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  • 1
    Electronic Resource
    Electronic Resource
    A comprehensive investigation of the sulfidation behavior of binary Co-Mo alloys (1990)
    Springer
    Oxidation of metals 33 (1990), S. 425-455 
    Details
    ISSN: 1573-4889
    Keywords: sulfidation ; duplex scale ; kinetics inversion ; MoS2 ; intercalation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract 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 Co1.62Mo6S8 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 Co1.62Mo6S8 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, Co1.62Mo3S4 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00666808
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  • 2
    Electronic Resource
    Electronic Resource
    Oxygen tracer study of the high-temperature oxidation of pure and impure cobalt (1989)
    Springer
    Oxidation of metals 32 (1989), S. 379-390 
    Details
    ISSN: 1573-4889
    Keywords: Oxidation ; cobalt ; duplex scales ; oxygen tracer ; SIMS
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Oxygen tracer experiments were used to examine the role of oxygen movement in the development of CoO scales at 1273 K and 1373 K. Samples were oxidized in various sequences of16O2 and18O2, and the resulting isotope distributions imaged in polished sections of scales using a Camec a SIMS. Scales that were simple (i.e., plane parallel and composed of large, pore-free columnar grains) grew without a significant contribution by movement of oxygen. However, oxygen circulation within the scale was essential to the growth of duplex scales that developed after extended periods of oxidation. The relatively coarsegrained inner oxide on the purer Co grew without macroscopic separation of the scale from the metal. The oxygen for this inner scale came from the development of large pores in the columnar part of the scale. The fine-grained inner oxide on the impure Co had a crudely layered structure due to repeated sequences of scale separation followed by growth of porous oxide on the metal back to the main scale. The oxygen for these inner layers came in part from the development of large pores, but also from a more general dissociation of the base of the columnar scale. Scale failures, which occurred primarily at corners of specimens, did allow penetration of oxygen from the atmosphere to the inner scale. However, these failures sealed and were not essential for continued scale growth.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00665445
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  • 3
    Electronic Resource
    Electronic Resource
    The Long-Term, Cyclic-Oxidation Behavior of Selected Chromia-Forming Alloys (1998)
    Springer
    Oxidation of metals 49 (1998), S. 373-399 
    Details
    ISSN: 1573-4889
    Keywords: CYCLIC OXIDATION ; CHROMIA-FORMER ; INTERNAL OXIDATION ; SCALE SPALLATION
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Long-term, cyclic-oxidation testing in still airfor about 2 years (720 days) at 982°C and 1 year(360 days) at 1093, 1149, and 1204°C has beenconducted on the commercial, high-temperaturechromia-forming HR-120®, HR160®, and 230® alloys(all trademarks of Haynes International, Inc.). Eachthermal cycle consisted of 30 days at temperaturefollowed by about 4 hr at ambient. The resultsdemonstrated the significant effects of alloy composition on long-term,cyclic-oxidation resistance. Each of the alloysexhibited scale spallation; however, the manner by whichspallation occurred varied between the alloys. The 230 alloy, which contains 0.02 wt.% La, exhibitedpartial scale spallation, thus allowing for the easierformation of a protective or semiprotectiveCr2O3-rich scale during subsequentoxidation. The HR-160 alloy exhibited complete spallation owinglargely to its relatively high silicon content (2.75wt.%). However, the silicon was also beneficial inpromoting protective or semiprotective scale formationwhen the exposed alloy was subsequently oxidized.The HR-120 alloy showed the poorest cyclic-oxidationresistance, due in part to poor scale adhesion and thetendency of the iron in this alloy (33 wt.%) toeventually oxidize and result in the formation of aless-protective scale. All of the alloys underwentinternal attack in the form of internal oxidation andvoid formation. In most cases, the extent of internalattack was significantly greater than that of metalloss.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1018874206733
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  • 4
    Electronic Resource
    Electronic Resource
    Beneficial Effects of Rhenium Additions on the Cyclic-Oxidation Resistance of β-NiAl + α-Cr Alloys (1998)
    Springer
    Oxidation of metals 50 (1998), S. 399-429 
    Details
    ISSN: 1573-4889
    Keywords: CYCLIC-OXIDATION ; MULTIPHASE ALLOY ; RHENIUM ADDITION ; β-NiAl
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior ofβ-NiAl + α-Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in theβ phase and causing the interdendritic α-Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the α-Cr precipitates fromthe subsurface region of the alloy. The maintenance of α-Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl2O3-rich scale.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1018804824847
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  • 5
    Electronic Resource
    Electronic Resource
    Phenomenological treatment of multilayer growth (1989)
    Springer
    Oxidation of metals 31 (1989), S. 415-429 
    Details
    ISSN: 1573-4889
    Keywords: multiple oxide layers ; phase-boundary reactions ; parabolic kinetics ; apparent rate constants
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A phenomenological treatment of multilayer growth has been carried out with three basic assumptions: (1) steady-state conditions existing at the phase boundary between two phases within the multilayer, (2) a well-defined stoichiometric composition and structure existing for each layer, and (3) the same kinetics growth law followed by each layer. The supply of matter for the growth of each layer within the multilayered structure was considered to be from solid-state boundary reactions; using the concept of boundary-regions, general equations for the apparent growth rate of the region were derived. The treatment has been applied to linear kinetics, parabolic kinetics, and logarithmic kinetics. Using the equations obtained from this treatment, the effect of neighboring layers on the apparent growth rate of a given layer is shown.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00666465
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  • 6
    Electronic Resource
    Electronic Resource
    The sulfidation behavior of Co-Mo alloys containing various ternary additions (1990)
    Springer
    Oxidation of metals 34 (1990), S. 123-150 
    Details
    ISSN: 1573-4889
    Keywords: sulfidation ; multiphase alloy ; transitional band ; Al0.55Mo2S4
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract 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, Al0.55Mo2S4, had to form within the inner portion of the scale.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00664342
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  • 7
    Electronic Resource
    Electronic Resource
    Effect of Nb on the high-temperature sulfidation behavior of cobalt (1989)
    Springer
    Oxidation of metals 31 (1989), S. 209-236 
    Details
    ISSN: 1573-4889
    Keywords: sulfidation ; duplex scale ; heterophasic inner layer ; NbS2 ; CoNb2S4
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract 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 α-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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00846687
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  • 8
    Electronic Resource
    Electronic Resource
    An extension of Wagner's analysis of competing scale formation (1991)
    Springer
    Oxidation of metals 35 (1991), S. 317-332 
    Details
    ISSN: 1573-4889
    Keywords: Competitive scale growth ; exclusive-scale formation ; moving boundary
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Wagner's criterion to predict the minimum concentration of solute (N B(min) 0 )necessary for the exclusive formation of its scale has been modified to incorporate both thermodynamic and kinetics requirements. The analysis presented here involves a receding alloy/scale interface for which the composition of the alloy at this interface is governed by thermodynamic equilibrium between the two competing scale phases and the base alloy. This is a more rigorous analysis than that of Wagner's, who assumed the alloy/scale interface to be immobile and the solute composition at the interface to be zero. A comparison is made between values of N B(min) 0 found experimentally and those predicted by Wagner's and the present criterion for a number of alloy-gas systems. It is shown that the criterion developed in the present analysis can often provide a better approximation of the actual N B(min) 0 .The expressions derived from the present analysis have been extended also in a semiquantitative manner to provide a criterion for the minimum amount of solute in the alloy required for the transition from internal to external scale formation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00738292
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  • 9
    Electronic Resource
    Electronic Resource
    A diffusional analysis of the oxidation of binary multiphase alloys (1991)
    Springer
    Oxidation of metals 35 (1991), S. 333-348 
    Details
    ISSN: 1573-4889
    Keywords: oxidation ; multiphase alloys ; stereology ; diffusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A nonsteady-state diffusional analysis has been applied to the oxidation of binary multiphase alloys. In addition to the solubility of the solute element in the continuous solid-solution phase, the volume fraction and the size of the second-phase particles have been found to be crucial for the exclusive formation of the solute oxide. By solving the diffusion equations, the oxidation behavior of a multiphase alloy has been quantitatively related to a microstructure parameter, P L ,which is a function of both the volume fraction and shape of the second phase. Moreover, the effectiveness of alloying for alloys with differently shaped second-phase particles on the oxidation of multiphase alloys has been discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00664707
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  • 10
    Electronic Resource
    Electronic Resource
    Oxidation of multicomponent two-phase alloys (1995)
    Springer
    Oxidation of metals 44 (1995), S. 211-237 
    Details
    ISSN: 1573-4889
    Keywords: multiphase alloy oxidation ; depletion zone ; reservoir effect ; diffusion path
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The high-temperature corrosion behavior of two-phase alloys presents a number of differences compared to that of single-phase alloys. These differences are mainly a consequence of the limitations that the presence of two phases impose on the diffusion of the alloy components. In this review, it is shown that the exclusive scale formation of the more stable, slow-growing oxide is more difficult on a two-phase alloy, requiring a higher concentration of the more reactive alloy component than for a corresponding single-phase alloy. The main types of corrosion behavior for binary two-phase alloys are also considered, showing that if diffusion in the alloy is slow the scale structure will closely reflect that of the starting material. When diffusion in the alloy is not negligible, the scale structure becomes similar to what forms on single-phase alloys. The oxidation of two-phase ternary alloys is shown to be even more complex than the two-phase binary alloys. The principal added complexity compared to the binary alloys is that diffusion in the ternary alloys may also occur in the presence of two metal phases, as a result of an extra degree of freedom in the ternary system. The oxidation behavior of two-phase ternary alloys is discussed in the context of a number of recent experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01046728
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