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.     

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys

The development of internal oxides and intergranular oxides in dilute NiCr alloys, containing 1–5% Cr, in NiNiO packs and in 1 atm oxygen at 800–1100°C has been investigated. The internal oxide particles were relatively coarse and widely spaced and were Cr₂O₃, except for a narrow band adjacent to the surface where NiCr₂O₄ particles were also present. Several types of intergranular oxide were developed in the Ni/NiO packs, with preferential penetration being more extensive in the higher chromium-containing alloys at the lower temperatures. Discrete intergranular oxide particles were formed deep in the alloy beneath bands of Cr₂O₃ which developed over intersections of the alloy grain boundaries with the surface, or beneath continuous or discontinuous grain-boundary oxides near the surface, possibly due to the development of a relatively flat oxygen profile and a steep chromium gradient in the subjacent alloy. In the presence of a thickening NiO external scale, preferential intergranular oxidation was much less extensive than in the Ni/NiO packs as the rapid growth of the scale prevented development of Cr₂O₃-rich surface bands.     

Development of preferential intergranular oxides in nickel-aluminum alloys at high temperatures

The development of intergranular oxides in dilute Ni-Al alloys containing 0.55–4.10% Al in Ni-NiO packs and in 1 atm oxygen at 800–1100°C has been examined. In the Ni-NiO packs, preferential intergranular oxide penetration as well as internal oxidation occurs in every case, except in the higher aluminum-containing alloys at 1100°C. Several different types of intergranular oxide morphology were observed, depending on alloy aluminum concentration and on temperature. The oxides in the more dilute alloys are thin and relatively continuous and are accompanied by preferential penetration of internal oxide particles in the adjacent grains. Thicker intergranular oxides are precipitated in the more concentrated alloys while, in some situations, numerous fine oxide particles are formed well ahead of the main intergranular oxide. The intergranular oxidation is facilitated by high stress development in the specimens due to increases in volume as internal and intergranular oxides are formed. These stresses create microvoids in the grain boundaries immediately ahead of the advancing internal and intergranular oxides, resulting in preferential nucleation and growth of further intergranular oxides. This is the case particularly at the lower temperatures where other stress-relief processes cannot operate. The resulting relatively continuous, incoherent intergranular oxide-metal interface allows a high flux of oxygen to the advancing intergranular oxide front. Preferential intergranular oxidation is much less extensive in the presence of a thickening external NiO scale, due to accommodation of the volume increases on internal oxide formation by vacancies injected into the alloy from the growing cationdeficient scale.British Nuclear Fuels, Windscale Works, Seascale, Cumbria, U.K.     

Corrosion of Alloy Haynes 230 in High Temperature Supercritical Carbon Dioxide with Oxygen Impurity Additions

The corrosion of Ni-based alloy Haynes 230 in supercritical carbon dioxide at temperatures of 650 and 750 °C at a pressure of 20 MPa was investigated. In high-purity research grade CO₂, the corrosion performance of the alloy was excellent with a thin, uniform, protective chromium-rich oxide layer forming on the surface. Introduction of 10 and 100 ppm O₂ impurity in the CO₂ environment noticeably enhanced oxidation with evidence of oxide spallation and nodule formation. In these oxygen impurity added tests, increased oxidation led to subsurface voids due to the more rapid outward diffusion of chromium as well as intergranular alumina and chromia. The oxygen concentration at the inlet and the outlet of the autoclave was measured and used to support the results of characterization of the surface oxide to develop a more holistic understanding of the role of oxygen impurity on the corrosion process. In all cases, there some carbon was observed, which manifested as slightly higher concentration of chromium–carbide phase at the grain boundaries compared to the unexposed alloy.     

Penetration of oxygen along grain boundaries during oxidation of alloys and intermetallics

Fast penetration of oxygen into grain boundaries and intergranular oxidation of -NiAl has been observed. Since the solubility of oxygen in NiAl is virtually nil, special ways of oxygen ingress at grain boundaries have to be presumed. Selective intergranular oxidation of binary alloys and fast penetration of oxygen along grain boundaries were analyzed by computer simulation. Interdiffusion caused by consumption of the less-noble component by oxidation at the metal-oxide interface leads to deviation of the alloy composition from the original value. When the diffusivity of the less-noble component is higher than the diffusivity of the other component, a grain-boundary Kirkendall effect may lead to void-chain formation. Experimental evidence for this phenomenon is presented. The deviation in composition and void formation were considered as processes influencing the effective oxygen diffusivity. Both processes were found to allow penetration of oxygen as fast as grain-boundary interdiffusion occurs. In addition, oxygen penetration during intergranular internal oxidation when oxides form at voids beneath the metal-oxide interface was analyzed and treated as a self-propagating process. In this case, fast oxygen penetration is accompanied by fast internal oxide formation and fast displacement of the metal-oxide interface.     

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

The morphological and structural development of internal oxides in nickel-aluminum alloys at high temperatures

The formation and development of internal oxides in Ni-Al alloys containing 1–4 wt.% Al in Ni-NiO packs and in 1 atm oxygen at 800 to 1100°C have been studied. The internal oxide particles were relatively fine, closely spaced, and mainly acicular, although more granular near the surface. They were identified as Al₂O₃ at the advancing front, but NiAl₂O₄ at the surface and at a significant distance from that surface. Growth of internal oxide particles resulted in the development of significant compressive stresses in the internal oxide zone when formed in Ni-NiO packs. These stresses led to grainboundary sliding at the higher temperatures and extrusion of weak, internal oxide-denuded zones adjacent to alloy grain boundaries. At the lower temperatures, these stresses also resulted in significant preferential penetration of oxides down grain boundaries and sub-grain boundaries. Stress development and resulting phenomena were much less significant during oxidation in 1 atm oxygen because vacancies injected from the external NiO scale accommodated the volume increase during growth of internal oxide particles.     

The oxidation behaviour of an aligned Ni-Al-Cr3-C2 eutectic alloy under isothermal and thermal cycling conditions

The isothermal and thermal cycling oxidation behaviour of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy at temperatures from 800° to 1200 °C in flowing air have been investigated using several physical techniques. At all temperatures an initial, protective, external layer of α-Al₂O₃ develops on the alloy surface. However, this breaks down mechanically during thermal cycling, enabling a less protective Cr₂O₃-rich scale to form. The time of retention of the α-Al₂O₃ layer at temperature decreases with increasing temperature, failing after between 30 min and 2 h at 1100° and 1200 °C. However, if platinum metal is introduced into the hot zone at these temperatures, this period is increased to about 48 h. Following formation of the external Cr₂O₃ scale, internal oxide penetration into the alloy can be considerable, involving preferential oxide penetration down the alloy/carbide fibre interfaces. Thermal cycling does not influence markedly the oxidation behaviour, although it does result in formation of a greater quantity of nickel-rich oxide nodules on the scale surface following crack development in the Cr₂O₃-rich scale. This crack development is assisted by differential thermal contraction stresses.     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

The mechanism of oxidation of dilute NiAl alloys at 800–1200°C

The oxidation behaviour of dilute NiAl alloys at 800–1200°C in flowing oxygen at 1 atm pressure has been studied using kinetic measurements, optical and scanning electron microscopy and electron probe micro-analysis. The oxidation rates of Ni0.5 to 4%Al alloys are greater than the corresponding values for nickel at 1000 and 1200°C, but less at 800°C. The increased rates at the higher temperatures are largely due to increases in the total cation vacancy concentration in the scale, although internal oxide formation can make a significant contribution to the oxidation rate. The decreased rates at 800°C are almost certainly due to a build-up of Al₂O₃ particles at the oxide/alloy interface. The roles played in the oxidation processes by doping, internal oxidation, blocking effects in the oxide, dissociation of NiO and gaseous transport of oxygen within the scale are considered in detail and related to the oxidation rates of the various alloys.     

Effects of cold work on the oxidation behavior and carburization resistance of Alloy 800

The structure of the oxide layer formed on Alloy 800 at 600 °C in superheated steam markedly indicates the role of the grain boundaries as easy diffusion paths of Cr and Mn to the alloy/oxide interface. Increasing the number of grain boundaries by 10-90% cold work leads to increasing Cr- and Mn-content in the scale and to decreasing oxide growth rates. Variation of the grain size by different annealing treatments leads – since the Cr-content in the scale is decreasing with the grain size – to a linear relation of growth rate and grain size. The effect of cold work was also demonstrated on the protectiveness of the oxide scale towards carbon uptake and carburization of Alloy 800. After preoxidation of differently deformed specimens at 900 °C, these were exposed to a CO-CO₂H₂O-H₂ mixture at 700 °C for long time. The gas mixture was tagged with ¹⁴C so that the C-ingress into the oxide scale and into the alloy could be sensitively monitored by autoradiography and (upon stepwise polishing) radioactivity measurements of the carbon penetration. The carbon uptake is effectively reduced with cold working; in contrast a non-deformed, electropolished and preoxidized specimen shows relatively high C-content after exposure. The investigations prove the highly favorable effect of mechanical pretreatment on the formation of the oxide scale on an austenitic Fe-Ni-Cr alloy. Cold work and other methods of surface deformation (grinding, polishing, sand blasting, shot peening) generate easy diffusion paths for fast Cr-diffusion to the surface and sufficient supply of Cr to form a protective oxide layer.     

Microstructure Evolution During Steam Oxidation of a Nb Stabilized Austenitic Stainless Steel

The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated at 500, 600, and 700 °C for a fixed oxidation time of 336 h. The samples were characterised using X-ray diffraction, reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500 °C, whereas continuous double-layered oxide scales covered the entire sample surface after oxidation at 600 and 700 °C. The major part of the inner oxide layer developed within the former alloy grains, whereas a Fe–Cr spinel formed along the former alloy grain boundaries. Transmission electron microscopy and electron energy loss spectroscopy investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe–Cr spinel embedded in a metallic Fe–Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. The thickness of the inner oxide zone at high humidity (46%) is not significantly affected by the type of carrier gas used, whereas this thickness at low humidity (8% H₂O) is sensitive for the carrier gas and increases in the following order: air < Ar+7% H₂ < Ar, indicating that the presence of oxygen or hydrogen in addition to a relatively low content of water vapour counteracts the effect of water vapour on the development of the inner oxide zone.     

Effects of 7·5 vol.-% water vapour on oxidation of nickel based alloy between 900 and 1100°C

Abstract

A Ni based SY 625 alloy was oxidised at 900, 1000 and 1100°C under dry and wet conditions. Water vapour has little effect on the oxidation rate and scale composition. At 900 and 1000°C, the outer scale is composed of Cr₂O₃, and a continuous NbNi₄–Ni₃Mo subscale is found at the oxide/alloy interface. At 1100°C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. Nevertheless, the oxide scale morphology differs between dry and wet conditions. Under dry conditions, the oxide scale appears to be compact, and chromia pegs are observed at the internal interface. The oxide scales formed under wet conditions show that porosities spread inside the scale, and the chromia grain size is smaller. At 1100°C, some scale spallation is observed under dry and wet conditions probably due to the molybdenum oxidation, leading to MoO₃ evaporation and void accumulation at the internal interface.     

The oxidation behaviour of a directionally solidified Co-12% VC eutectic

The isothermal oxidation behaviour of a directionally solidified Co-12%VC eutectic alloy in flowing air at 700° to 1000°C has been studied and subsequently examined using electron-optical techniques. The material oxidized rapidly at all temperatures and relatively thick cobalt-rich scales were formed. Considerable oxidation of the VC fibres in the adjacent alloy and occasional deep penetration of oxide down alloy grain boundaries and physical separation of the grains also occurred. Very extensive scale spallation invariably took place on cooling. The lack of suitability of this material in its present form for possible use in oxidizing environments at high temperatures is considered.     

TEM studies of cross sections of oxidized Fe-25Cr-6Al-La alloy

The oxidation behavior of the alloy Fe–25%Cr–6%Al-RE (rich in lanthanum) was investigated by means of TEM analysis. The results show that after 2 hr oxidation of the alloy, in pure oxygen at 1200° C, La precipitated in the oxide scale at the top of -Al₂O₃ grains and at the grain-boundary regions in the form of tiny particles of hexagonal La₂O₃. These tiny particles prevented aluminum from diffusing toward the surface and suppressed lateral growth of the oxide scale. The rare-earth constituents accelerated the internal oxidation of the alloy during thermal cycling between 1200° C and room temperature. They appeared as particles of aluminum oxide and lanthanum oxide. Particles of cubic La₂O₃ precipitated in the alloy matrix near the oxide scale-metal interface in a direction parallel to grain boundaries.     

The oxidation of nickel—tungsten alloys

The oxidation behaviour of NiW alloys and NiWCr alloys containing up to 40 wt%W has been studied in the temperature range 900–1200°C. The parabolic rate constant for oxidation increases with increasing tungsten content in the alloy. Addition of 10 or 15%Cr causes a significant reduction in the oxidation rate.In the Ni—7·5W alloy, spherical internal oxide particle of WO₃ are formed within the alloy, whereas as the tungsten content is increased the tendency to internal oxidation diminishes but the alloy/scale interface develops a highly irregular morphology. The roughened alloy/scale interface is less marked at the higher oxidation temperatures, and also when chromium is present in the alloy. The morphology of the interface is probably related to the relatively low interdiffusion coefficient in NiW alloys.     

Oxidation behaviour of an ODS NiAl-based intermetallic alloy

The oxidation behaviour of an oxide dispersion-strengthened (ODS) NiAl intermetallic, microalloyed with Ti and Mo and strengthened by a fine dispersion of Y₂O₃, is investigated in the temperature range of 900–1200°C for up to 200 h exposure. The results have shown that a nearly pure alumina scale is formed irrespective of the exposure time, which is consistent with previous results on oxidation of NiAl or doped NiAl alloys. From the analysis of the kinetics results and the surface scale morphology it follows that below 1100°C metastable alumina control the oxidation kinetics, even after long exposure times. The presence of yttria seems to account for this increase in the transient oxidation period. Preoxidation of the alloy (1200°C/3 h) leads to a substantial reduction of the oxidation rate and precludes void formation at the scale/gas interface. Furthermore, formation of uniform scale, which would diminish the probability of failure of the scale due to thermal stresses, is favoured.     

Effect of Titanium Addition on Alumina Growth Mechanism on Yttria-Containing FeCrAl-Base Alloy

FeCrAl-base alloys are well known for their excellent oxidation resistance due to formation of a slowly growing alumina surface scale during high-temperature service. The actual scale growth mechanism and especially adherence are strongly affected by the presence of oxygen-active elements such as yttrium, titanium or hafnium. In the present study, the effect of titanium addition on the scale growth mechanisms of an yttrium oxide dispersion-strengthened FeCrAl base alloy was studied during oxidation at 1200 °C in Ar–O₂. For microstructural characterization results of scanning electron microscopy and electron backscatter diffraction were combined with X-ray diffraction data. Scale growth mechanisms were investigated by two-stage oxidation using ¹⁸O tracer with subsequent scale analyses using secondary neutrals mass spectrometry. The scale on the alloy without intentionally added titanium grew virtually exclusively by oxygen diffusion along oxide grain boundaries and exhibited a columnar structure with the grain size increasing in growth direction. The addition of titanium resulted in formation of an outer oxide zone of equiaxed grains on top of the inner columnar part. The equiaxed grains increased in size with increasing exposure time. Comparison with the tracer studies revealed that the titanium-induced equiaxed zone was the result of outer scale growth. Mechanisms for the initiation of outward aluminium transport are discussed. Indications were found that the effect of titanium on the scale growth mechanisms already occurred for titanium additions as low as 0.02 wt%.     

Beneficial Effects of Ce Implantation into Preformed Cr2O3 Scales on the Subsequent Oxidation of Ni–20Cr Alloy

The influence of Ce implantation into preformed Cr₂O₃ scales with a dose of 1 × 10¹⁷ ions/cm² on the subsequent oxidation behavior of Ni–20Cr alloy at 1050°C in air has been investigated. The pre-oxidation was carried out at 1050°C in air for 0.5 and 1 hr respectively Cr₂O₃ and NiCr₂O₄ formed on Ni–20Cr alloy. The oxidation rate was decreased remarkably due to Ce implantation regardless of whether it was implanted into the alloy or into the pre-formed oxide scales, and the beneficial effect decreased with increasing pre-oxidation time, the alloy implanted directly with Ce had the lowest oxidation rate constant. During cyclic oxidation (350 cycles) Ce implantation played a similar benefical effect on the oxide-spallation resistance for blank and pretreated alloys. The result indicates that Ce incorporated into the oxide scale affected the diffusion of the reaction species and also the spallation resistance of the oxide scales. The change of the oxidation process is attributed to the segregation of Ce at the oxide grain boundaries