Duplex Oxide Layer Research Articles

XPS study of oxides formed on nickel‐base alloys in high‐temperature and high‐pressure water

AbstractThe aim of this work is to characterize the oxidation behaviour of different nickel‐base alloys exposed to high‐temperature and high‐pressure water for short oxidation times. The behaviour of Alloy 600 (74% Ni, 16% Cr, 9% Fe), Alloy 690 (60% Ni, 30% Cr, 9% Fe) and Alloy 800 (47% Fe, 32% Ni, 21% Cr) exposed to simulated, pressurised water reactor primary water at 325°C has been compared. From the combination of chemical and structural data obtained by XPS, nuclear reaction analysis and scanning electron microscopy, the chemical composition and the morphology of the oxide formed on the surfaces have been determined. All alloys present a duplex oxide layer composed of Fe‐rich crystals in the external layer and an inner Cr‐rich layer that is compact and continuous. The oxide thicknesses were 4–20 nm for Alloy 600 exposed for 0–400 h, 4–40 nm for Alloy 690 exposed for 0–400 h and 4–150 nm for Alloy 800 exposed for 0 to 150 h. The kinetics of oxidation of these samples most probably fit a logarithmic growth equation. Copyright © 2002 John Wiley & Sons, Ltd.

Oxidation, Diffusion and Segregation in CuNi(Mn) Films Studied by AES

The surface and in-depth compositions of sputter-deposited Cu0.57Ni0.42Mn0.01 thin films were studied by Auger electron depth profiling after thermal treatment. The samples were thermally cycled to maximum temperatures of 300 °C to 550 °C in air, argon and forming gas (N2, 5 vol. % H2). Linear least-squares fit to standard spectra and factor analysis were applied to separate the overlapping Auger transitions of Cu and Ni. Under bombardment by 4 keV argon ions, CuNi(Mn) layers display bombardment-induced surface enrichment of Ni in the same extent as binary CuNi alloys. At sufficiently high oxygen partial pressures, a duplex oxide layer is formed and a thick surface copper oxide overgrows the initial nickel oxide. In reducing atmosphere selective oxidation of manganese takes place. A capping NiCr layer prevents CuNi(Mn) from being oxidized, but the film configuration is degraded with increasing annealing temperature due to formation of a surface chromium oxide and diffusion of Ni from the CuNi(Mn) layer into the NiCr/CuNi(Mn) interface.

Electrochemistry of Chemical Vapor Deposited Tungsten Films with Relevance to Chemical Mechanical Polishing

The electrochemical behavior of chemically vapor deposited tungsten films in solutions of interest to tungsten chemical mechanical polishing has been investigated using dc potentiodynamic polarization, linear polarization, and Tafel methods. It was found that in the absence of an oxidizer, the tungsten surface was passivated most effectively at acidic pH values. At pH 2 or 4, a duplex oxide layer of less than 50 Aå thickness was detected over the tungsten layer by x‐ray photoelectron spectroscopy. The oxide layer formed at pH 2 was much thicker, and had better passivity compared to the oxide formed at pH 4. Addition of at pH 2 or 4 resulted in a dramatic increase in tungsten dissolution.

Characterization of Surface Barrier Effect upon Tritium Permeation through Stainless Steel with Auger Electron Spectroscopy

The surface barrier effect on tritium permeation through SUS-316 stainless steel was characterized with Auger electron spectroscopy for the surfaces which had been confirmed to have different barriers from our previous study. The surface which was prepared by heat treatment at 1273K for 1 hr in vacuum(10 -4 Pa) was not contaminated with oxygen and carbon but covered uniformly with a large amount of sulfur. The surface exposed to air at room temperature after the vacuum annealing was covered with duplex oxide layers: the top layer consisted of iron oxide and the inner layer consisted of chromium, iron and nickel oxides. The iron oxide in the top layer was easily reduced with hydrogen gas at elevated temperatures but inner oxide layer was not completely reduced under the present conditions. These results were correlated to the surface barrier effect on tritium permeation based on our previous experimental results concerning the dissolution rate of gaseous tritium into stainless steel.

Chemical states of the molybdenum disilicide (MoSi2) surface

The analysis of the oxidation of MoSi2 at room temperature was carried out using X-ray photoelectron spectroscopy. A clean surface of MoSi2 was obtained by sputter-etching the bulk material inside the spectrometer, and the chemical states of the surface were examined with both high and low take-off angles for the conditions of as-etched and exposed to the air for different times. The commercially pure MoSi2 powder was also investigated in the as-received condition. The analysis indicated that the exposure of a clean molybdenum disilicide to the air for just a few minutes led to the formation of SiO2 and MoO2, and this oxidation would persist at least for more than 24 h with the final surface of MoSi2 being covered by a duplex oxide layer of SiO2 + MoO3.

Microstructural development in the directed melt-oxidized (DIMOX) Al-Mg-Si alloys

Metal-ceramic composites produced via directed melt oxidation (DIMOX) of aluminium alloys are of recent interest. Thein situ composite forming method is based on the reaction of a molten alloy with a gaseous oxidant. In the present study, Al-Mg-Si alloys were subjected to directed melt oxidation and the progressive microstructural evolution was examined by interrupted growth experiments. In the early stages, liquid alloy oxidizes to form a duplex oxide layer (MgO+MgAl2O4) on the surface. The openings in these oxide layers allow the liquid alloy to wick through to form small nodules on the surface. When further wicking occurs through these nodules, a “cauliflower” type of colonies is formed. During the early part of the second stage, spinel growth dominates to form a multi-layered structure. In the final stage, as the magnesium reaches low levels, Al2O3 formation dominates the growth, and alumina crystals grow continuously for several tens of micrometres. The oxygen required for alumina formation is expected to come from two sources: (i) from the ingress of oxygen through microcracked oxide layers, and (ii) demixing of magnesium-containing oxides in the underneath layers.

Ellipsometry of iron hydrous oxide layers formed by potentiodynamic techniques

Hydrous iron oxide layers electrochemically formed on iron electrodes in different electrolytes (8.9 < pH < 12.6) have been investigated by ellipsometry. Data have been interpreted on the basis of a duplex oxide layer structure consisting of an inner barrier layer and an outer hydrous layer. The optical response of the hydrous oxide layer depends on the solution composition and in particular on the presence of carbonate ions and the applied potential.

Nucleation and growth of Al2O3/metal composites by oxidation of aluminum alloys

The nucleation and growth mechanisms during high temperature oxidation of liquid Al−3% Mg and Al−3% Mg−3% Si alloys were studied with the aim of enhancing our understanding of a new composite fabrication process. The typical oxidation sequence consists of an initial event of rapid but brief oxidation, followed by an incubation period of limited oxide growth after which bulk Al2O3/Al composite forms. A duplex oxide layer, MgO (upper) and MgAl2O4(lower), forms on the alloy surface during initial oxidation and incubation. The spinel layer remains next to the liquid alloy during bulk oxide growth and is the eventual repository for most of the magnesium in the original alloy. Metal microchannels developed during incubation continuously supply alloy through the composite to the reaction interface. During the growth process, a layered structure exists at the upper extremity of the composite, consisting of MgO at the top surface, MgAl2O4(probably discontinuous), Al alloy, and finally the bulk Al2O3composite containing microchannels of the alloy. The bulk oxide growth mechanism appears to involve continuous formation and dissolution of the Mg-rich oxides at the surface, diffusion of oxygen through the underlying liquid metal, and epitaxial growth of Al2O3on the existing composite body. The roles of Mg and Si in the composite growth process are discussed.

Effects of iron content on electrical resistivity of oxide films on Zr-base alloys.

Measurements of electrical resistivity were made for oxide films formed by anodic oxidation and steam oxidation (400°C/12 h) on Zr plates with different Fe contents. When the Fe content was higher than about 1,000 ppm the electrical resistivity of the steam oxide films was almost equivalent to that of the anodic oxide films, while at lower Fe content the former exhibited lower electrical resistivity than the latter by about 1~3 orders of magnitude. The anodic oxide film was an almost homogeneous single oxide layer. The steam oxide films, on the other hand, were composed of duplex oxide layers. The oxide layer formed in the vicinity of the oxide/metal interface had higher electrical resistivity than the near-surface oxide layer by about 1~4 orders of magnitude. The oxide layer in the vicinity of the interface could act as a protective film against corrosion and its electrical resistivity is one important factor controlling the layer protectiveness. The electrical resistivity of the oxide/metal interfacial layer was strongly dependent on the Fe content. One possible reason for Fe to improve the corrosion resistance is that Fe ions would tend to stabilize the tetragonal (or cubic) phase and consequently suppress the formation of open pores and cracks in the interfacial layer.

Oxidation of SUS-316 stainless steel for fast breeder reactor fuel cladding under oxygen pressure controlled by Ni/NiO oxygen buffer

Oxidation of SUS-316 stainless steel for a fast breeder reactor fuel cladding was examined in the temperature range of 843–1010 K under the oxygen pressure of 1017 t – 10 t-13 Pa hy use of an experimental technique of a Ni/NiO oxygen buffer. The formation of the duplex oxide layer, i.e. an outer Fe 3O 4 layer and an inner (Fe, Cr, Ni)-spinel layer, was observed and the oxidation kinetics was found to obey the parabolic rate law. The oxygen pressure and temperature dependence of the parabolic rate constant k p ( P O 2 , T) was determined as follows: k p ( P O 2 , T)/ kg 2 · m −1 · s −1 = 0.170( P O 2 / Pa) 0.141 exp[−114 × 10 3/( RT/ J)]. On the basis of the oxidation kinetics and the metallographic information, the outward diffusion of Fe in the outer oxide layer was assigned to be the rate-determining process.

The effect of dispersed nitrides on the oxidation of ferritic alloys

Using gas-metal equilibration, a nitrogen martensite case can be produced on mild steel which tempers rapidly at 500°C forming incoherent Fe 4N precipitates dispersed in a ferritic matrix. Under oxidizing conditions the finely distributed nitrides provide sites for oxide nucleation resulting in a fine-grained oxide layer. The dispersed nitrides also act as vacancy sinks for inwardly diffusing cation vacancies during the subsequent growth of the oxide scale, thus giving improved metal-oxide adherence. An oxide forming reaction occurs at the metal/oxide interface as well as the oxide/gas interface resulting in a duplex oxide layer and further enhancing metal-oxide adherence. Similar results are found with nitrided FeX alloys (where X is a substitutional alloying element—Cr, Ti or Mo). Nitriding of these alloys produces a dispersion of nitrides in a ferritic matrix, which subsequently influences the oxidation mechanisms. An analogy can be drawn with the influence of subsurface dispersions of incoherent stable oxide particles on the oxidation of superalloys.

Structure and Chemistry of Oxide Films Thermally Grown on Molybdenum Silicides

The oxides grown during thermal oxidation of molybdenum silicides were studied using electron microscopy, infrared absorption, and thermogravimetric techniques. Analysis of the oxidation mechanism based on thermochemical and diffusion rate data was correlated with experimental results. Because of the higher oxidation potential of silicon, it may be preferentially depleted from the silicide substrate to form a single-phase silica layer. Whether this occurs depends also on the relative diffusion rates in the oxide and substrate layers. Alternately, a duplex oxide layer containing both silica and molybdenum oxide phases will form. The molybdenum oxide phase is a necessary condition for the silicide pest, a low-temperature failure mechanism in which oxidation results in a nonadherent powder rather than in a protective film.