The corrosion behavior of Fe-Nb-Al alloys in H2/H2O/H2S atmospheres

The corrosion behavior of eight Fe-Nb-Al ternary alloys was studied over the temperature range 700–980°C in H₂/H₂O/H₂S atmospheres. The corrosion kinetics followed the parabolic rate law for all alloys at all temperatures. The corrosion rates were reduced with increasing Nb content for Fe-x Nb -3Al alloys, the most pronounced reduction occurred as the Nb content increased from 30 to 40 wt.%. The corrosion rate of Fe-30Nb decreased by six orders of magnitude at 700°C and by five orders of magnitude at 800°C or above by the addition of 10 wt.% aluminum. The scales formed on low-Al alloys (3 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with Al dissolved near the outer-/inner-layer interface) and an inner complex layer of Fe_xNb₂S₄(FeNb₂S₄ or FeNb₃S₆), FeS, Nb₃S₄ (only detected for Nb contents of 30 wt.% or higher) and uncorroded Fe₂Nb. No oxides were detected on the low-Al alloys after corrosion at any temperature. Platinum markers were found to be located at the interface between the inner and outer scales for the low-Al alloys, suggesting that the outer scale grew by the outward transport of cations (Fe and Al) and the inner scale grew by the inward transport of sulfur. The scales formed on high-Al alloys (5 wt.% Al) were complex, consisting primarily of Nb₃S₄, Al₂O₃ and (Fe, Al)_xNb₂S₄, and minor amounts of (Fe, Al)S and uncorroded intermetallics (FeAl and Fe₂Nb). The formation of Nb₃S₄ and Al₂O₃ blocked the transport of iron through the inner scale, resulting in the significant reduction of the corrosion rates.     

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.     

The effect of Al on the corrosion behavior of Ni-Mo in a H2/H2O/H2S gas mixture

The corrosion behavior of seven Ni-Mo-Al alloys was investigated over the temperature range of 600–950°C in a mixed-gas atmosphere of H ₂/H ₂O/H ₂ S. The parabolic law was followed at low temperatures, while linear kinetics were generally observed at higher temperatures. At a fixed Mo content, the transition from parabolic to linear kinetics shifted to higher temperature with increasing Al concentration. Double-layered scales generally formed on alloys having a low Al content, consisting of an outer layer of nickel sulfide and a complex inner scale. The thickness of the outer scale and the inner scale decreased as the Al content increased. The outer scale became porous and discontinuous with increasing Al content and temperature. Al ₂ O ₃ was detected in the scales of all alloys corroded at higher temperatures ( 800°C), even though the amount of Al ₂ O ₃ was very small in some cases. The decrease in corrosion rate with increasing Al content may be attributed to the formation of Al ₂ O ₃,Al _0.55 Mo ₂ S ₄,and Al ₂ S ₃ in the inner scale.     

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700–900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H₂/H₂S/H₂O mixture with varying sulfur partial pressures of 10^–7–10^–5 atm. and oxygen partial pressures of 10^–24–10^–2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and –10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe_1–xS) and an inner complex scale of FeAl₂S₄ and FeS. Alloys having intermediate Al contents (Fe-18 and –28 at.% Al) formed scales that consisted of mostly iron sulfide and Al₂O₃ as well as minor a amount of FeAl₂S₄. The amount of Al₂O₃ increased with increasing Al content. The Fe 40 at.% Al formed only Al₂O₃ at 700°C, while most Al₂O₃ and some FeS were detected at T800°C. The formation of Al₂O₃ was responsible for the reduction of the corrosion rates.     

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.     

Effect of Al on high-temperature corrosion of Co-15a/oNb alloys in H2-H2O-H2S environments

Co–15 at.% Nb alloys containing up to 15 at.% Al were corroded in gaseous H₂–H₂O–H₂S mixtures over the temperature range of 600–900°C. The corrosion kinetics followed the parabolic rate law at all temperatures. Corrosion resistance improved with increasing Al content except at 900°C. Duplex scales formed on alloys consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. A small amount of Al₂O₃ was found only on Co–15Nb–15Al. Contrary to what formed in Co–Nb binary alloys, neither NbS₂ nor NbO₂ were found in the inner layer of all alloys, but Nb₃S₄ did form. The absence of NbS₂ and NbO₂ is due to the formation of stable Al₂O₃ and Al₂S₃ that effectively blocked the inward diffusion of oxygen and sulfur, respectively, and to the reduction of activity of Nb by Al additions in the alloys. Intercalation of ions in the empty hexagonal channels of Nb₃S₄ is associated with the blockage of the transport of cobalt. An unknown phase (possibly Al_0.5NbS₂) was detected. Alloys corroded at 900°C were abnormally fast and formed a scale containing CoNb₃S₆ and Co. Pt markers were found at the interface between the inner and outer layers.     

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

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H₂/H₂S/H₂O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of -Nb and Nb₃Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO₂ layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO₂ and minor amounts of Nb₂O₅, NbS2, and -Al₂O₃, while the scales formed on Nb-25Al consisted of mostly Nb₂O₅ and some -Al₂O₃. The scales formed on Nb-75Al consisted of mostly -Al₂O₃ and Nb₃S₄ atT 900°C, and mostly -Al₂O₃ , Nb₃S₄ and some AlNbO₄ at 1000°C. The formation of -Al₂O₃ and Nb₃S₄ resulted in a significant reduction of the corrosion rates.     

High-temperature corrosion of Co-15Mo alloys containing ternary additions of Al in mixed-gas environments

The corrosion behavior of Co-15 at.% Mo alloys containing up to 20at.% Al in gaseous H ₂ -H ₂ O-H ₂ S mixtures was studied over the temperature range of 600–900°C. The corrosion kinetics of all alloys followed the parabolic rate law over the temperature range of interest. Corrosion resistance increased with increasing aluminum content. Complex scales formed on the alloys, consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. Al ₂ O ₃ formed only at high temperatures in alloys having aluminum additions of 15at.% or more. The absence of Al ₂ O ₃ in some cases is due to the small volume fraction of the intermetallic phase CoAl in the alloys and the nature of the slow growth rate of Al ₂ O ₃.Improvement in corrosion resistance is attributed to the presence of a ternary sulfide, Al _0.55 Mo ₂ S ₄,and Al ₂ O ₃ in the inner layer.     

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.     

The high-temperature corrosion of Fe-Nb alloys in a H2/H2O/H2S gas mixture

The corrosion of Fe-Nb alloys containing up to 40 wt.% Nb has been studied over the temperature range 600–980°C in a mixed gas of constant composition having sulfur and oxygen pressures ranging from 10^–8 to 10^–4 atm. and from 10^–27 to 10^–18 atm., respectively. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe₂Nb. The scales formed on the Fe-Nb alloys were duplex, consisting of an outer layer of iron sulfide (FeS) and an inner complex layer of Fe_xNbS₂(FeNb₂S₄ or FeNb₃S₆), FeS and unreacted Fe₂Nb. No oxides were detected at any temperature. The addition of Nb reduced the corrosion rate. The corrosion kinetics of Fe-Nb alloys followed the parabolic rate law, regardless of alloy composition and temperature. Platinum markers, attached to the original alloy surfaces, were always located at 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.     

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 corrosion behavior of Ni-Nb alloys in a mixed gas of H2-H2O-H2S

The corrosion behavior of Ni-Nb alloys containing up to 40 wt.% Nb was studied over the temperature range of 550–800°C in a mixed H₂/H₂O/H₂S gas. The scales formed on all alloys were multilayered. The outer scale was single-phase Ni₃S₂, while the structure and constitution of the inner scale depended on alloy composition and reaction conditions. Internal oxidation has been found in Ni-20Nb and Ni-30Nb, external oxidation has been observed on Ni-34Nb. Platinum markers were located at the interface between the outer scale and inner scale. The decrease in corrosion rate with increasing Nb content may be attributed to the presence of increasing amounts of Ni-Nb double sulfides as well as to the presence of Nb₂O₅ in the inner region of the scale.     

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.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

Corrosion Behavior of Y–Al Alloys in a H2/H2S/H2O Gas Mixture at 800–950°C

The corrosion behavior of pure Y and two Y–Al alloys containing 5 and 10 wt.% Al was studied over the temperature range 800–950°C in a H₂/H₂S/H₂O gas mixture. Both alloys had the two-phase structure of Y+Y₂Al. With the exception of Y–10Al, for which a kinetics inversion was observed between 800°C and higher temperatures (T 850°C), the parabolic rate constants generally increased with increasing temperature, but decreased with increasing Al content. The scales formed on pure Y and the Y–Al alloys were single but heterophasic, consisting of mostly Y₂O₃ and minor Y₂O₂S. XRD results showed no evidence of Al₂O₃ and pure sulfides. The formation of Y₂O₃ and Y₂O₂S on Y–10Al at 800°C resulted in a subsurface phase transformation from Y+Y₂Al to YAl₂ and broke the structural integrity of the scale, being responsible for the fast corrosion rate.     

The Corrosion Behavior of Sulfidation-Resistant Fe–Mo–Al Alloys in H2/H2S Atmospheres at 900°C

The corrosion behavior of Fe–22Mo–10Al (a/o, atom %),Fe–20.5Mo–15.7Al, and Fe–10Mo–19Al was examined inflowing H₂/H₂S gases of 4 Pa sulfur partial pressureat 900°C. Al₂O₃ was stable on all the alloys inthe atmospheres investigated. Fe–22Mo–10Al andFe–20.5Mo–15.7Al reacted slowly, following the parabolic ratelaw. Multilayered reaction products were formed on these alloys and it isuncertain which layer(s) provided the protection. Fe–10Mo–19Alreacted even more slowly, exhibiting two-stage parabolic kinetics. Duringthe early stage of this alloy's reaction, a preferential reaction zone,consisting of an oxide mixture, possibly Al₂O₃+FeAl₂O₄,and nonreacting Fe₃Mo₂, provided the protection. Duringthe later reaction stage, the formation of a continuous, externalAl₂O₃ layer further decreased the alloy reaction rate.     

Oxide scale adhesion and impurity segregation at the scale/metal interface

The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al₂O₃ and Cr₂O₃ scales formed between 900°C and 1100°C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.     

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.