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.     

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.     

Effect of internal oxidation pretreatments and Si contamination on oxide-scale growth and spalling

Internal oxidation pretreatments carried out in quartz capsule with a Rhines pack were found to have a profound effect on the subsequent oxidation behavior of alloys. Specimens of Co-15 wt.% Cr, Co-25 wt.% Cr, Ni-25 wt.% Cr, and Ni-25 wt.% Cr-1 wt.% Al were tested at 1100°C after pre-oxidation treatments. Even without the development of internal oxide particles, pretreated binary CoCr and NiCr alloys oxidized with significantly lower rates. Selective oxidation of chromium was observed on the non-Cr₂O₃-forming Co-base alloys, whereas on the Cr₂O₃-forming Ni-base alloys, elimination of base-metal oxide, reduction in the Cr₂O₃ growth rate, and better scale adhesion were found. These effects were more apparent with pre-oxidation temperatures greater than 1000°C and with longer pretreatment times. Contaimination of Si from the quartz is believed to be the cause.     

Establishment and Breakdown of α-Al2O3 Scales on Ni-Al Alloys at High Temperatures

Abstract

The isothermal oxidation behaviour of Ni-7% Al and Ni-12·5% Al in 1atm oxygen at 800, 1000 and 1200°c has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis and scanning electron microscopy. The ease of formation of an initially complete, protective, external α-Al₂O₃ scale is greater the higher the alloy aluminium content and the higher the temperature. Once developed, this external α-Al₂O₃ scale tends to fail mechanically in localised regions, the subsequent oxidation behaviour depending upon the composition of the exposed alloy. Under conditions favouring breakaway, Ni-7% Al yields rapid oxidation rates as NiO-rich nodules are formed on the exposed alloy substrate, although the rate declines later as a healing α-Al₂O₃ layer develops at their bases. If the α-Al₂O₃ fails on Ni-12·5% Al, the aluminium content of the revealed alloy is still high enough to ensure rapidproduction of a new external α-Al₂O₃ layer.     

The development of internal and intergranular oxides in nickel-chromium-aluminium alloys at high temperature

The development of internal oxides, intergranular oxides and internal voids in Ni-15.1Cr-1.1Al and Ni-28.8Cr-1.0Al during oxidation in 1 atm oxygen at 1000° to 1200°C has been studied. In both cases, the formation of an external Cr₂O₃-rich scale causes vacancies to be generated in the alloy due to the different diffusion rates of chromium towards the alloy-scale interface and of nickel back into the bulk alloy. At 1000°C, condensation of these vacancies at the alloy grain boundaries facilitates formation of intergranular oxides while, at 1200°C, the vacancies condense to give voids in the grains and grain boundaries. Internal oxides are formed at both temperatures. The internal and intergranular oxides are mainly α-Al₂O₃, although some Cr₂O₃-rich oxides are produced near the alloy-scale interface. Possible mechanisms for the development of the internal and intergranular oxides in these alloys are discussed and related to the observed oxide morphologies and compositions.     

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.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

Oxidation behavior of Ni-Cr-1ThO2 alloys at 1000 and 1200°C

The oxidation behavior of Ni-13.5-33.7Cr-1ThO₂ alloys in flowing oxygen at 150 Torr was investigated in the temperature range 1000–1200°C. Gravimetric measurements of the oxidation kinetics have been combined with microstructural studies of the reacted samples in order to evaluate the reaction mechanisms. The oxide products formed on the alloys were a function of Cr content, sample surface preparation, reaction time, and temperature. The presence of ThO₂ appears to produce two effects during alloy oxidation. First, enhanced Cr diffusion to the alloy surface results in rapid formation of a Cr₂O₃ subscale beneath NiO on Ni-13.5Cr-1ThO₂ and selective oxidation of Cr for Ni-22.6Cr-1ThO₂. Second, the mechanism of formation of Cr₂O₃ is apparently different from that for simple Ni-Cr alloys, resulting in about an order of magnitude reduction in the Cr₂O₃ growth rate. The oxidationvaporization of Cr₂O₃ to CrO₃ becomes rate controlling for the higher Cr alloys after only a few hours of exposure at 1200°C.     

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.     

Effects of Nano Metal Coatings on Growth Kinetics of α-Al2O3 Formed on Ni-50Al Alloy

The effects of pure metal coatings, including Ni, Fe and Cr, on long-term oxidation kinetics, surface morphology and structure were studied. Ni-50Al alloy and Ni-coated, Fe-coated and Cr-coated samples were pre-oxidized at 900 °C in air. They were then oxidized isothermally at 1,000 °C in air. The bare Ni-50Al alloy oxidized rapidly during the initial stage of oxidation due to the formation of θ-Al₂O₃, but the oxidation rate decreased after α-Al₂O₃ had developed. Oxidation of the Ni-coated sample was slow from the beginning of oxidation even though the θ-Al₂O₃ was predominated for a longer oxidation time. No θ-Al₂O₃ developed on the Cr and Fe-coated samples, but the oxidation rates of these samples were much faster than those of bare and Ni-coated samples. Cross-sectional images revealed that the grain size of α-Al₂O₃, which formed on Cr and Fe-coated samples, was smaller than those of bare and Ni-coated samples. These metal coatings affected the microstructure of α-Al₂O₃ and they showed a strong effect on the growth rate of α-Al₂O₃ in the steady-state oxidation stage.     

The Transition from Transient Oxide to Protective Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scale on Fe–Cr–Al Alloys During Heating to 1000 °C

The transition behavior of an Al-rich amorphous oxide layer to an external Al₂O₃ layer on Fe–(4, 24)Cr–(6, 10)Al (at.%) alloys was investigated during heating to 1000 °C at a heating rate of 50 °C/min, by means of in situ high-temperature X-ray diffraction measurement and TEM observation. In the alloy containing 6Al, internal amorphous Al₂O₃ was initially developed below the Al-rich amorphous surface layer. The amorphous internal precipitates transformed to be crystalline and grew laterally with time. The internal precipitates subsequently connected with each other to form a continuous α-Al₂O₃ scale. In the case of 10Al alloy, an Al-rich amorphous layer transitioned to a crystalline α-Al₂O₃ layer from the interface between transient/amorphous layers during heating. The Al₂O₃ scale developed on high Al alloys contained Fe and Cr with relatively higher contents, but that formed on low Al alloy contained low Fe and Cr. The effect of Cr on promoting an external Al₂O₃ scale formation was found to be weaker for alloys with higher Al content compared to the alloys with lower Al content, if Al₂O₃ scale was directly transitioned from the amorphous layer.     

Rapid Formation of α-Al2O3 Scale on an Fe–Al Alloy by Pure-Metal Coatings at 900 °C

Rapid formation of an α-Al₂O₃ scale on Fe–50 at.%Al by pure metal thin coatings of Ni, Al, Ti, Cr or Fe was investigated, and the effects of those elements on Al₂O₃-scale evolution were assessed. The oxidation behavior of samples with and without coatings could be divided into two groups: the samples with/without Ni and Al, and those with Ti, Cr and Fe. The mass gains of samples coated with Al and Ni were almost the same as that of non-coated Fe–50 at.%Al alloy. The mass gains of samples coated with Ti, Cr, and Fe were much lower than that of the Fe–50 at.%Al alloy. A stable α-Al₂O₃ scale was found to develop from the beginning of oxidation on the samples coated with Ti, Cr and Fe. However metastable θ-Al₂O₃ remained after long-time oxidation of non-coated and Ni- and Al-coated samples. The direct α-Al₂O₃ scale formation on the samples with Cr or Fe coatings was speculated to be due to sympathetic nucleation of α-Al₂O₃ on the surface of Al-supersaturated Fe₂O₃ for Fe-coated sample, and composition changes from (Cr,Al)₂O₃ to (Al,Cr)₂O₃ for the Cr-coated sample. Initial formation of an oxide having a corundum structure was inferred to provide a nucleation site for precipitation of α-Al₂O₃ without prior formation of a metastable Al₂O₃ scale.     

Cyclic oxidation behavior of Ni3Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025% B between 900 and 1100°C

A Ni₃Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al₂O₃, NiAl₂O₄ and ZrO₂ were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al₂O₃, ZrO₂)-containing oxides that keyed the oxide scale to the matrix alloy.     

Application of an acoustic-emission technique to the high-temperature oxidation of Fe-Cr-Al alloys

Acoustic-emission analysis combined with thermogravimetry has been used to investigate the oxidation behavior of undoped and doped Fe-Cr-Al alloys. It was demonstrated that acoustic-emission signals which were detected upon isothermal oxidation of undoped Fe-20Cr-5Al arise from buckling of the finegrain -Al₂O₃ layer. The acoustic-emission signals which were detected upon isothermal oxidation of Fe-18Cr-12Al are attributed to repeated cracking of the coarse-grain -Al₂O₃ layer. The mass-gain curve results from superimposed diffusion-controlled oxide growth and accelerated oxide growth after cracking of the oxide layer. The present study shows that acoustic-emission analysis is very useful as it complements thermogravimetric results. The advantage of acoustic-emission analysis is that it reveals cracking and spalling of the oxide layer, which is not recorded by thermogravimetry.     

The effect of aluminum as an alloying addition or as an implant on the high-temperature oxidation of Ni-25Cr

The oxidation behavior of aluminum-implanted Ni-25Cr and Ni-25Cr containing 1 wt.% Al has been studied at 1000°C and 1100°C in oxygen. As did Y alloying addition or Y-implantation, 1 wt.% Al added to Ni-25Cr prevented nodular formation of Ni-containing oxides, improved spalling resistance of the scale upon cooling to a similar degree, and eliminated the formation of large voids between the alloy and the scale at the oxidation temperature. However, the Al addition did not alter the rate of growth of the Cr₂O₃ scale, nor did it change the growth direction. Al-implantation produced no effect even when the maximum concentration and depth of penetration were adjusted to be identical with those of the yttrium in the Y-implanted alloy. The implications of these results concerning the reactive element effect are discussed.     

Transient Oxidation of a γ-Ni–28Cr–11Al Alloy

γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al₂O₃ scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr₂O₃ layer, and an α-Al₂O₃ layer at the oxide/alloy interface. The evolution of such an NiO/Cr₂O₃/Al₂O₃ layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr₂O₃ outer layer and a discontinuous inner layer of Al₂O₃ initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-Al₂O₃, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr₂O₃ and Al₂O₃ to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner α-Al₂O₃ layer.     

TEM Study of the Effect of Y on the Scale Microstructures of Cr2O3- and Al2O3-Forming Alloys

This study was undertaken to investigate and compare the effects of a yttrium addition on the oxide scale development of -Cr₂O₃- and -Al₂O₃-forming alloys under isothermal oxidation conditions. The alloys had a nominal composition (in wt.%) of Ni–30Cr, Ni–30Cr–0.5Y, Ni–30Cr–5Al, and Ni–30Cr– 5–Al–0.5Y. They were oxidized in air for 50 hr at 1000°C. The scale microstructures were characterized using cross-sectional transmission-electron microscopy combined with energy-dispersive X-ray spectroscopy. It was observed that the scale thickness decreases and the scale adherence increases due to the Y addition. The growth direction of -Cr₂O₃ scale changes from predominately outward to inward while countercurrent diffusion within -Al₂O₃ is replaced by inward diffusion due to Y modification. It is considered that the ability of Y to scavenge sulfur from the alloy and its segregation to the oxide grain boundaries primarily account for most of its beneficial effects.     

Analytical Electron-Microscopy Study of the Breakdown of α-Al2O3 Scales Formed on Oxide Dispersion-Strengthened Alloys

Alumina scales formed during cyclic oxidation at 1200°C on three Y₂O₃–Al₂O₃-dispersed alloys: Ni₃Al, -NiAl, and FeCrAl (Inco alloy MA956) were characterized. In each case, the Y₂O₃ dispersion improved the -Al₂O₃ scale adhesion, but in the case of Ni₃Al, an external Ni-rich oxide spalled and regrew, indicating a less-adherent scale. A scanning-transmission electron microscope (STEM) analysis of the scale near the metal–scale interface revealed that the scale formed an ODS FeCrAl showed no base metal-oxide formation. However, the scale formed on ODS Ni₃Al showed evidence of cracking and Ni-rich oxides were observed. The microstructures and mechanisms discussed may be relevant to a thermal-barrier coating with an Al-depleted aluminide bond coat nearing failure.     

On the transient oxidation of a Ni-15Cr-6Al alloy

Stages in the development of a protective -Al₂O₃ scale on a Ni-15Cr-6Al (wt.%) alloy have been examined. It is shown that prior to the formation of a continuous -Al₂O₃ layer, a transient stage of oxidation occurs that consists of a rapid uptake of oxygen with conversion of a thin surface layer of alloy to predominantly spinel and the subsequent development of a discrete layer of Cr₂O₃. It is also shown that during the transient period of oxidation metastable phases of aluminum oxide are formed which transform to -Al₂O₃ upon incorporation into the external oxide scale.     

The influence of aluminum on the transition from oxide to sulfide of aluminum-rich Fe-25Cr alloys with low-oxygen and high sulphur pressures

The simultaneous oxidation and sulfidation of Fe-25Cr, Fe-25Cr-5Al and Fe-25Cr-10Al alloys were studied at 1023, 1123, and 1223 K in H₂-H₂O-H₂S gas mixtures. Fe-25Cr and aluminum-rich alloys with 0–10 wt.% Al show, in H₂H₂O-H₂S gas mixtures at high temperatures, a transition from protective oxide-scale formation to the formation of a sulfide-rich corrosion product. The kinetics boundary, which indicates the transition from oxide formation with slow weight gains to sulfide formation with rapid weight gains, has been found in these three alloys. The critical oxygen partial pressures to stabilize oxide formation at the constant-sulfur partial pressures of aluminum-rich Fe-25Cr alloys were systematically below those of Fe-25Cr alloy. When the oxygen partial pressure is much higher than the critical one, the oxide scale formed on the Fe-25Cr alloy was mainly Cr₂O₃ with a small amount of FeCr₂O₄; on the other hand, the oxide scale formed on the aluminum-rich Fe-25Cr alloys was mainly Fe(Cr,Al)₂O₄ with a small amount of Al₂O₃ and Cr₂O₃. The thermodynamic stability diagrams for (Fe, Cr, Al) -S-O systems were constructed, and the experimental results which show the existence of Fe(Cr, Al)₂O₄ in the simultaneous sulfidation and oxidation of aluminum-rich Fe-25Cr alloys are explained by these diagrams. The reaction kinetics were measured by a stainless-steel spring balance, and the reaction products were characterized by x-ray diffraction, Auger spectroscopy, and scanning electron microscopy. The reaction rate usually decreased with an increase of the oxygen partial pressure at a constant sulfur partial pressure. The existence of aluminum plays an important role to suppress the sulfidation of Fe-25Cr alloys.