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The presence of a fine dispersion of a stable oxide is known to have a beneficial effect on the oxidation resistance of nickel- and cobalt-base heat-resisting alloys. This paper presents some preliminary experimental results relating to the hot-corrosion resistance of these alloys. Alloys forming CrO scales appear to be resistant to oxidation when coated with sodium sulfate, whereas an alloy normally forming an AlO scale suffers accelerated attack. During sulfidation some of the alloys suffer an accelerated degradation, with sulfur penetrating rapidly along what appear to be grain boundaries. The same effect is noted in sulfidation-oxidation experiments, when the CrO- forming alloys suffer accelerated oxidation, the effect of the dispersoid being apparently removed. An AlO- forming alloy resists this form of attack well. The sodium sulfate-coated test is probably a good guide to the behavior under weakly corroding conditions, whereas the sulfidation-oxidation test may give a better indication of the behavior under highly aggressive conditions. [ABSTRACT FROM AUTHOR]

As part of an overall research program to examine the possible development of sulfidation-resistant alloys, the corrosion behavior of binary iron-based alloys containing 5, 10, and 20 wt. % of aluminum has been studied in a hydrogenhydrogen sulfide atmosphere for the temperature range 800-1000°C. Details of the observed overall reaction kinetics, the scaling kinetics, and the scales' morphology and composition are given. By considering these results in relation to those reported in an earlier paper concerning the sulfidation of an iron- 5 wt. % aluminum alloy at a lower temperature range, 500-700°C, it has been possible to ascertain the usefulness of aluminum as an alloying addition whereby the sulfidation resistance of iron might be improved. It is concluded that although aluminum can be quite effective in this context at relatively low reaction temperatures at only the 5 % concentration level, at the elevated temperatures likely to be of more industrial importance its usefulness is not maintained, even at the 20% level. The possible reason(s) for the inferior high-temperature performance of these alloys are discussed. [ABSTRACT FROM AUTHOR]

Two fundamental errors are pointed out in a recent paper by Bridges and Fassell Jr. Both concern the application of the law of mass action to a heterogeneous equilibrium, the first in the case of the sorption equilibrium, the second for the external interfacial reaction. These errors lead to an incorrect derivation of the pressure law for the oxidation of nickel and cobalt. [ABSTRACT FROM AUTHOR]

This paper describes some oxidation studies of evaporated aluminum films. Resistance marker measurements were carried out between 230 and 400° C and it was concluded that the oxide grows by metal transport. From the sign of its Seebeck coefficient, the oxide was deduced to be n-type. The effect of applying an electric field across the growing oxide layer on aluminum was also investigated. A porous platinum layer evaporated onto the oxide surface was used as one electrode, the underlying metal being the other electrode. At all temperatures between 50 and 400° C the same field effect was observed. When the oxygen-oxide interface was biased negative with respect to the aluminum, an enhancement of the oxidation rate was achieved. These results have been interpreted in terms of the Mott-Cabrera theory. [ABSTRACT FROM AUTHOR]

Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces 'dangling' bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands. [ABSTRACT FROM AUTHOR]

This paper describes phase formation and mass transport during the reaction of iodine-refined titanium with nitrogen at 760 Torr and 800 to 1200°C. Each titanium-nitrogen phase is present within the composition and temperature range of its thermodynamic stability. Between 882 and 1110°C there are the nitrides, TiN and TiN, and solutions of nitrogen in α and β-titanium. There is no β-titanium below 882°C or TiN above 1110°C. The growth of each phase layer involves some favorably oriented grains from among the many that are nucleated. Nitrogen concentration gradients exist in all phases. Nitrogen is the predominant diffusing species in all phases but a small amount of outward diffusion of titanium occurs through the nitrides, producing α-phase porosity above 1100°C. There is some porosity in the outer regions of TiN. All phases remain mechanically sound and coherent with each other. Changes in the relative proportions of phases during cooling are identified, which enables corrections to be made to kinetic data for the reaction. [ABSTRACT FROM AUTHOR]

Some recent developments in the understanding of the oxidation of alloys at elevated temperatures are reviewed, with special reference to binary and ternary alloys upon which many commercial materials are based. Following an initial classification of alloy systems, certain basic principles and their limitations are considered, including factors determining whether an alloy displays surface scaling only, internal oxidation only, or both phenomena; the mode of distribution of cations in an oxide solid solution growing on an alloy; and doping and the Wagner-Hauffe rules. Previous alloy oxidation theory has concerned itself principally with steady-state oxidation but, in practice, behavior of the oxidizing sample in approaching the steady state, both upon initial exposure to the environment and after scale spalling induced isothermally or during thermal cycling, is often as important. Such behavior is illustrated in terms of scale establishment and with respect to compositional changes in the subjacent alloy, the importance of certain alloy and oxide properties being emphasized. After further brief consideration of steady-state scaling, the causes and consequences of breakaway oxidation are presented. The role of major and minor concentrations of third elements on scaling behavior is discussed. The paper concludes with brief reference to situations not considered in detail and with aspects requiring further study. [ABSTRACT FROM AUTHOR]