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 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.     

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.     

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 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 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 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.     

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 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 high-temperature corrosion behavior of Co-Nb alloys in mixed-gas atmospheres

The corrosion of Co-Nb alloys containing up to 30 wt.% Nb in H₂-H₂S-H₂O gas mixtures was studied over the temperature range of 600–800°C. The gas composition falls in the stability region of cobalt sulfide and Nb₂O₅ in the phase diagrams of the Co-O-S and Nb-O-S systems at all temperatures studied. Duplex scales, consisting of an outer layer of cobalt sulfide and a complex, heterophasic inner layer, were formed at all temperatures studied. In addition to cobalt sulfide and CoNb₃S₆, a small amount of NbO₂ was found in the inner layer. The reason for the formation of NbO₂ over that of Nb₂O₅ in the scale is that the outer sulfide scale lowers the oxygen activity within the scale into the NbO₂-stability region. Two-stage kinetics were observed for all alloys, including an initial irregular stage usually followed by a steady-state parabolic stage. The steady-state parabolic rate constants decreased with increasing amounts of Nb, except for Co-20Nb corroded at 700°C. Nearly identical kinetics were observed for Co-20Nb corroded at 600°C and 700°C. The presence of NbO₂ particles leads only to a limited decrease of the available cross-section area for the outward-diffusing metal ions. The activation energies for all alloys are similar and are in agreement with those obtained in a study of the sulfidation of the same alloys. The primary corrosion mechanism involves an outward Co transport.     

The high-temperature oxidation behavior of vanadium-aluminum alloys

The oxidation behavior in air of pure vanadium, V-30Al, V-30Al-10Cr, and V-30Al-10Ti (weight percent) was investigated over the temperature range of 700–1000° C. The oxidation of pure vanadium was characterized by linear kinetics due to the formation of liquid V₂O₅ which dripped from the sample. The oxidation behavior of the alloys was characterized by linear and parabolic kinetics which combined to give an overall time dependence of 0.6–0.8. An empirical relationship of the form: W/A=Bt + Ct^1/2 + D was found to fit the data well, with the linear contribution suspected to be from V₂O₅ formation for V-30Al and V-30Al-10Cr, and a semi-liquid mixture of V₂O₅ and Al₂O₃ for V-30Al-10Ti. The parabolic term is presumed related to the formation of a solid mixture of V₂O₅ and Al₂O₃ for V-30Al and V-30Al-10Cr, and TiO₂ for V-30Al-10TiThe addition of aluminum was found to reduce the oxidation rate of vanadium, but not to the extent predicted by the theory of competing oxide phases proposed by Wang, Gleeson, and Douglass. This was attributed to the formation of a liquid-oxide phase in the initial stages of exposure from which the alloys could not recover. Ternary additions of chromium and titanium were found to decrease the oxidation rate further, with chromium being the most effective. The oxide scales of the alloys were found to be highly porous at 900° C and 1000° C, due to the high vapor pressure of V₂O₅ above 800° C.     

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 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.     

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.     

The formation of aluminum oxide scales on high-temperature alloys

This paper is a brief review of the extensive literature relating to the formation of protective —Al₂O₃ scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.     

High-temperature corrosion of Cr2O3-forming alloys in CO-CO2-N2 atmospheres

The corrosion of Fe–28Cr, Ni–28Cr, Co–28Cr, and pure chromium in a number of gas atmospheres made up of CO–CO₂(–N₂) was studied at 900°C. In addition, chromium was reacted with H₂–H₂O–N₂, and Fe–28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H₂–H₂O–N₂ produced a single-phase of Cr₂O₃. In a CO–CO₂ mixture, a sublayer consisting of Cr₂O₃ and Cr₇C₃ was formed underneath an external Cr₂O₃ layer. Adding nitrogen to the CO–CO₂ mixture resulted in the formation of an additional single-phase layer of Cr₂N next to the metal substrate. Oxidizing the binary alloys in CO–CO₂–N₂ resulted in a single Cr₂O₃ scale on Fe–28Cr and Ni–28Cr, while oxide precipitation occurred below the outer-oxide scale on Co–28Cr, which is ascribed to the slow alloy interdiffusion and possibily high oxygen solubility of Co–Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe–28Cr, but varied between the different materials, decreasing in the order chromium >Fe–28Cr>Ni(Co)–28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr₂O₃ is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affects the permeability of the oxide.     

The corrosion behavior of two-phase Ni-Dy alloys in a eutectic (Li,K)2CO3 mixture at 650 °C

The corrosion behavior of binary Ni-Dy alloys containing 1, 3 and 5 wt.% Dy in a eutectic (0.62Li,0.38K)₂CO₃ mixture at 650 °C has been investigated. The alloys are two-phase composed of α-Ni solid solution and intermetallic compound Dy₂Ni₁₇. The experimental results indicate that the corrosion rate of the three Ni-Dy alloys is significantly lower than that of pure Ni, forming a scale mainly composed of NiO together with a small amount of dysprosium oxides. The weight gain of the alloys changes little with the content of dysprosium, while the thickness of the scale decreases with increasing content of dysprosium. The presence of Dy can promote the lithiation process of NiO during immersion in the melt. The corrosion mechanism of the alloys is also discussed.     

18O Tracer studies of Al2O3 scale formation on NiCrAl alloys

Diffusion processes in Al ₂ O ₃ scales formed on NiCrAl + Zr alloys were studied by the proton activation technique employing the ¹⁸ O isotope as a tracer. The ¹⁸ O profiles identified a zone of oxide penetration beneath the external scale. Both this subscale formation and the outer Al ₂ O ₃ scale thickness were shown by this technique to increase with Zr content in the alloy. Estimated k _p 's from scale thicknesses were in agreement with gravimetric measurements for various Zr levels. Alternate exposures in O and ¹⁸ O revealed that oxygen inward transport was the primary growth mechanism. A qualitative analysis of these ¹⁸ O profiles indicated that the oxygen transport was primarily via short-circuit paths, such as grain boundaries.     

Effect of Al2O3 reinforcement and precipitates on corrosion behaviour of 2618 and 6061 aluminium MMCs

ABSTRACT

The corrosion resistance features of two different Metal Matrix Composites based on 6061 and 2618 aluminium alloys reinforced by 20% Al₂O₃ particles by stir casting process, were studied in 3.5% NaCl and compared. The composites and their respective base alloys were characterised in terms of microstructure by optical and scanning electron microscopy and in terms of corrosion resistance by polarisation curves and electrochemical impedance. Results show how the presence of Al₂O₃ has different effects depending on matrix composition. In 6061 MMC, Al₂O₃ promotes the formation of Mg₂Si which has a cathodic behaviour altering the pitting susceptibility of the composite. In 2618 MMC, the high content of Cu induces a strong uniform attack. The subsequent increase in the Open Circuit Potential caused a pitting attack.     

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.