The Internal Oxidation of Ternary Alloys. V: The Transition from Internal to External Oxidation of the Most-Reactive Component under Low Oxidant Pressures

This paper examines the conditions for the transition from internal to external oxidation of the most-reactive component C of ternary A–B–C alloys by a single oxidant under gas-phase oxidant activities below the stability of the oxide of the two most-reactive components using Wagners criterion. For this, approximate relations between the solubility and diffusivity of oxygen and the composition of the binary A–B alloy matrix in the zone of internal oxidation, already developed previously, are used. The critical C content needed for the transition in ternary alloys is calculated as a function of the many parameters involved. At variance with the behavior of binary alloys, for ternary alloys this critical C content depends also on the ratio between the concentrations of A and B in the bulk alloy. The results calculated for ternary alloys are compared with those obtained for binary A–C and B–C alloys under the same values of all the relevant parameters. Finally, complete oxidation maps for ternary alloys under low oxidant pressures,including the condition for the stability of external scales of the C oxide, are also presented.     

The Internal Oxidation of Ternary Alloys I: The Single Oxidation of the Most-Reactive Component Under Low Oxidant Pressures

The internal oxidation of the most-reactive component C of ternary A–B–C alloys by a single oxidant is examined assuming a gas-phase oxidant pressure below the stability of the oxides of the other two components. The precipitation of the most-stable oxide leaves behind a matrix composed of a binary alloy of the two less-reactive components, whose composition affects the solubility and diffusivity of the oxidant within the region of internal oxidation, with an effect on the reaction kinetics. Approximate relations between these properties are proposed and used to predict the kinetics of internal oxidation of C under the assumption of parabolic rate law. The results obtained for the ternary alloys are compared with the behavior of binary A–C and B–C alloys with the same C content. A new important factor in establishing the difference between the internal oxidation in ternary A–B–C alloys and in binary A–C and B–C alloys under a fixed gas-phase oxygen pressure and C content is the ratio between the concentrations of A and B in the bulk ternary alloy.     

The Internal Oxidation of Ternary Alloys. VI: The Transition from Internal to External Oxidation of the Most-Reactive Component under Intermediate Oxidant Pressures

The conditions for the transition from internal to external oxidation of the most-reactive component C of ternary A–B–C alloys are examined, assuming the presence of external scales of the oxide of the component of intermediate reactivity B. For this, approximate expressions for the diffusion coefficient of oxygen and for the concentration of oxygen dissolved in the binary A–B alloy matrix within the zone of internal oxidation as functions of the composition of the metal matrix within the zone of internal oxidation are used. Numerical calculations of the critical content of C needed for this transition are carried out for different combinations of values of the various parameters involved. The results obtained for the ternary alloys are compared with the corresponding data calculated for the binary A–C and B–C alloys under oxygen pressures insufficient to oxidize the most-noble alloy component. This allows to predict the possibility of existence of a third-element effect under intermediate oxidant pressures.     

The Internal Oxidation of Ternary Alloys. VIII: The Transition from the Internal to the External Oxidation of the Two Most-Reactive Components Under High-Oxidant Pressures

Ternary A–B–C alloys, where A is the most-noble and C the most-reactive component, exposed to oxygen pressures above the stability of the least stable oxide AO (high-oxidant pressures) may present two different types of internal oxidation, i.e. either a coupled internal oxidation of both B and C beneath an external AO scale or a single internal oxidation of C beneath an external scale of the oxide of B. This paper examines the conditions required to avoid the coupled internal oxidation of B plus C beneath external AO scales, considering both the case of formation of a single and a double front of internal oxidation. The analysis, based on an extension to ternary alloys of the criterion defined by Wagner for the transition between the internal and external oxidation of the most-reactive component of binary alloys, shows that the addition of B to binary A–C alloys is very effective in reducing the C content needed for this transition in comparison with binary A–C alloys. This results provides a basis for a possible explanation of the third-element effect. However, the actual possibility of its occurrence depends also on the ability to avoid other oxidation modes, not examined here.     

The Internal Oxidation of Ternary Alloys. III: The Coupled Internal Oxidation of the Two Most-Reactive Components under High Oxidant Pressures

The kinetics of the simultaneous internal oxidation of the two most-reactive components B and C in ternary A–B–C alloys, where A is the most-noble and C the most-reactive component, in the presence of an external layer of the A oxide, AO, (high oxidant pressures) are examined assuming the existence of either a single or a double front of internal oxidation. For a single front a unique solution is obtained for the parabolic rate constant of internal oxidation under assigned values of all the parameters involved. When the front is double, a finite range of solutions is allowed with an upper limit equal to the single-front solution and a lower limit equal to the rate constant for the growth of external AO scales. In the case of a single front of internal oxidation, an increase of the rate constant for the growth of the external scales produces an increase of the rate constant for the internal oxidation but a decrease of the degrees of enrichment of the components being oxidized internally within the region of internal oxidation. The behavior in the case of a double front is more complex because it depends also on the actual value of the ratio between the rate constants of internal oxidation for the two fronts.     

An Approximate Analysis of the External Oxidation of Ternary Alloys Forming Insoluble Oxides. I: High Oxidant Pressures

The general properties of the isothermal thermodynamic phase diagrams for ternary A–B–C alloys exposed to a single oxidant at high temperatures are examined, assuming, for simplicity, the formation of insoluble p-type oxides, an ideal behavior of the alloy, and a common value of the self-diffusion coefficient for the alloy components, independent of the alloy composition, but disregarding the possibility of internal oxidation. Simplified two-dimensional phase diagrams are produced by projecting the lines and points of equilibrium of the complete three-dimensional diagrams on the base triangle, which gives the composition of the system in terms of the metal components only. Similar kinetics diagrams concerning the nature and the growth kinetics of the scales formed on ternary alloys as functions of the bulk substrate composition are also calculated for oxidant pressures above the stability of the oxides of all the alloy components and some of their general features are examined. The kinetics diagrams for ternary alloys exposed to a single oxidant contain seven regions, three of which correspond to the areas of stability of each single oxide, three to the possible mixtures of two oxides, and one to a mixture of all three oxides.     

The Effect of Supersaturation on the Internal Oxidation of Binary Alloys

According to the theory of Bohm and Kahlweit ofthe internal oxidation of binary A-B alloys, theparabolic rate constant for the formation of reasonablystable internal BO oxides as well as theconcentrations of O and B at the oxidation front arecontrolled only by the degree of supersaturationnecessary for the nucleation of new oxide particles. Theeffects of this factor on the previous parameters arecalculated for various values of the solubility product ofthe oxide and of the diffusion coefficients of O and B.Moreover, an alternative procedure for the calculationof the critical degree of supersaturation behind the precipitation front required for oxideprecipitation, which is a function of the concentrationof the reactants at the internal oxidation front, isproposed. A simple modification of Wagner's theory of internal oxidation is also presented, andits results are compared with those of the treatment byBohm and Kahlweit. Finally, the limitations of the twomethods are examined.     

低氧压下三元合金最活泼组元单一内氧化的理论分析

对含单一氧化剂的气氛中氧分压低于A与B两组元氧化物分解压的情况，三元A-B-C合金系中最活泼组元C内氧化过程进行了理论分析.由于最活泼组元C内氧化生成氧化物的沉淀析出，令内氧化带的基体变成富集A、B两组元的合金.该合金的组成将影响氧在其 中的溶解度和扩散，进而影响内氧化反应动力学过程.对于内氧化遵循抛物线律的情况，我们提出了描述这种影响的一个近似关系式，并据此预测组元C的内氧化动力学.同时，对三元合金的预测结果与含有等量组元C的多种二元A-C 和 B-C合金的氧化行为进行了比较，提出并定义了“三元合金中A与B的浓度比” 的新参数.对于任何给定氧压和组元C含量的情况，借助该参数可以方便地界定三元A-B-C与二元A-C或B-C合金的内氧化行为间的差别.  〖HT5”H〗中图分类号：〖HT5”SS〗〓〓     

Internal oxidation of ternary alloys. Part II: Kinetics in the presence of an external scale

Classic kinetics equations of internal oxidation of binary alloys associated with the formation of an external oxide scale of the base metal have been simplified by applying mathematical asymptotical analysis. An analysis for the case of ternary alloys is also presented. The analysis involves simultaneous internal oxidation of the two solutes in the ternary alloys below the external oxide scale of the base metal. The kinetics equations derived are applied to calculate depths of internal oxidation zones of Ni-4 at.% Al-1 at.% Si alloys oxidized in 760 torr of oxygen at 800°C.     

An Approximate Analysis of the External Oxidation of Ternary Alloys Forming Insoluble Oxides. II: Low Oxidant Pressures

The construction and the properties of three-dimensional diagrams showing the regions of stability of the various compounds, which can form as a result of the oxidation of ideal ternary A–B–C alloys by a single oxidant at a constant temperature (kinetics diagrams) are examined for oxidant pressures insufficient to oxidize all possible alloys within the system (low oxidant pressures). For the calculation it is assumed that the various oxides do not dissolve into each other and do not form double oxides and that the alloy has an ideal behavior, while internal oxidation of the most-reactive components is disregarded. The range of meaningful oxidant pressures is divided into six intervals, which correspond to the formation of different types of scales. The simplified two-dimensional (2D) kinetics diagrams presented are obtained by projecting the appropriate three-dimensional (3D) lines of equilibrium between the alloy and the various oxides on the base triangle, which gives the composition of the system in terms of the three metal components only. The kinetics diagrams are correlated with the corresponding equilibrium phase diagrams for the same quaternary A–B–C–O systems.     

Complete Maps for the Internal Oxidation of Ideal Ternary Alloys Forming Insoluble Oxides under High Oxidant Pressures

This paper presents an analysis of the conditions of stability of the different forms of internal oxidation of ideal ternary A-B-C alloys, where A is the most noble and C the most reactive component, forming insoluble oxide and exposed to high pressures of a single oxidant. The treatment, based on an extension to ternary alloys of Wagner's criterion for the transition from internal to external oxidation in binary alloys, allows to predict the existence of three different forms of internal oxidation. In fact, in addition to the most common kinds of internal attack, involving the coupled internal oxidation of B+C beneath external AO scales and the internal oxidation of C beneath external BO scales, a third mode, involving the internal oxidation of C beneath external scales composed of mixtures of AO+BO, becomes also possible under special conditions. A combination of the boundary conditions for the existence of these different types of internal oxidation allows to predict three different kinds of complete maps for the internal oxidation in these systems, one of which involves only two modes, while the other two involve all the three possible modes of internal oxidation.     

The Internal Oxidation of Ternary Alloys. VII: The Transition from the Internal to the External Oxidation of the Two Most-Reactive Components Under Intermediate Oxidant Pressures

The conditions for the transition between the coupled internal oxidation of two most-reactive components and the formation of external scales in the scaling of ternary alloys under oxidant pressures below the stability of the oxide of the most-noble component, denoted as a situation of intermediate oxidant pressures, are examined under a number of simplifying conditions which allow to develop an approximate analytical treatment. If the precipitation of the two oxides occurs at the same front of internal oxidation, the kinetics of internal oxidation as well as the critical B and C contents needed for the transition have a single solution under fixed conditions of all the parameters involved. On the contrary, in the presence of two different fronts, when the most-stable oxide forms at the innermost front, a whole range of possible solutions is predicted. In both cases, the critical-C content needed to avoid the simultaneous internal oxidation of B plus C is progressively reduced by the addition of B. This behavior provides the basis for a possible interpretation of the “third-element effect”. However, the existence and the magnitude of this effect are complicated by the occurrence of other modes of oxidation for these systems. Thus, a general treatment of the third-element effect under intermediate oxidant pressures requires an exhaustive analysis of all the oxidation modes permitted for ternary alloys under these conditions.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2 at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5at.% aluminum has been studied at 800 and 900°C under 1atm O₂. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.     

Internal Oxidation of Ternary Alloys Forming a High Oxygen Conductive Oxide

Five ternary alloys consisting of a noble base metal (Ni, Co, Fe, Cu) and two reactive metals (Zr + Y, Ce + Gd) being able to form a high oxygen ion conductive oxide were internally oxidized under low oxygen partial pressures. All alloys developed either a continuous yttria-stabilized zirconia phase or a continuous gadolinia-doped ceria phase behind the front of internal oxidation. A Ni–Ce–Gd alloy showed extraordinarily high internal oxidation rates of up to 120 µm²/s at 900 °C. High internal oxidation rates in these ternary alloys were not limited to low concentrations of the reactive metals. The type of the internal oxide phase was found to be more important for the internal oxidation kinetics than the noble base metal.     

The Oxidation of cobalt-tantalum base alloys containing carbon

The oxidation of cobalt-tantalum carbon alloys, containing 10 and 15 wt.% Ta and carbon in the range 0–1 wt%, was carried out in oxygen and air at atmospheric pressure at 900, 1000 and 1100°C. The alloys oxidised according to the parabolic rate law with activation energy of about 38 Kcal/mole. In general, the addition of tantalum decreases the oxidation rates, in comparison with cobalt and with the same mass of chromium added to cobalt. Again, the presence of carbon in the Co-Ta alloys decreases its oxidation rates in comparison with carbon-free alloys. The scales formed on Co-Ta and Co-Ta-C alloys consist mainly of an outer layer of cobalt oxide, CoO, and an inner porous layer of mixture of oxides: cobalt oxide; CoO, tantalum oxide; Ta₂O₅, and solid solution of these two oxides; CoTaO₄ at all temperatures in the range of 900°-1100°C. The binary Co ?10% Ta and Co ?15% Ta show an internal oxidation along the internal phase, increasing of alloy tantalum content increases the density of the internal phase. The presence of carbon in the ternary Co-Ta-C alloys has little effect and there is no apparent preferential penetration along the tantalum carbide network. In contrast to carbide present in Co-Cr-C alloys, where these carbides were preferentially attacked, the outer scale was disrupted, due to the formation of carbon gaseous oxides.     

Oxidation of Two Ternary Fe–Cu–5 at.% Al Alloys in 1 atm of Pure O2 at 800°C

The oxidation of two two-phase ternary Fe–Cu–Al alloys containing about 5 at.% aluminium, one Fe-rich and one Cu-rich, has been studied at 800°C under 1 atm O₂. The Fe-rich alloy (Fe–15Cu–5Al) shows two parabolic stages, with a large decrease of the parabolic rate constant after about 2 hr. The presence of 5 at.% Al reduces significantly the oxidation rate of this alloy with respect to a binary Fe-Cu alloy of similar composition by forming an external alumina scale. Moreover, the addition of 15 at.% Cu is able to reduce the critical aluminium content needed to form alumina scales with respect to binary Fe–Al alloys. On the contrary, the Cu-rich Fe–85Cu–5 Al alloy presents a single parabolic stage and forms a thick and porous external scale composed of an outermost layer of copper oxides and an inner region containing a mixture of copper and Fe–Al oxides, coupled to the internal oxidation of iron and aluminium. As a result, the oxidation of the Cu-rich ternary alloy at 800°C is much faster than that of the Fe-rich ternary alloy.     

Oxygen solubility and a criterion for the transition from internal to external oxidation of ternary alloys

Smith's model is expanded in order to derive expressions to quantitatively describe the oxygen-solubility behavior in ternary alloys as a function of alloy composition. Multicomponent-diffusion theory is used to establish a criterion for the onset of internal oxidation beneath the external scale when oxidizing conditions favor formation of the oxide of the least-noble metal in a ternary alloy. The oxygen-solubility model and the criterion are applied to the oxidation of Ni–Cr–Al alloys in 76 torr of oxygen at 1100 and 1200°C, predicting the minimum Al concentrations required to form a protective Al₂O₃ scale. It shows that sufficient Cr additions would significantly reduce the oxygen solubility and also alter the oxygen distribution in the ternary alloys, avoiding the oxygen supersaturation necessary for the onset of internal oxidation. These two factors make it easier to establish the protective Al₂O₃ scale.     

Enhanced Oxygen Diffusion Along Internal Oxide–Metal Matrix Interfaces in Ni–Al Alloys During Internal Oxidation

A study of the internal oxidation of dilute Ni–Al alloys in an NiO/Ni Rhines pack was performed at 800, 1000, and 1100°C. Considerable deviations from the classical internal oxidation model have been observed. The rate of internal oxidation depends not only on the concentration of the alloying element but also on its nature, which contributes to determining the size, shape, orientation and distribution of the internal oxide precipitates. For instance, the precipitates in the Ni–Al alloys are continuous rods, arranged in a cone-shaped configuration that extends from the surface to the internal oxide front. The observed depths of internal oxidation for the various concentrations of aluminum are discussed and related to the morphologies of the internal oxide precipitates. The apparent N^(s) _oD_o values determined from internal oxide penetrations increase with increasing solute content in the alloy. It is postulated that diffusivity of oxygen is enhanced along the internal oxide–metal matrix interface compared with that in the metal matrix.     

The effect of supersaturation on the parabolic rate constant for internal oxidation with production of a pure oxide: An approximate analytical treatment

Internal oxidation in binary alloys leading to the precipitation of a pure oxide is examined, taking into account the finite value of the solubility product of the oxide in the base metal, and approximate analytical expressions for the profiles of the concentrations of oxygen and the less noble component are derived. In contradiction with the original analysis by Wagner, it is found that the knowledge of fundamental parameters like the solubility of oxygen and the diffusivity of oxygen and the oxidized metal in the alloy are not sufficient to calculate the parabolic rate constant for internal oxidation. This in fact depends also on the extent of supersaturation required to nucleate new oxide particles in front of the internally oxidized region. In absence of this information only an upper limiting value for the rate constant may be obtained, corresponding to zero supersaturation, while use of the experimental value of the rate constant enables the critical value of the supersaturation for the system examined to be calculated. In addition, the effect of ternary interactions on the oxygen diffusion in the alloy is examined and it is shown how the apparent product of the solubility and diffusivity of oxygen in the base metal as measured from internal oxidation experiments according to Wagner's formulas may be a function of the alloy composition, a fact which is not predicted by simpler treatments.