The corrosion behavior of Ni-Mo alloys in a H2/H2O/H2S gas mixture

The corrosion behavior of Ni-Mo alloys containing up to 40 wt.% Mo was studied over the temperature range of 550–800C in a mixed gas of H₂/H₂O/ H₂S. The scales formed on all alloys contained only sulfides and were doublelayered. The outer scale was single-phase Ni₃S₂. Depending on the alloy composition and reaction conditions, the inner scale was: (1) a mixture of MoS₂ plus Ni₃S₂ with/without Ni, (2) MoS₂, or (3) MoS₂ plus intermetallic particles and/or double sulfide Ni_2.5Mo₆S_6.7. Neither internal oxidation nor internal sulfidation were observed at lower temperatures. Internal sulfidation was however observed at higher temperature when the scale apparently melted. The parabolic law was generally obeyed for the most concentrated alloys. For the two more-dilute alloys the kinetics were mostly linear. A decrease in the corrosion rate occurred with increasing Mo content of the alloy and may be attributed to the presence of increasing volume fractions of MoS₂ and/or of a double Ni-Mo sulfide in the inner region of the scale. For the two most concentrated alloys this may also be due to the presence of a number of particles of the unsulfidized intermetallic compound, which is Ni₃Mo for Ni-30Mo, but NiMo for Ni-40Mo.     

High-temperature sulfidation behavior of Ni-Nb alloys

The sulfidation properties of Ni-Nb alloys containing additions of niobium up to 40 wt.% have been studied at atm over the temperature range 550-700 °C. The sulfidation reactions followed the parabolic rate law; the sulfidation rates decreased with increasing amounts of niobium. An Arrhenius plot of the rate constants gave activation energies of 25.0+3.5 kcal/ mole. The scales formed on Ni-Nb alloys were multilayered, generally consisting of an outer layer of nickel sulfide ( NiS_1+x and Ni₃S₂) and an inner complex layer of NiNb₃S₆ plus NbS₂. The position of the original metal surface was notedy platinum-wire marker experiments to be the interface between the inner andouter layers. The location of the marker indicates that the outer layer, generally greater in thickness than the inner layer, grew by outward diffusion of the nickel cations, and the inner layer formed probably by the inward diffusion of sulfur. Neither preferential sulfidation nor internal sulfidation was observed. The development of the scale structures from the transient stage to steady state was also studied.     

Sulfidation behavior of Ni-Cr-Mo alloys at 700°C

The effect of molybdenum additions 5, 10, 15, and 20 wt. %, on the sulfidation behavior of Ni-20Cr, and the effect of chromium additions, 5, 10, 15, and 20 wt.%, on the sulfidation of Ni-20Mo were studied in pure sulfur vapor at 700°C. In general, the alloys followed a linear or near-linear rate law, the sulfidation rate of Ni-20Mo being slightly less than that of Ni-20Cr. The alloys having the lowest ternary addition, e.g., Ni-Cr-5Mo and Ni-20Mo-5Cr. exhibited the most rapid reaction rates. The highest alloying additions of 20 wt.% had no appreciable benefit on reaction rates. Scale structures were complex but generally consisted of several layers. The outer layer was always NiS_1.03, although both binaries formed Ni₃S₂ within the NiS_1.03. An inner layer of Cr₃S₄ existed in which there was considerable dissolved molybdenum. A thin, intermediate layer of Cr₂S₃ generally formed between the Cr₃S₄ and the outer nickel sulfide. An innermost layer of MoS₂ formed on all alloys containing more than 10 wt. % Mo, and a second phase of Mo₂S₃ formed within the MoS₂ on Ni-20Mo. Although the scales changed with alloy composition, no significant changes in reaction rate were observed. Notable differences in both scale structure and reaction kinetics between this study and previous studies were apparent. The differences and possible reaction mechanisms are discussed.     

Sulfidation kinetics and mechanism of some Ni-Mo-Al and Ni-Al alloys over the temperature range 600–800°C

The sulfidation behavior of five Ni-Mo-Al ternary alloys and two Ni-Al binary alloys was studied over the temperature range 600–800°C in sulfur vapor of 10^–2 atm. The effect of sulfur pressure was also investigated at and10^–4 atm. using two Ni-Mo-Al alloys. The sulfidation of all Ni-Mo-Al andNi-Al alloys followed the parabolic rate law. The sulfidation rate decreasedwith increasing Al content for a given Mo content for Ni-Mo-Al alloys. Twobinary alloys, Ni-13.5Al and Ni-31Al, sulfidized at comparable rates toNi-30Mo-7.5Al, which has excellent sulfidation resistance. The activationenergies for ternary alloys range from 44.8–50.8 kcal/mol, whereas those forNi-13.5Al and Ni-31Al are 41.5 and 39.1 kcal/mol, respectively. Complexscales formed on all Ni-Mo-Al alloys, consisting of an outer layer of nickelsulfide and an inner layer of MoS₂, A1₂S₃, and Al_0.55Mo₂S₄. Sulfide scalesformed on Ni-Al alloys were bilayered, consisting of an outer layer of nickelsulfide and an inner layer of A1₂S₃. The low sulfidation rate of the ternaryalloys was attributed to the combined presence of both A1₂S₃ and Al_0.55Mo₂S₄.The sulfidation kinetics of two Ni-Mo-Al alloys are independent of sulfurpressure, suggesting that the growth of the inner layer was the dominant process.     

Effect of some ternary additions on the sulfidation of Ni-Mo alloys

Five ternary additions, Cr, Ti, Mn, V, and Al were studied at equi-atomicpercent levels (17 a/o) for their effect on the sulfidation behavior of Ni-19a/o Mo (28–30 w/o) over the range of 600–800°C in 0.01 atm S₂. Al was by far the most effective addition. A linear decrease in log k_p vs. Al content was observed up to 7.5w/o Al, beyond which no further change was observed. All alloys followed the parabolic rate law. Arrhenius plots gave activation energies of 36.9–41.2 Kcal/mol for alloys containing Ti, Cr, Mn, and V, whereas the activation energies for Al-containing alloys were 47.2 Kcal/mol, indicating that a different diffusion process was involved. Complex scales were formed on all alloys, consisting of an outer layer of Nis_1+x and complex inner layers which depended upon alloy composition. Two alloys, those with Cr and Mn, formed intermediate layers of Cr₂S₃ and MnS, respectively, but these layers had little effect on the kinetics. MoS₂ was a constituent of the inner scales except for the alloys with Al. A ternary sulfide, Al_0.55Mo₂S₄ and Al₂S₃ were observed. The presence of the mixed sulfide was always associated with the low sulfidation rates. The formation of MoS₂ on alloys results in a different, less-protective behavior than for MoS₂ formed on pure Mo. This effect is due to the intercalation of Ni into MoS₂ in octahedral positions between the weakly bonded layers of covalently bonded sheets of trigonal prisms. The size of Al⁺³ is too small to be intercalated, and thus MoS₂ is destabilized by Al.     

A comprehensive investigation of the sulfidation behavior of binary Co-Mo alloys

The sulfidation behavior of Co-Mo alloys containing up to 40 wt.% Mo was studied over the temperature range 600–900°C in both 10^–2 and 10^–4 atm. sulfur vapor. All of the alloys were two-phase, with the alloys containing up to 30Mo consisting of Co₃Mo plus solid-solution Co, and the Co-40Mo alloy consisting of the two intermetallic compounds, Co₃Mo and Co₇Mo₆. The sulfide scales which formed were duplex, with an outer layer of cobalt sulfide and a complex, heterophasic inner layer whose phases were both composition- and temperature-dependent. The parabolic rate constant for the sulfidation kinetics decreased with increasing Mo content at all temperatures investigated. Three activation energies, all different from that of pure Co, were observed. Furthermore, Co-30Mo exhibited a kinetics inversion between 800 and 850°C. This inversion was largely the result of the formation of an innermost layer of Co_1.62Mo₆S₈ at the high temperatures. Specifically, the presence of this sulfide in the inner scale caused a significant decrease in the growth rate of the outer layer of cobalt sulfide. In fact, formation of a more compact, innermost layer of Co_1.62Mo₆S₈ at 900°C compared to that at 850°C resulted in a negative activation energy for the growth of the cobalt sulfide in this temperature range. The variation in the activation energies was due to both the duplex nature of the scales which formed and the phase constitution of the inner scale. A simple model has been developed to explain the changes in the activation energies. At 800°C the sulfidation rate of the Co-Mo alloys was essentially the same at the two sulfur pressures studied. The predominant phase in the inner layer of Co-10Mo and Co-20Mo was CoMoS₃, while for Co-30Mo and Co-40Mo it was MoS₂. However, in the case of the latter alloys, Co_1.62Mo₃S₄ formed in the region of the alloy/scale interface at temperatures 850°C and above. Although the MoS₂, which had formed on Co-40Mo, appeared to be a continuous layer, it was in fact found to be relatively nonprotective. Platinummarker experiments revealed the position of the original metal surface to be the interface between the inner and outer scales.     

High-temperature sulfidation of Fe-30Mo alloys containing ternary additions of Al

Fe-30Mo alloys containing up to 9.1 wt% Al were sulfidized at 0.01 atm sulfur vapor over the temperature range of 700–900°C. The sulfidation kinetics followed the parabolic rate law for all alloys at all temperatures. For alloys containing small and intermediate amounts of Al (<4.8 wt.%), a duplex sulfide scale formed. The outer layers of the scales were found to be relatively compact FeS in all cases; whereas the inner layers were composed of the layered compound MoS ₂ (intercalated with iron), the Chevrel compound Fe _x Mo ₆ S ₈,a spinel double sulfide Al _x Mo ₂ S ₄,depending on the Al content of the alloy and the sulfidation temperature. Extremely thin scales were found on the alloys with higher Al contents. Accordingly, extremely slow sulfidation rates were observed—even slower than the sulfidation rate of pure Mo. The transition of the sulfidation kinetics from a high-rate active mode to a low-rate passive mode requires both a critical Al content in the alloy and a critical Mo content. Because of the two-phase nature of the alloys, the latter requirement implies a critical volume fraction of the intermetallic second-phase in the alloy, which has been known as the multiphase effect. Interestingly, the multiphase effect in these alloys was also a function of the Al content in the alloys.     

Kinetics and mechanism of the sulfidation of Fe-Mo alloys

Iron-molybdenum alloys containing up to 40 wt.% molybdenum were exposed to sulfur vapor at a partial pressure of 0.01 atm at temperatures of 600–900°C. Sulfidation kinetics were measured over periods of up to 8 hr using a quartz-spring thermogravimetric method. The sulfidation kinetics of all alloys studied obeyed the parabolic rate law. The sulfidation rate of iron was found to be reduced by factors of 60 at 900°C and 120 at 600°C by the addition of 40 wt.% molybdenum. Duplex sulfide scales formed on all alloys at all temperatures, the scales consisting of an inner layer of mostly MoS₂ and an outer layer of FeS. Platinum markers were located at the interface between the outer and inner scales, showing that outward iron diffusion and inward sulfur diffusion through the inner layer occurred. The improved sulfidation resistance was attributed to the formation of the MoS₂, which acted as a partially protective barrier to the diffusion of the reacting species. Sulfidation activation energies were found to range from 24.3 to 28.5 kcal mole for the alloys compared to 20.6 kcal/mole, for pure iron. The rate-controlling step was outward iron diffusion through the outer iron sulfide layer.     

Effect of Nb on the high-temperature sulfidation behavior of cobalt

The sulfidation behavior of Co-Nb alloys containing up to 30wt.% Nb was studied in sulfur vapor at a pressure of 0.01 atm in the temperature range of 600–700°C. Increasing niobium content decreased the sulfidation rate, following the parabolic rate law. An activation energy of 25.6 kcal/mole was obtained for Co-10Nb, Co-20Nb, and Co-25Nb, while a value of 20.5 kcal/mole was found for Co-30Nb. All were two-phase alloys, consisting of solid solution -Co and the intermetallic compound, NbCo₃. The two-phase alloys formed a rather thick outer layer of cobalt sulfides and a heterophasic inner layer that was complex. The inner layer always contained the mixed sulfide CoNb₂S₄ which, depending on the alloy composition, coexisted with cobalt sulfide, NbS₂, and / or NbCo₃ particles. Short-time sulfidations showed that the solid solution initially sulfidized rapidly to form nodules of cobalt sulfide, whereas the NbCo₃ phase formed a thin protective layer of NbS₂. The nodules grew laterally until they coalesced into the continuous, outer thick layer, while the NbS₂ completely or partially reacted with the cobalt sulfide to form CoNb₂S₄. Platinum markers were always found at the interface between the inner and outer scales, the location of the original metal surface.     

The corrosion of Fe-Mo alloys in H2/H2O/H2S atmospheres

The corrosion of Fe-Mo alloys containing up to 40 wt.% Mo was studied over the temperature range 600–980C in a H₂/H₂O/H₂S mixture having a sulfur pressure of 10^–5 atm. and an oxygen pressure of 10^–20 atm. at 850C. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe₃Mo₂. The scales formed on Fe-Mo alloys were bilayered, consisting of an outer layer of iron sulfide (FeS) and of a complex inner layer whose composition and microstructure were a function of the reaction temperature and of the Mo content of the alloys. No oxides formed under any conditions. The corrosion kinetics followed the parabolic rate law at all temperatures. The addition of Mo caused only a slight decrease of the corrosion rate. Platinum markers were always located at the interface between the inner and outer scales, indicating that outer scale growth was primarily due to outward diffusion of iron, while the inner scale growth had a contribution from inward diffusion of sulfur.     

The corrosion behavior of Co-Mo alloys in H2-H2O-H2S environments

The corrosion behavior of Co alloyed with up to 40 wt.% Mo alloys was studied in H₂-H₂O-H₂S gas mixtures over the temperature range between 600C and 900C. The parabolic rate constants for corrosion decreased with increasing amounts of Mo. The compositions of all gas atmospheres fall in the sulfide(s stability region of the ternary M-O-S phase diagrams at all temperatures investigated. All the corrosion scales were composed of sulfides, while no oxide was detected. The sulfide scales formed were duplex at all temperatures except at 900C. The outer layer consisted primarily of cobalt sulfide, while the inner layer was complex and heterophasic, the phases formed being highly composition dependent. MoS₂ predominated in the inner layer for all alloys. However, a metallic Mo layer was formed in the innermost layer of Co-40 Mo. Activation energies were different for all alloys, increasing with increasing Mo content. Identical kinetics were observed for Co-30Mo corroded at 700–800C. A Chevrel-phase Co_1.62Mo₆S₈ was present in scales formed on the samples exhibiting the temperature-independent kinetics. A possible model in which Co_1.62Mo₆S₈ forms preferentially in H₂-containing mixed gas is suggested. Alloys corroded at 900C formed a lamellar-structure scale which contained Co and CoMo₂S₄ layers perpendicular to the alloy surface. A eutectoid decomposition of an unknown Co-Mo sulfide may be responsible for the presence of the lamellar structure.     

The sulfidation behavior of Co-Mo alloys containing various ternary additions

The sulfidation behavior of Co-Mo-X alloys, where X is Al, Cr, Mn, or Ti, has been studied over the temperature range 600 or 700°C to 900°C in 10^–2 atm. sulfur vapor to determine the effectiveness of the various ternary elements at reducing the sulfidation rate relative to Co-Mo alloys. For comparative purposes, each ternary alloy contained a constant atomic proportion (i.e., 55Co, 20Mo, and 25X). All of the alloys were multiphase, and sulfidized to form complex, multilayered scales. The scales usually consisted of an outer layer of cobalt sulfide, an intermediate layer that contained primarily the ternaryelement sulfide, and an inner layer which was heterophasic. Usually, each phase within the multiphase alloy sulfidized independently of one another. In the region of the alloy/scale interface there was often a narrow band of fine porosity (transitional band) together with fine precipitates that separated the inner sulfide from the base alloy. It was found that Al and Cr improved the sulfidation resistance of the Co-Mo binary alloy, whereas Mn had the opposite effect. The Ti-containing alloy underwent a mixed sulfidation/oxidation process, so that its kinetics were inapplicable. Aluminum was found to exert the most beneficial effect. The sulfidation behavior of Co-Mo-Al alloys containing a range of Al concentrations was studied at both 700 and 900°C. It was found that for Al to be effective, a sufficient amount of the spinel, Al_0.55Mo₂S₄, had to form within the inner portion of the scale.     

Sulfidation properties of nickel-20 wt.% molybdenum alloy in hydrogen-hydrogen sulfide atmospheres at 700°C

The sulfidation kinetics and morphological development of the reaction products for a Ni-20wt.% Mo alloy exposed at 700° C to H₂-H₂S atm at sulfur pressures in the range 1×10^–11 to 2×10^–2 atm are reported. At Ps₂5×10^–11 atm, the reaction product was Mo₂S₃ which grew as an external scale by parabolic kinetics. For 1×10^–1024×10^–10 atm, simultaneous internal precipitation and external growth of MoS₂ occurred by linear kinetics. An external duplex scale was formed at sulfur pressures 2×10^–822×10^–2 atm in which the inner layer was a two-phase mixture of MoS₂ and nickel sulfide, and the outer layer contained solid nickel sulfides and a liquid Ni-Mo sulfide phase. Catastrophic linear kinetics occurred under these latter condition.     

Effect of yttrium on the sulfidation behavior of Ni-Cr-Al alloys at 700°C

The sulfidation of Ni-10Cr-5Al, Ni-20Cr-5Al, and Ni-50Cr-5Al, and of the same alloys containing 1% Y, was studied in 0.1 atm sulfur vapor at 700°C. The sulfidation process followed linear kinetics for all the alloys except Ni-50Cr-5Al-1Y, and possibly Ni-50Cr-5Al, which followed the parabolic law. The reaction rates decreased with increasing chromium content in alloys without yttrium, and the addition of yttrium reduced the rates by at least a factor of two for the alloys containing 10 and 20% Cr and by an order of magnitude for Ni-50Cr-5Al. Alloys containing 10 and 20% Cr (with and without yttrium) formed duplex scales consisting of an outer layer of NiS_1.03 and an inner lamellar layer of a very fine mixture of Cr₂S₃ and A1₂O₃ in a matrix of NiS_1.03. The two alloys containing 50% Cr formed only a compact layer of Cr₂S₃, which was brittle and spalled during cooling. The lamellae in the duplex scales were parallel to the specimen surface and bent around corners. The lamellae were thicker than those on Ni-Al binary alloys. The lamellae were also thicker in scales on the 20% Cr alloy than on the 10% Cr alloy. The presence of yttrium refined the lamellae and increased the lamellae density near the scale/metal interface in the 10% alloy, but in the 20% Cr alloy the lammellae were thicker and more closely spaced. Platinum markers were found in the inner portion of the exterior NiS_1.03 layer close to the lamellar zone. A counter-current diffusion mechanism is proposed involving outward cation diffusion and inward sulfur diffusion, although diffusion was not rate controlling for alloys containing 10 and 20% Cr. Auger analysis of scales formed on Ni-50Cr-1Y showed an even distribution of yttrium throughout the layer of Cr₂S₃, suggesting that some yttrium dissolved in the sulfide. The reduced sulfidation rate of samples containing yttrium is explained by the possible dissolution of yttrium as a donor. The presence of Y⁴⁺ would then decrease the concentration of interstitial chromium ions in the N-type layer of Cr₂S₃, which would decrease the reaction rate.     

Sulfidation Behavior of Inconel 738 Superalloy at 500–900°C

The high-temperature sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500–900°C in pure sulfur vapor over the range 10²–10⁴ Pa. The sulfidation kinetics followed the parabolic rate law in all cases. The sulfidation rates increased with increasing temperature and sulfur pressure. The scales formed were bilayered and temperature-dependent. At T700°C, the outer scale consisted of mostly NiS (with dissolved Co) and minor (CoS₂ and NiCo₂S₄, while the inner layer was a heterophasic mixture of NiS, NiCo₂S₄, and minor amounts of Al₂S₃ and chromium sulfide (Cr₂S₃/Cr₃S₄). At T750°C, the outer scale consisted of mostly Ni₃S₂ (with dissolved Co) and minor amounts of Co₃S₄ and Cr₂S₃/Cr₃S₄, while the inner layer was a complex, heterophasic mixture of Ni₃S₂, Cr₂S₃/Cr₃S₄, CoCr₂S₄, and minor Al₂S₃. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations and the inner scale grew by the inward transport of sulfur. The formation of Al₂S₃ and Cr₂S₃/Cr₃S₄ partly blocked the transport of cations through the inner scale and consequently reduced the sulfidation rates as compared to pure nickel.     

The corrosion behavior of Ni-Al alloys and Ni-Nb-Al alloys in a H2/H2O/H2S gas mixture

The corrosion behavior of two Ni-Al alloys and four Ni-Nb-Al alloys was studied over the temperature range of 600° C to 1000° C in a mixed-gas of H₂/H₂O/H₂S. The parabolic law was generally followed, although linear kinetics were also observed. Multiple-stage kinetics were observed for the Ni-Al alloys. Generally, the scales formed on Ni-13.5Al and Ni-Nb-Al alloys were multilayered, with an outer layer of nickel sulfide with or without pure Ni particles and a complex inner scale. The outer scale became porous and discontinuous with increasing temperature. Very thin scales formed on Ni-31Al. The reduction in corrosion rate with increasing Al content is ascribed to the formation of Al₂O₃ and Al₂S₃ in the scale. Platinum markers were found at the interface between the outer and inner scales.     

High-temperature sulfidation of Fe-Nb alloys

The sulfidation behavior of Fe-Nb alloys containing up to 30 w/o Nb was studied over the range of 600–900°C in 0.01 aim. S₂ vapor. All alloys were two-phase, consisting of an Fe-rich solid solution and Fe₂Nb, and followed the parabolic rate law at all temperatures. Scales consisted of two layers-an outer layer of FeS and an inner, complex layer which contained some FeS, FeNb₂S₄ (possibly some FeNb₃S₆), NbS₂, and intermetallic particles which were either completely or only partially sulfidized. Platinum markers were located always at the interface between the two layers, which corresponded to the original metal surface. Activation energies were 18±3 kcal/mol in close agreement with the 19.8 reported for pure iron. The sulfidation rate decreased markedly with increasing Nb content of the alloys. The decrease is attributed to increasing amounts of Fe₂Nb with increasing Nb, the net effect being that the diffusion path for outward iron diffusion through the inner layer is reduced as the Nb content increases. An analysis of the structure of NbS₂ reveals that it is easily intercalated with Fe between loosely bonded layers of S-Nb-S. The S-Nb-S layers are covalently bonded which results in very low diffusivities of either S or Nb in pure NbS₂. Although intercalated Fe tends to change the Van der Waal's type bonding between layers to more ionic or covalent, Fe diffuses readily between the layers in NbS₂. Intercalation of Fe also increases the concentration of sulfur defects in NbS₂, which in turn increases the diffusivity of sulfur. Nb was observed to be immobile. Thus, it is thought that either outward iron diffusion or inward sulfur diffusion in the inner layer is the rate-controlling step, in spite of the close agreement of activation energies with that of the sulfidation of pure iron.     

Studies of the kinetics of nickel sulfidation in H2S-H2 mixtures in the temperature range 450–600°C

The reaction between pure nickel and H₂S-H₂ mixtures containing 1–65% H₂S has been studied over the temperature range 450–600°C. The sulfidation of nickel in the temperature range 560–600°C has been found to follow a linear rate law at low concentrations of H₂S and a parabolic rate law at higher concentrations (10% and 65% H₂S); X-ray examination of the scale formed on the metal showed it to be almost entirely -Ni₃S₂. On the basis of the kinetics and marker studies it can be concluded that the sulfide scale on nickel is formed by the outward transport of the metal and the inward transport of sulfur. In the temperature range 450–500°C the sulfidation of nickel follows a parabolic rate law. In mixtures containing 10% H₂S the scale formed contains voids, the occurrence of which is connected with formation of Ni₇S₆. It has also been shown that the rate of transport through the Ni₃S₂ layer has an essential influence on the formation of a continuous layer of Ni₇S₆. Marker studies have shown that both nickel and sulfur appear to be mobile in -Ni₃S₂.     

The high-temperature corrosion behavior of Fe-Mo-Al alloys in H2/H2O/H2S mixed-gas environments

The corrosion behavior of 11 Fe-Mo-Al ternary alloys was studied over the temperature range 700–980°C in H₂/H₂O/H₂S mixed-gas environments. With the exception of Fe-10Mo-7Al, for which breakaway kinetics were observed at higher temperatures, all alloys followed the parabolic rate law, despite two-stage kinetics which were observed in some cases. A kinetics inversion was observed for alloys containing 7 wt.% Al between 700–800°C. The corrosion rates of Fe-20Mo and Fe-30Mo were found to be reduced by five orders of magnitude at all temperatures by the addition of 9.1 or higher wt.% aluminum. The scales formed on low-Al alloys (5 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with some dissolved Al) and a complex inner of Al_0.55Mo₂S₄, FeMo₂S₄, Fe_1.25Mo₆S_7.7, FeS, and uncorroded FeAl and Fe₃Mo₂. Platinum markers were always located at the interface between the inner and outer scales for the low-Al alloys, indicating that outer-scale growth was due mainly to outward diffusion of cations (Fe and Al), while the inner scale was formed primarily by the inward flux of sulfur anions. Alloys having intermediate Al contents (7 wt.%) formed scales that consisted of FeS and Al₂O₃. The amount of Al₂O₃ increased with increasing reaction temperature. The high-Al-content alloys (9.1 and 10 wt.%) formed only Al₂O₃ which was responsible for the reduction of the corrosion rates.     

Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al₂S₃ or Cr₂S₃ layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al³⁺ ions into NbS₂ and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al₂S₃ layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr₂O₃ layer in the oxide scales is formed on Cr-rich Cr-Nb and Cr-Ta alloys due to outward diffusion of Cr³⁺ ions. In contrast, continuous alumina layer cannot be formed on the Al-Nb and Al-Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.