B. D. Bastow

The transport of oxygen to the advancing internal oxide front during internal oxidation of nickel-base alloys at high temperature

Abstract For Ni-Al alloys (containing 0.5 to 4 wt% Al), the depths of penetration of internal oxide precipitates on exposure to Ni/NiO packs at 800–1100°C are relatively independent of alloy composition. However, for Ni-Cr alloys (containing 1 to 5 wt% Cr), the depths decrease with increasing alloy chromium concentration, although generally to a lesser extent than expected from classical theory. At all temperatures, the depths for the two systems approximately coincide at the lowest alloy aluminium and chromium concentrations. The results can be related to the sizes, shapes and distributions of the internal oxide precipitates and accounted for in terms of enhanced diffusion of oxygen along the incoherent interface between the oxide and the alloy matrix. In particular, the precipitates in the Ni-Al system are continuous rods which extend from the surface to the internal oxide front while those in the Ni-Cr system are discrete, often relatively spherical, particles. The model accounts for the observations if the ratio of the diffusion coefficient of oxygen along the incoherent internal oxide interface to that through the bulk nickel lattice is approximately 1.5×10 4 at 800°C, decreasing with increasing temperature to approximately 6×10 3 at 1100°C.

Alloy depletion profiles resulting from the preferential removal of the less noble metal during alloy oxidation

The assumptions involved in Wagners original treatment of alloy depletion profiles are examined and found to be acceptable for many situations. Finite difference analyses do not result in profiles which are significantly different from those obtained by the much simpler analytical solution once steady-state parabolic growth is established. Consequently an analytical solution is preferred and its combination with the classical Wagner expression for scale growth leads to a unified description of alloy oxidation when only the least noble metal is oxidized. The description is tested for an Fe-27.4wt.% Cr alloy oxidized at 1273°K and agreement between theoretical and experimental results is satisfactory. Alternative treatments of alloy oxidation which require that there be no recession of the alloy-scale interface are discussed and it is concluded that this assumption is unnecessarily restrictive in many cases. Suggestions that the oxidation of austenitic steels is controlled by diffusion in the alloy and that an interfacial transfer step is of importance in determining the oxidation rate in some cases are shown to be based on invalid assumptions. An analytical solution to the diffusion equation is developed for the case when a phase change occurs in the alloy because of less noble metal depletion and an expression is also presented for the profile developed in the limiting case where depletion is determined by scale evaporation.

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

Abstract 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.

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Abstract Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr 2 O 2 scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.

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.

Morphologies of Uniform Adherent Scales on Binary Alloys

The various scale morphologies arising from the oxidation and sulfidation of binary alloys are summarized. The relationships between simple single-phase morphologies and more complex multiphase, multilayer cases are arranged diagrammatically by defining qualitatively the restrictions on the thermodynamic stabilities and transport properties of the oxidation products which result in increasing scale complexity. Factors which can cause the formation of nonuniform scales are considered briefly. Oxide and sulfide scales on binary alloys of Fe, Co, Ni, Cu, Mn, Cr, and Al are discussed semiquantitatively in an attempt to identify the important properties which cause changes in oxidation rates and morphologies.

Multilayer scale formation during the sulfidation of nickel

The reaction between pure nickel and sulfur vapor has been studied over the temperature range 380–475° C. A scale consisting of four separate layers is formed and the growth of the three outer layers obeys parabolic kinetics. The innermost layer grows only in the very early stages, after which the thickness remains approximately constant. The layers have been identified as three different sulfides-Ni3S2, which forms the two inner layers, Ni7S6, and NiS. The three outer layers grow with preferred orientations; growth of the Ni3S2 and NiS layers is with the basal planes of their hexagonal structures parallel to the nickel substrate. The formation of the scale can be described in terms of a diffusion-controlled process and the same basic process continues after scale-metal separation occurs. The description of scale growth is combined with experimental data to calculate the diffusion coefficient of nickel ions in each phase.

The segregation of alloy components in scales and subscales formed by binary alloys of Mn, Fe, Co and Ni

Abstract The basic equations describing the growth of ternary solid solution scales by cation diffusion are summarized and the reliability of available analytical and numerical solutions are evaluated. The published diffusion and thermodynamic data are summarized for the evaluation of these equations for oxide scales on binary alloys of manganese, iron, cobalt and nickel and comparisons between theoretical calculations and experimental measurements illustrate the applicability of the theoretical treatment. The possible effects of anion diffusion and short-circuit cation diffusion are briefly considered and related diffusion-controlled segregation phenomena in oxides are examined. The evolution of more complex microstructures, which cannot be described quantitatively by available theoretical treatments, is examined in terms of diffusion paths on an oxide stability phase diagram. The theoretical treatment of the development of segregation during the formation of subscales composed of oxide solid solutions is outlined. Limitations to the applicability of recent theoretical treatments are demonstrated by the interpretation of previously published data on subscale compositions.