Effect of Oxide Growth Strain in Residual Stresses for the Deflection Test of Single Surface Oxidation of Alloys

Residual stresses developed in FeCrAlY and Ni₈₀Cr₂₀ alloys have been predicted considering growth strain and creep strain in oxide layer and creep strain in alloy or metal. Such stresses, a net compressive stress developed in oxide scales and a net tensile stress developed in alloy strip, produce deflection of a single surface oxidized specimen during high temperature isothermal oxidation. Stresses generated in these alloys and oxide scales were compared with creep deflections. Introducing oxide growth strain in the oxide scales increase the oxide stress value during the initial oxidation stage, during which creep analysis lacks prediction. Oxide stress reaches maximum value at certain oxidation time in the initial oxidation stage. After that oxidation time relaxation of oxide stress occurs considerably in later oxidation stage.     

Mechanical stresses developed in austenitic Fe-Cr-Ni alloys by oxidation in a CO2 atmosphere

The stresses developed during oxidation of Fe-Cr-Ni alloys in a CO ₂ atmosphere at 600 and 700°C have been estimated by measuring the deflection of thin foil specimens oxidized on one side only. One side of the specimen was protected from oxidation by an Al-Au film which was oxidized prior to the deflection experiment. The character and magnitude of the stresses measured are explained by electron microscope and x-ray measurements. During the initial stage of oxidation, high stresses are formed due to epitaxial growth of the oxide. These stresses are high enough to plastically deform the alloy. As oxidation proceeds, the stress decreases and eventually reaches a steady-state value. During this stage, the alteration in composition and molecular volume of the oxide, the formation of carbides, and the growth of whiskers determine the stresses.     

The influence of silicon on the high-temperature oxidation of nickel

An investigation of the oxidation of nickel-silicon alloys has been carried out in order to ascertain the mode of development of partially or fully protective SiO₂ layers. The addition of 1% Si has little effect on the oxidation rate of nickel at 1000°C but is sufficient for partial-healing layers of amorphous SiO₂ to be established. These layers are incorporated into the inner part of the duplex NiO scale but do not react with the oxide to form a double oxide. Increasing the silicon concentration to 4% or 7% facilitates the development of apparently continuous amorphous SiO₂ layers at the base of the NiO scale, resulting in reduced rates of oxidation. However, these layers develop imperfections, possibly microcracks resulting from oxide growth stresses, and are unable to prevent some continued transport of Ni²⁺ ions into the NiO scale and oxygen into the alloy, particularly for Ni-4% Si. Although the formation of SiO₂-healing layers can reduce the rate of oxidation of nickel, they provide planes of weakness that result in considerable damage under the differential thermal contraction stresses during cooling. In particular, severe scale spalling occurs for Ni-4% Si and Ni-7% Si as failure occurs coherently within the SiO₂ layer.     

On the elastic and creep stress analysis modeling in the oxide scale/metal substrate system due to oxidation growth strain

Considering the case of asymmetric oxidation, new elastic and creep analysis models are developed to elucidate the stress evolutions in an oxide scale/metal substrate system during isothermal oxidation, due to the oxidation growth strain in oxide scale. The theoretical works allow for the experimental inference of growth strain and stresses from the curvature measurements during oxidation. Moreover, they provide ways to explore and identify the main mechanisms for oxidation. Two sets of published experimental data are employed and analyzed by the elastic and creep analytical approaches. A novel simple determination method of the growth parameter is proposed and validated.     

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.     

Oxidation of Fe–22Cr Coated with Co3O4: Microstructure Evolution and the Effect of Growth Stresses

The oxidation behavior of a commercially available Fe–22Cr alloy coated with a Co₃O₄ layer by metal organic—chemical vapor deposition was investigated in air with 1% H₂O at 1,173 K and compared to the oxidation behavior of the non-coated alloy. The oxide morphology was examined with X-ray diffraction, electron microscopy, and energy dispersive X-ray spectroscopy. Cr₂O₃ developed in between the Co₃O₄ coating and the alloy, while alloying elements of the substrate were incorporated into the coating. Particular attention was devoted to possible sources of growth stresses and the effect of the growth stresses on microstructure evolution in the scales that developed on the non-coated and the coated Fe–22Cr alloy. Microstructural features suggested that scale spallation on coated Fe–22Cr occurred as a result of superimposing thermal stresses during cooling onto the growth stresses, that had developed during oxidation.     

Surface analysis of nickel-oxide films modified by a reactive element

The surface morphology of oxide grown in the temperature range of 873–1173K on modified high-purity nickel has been observed and quantitatively analyzed by atomic-force microscopy. The modifications included one of two finishing techniques and either CeO₂ sol-gel coatings or Ce-ion implantation. There is an essential difference in the surface morphology and grain size for oxides formed on Ce/CeO₂-modified nickel in comparison to pure NiO. The oxide topography also depends on the substrate-surface-finishing technique and the method of applying the cerium. Both Auger electron spectroscopy and Rutherford backscattering spectrometry were used to evaluate the depth composition of the oxide. In the modified oxide formed on chemically polished substrates the Ce generally appeared to be located close to the outer surface, even after long oxidation times. In oxide grown on mechanically polished substrates Ce was detectable by these techniques only during the very early stages of oxidation. A general correlation exists between the parameters describing the oxide-surface morphology and the Ce-depth distribution, and the reduction of the NiO growth rate achieved by applying the reactive element. Detailed analysis of the oxide surface, internal microstructure, and chemistry gives new insights into the problem of the characterization of thin oxide films and the mechanism of oxide formation.     

Role of Growth Stresses on the Structure of Oxide Scales on Nickel at 800 and 900°C

The development and relief of intrinsic growth stresses in oxide scales on nickel of different purity have been investigated by combining the deflection test in monofacial oxidation (DTMO) with acoustic-emission analysis (AE). Parallel metallographic analysis gave information about the development of the physical- defect structure and all other structural features. The investigations were performed for 100 hr at 800 as well as 900°C in air. The assumption of elastic behavior led to the best correlation between models, literature data, and results of the present investigations. Microcracks are responsible for the relief of growth stress and inward oxygen penetration leads to the typical NiO duplex scale. The growth of the microcracks is initiated at large pore populations at the oxide–metal interface that most probably form due to outward cation-diffusion and vacancy condensation. The pore formation is increased by the presence of impurities. An equilibrium of microcracking and crack healing is finally reached, leading to a continuous growth of the inner and outer NiO layers of the duplex scale. The scale growth stresses are mainly compressive and can reach maximum values of –560 MPa at 900°C. An estimation of the fracture toughness of the oxide–metal interface assuming a wavy interface led to a K_Ic value of about 2 MNm^-3/2 at 900°C     

Oxide phase development upon high temperature oxidation of γ‐NiCrAl alloys

The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr₂O₃ and an inner zone of isolated α‐Al₂O₃ precipitates in the alloy. The amounts of Cr₂O₃ and NiO in the oxide scale attain their maximum values when successively continuous Cr₂O₃ and α‐Al₂O₃ layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al₂O₃) in the oxide scale maintain at an approximately constant value. The formation of NiCr₂O₄ and subsequently NiAl₂O₄ happens as a result of solid‐state reactions between the oxide phases within the oxide scale.     

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.     

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.     

Competition Between Stress Generation and Relaxation During Oxidation of an Fe-Cr-Al-Y Alloy

The residual biaxial stress inAl₂O₃ scales formed duringoxidation of Fe-22.0%Cr-4.8%Al-0.3%Y in the temperaturerange 1000-1300°C have been measured usingCr³⁺ luminescence piezospectroscopy. Thestress measurements together with substantialdimensional changes of the specimens show that oxidationproduces high compressive growth stresses in theAl₂O₃ scale in addition tothermal-mismatch stresses that arise during cooling from the oxidationtemperature. The magnitude of the growth stress isdetermined by two opposing processes: stress generation,associated with lateral growth of the scale, and stress relaxation in both the metal and oxide. Thescale lateralgrowth strain continuously increasesconcurrently with the scale thickening. Creep in theoxide is a major relaxation process when the scale isthin, while metal deformation becomes significantduring the later stages of oxidation at highertemperatures.     

Das Oxydationsverhalten von Nickel-Vanadium-Legierungen in Luft

The oxidation behaviour of nickel-vanadium alloys in air With oxidation tests carried out on pure nickel in air at 1000°C, a simple oxide film of NiO is formed. With the oxidation of nickel-vanadium alloys, however, several layers of different composition are formed. the outermost layer contains mainly NiO and a small content of nickel vanadate, Ni(VO₃)₂. Below it is a second oxide layer which has, on the outside, a strong concentration of vanadium. On the side facing the metal, this second layer has a low content of either metal, and is porous. This is followed by an inner oxidation zone which projects into the matrix in the form of conic islands with concentrations of V₂O₃. In the temperature range from 800 to 1200 °C, the scale constants indicating the reactions of the nickel-vanadium alloys are of an order of magnitude above that of unalloyed nickel. The oxidation reactions obey parabolic laws for the formation of the outer NiO layers with nickel and of NiO and Ni(VO₃)₂ with the nickel-vanadium alloys. The growth of the inner oxidation zones is governed by a logarithmic law. The activation energy of the oxidation in air, for nickel and for the nickel-vanadium alloys investigated, is of the order of magnitude of 50kcal/Mol.     

On the Influence of Non-Protective CuO on High-Temperature Oxidation of Cu-Rich Cu-Ni Based Alloys

A number of copper-rich Cu-Ni alloys wereoxidized from 750 to 1000^°C at differentoxygen pressures. The oxide scales formed consist of anouter copper oxide layer and an inner porous layer where internal oxide-derived NiO particles aredispersed in a copper oxide matrix. The copper oxide maybe both single-phase CuO and a two-phase(CuO+Cu₂O). At the lower part of thetemperature range, the oxidation kinetics and oxidemorphology depend strongly upon the formation of CuO.The CuO layer is nonprotective and further oxidation ofCu₂O, forming CuO, therefore changes the oxidation from being approximately parabolic tohaving a breakaway-like behavior. The relative thicknessof nonprotective CuO increases with increasing NiO andreflects that the solid-state flux of copper across the Cu₂O decreases due to abarrier effect of the NiO particles and porosity in theoxide and NiO particles in the alloy. The beneficialeffect of Ni in reducing the oxidation rate is lost due to the extensive formation ofnonprotective CuO.     

Corrosion behavior of Hastelloy C-276 in supercritical water

The corrosion behavior of a nickel-based alloy Hastelloy C-276 exposed in supercritical water at 500–600 °C/25 MPa was investigated by means of gravimetry, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. An oxide scale with dual-layer structure, mainly consisting of an outer NiO layer and an inner Cr₂O₃/NiCr₂O₄-mixed layer, developed on C-276 after 1000 h exposure. Higher temperature promoted oxidation, resulting in thicker oxide scale, larger weight gain and stronger tendency of oxide spallation. The oxide growth mechanism in SCW seems to be similar to that in high temperature water vapor, namely solid-state growth mechanism.     

Scale stresses during protective and breakaway corrosion of iron and rimming steel in CO2

Foil extension experiments have been used to determine the stresses arising in oxide scales growing on iron and rimming steel in CO₂ during the protective and breakaway stages of oxidation. There is no definite association of either tensile or compressive stresses with protective oxidation, and scale stresses cannot be invoked to explain the mode of protective growth or the nucleation of breakaway oxide. Breakaway oxidation generates compressive stresses in the scale which cause tensile deformation of thin foils. Metallographic observations suggest that significant factors in the transition from protective to breakaway oxidation are prior formation of a duplex protective scale and carburization of the underlying metal.     

High-Temperature Oxidation of Ni–20 wt.% Cu from 700 to 1100°C

Ni–20 wt.% Cu was oxidized in different oxygen pressures from 1×10^–5 to 1 atm at 700–1100°C. The oxidation consisted of an initial transient period in which a composite scale of NiO and Cu oxides formed, and a subsequent quasi steady-state regime during which parabolic growth of NiO determined the overall oxidation rate. Based on the oxide composition and the oxygen- pressure dependence of the parabolic rate constant, it is concluded that outward transport of Ni via vacancies determines the growth rate of the oxide during the steady-state period, either in the grain boundaries or in the lattice. The influence of Cu dissolved in NiO on the oxidation rate and the oxygen-pressure dependence of the parabolic rate constants is discussed.     

Effect of Water Vapor on the High-Temperature Oxidation of Pure Ni

The high-temperature oxidation behavior of pure Ni in air and Ar with and without 30?vol%H₂O at 1,000?°C was investigated to understand the effects of water–vapor on the resulting oxidation kinetics and scale structures. It was found that water–vapor significantly affected the morphology and scale structure of NiO. A duplex NiO scale with a powder-like outer and dense inner NiO layer developed when the Ni was oxidized in atmospheres containing water–vapor. The grain size of the dense inner NiO layer was much smaller than that formed in dry atmospheres. The growth of the powder-like NiO required outward diffusion of Ni and its continued formation occurred at the interface between the powder and dense NiO layers. The dense inner NiO layer grew outward and incorporated the powder-like NiO particles and the resulting grain size of the inner layer was smaller in the presence of water–vapor. The water–vapor is speculated to have prevented sintering of NiO particles during growth of the NiO scale.     

Breakaway oxidation of Fe10%Cr and Fe20%Cr at temperatures up to 600°C

In the oxidation of Fe10%Cr and Fe20%Cr in air at 600°C, nucleation is followed by a period when growth rates are extremely low, and this ends in a transition to a fast linear rate. Microstructural and micro-analytical studies of the thin protective oxide (2–50nm) indicate that it is a duplex layer of Fe₂O₃ and M₃O₄ of rather variable thickness and composition. The heterogeneity of the oxide layer results from differences in diffusion rates in both the oxide and alloy. The transition to higher oxidation rates arises because of a build-up of compressive growth stresses which disrupts the protective layer. Protection cannot be re-established because during failure the alloy close to the alloy-oxide interface is deformed, causing growth of a non-coherent chromium-containing oxide.     

Duplex scale formation during high-temperature oxidation of Ni-0.1 wt.% Al alloy

The development of a duplex NiO scale microstructure on a Ni-0.1 wt.% Al alloy at 900°C has been examined, principally using secondary-ion mass spectrometry and analytical transmission electron microscopy. The¹⁸O-tracer distribution following sequential oxidation in¹⁸O₂/¹⁸O₂ showed that the inner NiO layer formed as a result of gaseous-oxygen penetration of the scale. The provision of pathways for oxygen transport as well as the role of Al, Si, and Ce segregation at oxide grain boundaries in influencing the growth rate and spallation behavior of the scale are discussed.