions. Short-term tests in this environment at 750° C led to considerable sulphidation of the uncoated, Cr₂O₃-forming alloys. The aluminized alloys were much more resistant to sulphidation than the uncoated materials, with relatively little degradation being observed after 200 hr at 750°C. Even at 1000°C, with the exception of the ferritic steel, the coated systems showed reasonable degradation resistance for 500 hr. Eventual sulphidation resulted from back diffusion of aluminum into the substrate and dilution of the coating surface in this element until an Al₂O₃ scale was unable to reform and base metal sulphides could develop. The composition of the substrate was important in determining the rate of aluminium depletion from the coating, with interdiffusion being faster in ferrite-rich matrices than the austenite matrices. Thus, the higher nickel-containing alloys developed the most effective coating systems for use in such environments. The structures of the various aluminized systems are presented and their mechanisms of protection and breakdown are discussed and correlated with their performances under these high-temperature conditions.     

Sulphidation of someMCrAlYX-type alloys

The sulphidation behavior of a series ofMCrAlYX-type alloys, where M is Co or Co and Fe, and X is V, Nb, Mo, or W, added in combination at various levels, has been studied at 750°C in an atmosphere of 10^–1 Pa and 10^–18 Pa. Sulphidation kinetics followed a parabolic rate law for each alloy over prolonged periods of exposure (up to 240 hr). Inclusion of Mo in the alloy containing both V and Nb, and particularly the combined additions of Mo and W, greatly enhanced sulphidation resistance. Partial replacement of Co by Fe also provided superior sulphidation resistance. The scales formed on the Co-based alloys consisted of three subscale layers-an outer layer of Co₉S₈ and Co₃S₄, a mid-layer of Cr₃S₄ and an inner layer of chromium and refractory-metal sulphides. In the case of (Fe, Co)-based alloys, the scales were duplex in nature with an outer layer of Fe₃S₄, Co₉S₈ and Cr₃S₄ and a compact inner layer consisting of sulphides of chromium and refractory metals. For both categories of alloys, the inner layer of refractory-metal sulphides was decorated by an alumina film which inhibited the outward migration of the base elements.     

Metal dusting of ferritic Fe‐Al‐M‐C (M = Ti,V, Nb,Ta) alloys in CO‐H2‐H2O gas mixtures at 650 °C

Iron aluminides are known for their resistance to high temperature oxidation and sulphidation. Only little information is available about carburisation and metal dusting of Fe‐Al alloys. Metal dusting experiments with Fe‐15Al and Fe‐15Al‐2M‐1C alloys (in at.%) with M = Ti, V, Nb, or Ta were conducted at 650°C in CO‐H₂‐H₂O gas mixtures with the carbon activity a_c = 28. The kinetics of the carbon transfer was measured using thermogravimetric analysis (TGA). It is shown that the mass gain kinetics decreases by adding the alloying elements Nb, Ta, V, or Ti with C. Alloying with titanium and carbon leads to the most significant decreasing effect. The metallographic cross section observation showed a general metal wastage for Fe‐15Al, but local pitting for the Fe‐15Al‐2Nb‐1C and Fe‐15Al‐2Ta‐1C alloys. For the Fe‐15Al‐2V‐1C and Fe‐15Al‐2Ti‐1C alloys no significant attack was observed. Needle‐ or plate‐like Fe₃AlC_x precipitates were detected in the carburised samples. The existence of this ternary carbide with perovskite structure was predicted by thermodynamic calculations using the software Thermo‐Calc. The morphology of graphite on the surface was analysed by scanning electron microscopy (SEM). Mainly fine filaments with iron containing particles were detected. Cementite was detected in the coke layer by X‐ray diffraction analysis (XRD).     

A kinetic and morphological study of the sulphidation of Ni/23 Cr and Ni/33 Cr alloys in S2 vapour and hydrogen sulphide

The corrosion of Ni/23 Cr and Ni/33 Cr alloys has been studied in the presence of S₂ vapour and H₂S/H₂ mixtures between 600 and 900°C. The reaction progress curves Δm/q = f(t) are parabolic and the transformed ones (Δm/q)² = f(t) are linear. At low temperatures, the activation energy value is 92 < E < 260 kJ/mol depending on the nature of the sulphurizing agent and the composition of the alloy. The formation of two distinct sulphide layers is shown: the outer layer of N₃S₂, the underlying layer of Cr₂S₃. The formation of NiCr₂S₄ from both sulphides present is also observed. At higher temperatures, chromium sulphide alone develops and pure nickel blocks remain. The sulphidation rate does not obey the Arrhenius law.     

Hot Corrosion of Nickel-base Alloys containing Aluminium and Molybdenum

The effects of pre-sulphidation in H₂- 10%H₂S mixtures, where the sulphur activity is sufficient to form nickel and chromium sulphides, on the oxidation behaviour of a series of Ni-Cr, Ni-Cr-Al, Ni-Cr-Mo and Ni-Cr-AI-Mo alloys has been investigated. In binary Ni-15Cr alloys the presence of dispersed chromium sulphides in the alloy precludes the formation of a continuous Cr₂O₃ layer. The presence of molybdenum and to a lesser extent aluminium in the alloy increases the extent of attack after pre-sulphidation, particularly the depth of penetration of internal sulphides. Neither element is particularly deleterious under direct oxidation conditions, at least in flowing atmospheres. The technique of a brief pre-sulphidation treatment followed by oxidation is able to produce corrosion morphologies strikingly similar to those observed in practical hot corrosion conditions and it seems probable that the role of sulphur has been under- estimated in recent mechanistic investigations.     

Brass sulphidation by hydrogen sulphide and sulphur vapour at low pressure: Morphological and kinetic aspects

The effect of alloy composition on the sulphidation of brass in H₂S/H₂ mixtures and sulphur vapour is shown. In all cases the corrosion rate is determined by cationic diffusion through one of the sulphide layers formed. The different morphologies observed at the end of sulphidation are explained on the basis of zinc content of the alloys.     

Corrosion of Fe-(4.8, 9.2, 14.3) Wt%Al alloys at 700 and 800 °C in N2/H2O/H2S gases

Fe-(4.8, 9.2, 14.3)wt%Al alloys were corroded at 700 and 800 °C for up to 70 h in 1 atm of N₂/H₂O and N₂/H₂O/H₂S gases. Oxidation prevailed in N₂/H₂O gases. Fe-(4.8, 9.2)Al alloys formed a duplex scale that consisted of an outer iron oxide layer and an inner (Fe, Al, O)-mixed layer. The Fe-14.3Al alloy formed a thin layer consisting of α-Al₂O₃. Sulfidation dominated in N₂/H₂O/H₂S gases, resulting in rapid corrosion. Fe-(4.8, 9.2)Al alloys formed a duplex scale that consisted of an outer FeS layer and an inner (Fe, Al, S, O)-mixed layer. The high growth rate of FeS impeded the formation of a continuous, protective aluminium-rich oxide. The Fe-14.3Al alloy formed a thin layer consisting of α-Al₂O₃ that was incorporated with a bit of sulfur.