M. J. Graham

The formation of aluminum oxide scales on high-temperature alloys

This paper is a brief review of the extensive literature relating to the formation of protective α—Al2O3 scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal

The oxidation behavior at 900°C of pure Cr and Cr implanted with 2×1016 Y ions/cm2 was studied. The kinetics of oxidation were measured thermogravimetrically and manometrically. The mechanisms of oxide growth were studied using18O-tracer oxidation experiments, and the composition and microstructure of the oxide scales were characterized by TEM and STEM. Segregation of Y cations at Cr2O3 grain boundaries was found to be the critical factor governing changes in the oxidation behavior of Cr upon the addition of Y. In the absence of Y, pure Cr oxidized by the outward diffusion of cations via grain boundaries in the Cr2O3 scale. When Y was present at high concentration in the scale, as when Cr implanted with 2×1010 Y ions/cm2 was oxidized, anion diffusion predominated. It is concluded that strain-induced segregation of Y at grain boundaries in the oxide reduced the cation flux along the grain boundaries. The rate of oxidation was reduced because the grain-boundary diffusivity of cations became lower than the grain-boundary diffusivity of the anions, which then controlled the rate of oxidation. Changes in the relative rates of Cr3+ and O2− transport, as well as a solute-drag effect exerted by Y on the oxide grain boundaries, resulted in changes in the microstructure of the oxide.

The oxidation of iron-aluminum alloys

This paper briefly summarizes studies related to the oxidation of Fe-Al alloys. Emphasis is placed on oxide phase composition, morphology, and the development of protective α-Al2O3 scales on oxidation-resistant Fe-Al alloys both with and without ternary additions.

The influence of implanted yttrium on the mechanism of growth of Cr/sub 2/O/sub 3/ on Cr

One of the apparent effects of Y on the growth of Cr/sub 2/O/sub 3/ scales, based on inert marker studies, is a change in growth mechanism from predominant cation diffusion to predominant anion diffusion. The interpretation of inert marker studies has always been open to question, and a recent study of the mechanisms of growth of ..cap alpha..-Al/sub 2/O/sub 3/ scales using /sup 18/O tracer techniques has demonstrated that the results of inert marker studies can be misleading. Results of /sup 18/O tracer oxidation experiments conducted with Y-implanted and Y-free Cr are reported in this paper, which demonstrate that Y implantation does change the mechanism of oxide growth.

The effect of yttrium on the growth process and microstructure of α-Al2O3 on FeCrAl

The oxidation behavior of undoped and Y-dopedhigh-purity FeCrAl at 1200°C was investigated. Thescale-growth processes were studied using two-stage16O/18O experiments andhigh-resolution imaging SIMS. SEM, TEM, and scanning transmission electronmicroscopy-electron-dispersive X-ray spectrometry(STEM-EDS) studies were performed in order to determinethe microstructure of the oxide and the chemistry of the metal-oxide interface. An equiaxedα-Al2O3 scale was found togrow by simultaneous outward Al and inward oxygendiffusion on the undoped FeCrAl. In contrast, a columnarα-Al2O3 grain structure on the Y-doped FeCrAl was produced by inwardoxygen diffusion; outward Al diffusion was suppressed.Y was shown to segregate toα-Al2O3 grain boundaries andto the metal-oxide interface.

Characterization of Gd2 O 3 Films Deposited on Si(100) by Electron-Beam Evaporation

Gadolinium oxide films were deposited on Si(100) substrates from a rod-fed electron beam evaporator using a pressed-powder Gd 2 O 3 target. Films 25 nm thick were shown to he stoichiometric Gd 2 O 3 by Rutherford backscattering and had a dielectric constant at 100 kHz of 16.0 ± 0.3. Transmission electron microscopy and X-ray reflectivity measurements showed that films 7-13 nm thick annealed in oxygen consisted of three distinct layers, an interfacial silicon dioxide layer next to the substrate, a second amorphous oxide layer containing silicon, gadolinium, and oxygen above this, and a polycrystalline Gd 2 O 3 layer on top. Annealing in oxygen reduced the leakage currents, increased the thickness of the silicon dioxide layer, and increased the grain size of the top Gd 2 O 3 layer. The characteristics of the leakage currents through the gadolinium oxide were consistent with a Frenkel-Poole conduction mechanism with a silicon-Gd 2 O 3 band offset of 1.8 V. Interfaces with excellent electrical properties, Characteristic of good SiO 2 , were obtained after annealing in oxygen Annealing of the films in vacuum prior to oxygen annealing reduced the thickness of the interfacial silicon dioxide.

Oxidation mechanisms of β-NiAl + Zr determined by SIMS

Abstract The oxidation mechanisms of single crystal Zr-doped β-NiAl from 800 to 1200 °C were determined using primarily secondary ion mass spectrometry (SIMS) imaging and depth profiling. High spatial resolution SIMS imaging provides a means for critically assessing the effects of diffusion through the boundaries of the various morphologies of α-Al 2 O 3 scales that form on β-NiAl. Transient oxidation occurs by outward Al diffusion through transition Al 2 O 3 scales, θ-Al 2 O 3 in this case, for temperatures from 800 to 1100 °C. Steady-state oxidation occurs by counter-diffusion of oxygen inward and both Al and Zr outward through boundaries in the α-Al 2 O 3 ridge morphology.

The influence of reactive-element coatings on the high-temperature oxidation of pure-Cr and high-Cr-content alloys

The influence of various reactive-element (RE) oxide coatings (Y2O3, CeO2, La2O3, CaO, HfO2, and Sc2O3) on the oxidation behavior of pure Cr, Fe−26Cr, Fe−16Cr and Ni−25Cr at 900°C in O2 at 5×10−3 torr has been investigated using the18O/SIMS technique. Polished samples were reactively sputtercoated with 4 nm of the RE oxide and oxidized sequentially first in16O2 and then in18O2. The effectiveness of each RE on the extent of oxidation-rate reduction varied with the element used. Y2O3 and CeO2 coatings were found to be the most beneficial, whereas Sc2O3 proved to be ineffective, for example, for the oxidation of Cr. SIMS sputter profiles showed that the maximum in the RE profile moved away from the substrate-oxide interface during the early stages of oxidation. After a certain time the RE maximum remained fixed in position with respect to this interface, its final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer also varied with the substrate composition. For all coatings18O was found to have diffused through the oxide to the substrate-oxide interface during oxidation, the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxidegrowth mechanism to predominantly anion diffusion. The RE most probably concentrates at the oxide grain boundaries, generally as the binary oxide (RE) CrO3. Cr3+ diffusion is impeded, while oxygen diffusion remains unaffected.

A surface analytical and electrochemical study on the role of cerium in the chemical surface treatment of stainless steels

Abstract The mechanism of oxide layer formation and modification during chemical cerium nitrate treatment of stainless steel has been investigated. The aim of the work was to study the role of cerium in modifying the oxide layer properties, especially the kinetics of the cathodic reactions. For this, electrochemical and surface analytical studies were carried out. During exposure to hot (90 °C) cerium nitrate solution, oxide film formation by chromium passivation and an accompanying dissolution of iron oxide takes place, leading to an enrichment of chromium in the oxide layer. Further, insoluble cerium species are precipitated at the cathodic sites of the surface. The oxygen reduction reaction is inhibited on these films. The effect of the cerium treatment cannot be solely attributed to the formation of a chromium-rich oxide layer, since the cathodic reactions are more strongly inhibited on the cerium-treated stainless steel than on passivated pure chromium. Moreover, the cerium treatment is efficient in retarding the cathodic kinetics on pure chromium. Studies with a redox couple present in the electrolyte clearly show that the inhibition of the oxygen reduction reaction is not due to a lower electron conductivity of the oxide layer. The cathodic inhibition effect can be attributed to a high resistance against reductive dissolution. This is partially due to the chromium enrichment and in addition to the cerium precipitation at the weak sites of the oxide layer which otherwise under cathodic polarization would lead to reductive dissolution, thus providing current paths for electrons participating in the oxygen reduction reaction. Treatment parameters such as time, alloy composition, solution chemistry and potential during treatment were studied. Clearly, all factors leading to a maximum chromium enrichment and/or cerium precipitation increase the cathodic inhibition efficiency.

The growth and stability of passive films

This paper will consider the growth and breakdown of passive oxide films on metals and alloys. Emphasis is placed on the use of surface-analytical techniques, particularly secondary ion mass spectrometry (SIMS) to characterize oxides formed on Fe and Fe-Cr alloys in18O-containing solutions, and to determine the oxides stability to subsequent air exposure. It is found that the stability of passive films towards air exposure decreases with increasing Cr content of the alloy. Films formed on Fe in the passive potential range in borate buffer solution are stable towardsex situ exposure, whereas films formed on Fe-26Cr alloys are not stable to air exposure after passivation at any potential in the passive region. This result has implications for examining the thickness and composition of passive films on high-Cr alloys usingex situ techniques. Passive films play a critical role in the initiation of pitting and the processes which influence film breakdown on Fe in Cl−-containing solutions are discussed. The results indicate that pitting is associated with a critical stage in the development of passive oxide films.