Showing papers in "Oxidation of Metals in 1980"

Corrosion of metals and alloys in mixed gas environments at elevated temperatures

Abstract: The attack of nickel, cobalt, iron, and alloys of these metals containing chromium and aluminum, by gases containing sulfur-oxygen, carbon-oxygen, and nitrogen-oxygen has been studied at temperatures of 600 and 900°C. The degradation of these metals and alloys was characterized by using standard analytical techniques with emphasis on optical metallography. Three types of accelerated degradation were identified for the attack of alloys by gases containing another oxidant in addition to oxygen. One type of degradation occurred because of the formation of reaction products composed of mixtures of phases involving both of the oxidants. Another type resulted from the reaction of second oxidant phases with oxygen. The third form of degradation involved the development of less protective phases due to thermodynamic instabilities. Thermodynamic stability diagrams are used to help account for the effects produced by different elements in the alloys.

The early stages of development of α-Al2O3 scales on Fe-Cr-Al and Fe-Cr-Al-Y Alloys at high temperature

Abstract: The development ofthe oxides on Fe-14%Cr-4%Al, Fe-27%Cr-4%Al, and similar alloys containing 0.008% Y, 0.023% Y, and 0.8% Y has been investigated during the early stages of oxidation in 1 atm oxygen at 1000 and 1200°C. In all cases, a steady-state α-Al2O3layer is established rapidly, after some initial formation of transient oxides rich in iron and chromium. For the yttrium-free alloys the steady-state situation is achieved more rapidly for the higher chromium-containing alloy and at the higher temperature. The amount of transient oxide formed is also determined by the specimen surface topography since the development of the α-Al2O3 layer is less rapid at the base of alloy asperities than at a flat alloy-oxide interface. Following establishment of the complete α-Al2O3layer, the oxide develops a convoluted oxide morphology at temperature, due to high compressive growth stresses in the oxide. These arise following reaction between oxygen ions diffusing inward down the oxide grain boundaries and aluminum ions diffusing outward through the bulk oxide. This results in lateral growth of the oxide and plastic deformation and movement of the alloy in a direction parallel to the alloy-oxide interface. The addition of yttrium to the alloys promotes the selective oxidation of aluminum. Also, the yttrium is incorporated into the growing oxide where it changes the mechanism of growth, reducing the production of the high compressive growth stresses and thus the development of the convoluted oxide morphology.

The development of localized pits during stainless steel oxidation

Abstract: A study of the localized pitting attack of 20% Cr/25%Ni stainless steel and a similar alloy containing a dispersion of titanium nitride particles has been made over the temperature range 1023 to 1173 K. Pitting is initiated when localized spoiling of the protective chromic oxide film occurs. Rapid oxidation of the chromium-depleted substrate then proceeds with the formation of an iron-rich oxide mound on the alloy surface and spinels containing nickel, chromium, and iron within the pit itself. A silica layer which, in general, remains on the alloy surface acts as a diffusion barrier during this stage of the reaction. With increasing depth of attack the local chromium concentration in the alloy at the base of the pit attains a critical value (∼16%) for a protective chromium oxide film to reform; the pitting attack then effectively ceases, although a a subsequent slow rate of growth continues through the protective film at the base. The observed maximum depth of pitting as a function of time is consistent with the parabolic variation predicted by the proposed mechanism. There is no significant difference in the kinetics of attack between the alloys examined.

Influence of small Pt additions on Al2O3 scale adherence

Abstract: The effects of small Pt additions (1 or 3 wt.%) on the oxidation behavior of Co-10Cr-11Al and a similar alloy containing Hf have been studied. An intermetallic phase was present in the alloy containing Hf and Pt but not in that containing Pt alone. The size and distribution of the intermetallic was comparable to that of similar alloys containing oxide dispersions produced by a controlled internal oxidation treatment. As a consequence it promoted the formation of inwardly growing Al2O3 pegs that helped key the surface scale to the substrate and improve the scale-metal adhesion in both isothermal and cyclic oxidation tests. The improvement in overall oxidation resistance relative to an addition-free alloy was considerable, and similar to that of the best oxide dispersion-containing alloys.

Application of auger electron spectroscopy and inert metal marker techniques to determine metal and oxygen transport in oxide films on metals

Abstract: 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.

Correlation between the oxidation mechanism of titanium under a pure oxygen atmosphere, morphology of the oxide scale, and diffusional phenomena

Abstract: The oxidation behavior of titanium was studied over the temperature range of 600 to 800°C in a pure oxygen atmosphere. A parabolic kinetics period is made up of a succession of short parts whose weight gain is nearly constant and whose beginning is characterized by an alteration of the oxidation rate. The kinetics curve is closely correlated with the morphology of the oxide scale which contains several layers separated by short cracks. These cracks decrease the diffusional flows of oxygen and titanium and alter the oxidation rate. A parabolic-linear kinetics transition is attributed to the formation of a continuous crack between the oxide scale and the matrix. The oxide scale then forms a porous barrier of constant thickness which induces a linear rate law due to the steady-state oxygen diffusion. Titanium cannot diffuse across the continuous crack. During the linear kinetics period several layers of constant thickness, separated by a continuous crack, spread out.

The mechanism of corrosion of Fe-9%Cr alloys in carbon dioxide

Abstract: Oxide layers have been grown on Fe-9% Cr, Fe-9% Cr-0.3% Si, and Fe 9% Cr-0.6% Si alloys in carbon dioxide at 853 °K. It is known that such oxides are duplex, the outer layer being magnetite, formed by iron transport. The inner layer is Fe-Cr spinel but little is known about its growth mechanism so this has been investigated using oxygen-18 as a tracer. Oxides were grown first in C16O2 and then in C18O2 and the distribution of oxygen-18 in the scale measured using nuclear techniques. For all the alloys used, significant amounts of oxygen-18 were observed within the inner layer in addition to growth of18O-rich magnetite at the outer surface. The two possibilities of the oxygen-18 being present as a consequence of isotopic exchange or because new oxide had formed within the spinel layer are discussed. Our conclusion is that it is very unlikely that significant isotopic exchange had occurred in any part of the scale, and we deduce that at least a substantial amount of the oxygen-18 in the inner layer was deposited as a result of new oxide formation within that layer. The results also indicate that the location of growth sites within the inner layer differed between the alloys.

Effect of carbon on cavity formation during the high-temperature oxidation of Ni

Abstract: Ni containing 0.00002 to 0.003 wt.% carbon was oxidized in 1 atm O2 at 700 to 1270°C for 1 min to 20 hr. Cavity formation in the metal and oxide and at the oxide-metal interface was affected by the carbon content. Extensive cavitation developed at the grain boundaries of Ni containing as little as 0.0001 wt.% carbon but no cavitation occurred in decarburized Ni. Metal cavitation is dependent on the local concentration of carbon segregated at the Ni grain boundaries rather than on the overall carbon content. The cavities arise from hot deformation or creep of the metal substrate induced by the oxidation process. The cavities in the oxide formed on decarburized Ni remain near the oxide-metal interface; on Ni containing appreciable carbon the oxide cavities migrate outward by a dissociative mechanism assisted by the gaseous transfer of oxygen across the cavities with CO-CO2 acting as the carrier.

Oxidation of Ni-Cr-W Ternary Alloys

Abstract: The oxidation behavior in air at 1000–1250°C of four Ni-Cr-W alloys containing sufficient chromium content (∼22 at. % Cr) for protective Cr2O3 formation in a binary Ni-Cr alloy is reported. Generally for alloys high in W (10 and 16 at.% W), the rejection of tungsten into the alloy beneath the scale introduced a steep Cr concentration gradient and slower Cr diffusion such that continuous precipitation of Cr2O3 internal oxides prevented the formation of a Cr2O3 protective scale. The alloy most dilute in W (1.6 at. % W) formed a protective scale at short times with little outer NiO scale, but scale fractures led to internal oxidation and rapid nonprotective kinetics. After an initially rapid oxidation increment to form NiO, the 3 at.% Walloy formed a protective Cr2O3 scale with about the same steady-state parabolic kinetics as a binary Ni-30Cr alloy. The effect of ternary Wadditions on the development of Cr2O3 scales on Ni-Cr-W alloys is considered as a ternary analog to Wagner's description of the oxidation of Cu-Pt or Cu-Pd alloys.

High-temperature sulfidation of Fe-Mn alloys

Abstract: The complete range of binary Fe-Mn alloys were sulfidized in pure sulfur vapor (1 atm) at 700–1000°C. It was found that the corrosion kinetics could be divided into four groups, depending upon the alloy concentration. The first group, up to 11 w/o Mn, had a constant corrosion rate. The second group, up to about 63 w/o Mn, had an exponentially decreasing rate of corrosion with increasing manganese content. In the third group, about 63–80 w/o Mn alloys, the corrosion rate did not follow the parabolic law. The corrosion rates became constant above 80 w/o Mn. These corrosion kinetics were explained by considering the scale structure and alloy substrate.

High-temperature corrosion of pure chromium in SO2 (700–1000°C)

Abstract: The corrosion of chromium under 1 atm of pure SO2 has been studied over the range 700–1000°C at 100°C intervals. The reaction rate is higher than in pure oxygen and the scale does not contain sulfides, but only a small concentration of sulfur, in contrast with the behavior of other transition metals, even though the thermodynamic conditions are similar. Possible causes for this peculiar behavior are examined and discussed with reference to the results of the corrosion of chromium in pure oxygen.

The influence of yttrium additions on the oxidation resistance of a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Abstract: Isothermal oxidation of a directionally solidified Ni-Al-Cr3C2 eutectic alloy results in development of an external α-Al3O3-rich scale. However, this scale breaks down after relatively short times at temperature and a less protective Cr2O3-rich scale is formed, together with substantial internal oxide in the alloy. In an attempt to maintain the external α-Al2O3-rich scale and prevent damaging subscale oxidation, modified yttrium-containing directionally solidified alloys have been developed. The oxidation resistance of these alloys at 1000 and 1100°C in flowing air has been investigated and found to be considerably better than that of the corresponding yttrium-free alloy. At both temperatures an external α-Al2O3-rich scale is produced and is retained for much longer periods than on the yttrium-free alloys during isothermal and thermal cycling oxidation. Some scale breakdown does occur during thermal cycling at 1100°C, but α-Al2O3 is able to re-form as the surface oxide. However, although external α-Al2O3-rich scales are retained for long periods on these alloys, some oxide penetration into the alloy beneath these scales does occur where coarse carbide fibers intersect the alloy surface. This is associated with relatively poor scale integrity at these intersections.

Comparison of the kinetics and morphologic properties of titanium, Ti-1.5Ni and Ti-2.5Cu during oxidation in pure oxygen between 600 and 820°C

Abstract: The kinetics and morphologic oxidation properties of titanium, Ti-1.5Ni and Ti-2.5Cu were compared. Titanium and Ti-1.5Ni have a similar behavior, concerning the kinetics and the oxide micro structure. Copper additions decrease the oxidation rate of titanium. The oxide scales formed on Ti-Cu are thinner and less cracked than those obtained on Ti or Ti-1.5Ni. Copper is found in the oxide scale of Ti-Cu, whereas nickel is not found in the oxide scale of Ti-Ni. The oxidation of titanium and its alloys is controlled chiefly by diffusional phenomena in the oxide scale. Thus the alterations of the oxide scale structure and the slower oxidation rate of Ti-2.5Cu can be attributed to the copper which diffuses towards the gas-oxide interface.

High-temperature oxidation of Ti-4.32 wt.% Nb alloy

Abstract: The oxidation kinetics of Ti-4.32 wt.% Nb (2.27 at.%) alloy in either air or oxygen under 1 bar pressure have been investigated in the temperature range of 1255–1471 K. For oxidation in oxygen the kinetics follow consecutively decreasing parabolic rate laws. The kinetics become nonparabolic after prolonged oxidation in air. X-ray data reveal the presence of TiO2 (rutile) as the main oxide. The presence of Ti2O, δ-TiN, and ɛ-TiN is found on the metal surface after oxidation in air. Microhardness and electron microprobe measurements are used to determine Ti, Nb, O, and N profiles across the oxidized alloy. The oxygen diffusion coefficient in the alloy can be expressed byD=65.6 exp(−234.3/RT) with the activation energy in kilojoules per mole. The oxidation behavior of the alloy is explained.

The role of thermal shock in cyclic oxidation

Abstract: The effect of thermal shock on the spalling of oxides from the surfaces of several commercial alloys was determined. The average cooling rate was varied from approximately 240°C/sec to less than 1.0°C/sec during cyclic oxidation tests in air. The tests consisted of 100 cycles of 1 hr at the maximimum temperature (1100 or 1200°C). The alloys were HOS-875, TD-Ni, TD-NiCrAl, In-601, In-702, and B-1900 plus Hf. All of these alloys exhibited partial spalling within the oxide rather than total oxide loss down to bare metal. Thermal shock resulted in deformation of the metal, which in turn resulted, in most cases, in change of the oxide failure mode from compressive to tensile. Tensile failures were characterized by cracking of the oxide and little loss, while compressive failures were characterized by explosive loss of platelets of oxide. This behavior was confirmed by examination of mechanically stressed oxide scales. The thermally shocked oxides spalled less than the slow-cooled samples with the exception of TD-NiCrAl. This material failed in a brittle manner rather than by plastic deformation. The HOS-875 and the TD-Ni did not spall during either type of cooling. Thus, the effect of thermal shock on spalling is determined, in large part, by the mechanical properties of the metal.

The Influence of Ternary Additions of W, Mo, Ti, Ta, and Nb on the Isothermal and Cyclic Oxidation of Ni-10Cr Alloy

Abstract: Isothermal and cyclic oxidation studies have been carried out on Ni-10Cr binary alloy and ternary alloys containing respectively 8.0 wt.% Mo, 14.3 wt% W, 4.2 wt.% Ti, 7.7 wt.% Nb, and 14.1 wt.% Ta (i.e., ∼5 at.%, respectively). Tests were carried out for a total duration of 100 hr at 1000°C in a flowing simulated flue gas mixture apportioned to 81 vol. % N2, 16 vol. % O2, and 3 vol. % CO2. Various cycling frequencies were used in the thermal cycling studies. Alloys containing Ta (especially) and Nb behaved better during both isothermal and cyclic oxidation, while additions of Mo and W resulted in poor oxidation properties and spallation, especially at higher cycling frequencies. Self-healing oxides rich in Cr were found to be significant in the former cases, there being some benefit to be derived from Ta- or Nb-containing oxides. Ti which was of benefit isothermally was of little use under cyclic conditions.

The oxidation of Ni-70 wt.%Cr in oxygen between 1073 and 1473°K

Abstract: Oxidation of the relatively simple, two-phase alloy Ni-70 wt.%Cr in oxygen between 1073 and 1473°K results in the formation of a Cr2O3 scale containing less than O.5 wt.% Ni in solid solution. The oxidation kinetics are irreproducible for an initial period, which is brief at 1073 and 1273°K but much more pronounced at 1473°K, both in duration and degree. This behavior is associated with the failure of the protective Cr2O3 scale. However, after longer periods a compact layer of Cr2O3 becomes established under isothermal conditions and results in a change to more reproducible kinetics, especially at 1073 and 1273°K. Oxidation causes chromium depletion and the formation of a single-phase zone which separates the scale and the two-phase bulk alloy. The depth of Cr2O3 internal oxide coincides with this zone. The oxidation behavior is compared with that of more Ni-rich, single-phase Ni-Cr alloys, with particular reference to the effects of the constitution of the underlying alloy and the integrity of the protective oxide.

High-temperature corrosion of manganese in pure SO2

Abstract: The corrosion of manganese in 1 atm of pure SO2 has been studied from 700 to 900°C. The reaction shows two approximately parabolic stages, the first being faster; the reaction products are MnO and MnS. In the early moments of the reaction a fine mixture of oxide and sulfide is formed directly at the scale surface, which is not in equilibrium with the gas phase due to rate control by the surface reaction. This process ends quickly due to the lowering of the Mn activity at the scale surface, and then an outer region of MnO surmounting a dispersion of coarse MnS particles in the oxide develops. The rate control shifts to solid-state diffusion, and the scale surface equilibrates with the gas. The sulfide is produced in this stage in the interior of the scale as a consequence of sulfur penetration by two mechanisms whose relative importance is discussed. The reaction produces a protective scale, and the rate is close to that for oxidation of Mn in 1 atm O2 at the same temperature.

Reequilibrium kinetics of NiO-Cr2O3 solid solutions

Abstract: Electrical conductivity has been measured to monitor the reequilibration kinetics for single crystals of NiO-Cr2O3 solid solutions. It has been found that the rate for the reduction process is higher than that for the oxidation runs, thus indicating that the obtained kinetic data are not purely bulk controlled. The following expressions for the apparent chemical diffusion coefficient have been obtained within the temperature range 900–1200°C and oxygen partial pressure range 1–10−5 atm: $$\begin{gathered} \tilde D_{1 red} = 1.22 \times 10^{ - 2} exp \left( {\frac{{24,420 \pm 1210 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{1 oxid}^* = 1.44 \times 10^{ - 2} exp \left( {\frac{{27,340 \pm 700 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{2 red} = 2.29 \times 10^{ - 2} exp \left( {\frac{{25,340 \pm 2230 calmole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{2 oxid}^* = 0.109 exp \left( {\frac{{29,610 \pm 3200 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{3 red} = 3.16 \times 10^{ - 2} exp \left( {\frac{{26,020 \pm 2430 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{3 oxid}^* = 0.202 exp \left( {\frac{{31,500 \pm 2640 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \end{gathered} $$ .

High-temperature oxidation of directionally solidified Ni-Al-Cr3C2 eutectic

Abstract: The isothermal oxidation behavior of a directionally solidified Ni-Al-Cr3C2 eutectic alloy in flowing air at 1100° C has been studied using various physical techniques. Initially, the alloy oxidizes relatively slowly as a protective α- Al2O3 layer develops on its surface. However, this layer breaks down mechanically at temperature, enabling a more rapidly growing Cr2O3 -rich scale to develop, the carbide fibers supplying chromium for the oxidation process. The extent of oxidation is further increased by the development of substantial amounts of internal oxide in the alloy beneath this scale, with some preferential oxidation down the alloy-carbide fiber interfaces. The influence of carbide fiber orientation on the oxidation behavior is considered and discussed in relation to proposed oxidation mechanisms.

Sulfate-induced high-temperature corrosion of nickel

Abstract: Corrosion of high-purity nickel with deposits of sodium sulfate has been studied at 900°C in O2+4.2 % SO2 at gas pressures ranging from 700 to 0.5 Torr. Corrosion rates in the gas mixture are faster than in oxygen or sulfur dioxide alone. At the higher pressures the initial reaction is parabolic, but subsides after extended reaction. The parabolic reaction is concluded to comprise a diffusion-controlled transport of oxygen atoms and sulfur dioxide molecules through the molten sulfate layer with dissolved nickel oxide. Ni-S liquid solution is formed at the metal surface during the reaction. At reduced gas pressures the initial reaction is nonparabolic (linear and subsequent gradually increasing rate) prior to the parabolic stage. After extended reaction at sufficiently low partial pressure of SO2 or reaction in 1 atm O2 significant amounts of Ni-S liquid solution cannot be detected at the metal surface below the sulfate melt. The reaction mechanism is illustrated and discussed in terms of the stability diagrams of the Na-O-S and Ni-O-S systems.

Phase relations in the Fe-Cr-O system at 1200 degrees C investigated by means of a diffusion couple technique

Abstract: The morphology and composition of diffusion layers formed in multiphase ternary diffusion couples of Cr or Cr-Fe alloy vs. FeO or FeCr2O4 have been studied by electron probe microanalysis, X-ray diffraction, and microscopy. A new technique for annealing of the couples has been developed. Phase relations at 1200° C of the ternary system Fe-Cr-O are discussed, and diffusion paths of the investigated couples are plotted on the isothermal section of the ternary phase diagram.

Oxidation characteristics of Ti-4.37 wt.% Ta alloy in the temperature range 1258–1473 K

Abstract: Oxidation kinetics of Ti-4.37 wt.% Ta (1.19 at.%) alloy in either air or oxygen have been investigated in the temperature range 1258–1473 K and at three pressure levels: 0.013, 0.133, and 1.0 bar. X-ray data reveal only TiO2 (rutile) as the main oxide and, in addition, TiO2, S-TiN, and ɛ-TiN at the metal surface for oxidation in air at 1273 K. The ɛ-TiN phase was not detected at higher temperatures. Electron microprobe analyses are used to determine the concentration profiles of Ti, Ta, O, and N across the oxide scale and in the metal. Microhardness traverses follow the oxygen distribution in the alloy. The oxygen diffusion coefficient,D = 60.8 exp(−235.1/RT), as determined from weight-gain measurements, is in reasonable agreement with that calculated from microhardness data. The oxidation mechanisms for oxidation of the alloy in air or oxygen are proposed.The existence of interstitial Ti cations in the oxide is suggested.

ESCA study of oxidation and hot corrosion of nickel-base superalloys

Abstract: A study of the high-temperature oxidation and Na2SO4-induced hot corrosion of nickel-base superalloys has been accomplished by using ESCA to determine the surface composition of the oxidized or corroded samples Oxidation was carried out at 900 or 1000 C in slowly flowing O2 for samples of B-1900, NASA-TRW VIA, 713C, and IN-738 Hot corrosion of B-1900 was induced by applying a coating of Na2SO4 to preoxidized samples, then heating to 900 C in slowly flowing O2 For oxidized samples, the predominant type of scale formed by each superalloy showed a marked surface enrichment of Ti For corroded samples, the transfer of significant amounts of material from the oxide layer to the surface of the salt layer was observed before the onset of rapidly accelerating weight gain Marked changes in surface composition coincided with the beginning of accelerating corrosion, the most striking of which were a tenfold decrease in the sulfur to sodium ratio and an increase in the Cr(VI) to Cr(III) ratio

The effects of gaseous pretreatment and oxide sintering on oxidation of Ti-4.32 wt.%Nb alloy

Abstract: Oxidation kinetics in either air or oxygen of prenitrided Ti-4.32 wt.% Nb alloy are investigated in the temperature range of 900–1200°C. Based on X-ray and electron microprobe analyses, thermogravimetric measurements, microhardness data, and sintering experiments, a quantitative oxidation model is developed to explain the gas-metal reactions. Temperature cycling experiments, in the temperature range of 900–1200°C and for a 12-hr duration, are performed in an attempt to reduce the oxidation rates of the alloy in air or oxygen. The oxidation resistance of nitrogen pretreated alloy is comparable to that in air and it is considerably higher than in oxygen alone.

The influence of an intermediate annealing treatment on the oxidation of copper and nickel

Abstract: A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics of copper and nickel in 1 atm oxygen at 800 and 1100°C, respectively, using a semiautomatic microbalance. Changes in scale morphology and composition have been investigated using various physical techniques. The outer CuO layer formed on copper during oxidation dissociates very rapidly on annealing to give CU2O and oxygen since the partial pressure of oxygen in the gas is below the dissociation pressure of CuO but above that of Cu2O at 800°C. The CuO layer is quickly re-formed on reoxidation in oxygen. There are relatively few other changes in the oxide morphologies of either metal during annealing, although the small grains present in the scale adjacent to the metal after oxidation are able to grow. During reoxidation both metals show a reduction in oxidation rate constant because of the decrease in total cation vacancy concentration in the scale and the reduced cation vacancy gradient across the scale brought about by the reduction in oxygen partial pressure at the oxide-gas interface during annealing. The reoxidation rate constants following annealing approach those recorded prior to annealing as the equilibrium cation vacancy levels in the scales are reestablished in the oxidizing environment. Rosenberg's method for analysis of the kinetics of reoxidation has enabled the equilibrium concentrations and diffusion coefficients of cation vacancies in CU2O and NiO during oxidation in 1 atm oxygen at the appropriate temperatures to be estimated approximately. These show reasonable agreement with literature values.

“Anomalous” oxide growth during transient-temperature oxidation of zircaloy-4

Abstract: Oxidation tests of Zircaloy-4 pressurized water reactor tube specimens in steam were conducted for several types of temperature excursions. These tests were used to evaluate the accuracy of predictions of the oxidation behavior based on ideal models that employ isothermal kinetic data. While oxidation during simple transient temperature excursions, where the peak temperature was between 1000 and 1400°C, could be predicted with reasonable accuracy, certain “two-peak” transients resulted in oxide layers considerably less than those anticipated. On the basis of our experiments, we propose that this effect is related to the manner in which the monoclinic-tetragonal phase transformation occurs in the oxide and the fact that the normal (bulk) transformation hysteresis is altered for a growing oxide film.

Oxidation resistance of aluminized coatings on a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Abstract: Although a directionally solidified Ni-Al-Cr3C2 eutectic alloy has good high-temperature mechanical properties, it does not have adequate oxidation resistance for prolonged exposure to high surface temperatures Thus the oxidation behavior of several aluminized coating systems on this alloy in flowing air at temperatures of 900 to 1100°C under isothermal and thermal cycling conditions has been investigated Attempts to produce an oxidation-resistant system by direct aluminizing have not been successful since removal of carbide fibers results in a porous coating which gives little protection to the alloy The deposition of a layer of nickel or a Ni-20%Co-10%Cr-4%Al alloy on the eutectic prior to aluminizing gives improved isothermal oxidation resistance for prolonged exposure to high surface temperatures Thus the eutectic alloy substrate occur during thermal cycling A more successful system has been produced by depositing a thin layer of platinum on the eutectic alloy prior to aluminizing Protective α-Al2O3 scales are formed and maintained during isothermal and thermal cycling oxidation at 900 and 1000°C Similar scales are developed at 1100° C although these do break down during thermal cycling However, surface α-Al2O3 scales are able to re-form rapidly, thereby preventing excessive oxidation of the coating

Stress relief and plastic flow in nickel oxide at elevated temperatures

Abstract: Stress relief and plastic flow modes in nickel oxide over the temperature range 1323–1473 K have been studied, involving an assessment of its creep-in-bend behavior in the form of small sintered beams, of average grain sizes in the range 2–10 μ, and of densities of ∼70% of the theoretical maximum; conditions of three-point loading were adopted involving a range of maximum fiber stress of 2 × 106 −8×107 N · m−2, that is, of the magnitudes likely to arise during the growth of nickel oxide scales on metallic nickel. After subtraction of any observed threshold stresses, most specimens were found to exhibit a stress index of about unity. The grain-size index was determined to be −2.5. It is believed that the dominant creep mechanism was probably that of Nabarro-Herring.

Hot corrosion of an aligned co-based, TaC-Strengthened eutectic alloy

Abstract: A study of morphology of simple oxidation and hot corrosion of the aligned Co-10 wt.% Cr-14 wt.% TaC alloy was performed at 900°C in still air. The morphology of simple oxidation was found to correlate well with previous results on similar alloys. Hot corrosion tests were performed by coating the samples with a thin layer of Na2SO4 and exposing them to still air at 900°C. A radically different morphology was found and extensively analyzed using optical microscopy and the electron microprobe. These results are then explained using a localized reduction model of hot corrosion.