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Cerium ions were implanted into a high purity polycrystalline Ni to a dose of 2 × 1016 Ce+/cm2. The radiation damage and distribution of Ce in Ni substrate were modified by postimplantation vacuum annealing. The Ce implants significantly decreased the NiO growth rate at 973 K. Thin oxide films formed on implanted Ni were composed of three well-defined sublayers, each with an essentially different microstructure. Cerium was present in the near surface region of the oxide in the form of CeO2 particles, randomly distributed in NiO matrix, and Ce ion segregants at NiO grain boundaries. The size of CeO2 particles, formed during the initial stages of exposure to oxygen, affected the inhibition of oxide growth. Vacuum annealing following implantation decreased the beneficial effect of Ce implants by increasing the size of CeO2 particles. The role of Ce implants in inhibiting grain boundary diffusion in NiO is analyzed. As a result of this study, the growth mechanism of thin NiO films on Ce-implanted Ni is proposed. more...

The effect of CeO{sub 2} ceramic coatings with thicknesses in the range of 14 to 42 nm on the oxidation of high purity polycrystalline nickel at 973 K in 1 atm O{sub 2} has been studied. The ceria coatings decrease the Ni oxidation rate after 125 h by a factor up to 45, which is significantly higher than the reduction achieved during early stages. Growth features, including oxide thickness, surface morphology, and the texture for both the pure and CeO{sub 2} modified NiO, demonstrate a definite dependence on the crystallographic orientation of the Ni substrate. Oxides with thicknesses of up to 1 {micro}m developed on CeO{sub 2}-coated Ni consisted of three sublayers. The Ce-rich part, composed of small NiO grains and CeO{sub 2} particles, was located inside the scale and shifted deeper into the scale with increased oxidation time. The growth kinetics and microstructural evolutions of modified NiO are discussed in terms of the mechanism of inhibition of grain boundary diffusion in NiO by Ce{sup +4} ions. more...

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. more...

The formation and development of oxides in Ni−4Al and Ni−4Al−xSi (at.%, x=1, 3, 5) alloys at 5–9×10−6 and 1 atm oxygen pressure at 1073 K have been studied. The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl2O4 and Ni2SiO4) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni−Al−Si alloys. In Ni−4Al alloy where no complete intermediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher oxidation rate than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO. more...

The influence of 14-nm thick ceria ceramic coatings deposited by the sol-gel technique on the early-stage oxidation of polycrystalline nickel at 973 K was studied by analytical electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry and X-ray diffraction. The size of the ceria particles in the coating was modified prior to oxidation by vacuum annealing. It was found that ceria particle size is a crucial factor affecting the oxidation kinetics, oxide microstructure, and distribution of cerium within the oxide film. Coarse ceria particles applied to the nickel surface were ineffective in the inhibition of oxidation and were spread throughout the whole oxide. Coatings with small ceria particles markedly improved the oxidation resistance. After oxidation such particles were present in the surface region of nickel oxide, acting as the sources of cerium ions segregated at the nickeloxide grain boundaries. The stereological analysis of oxide microstructure as well as microchemical examination supported the predominant role of grain-boundary segregation of cerium ions decreasing the oxidation rate. The results are discussed in terms of reactive-element effect on the development of microstructure of nickel oxide film during initial stages of oxidation. more...

CeO[sub 2] sol with particle diameter of 5 nm was applied to the surface of both chemically and mechanically polished polycrystalline nickel. The oxidation kinetics during early stages of NiO formation have been measured with an ultrahigh vacuum manometric system in the temperature range of 873 to 1073 K and oxygen pressure 5 [times] 10[sup [minus]3] Torr. The ceria coatings reduced the nickel oxidation rate by up to a factor of 4, but the extent of the effect depended on coating thickness, substrate surface finish, and preoxidation prior to coating. After oxidation the Auger electron spectroscopy sputter profiles revealed the cerium concentration maximum in the vicinity of the gas-oxide interface. As was determined by scanning and transmission electron microscopy, ceria coatings resulted in formation of more uniform oxide films, with grain size much smaller than that observed on pure nickel. Ceria particle with diameter less than 10 nm were detected both inside nickel oxide grains and at oxide grain boundaries. more...

The interaction between deuterium and irradiation defects in nickel is studied using the gas-phase permeation technique. Irradiation defects were introduced using 90 keV Ni + ions with fluences of 10 14 −10 17 ions cm −2 , or 30 keV 4 He + ions with fluences of 10 15 −5 × 10 16 ions cm −2 . Permeation results show that crystalline defects such as dislocation loops produce no measurable effect on deuterium permeation in the temperature range 370–570 K. By comparison, helium defects decrease the apparent diffusivity by four to six orders of magnitude, indicating a strong trapping effect for deuterium. These results are discussed using a simple trapping model. more...

Sulfidation of an Fe-23.4Cr-18.6Al (at.%) alloy was investigated in H2S-H2 atmospheres,\(10^{ - 6} \leqslant {\text{P}}_{{\text{S}}_{\text{2}} } \leqslant 10^2 \) Pa, at 973 K. It was found over this pressure range that sulfidation after an early transient period followed the parabolic rate law, being diffusion controlled. An investigation was carried out of the scales formed during early transient sulfidation over the sulfur pressure range\({\text{p}}_{{\text{S}}_{\text{2}} } = 10^{ - 6} - 10^{ - 2} \) Pa. Fully developed scales were multilayered consisting of an inner compact layer of equiaxed grains, an intermediate layer of equiaxed and columnar grains exhibiting a small degree of porosity, and an outer porous layer of distinct plates and needles. The grains of the inner and intermediate layers contained quarternary sulfide phases. The following phases were identified: spinels (CrFe)Al2S4 and (FeAl)Cr2S4, hexagonal (FeCr)Al2S4, (CrAlFe)2S3, and (CrAlFe)5S6. The plates and needles were composed of hexagonal (FeCr)Al2S4 and (CrAlFe)2S3 at\({\text{p}}_{{\text{S}}_{\text{2}} } \geqslant 10^{ - 6} \) and 10−5 Pa from which pyrrhotite, FeS, grew at\({\text{p}}_{{\text{S}}_{\text{2}} } \geqslant 10^{ - 4} \). more...

The sulfidation behavior of an Fe-26.4Cr-8Al-6Mn (at.%) alloy in H2S-H2 atmospheres,\(10^{ - 4} \leqslant {\text{P}}_{{\text{S}}_{\text{2}} } \leqslant 10^2 {\text{Pa}}\).

Oxidation properties of Ni‐2 and 6 w/o Al alloys were investigated in 1 atm oxygen at temperatures in the range 1273°–1573°K. Ni‐2 w/o Al oxidized parabolically, one order of magnitude more rapid than pure nickel, to a duplex scale by metal diffusion consisting of outer and inner layers of Al‐doped and , respectively, and an internal zone of rodlike precipitates by oxygen diffusion through the Al‐depleted alloy. Oxidation kinetics of Ni‐6 w/o Al alloy were irreproducible due to early‐stage formation of an imperfect scale containing nodules. Impingement of favorably oriented rodlike precipitates growing beneath the nodules accompanied with lateral diffusion of aluminum to react with oxygen at the precipitation front eventually led to transition from internal oxidation to development of a multilayered scale. more...

The transition metals when exposed to oxygen at low and intermediate temperatures form thin, protective oxide films of up to some thousands of Angstroms in thickness. At higher temperatures, thick (> 1/z) oxide scales are generally formed. The kinetics of the high-temperature process are well established, whilst those of the low-temperature reaction are a matter of contention in the literature. Our discussion reflects this difference by incorporating a detailed consideration of lowtemperature kinetics. Oxidation kinetics are, of course, determined by the reaction mechanism. The mechanism is in turn determined by the properties of the solid oxide reaction product. It should therefore be possible to predict the kinetics of an oxidation reaction from the appropriate properties of the solid oxide or, conversely, to determine those properties from kinetic measurements. This discussion therefore commences with a review of the relevant structural, thermodynamic and transport properties of transition metal oxides. A confrontation of theory and experiment for both the lowand hightemperature r6gimes then follows, and both the successes and the inadequacies of oxidation theories are pointed out. Finally, the model employed for the oxide product is extended to include line defects and internal surfaces, and the importance of these parameters on transition metal oxidation is assessed. more...

Equilibrium oxygen pressures of the univariant systems , and were determined between 1123 and 1423 K using galvanic cells with calcia‐stabilized zirconia as the solid electrolyte. Slipcast tubes of a two‐phase mixture of were used as electrolyte in cells to determine the equilibrium oxygen pressures of the univariant between 923 and 1173 K and the dissociation pressure of equilibrated with (Ni‐Al) alloys containing 0.17, 1.19, and 2.36 atomic percent Al at 1213 K. The free energy of formation of was determined between 923 and 1423 K. An electron probe microanalyzer was used to determine the composition of oxides and alloy phases of the ternary Ni‐Al‐O system that coexist with each other at 1273 K. The Ni‐Al‐O isotherm at 1273 K and Ni‐Al‐O equilibrium oxygen pressure diagrams at 1213 and 1273 K are presented. more...

Growth of layers on the (100) surface of single crystal was investigated in air at 700° and 800° C. These layers were of duplex structure due to simultaneous inside and outside growth. Measurements on the position of inert gold markers demonstrated that cobalt migration outward in was the dominant diffusion mechanism. The oxidation kinetics were parabolic; the parabolic rate constant at 800°C was used to obtain an estimate of for the self‐diffusion coefficient of cobalt in . more...

Auger electron spectroscopy in combination with depth profiling by Ar ion sputtering was employed as a surface analytical technique to determine the positions of thin (3–5 nm) inert Pd markers during growth of oxide films on Al, Ni, and β-NiAl. Alumina films 35 and 120 nm thick formed on Al and β-NiAl, respectively, grew by inward diffusion of oxygen. Metal migration accounted predominantly for the growth of a NiO film 150 nm thick on the (111) crystal face of Ni. However, an assessment could not be made with respect to oxygen diffusion, if any, in this film due to the limitations imposed on precise depth profiling of the marker position caused by uneven sputtering of the oxide and metal phases. more...

Equilibrium oxygen pressures of the univariant ternary coexistences hematite + spinel + alumina, alloy + wustite + spinel, and alloy + spinel + alumina were determined at temperatures in the range 1123 to 1423 K using solid state electrochemical cells. The compositions of alloys and oxide phases of this Fe-Al-0 system that equilibrated with one another at 1273, 1423, and 1573 K were determined. These results and those from the literature are used to determine stabilities of the ternary oxide phases as a function of oxygen pressure and composition by constructing equilibrium oxygen pressure diagrams at 1073, 1173, and 1273 K. more...

Diffusion coupling of an Fe6Al alloy with FeS in the form of compacts results in periodic precipitation of FeS + FeAl 2 S 4 bands in the alloy. The thickness of sulfide bands increases with depth satisfying the Jablczynski spacing relationship for Liesegang-type of precipitation. The initial stage of sulfide band formation is explained by a model involving precipitation of Al 2 S 3 which is subsequently converted to FeAl 2 S 4 and FeS. Growth of each sufide band is supported by rapid diffusion of iron and aluminum in the FeS phase. It is suggested that aluminum depletion from the alloy in front of the innermost sulfide band ultimately leads to its cessation of growth, a new sulfide band beginning to form in the alloy at a distance where the concentration product for the precipitation of Al 2 S 3 is satisfied. more...

A ternary diffusion model is invoked to interpret growth of the scale on alloys containing greater than 40 atomic percent (a/o) Fe in atmospheres, , at temperatures 533°– 980°C. Concentration profiles in the solid phases were determined from electron probe microanalyses; the iron concentration across a scale continuously decreased from its highest value at the scale/alloy interface. Computations demonstrated that the relative self‐diffusivity of cobalt to iron in was dependent on concentration increasing asymptotically from a value of 1.3 in essentially pure to a value of . A numerical computing method capable of determining sulfidation rates of a binary alloy from diffusional and thermodynamic data was used to demonstrate that calculated metal composition profiles in the scales and values of the parabolic sulfidation rate constants were in good agreement with the experimental determinations. more...

Sulfidation properties of Fe‐6, 9, 18, and 28 atomic percent (a/o) Al alloys were investigated at 1173 K in sulfur vapor at the dissociation pressure of . The Fe‐6 Al alloy exhibited internal sulfidation; the Fe‐9 and 18 Al alloys exhibited internal sulfidation along with formation of an external scale. Compositions of the internal sulfide precipitates corresponded to in the Fe‐6 Al alloy and to in the alloys containing 9 and 18 a/o Al. The Fe‐28 a/o Al alloy upon sulfidation only formed an external scale. The apparent permeability of sulfur in the alloy of the internal sulfidation zone as defined by product increased linearly with alloy composition. Growth of the sulfide precipitates which extended as plates across the internal sulfidation zones is satisfactorily interpreted by a model involving enhanced sulfur diffusion along the interfaces between the internal sulfides and alloy matrix. Transition from internal sulfidation to internal sulfidation plus external scale formation and finally to only external scale formation based upon the alloy aluminum composition is discussed. more...

High temperatures associated with arcing cause Cu–Ag alloys to oxidize and acquire increased resistivities when utilized in electrical switches. This oxidation involves formation of copper oxides at the surface and as precipitates randomly distributed in the arced layer and bulk of the alloy. As such, the change of resistivity with the extent of oxidation resembles a percolation process: specimens with an initial good electrical conductivity exhibit an abrupt transition to a poor conductivity when the buildup of oxide precipitates blocks paths in the conductive metal matrix. more...

An investigation is reported on the sulfidation properties of Fe-Cr alloys over their complete compositional range in H2S -H2atmospheres at 1073 K. Sulfidation of alloys containing less than 60 at. % Cr obeyed parabolic kinetics and alloys of larger chromium content and chromium followed linear kinetics. Scales were composed of two distinct layers: an outer columnar grained sulfide and an inner equiaxed grained sulfide layer of increased relative thickness and porosity at larger alloy chromium contents. The scales on quenched specimens were composed of either (CrFe)5S6or (FeCr)S plus small amounts of (FeCr)3S4at\(P_{S_2 }\) =10−2Pa and of (CrFe)5S6at\(P_{S_2 }\) =10−5Pa. Internal sulfidation was most pronounced in the high chromium content alloys and at the low sulfur pressure. The parabolic sulfidation rate at\(P_{S_2 }\) =10−2Pa was large and of practically constant value irrespective of alloy composition. These kinetics at\(P_{S_2 }\) =10−5Pa, however, increased from a very low value by three orders of magnitude as the alloy chromium content was increased from 5–60 at.%. more...

The structures of films of 300A‐15μ thick formed on (100), (110), and (111)Ni crystal faces at temperatures in the range 500°–800°C were investigated by electron microscopy and x‐ray diffraction. The oxide was initially polycrystalline but rearranged as the oxide thickened to major epitaxial relationships with the metal substrate. These relationships were characterized by coincidence between the close‐packed directions of the metal and oxide; the number and type of oxide orientations were correlated with the number of close‐packed directions in the metal faces. Oxide thicker than 1μ exhibited preferred {100} and {111} orientations. The oxide on (100) and (111)Ni exhibited the highest preferred orientation and a duplex layer structure consisting of equiaxed crystallites in the inner layer and columnar ones in the outer layer. These findings are rationalized in terms of a diffusional model for the growth of these oxide layers in which the reaction rate is determined by boundary and lattice diffusion of nickel through nickel oxide. more...

EDAX spectrometry was employed as a semiquantitative analytical technique to determine the penetration depth of NiAl 2 O 4 into the internal oxidation zone of Ni-5 wt. % Al alloy oxidized in air at 1273 K. Depth profiles obtained by spot counting Al to Ni K α peak intensities demonstrated that a sharp NiAl 2 O 4 -Al 2 O 3 phase boundary exists in this internal oxidation zone at a normalized fractional depth which can be determined to an accuracy of 0.70±0.05. X-ray reflection diffraction analyses and color examination of chemical deep-etched tapered cross sections substantiated these results. more...

Surface segregation measurements were made on three Co-Rh alloys (19.3, 47.8 and 84.7 at% Rh) in the temperature range 623–1123 K using AES. Cobalt segregation occurred on the Co-84.7Rh alloy with a heat of segregation of −3.7 ± 7 kJ mol −1 while rhodium enrichment was observed on the Co-19.3Rh alloy with a heat of segregation of 1 ± 1 kJ mol −1 . No variation from the bulk composition could be conclusively demonstrated on the surface of the Co-47.8Rh alloy. There is good quantitative agreement between these results and the combined bond breaking and lattice strain theories of segregation. more...

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of α-Al2O3 in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous α-Al2O3 film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective α-Al2O3, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective α-Al2O3 scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the α-Al2O3 grains playing an essential role during growth of this protective oxide layer. more...

The effect of molybdenum additions to γ-phase Ni-Cr on the reaction of the alloys with H2/H2S atmospheres at 700°C has been investigated. The effect is to reduce the rate of internal sulfidation of the alloys, diminish greatly the extent of liquid sulfide formation and slow the growth rate of external solid sulfide scale. The first two effects are thermodynamic in origin whilst the last effect is due to favorable doping of Cr2S3 and to the formation within a Cr3S4 scale of MoS2 precipitates which partially block outward metal diffusion. more...

The sodium oxide activity in the coexistence was determined between 823 and 1198 K by relating measured values of the electromotive force of cells of the typeand the Gibbs energy change of the cell reaction which can be written aswhere refers to halide or sulfide anion. The dependence of activity, , on temperature, , isAccordingly, the variation of the sodium vapor pressure of the coexistence, (atm), with temperature and oxygen pressure, (atm), follows the relationExtrapolation of this relation to higher temperatures demonstrates that attains 1 atm at 1350 K in the environment of the imposed oxygen pressure. more...

The oxidation kinetics of Co-Fe alloys containing up to 70 wt.% Fe have been determined at 1200°C in oxygen at pressures ranging from 10−4 to 1 atm. These kinetics, which were parabolic, resulted from metal diffusion through the scales as the rate-controlling process. The scales were either one-phase cobalto-wustite (CoFe)O or two-phase, containing growth of spinel cobalt ferrite (CoFe)3O4 in the cobalto-wustite matrix. The particular shape of the relationship, exhibiting a maximum for the parabolic oxidation constant vs. the iron alloy composition, is interpreted as being caused by the combined opposing effects of more rapid metal diffusion in cobalto-wustite and diffusional resistance of the spinel growths in cobalto-wustite with increasing iron alloy content. more...

Iron and iron-nickel alloys containing up to 40 wt. % nickel were oxidized in carbon dioxide-carbon monoxide atmospheres at 1000°C. Wustite was the only oxide formed in the superficial scales and subscales which exhibited preferential formation at alloy grain boundaries. The linear rate constants for the overall reaction kinetics showed a proportional dependence on the partial pressure of carbon dioxide. Equations based upon detailed balance of the surface reaction steps for dissociation of carbon dioxide and incorporation of chemisorbed oxygen into the wustite lattice accounted for the dependence of the reaction kinetics on the atmospheric and alloy compositions. more...

An investigation has been carried out on the morphological development and structure of the wustite-fayalite scale formed on a ferritic Fe-1.5 wt.% Si alloy exposed to carbon dioxide-carbon monoxide atmospheres at 1000°C. The amorphous silica film formed on the metallographically polished specimens crystallized to β-cristobalite at the reaction temperature. Wustite and fayalite developed within nodules which grew laterally to cover the alloy surface. Concurrent with growth of the nodules externally, oxygen diffusion into the underlying alloy led to precipitation of silica as α-tridymite. This internal oxidation zone was sufficiently depleted in silicon for its transformation to an austenitic phase. A fully developed scale was composed of an external wustite layer and an inner wustite-fayalite conglomerate layer, interspersed with discontinuous fayalite bands. more...

The diffusivity of oxygen in the monoclinic zirconia scale formed on zirconium has been determined at 750, 850 and 950°C by an oxygen-18 exchange technique. A comparison of these determinations to those determined for the self-diffusion of oxygen in monoclinic zirconia demonstrated that oxygen migration in the columnar structured scale occurred largely by short-circuit diffusion. A previously proposed two-phase diffusion model for concurrent growth of a superficial zirconia scale and oxygen solution in the metal accounts for parabolic oxidation kinetics insofar as the structure of the scale remains a time independent parameter. It was possible to demonstrate, by calculations from this model, firstly, that the oxygen concentration drop across the oxide scale was relatively small and of the order 4 × 10 −2 g / cm 3 and, secondly, that the magnitudes of the overall parabolic oxidation kinetics would be significantly decreased at temperatures less than 700°C if a zirconia scale exhibiting only lattice diffusion of oxygen could be formed on alpha-zirconium. more...

A study has been made of the growth of nickel oxide layers on the (100), (110), and (111) crystal faces of nickel exposed to oxygen for periods up to 100 hr at temperatures in the range 500–800°C, These layers grow at a rate dependent upon the crystallographic orientation of the nickel face even at thicknesses within the scaling range. The (100) face oxidized more rapidly than the (110) and (111) nickel faces while the latter faces oxidized at different relative rates dependent upon temperature. A diffusional model is employed to interpret the oxidation kinetics wherein nickel transport proceeds in nickel oxide both by short-circuit diffusion at grain boundaries and by lattice diffusion. more...

An investigation of the parabolic-linear oxidation of zirconium at 800° and 850°C is reported. It was found that irrespective of the oxygen concentration in the metal substrate, parabolic kinetics represent the growth of the initial compact oxide scale while linear kinetics represent the later stage s of growth. Metallographic observations indicated a transition from a compact to compact plus porous oxide scale. Accompanying hardness measurements demonstrated that oxygen penetrated into the metal via a steady concentration profile during linear oxidation. An exponential solution of the diffusion equation fitted to the concentration profile gave evaluations of the oxygen diffusion constant in the metal which are in good agreement with such evaluations from diffusion experiments. Several features of the oxidation kinetics of zirconium and titanium were correlated as further elucidation of the properties of metals in Periodic Subgroup IVB. more...

An investigation is reported on the oxidation properties of β-zirconium at 950°C in oxygen for periods extending to 100 hr. The kinetics were determined by a volumetric technique and may be represented by a parabolic relationship after a period of rapid oxidation. The thicknesses of the zirconium dioxide scale and the underlying layer of α zirconium-oxygen solid solution were determined by metallographic techniques. Parabolic diffusion relationships derived from a three phase oxide-metal model account for the kinetics of oxidation, scale growth and oxygen solution in the metal. more...

The principles governing formation of superficial oxides on metal surfaces are surveyed. Oxide crystals grow on a metal surface by rearrangement and place exchange within metal-oxygen substructures arising from the accumulation of oxygen by adsorption processes. The rate-controlling steps for growth of films are discussed by means of film models describing ion and electron migration under the influence of electrical potential gradients. These condiderations give rise to inverse logarithmic, direct logarithmic, cubic and parabolic time equations for film growth. The most rigorously tested theoretical basis in metal oxidation is that for the parabolic growth of scales based upon ambipolar diffusion of the reactants by point defects in oxides such as interstitials and vacancies. In several instances low resistance diffusion paths in oxides, such as grain boundaries and dislocations, cause leakage paths in films and short-circuit paths in scales. Several cases are discussed, where oxidation is control... more...

A study has been made on the influence of surface preparation involving abrasion, electropolishing, and vacuum annealing on the physical nature of the (100) nickel crystal face and on the morphological development of the oxide film. The structure of the initial surface was dependent upon the method of surface preparation, but electropolished specimens subjected to conventional or high vacuum anneals at 10−6 and 10−10 Torr, respectively, and temperatures of 800 and 700° C were structurally identical. Thermal faceting of surfaces prepared to correspond to the (100) plane was negligible. Oxidation at 500° C and over the oxygen pressure range 10−5−400 Torr led to formation of polycrystalline oxide crystallites randomly distributed over the surface during early stage exposures; the crystallites were only a few hundred Angstroms in size upon coverage of the surface by a nickel oxide film. These films were relatively uniform for thicknesses up to 2000 A. Crystallite growth processes led to three major epitaxial relationships between oxide and metal: (100) NiO , (111) NiO , and (211) NiO ∥ (100) Ni . more...

A transmission electron microscopy investigation was made of the structural defects in nickel oxide films formed at 500° and 600°C on metallographically polished polycrystalline nickel. These films within the thickness range 500–3000 A were composed of small crystallites. For a given film orientation, the crystallites increased in size with increasing oxidation time. This increase in crystallite size may be considered as a recrystallization reaction occurring during film growth. At later stages of film growth, the preferential migration of some boundaries led to the formation of large oxide grains. These latter films contained dislocations whose distribution varied from a random arrangement to well-defined subboundaries containing dislocations in low energy arrays. All observations were consistent with the conclusion that incoherent boundaries and dislocations in the oxide lying normal to the metal surface acted as short-circuit paths for selective diffusion of nickel. An oxide film model is advanced for correlation of the reaction kinetics to the observed structural defects. more...