The Corrosion Performance of Nickel-Based Alloys containing group II-V elements in a gas turbine material test rig

The corrosion of eight Ni-based ternary alloys (15 w/o Cr plus 1 w/o X, where X = Mg, La, Ce, Gd, Th, U, Zr or V), and a binary control alloy (15 w/o Cr) has been assessed in a rig designed to simulate combustion conditions in gas turbines operating on a contaminated Diesel fuel. Performance has been evaluated in terms of metal loss (radius measurements), scale thicknesses and depths of internal corrosion after 10 and 200 h exposure: the morphology and composition of corrosion products have also been examined. Although the pattern of oxide scale formation was broadly similar from alloy to alloy, considerable differences in the type and depth of the internal corrosion taking place were apparent. The possibility of the use of some of these addition elements in future turbine materials, particularly coatings, is considered in the light of these observations.     

A comparison of the high temperature nitridation and oxidation behaviour of metals

Published information concerning the nitridation behaviour of a large number of elements from Groups II–VIII of the Periodic Table is summarized. Analogies and differences between this type of gas/metal reaction, and the process of oxidation, are highlighted and interpreted by compiling and considering fundamental factors which determine the nature of such phenomena—the physicochemical properties of nitrides and oxides (and especially the bonding, non-stoichiometry and defect structures of nitrides), diffusion data for reactive species in the compounds, the solubilities of the gases and their diffusion rates in metal substrates. It is concluded that while many factors are associated with observed differences in reaction kinetics, particularly as exemplified by the behaviour of metals from within Groups IV and V, it is the often-unique physico-chemical properties of nitrides which are primarily responsible for such differences.     

The creep of CoO scales — the influence of stoichiometry,stress and temperature

The plastic deformation of cobalt monoxide scales formed by complete thermal oxidation of high purity cobalt sheet has been assessed in the temperature range 925 to 1050° C at different oxygen pressures. By observing the creep-in-bend behaviour using a three-point-loading technique it has been demonstrated that these scales undergo considerable plastic deformation, both primary and secondary creep being exhibited. It has been shown that the creep behaviour of the oxide is strongly dependent on oxide composition, the scales becoming more plastic as the non-stoichiometry of the material as defined by the ambient oxygen pressure surrounding the specimen, is increased. In particular, the secondary creep rates increase linearly with increasing defect concentration. Measurements of the stress dependence of the secondary creep rates suggest that the deformation involves a process other than the Nabarro-Herring mechanism of stress-directed vacancy diffusion. An activation energy for creep of 46 kcal/mole was obtained.     

High temperature corrosion of alloys containing rare earth or refractory elements: a review of current knowledge and possible future developments

The beneficial effects of minor (≤1%) additions of rare earth/reactive elements on the oxidation of HT alloys have been known for many years. Much interest has recently centred on the influence of certain refractory elements present at levels of several weight percent in a number of such alloys. The reasons why minor additions of these reactive elements, especially the rare earths, have beneficial effects have been the subject of systematic studies over the past 10–15 years. Ideas have been influenced by the understanding of the mechanisms of the development of protective chromia or alumina scales; in particular, the role such additions play in assisting the more rapid nucleation and growth of oxides on HT alloys, reducing the period of transient oxidation and the earlier attainment of steady-state scaling conditions. Much attention has also been paid to the mechanisms whereby such additions appear to promote improved scale adherence.

The various theories of high temperature corrosion of such alloys and supporting experimental evidence are reviewed in this paper. The reported information concerning the beneficial role or otherwise of the refractory metals, particularly with respect to hot corrosion in gas turbines, is appraised. Attention is drawn to the potentially beneficial effects of additions of certain of these Group III-VI elements in the development of novel alloys or coatings to resist corrosion in the complex environments of low oxygen potential, which occur in plants to produce synthetic natural gas from oil or coal. Areas for further research are also identified.     

A Comparison of the Growth Kinetics and Scale Morphology for Three Superalloys at 930°C in Air and Low PO2 Environments

This study assessed the oxidation behavior of three commercial alloys in airand low partial pressures of oxygen roman (P_{O 2}. The kinetics ofoxidation in air were compared with values obtained in an atmosphere of lowroman P_{O 2}. The low partial pressure of oxygen was of the order of10–16 atm at 930°C and was generated using an H₂/4% H₂Omixture. The nature of the corrosion products, structure, morphology, andcomposition were assessed and characterized using scanning electronmicroscopy (SEM), X-ray mapping, and scanning Auger microscopy (SAM). Theresults were compared with those obtained for 99.99% pure chromium. Parabolickinetics were exhibited by all of the alloys, with the overall k_p valuesbeing of the order constant for chromia-forming alloys. Large variations inthe morphologies of the oxide scales were observed as a result of oxidationin the high and low roman P_{O 2} environments.     

Studies of the oxidation of titanium in carbon dioxide—II. Characteristics at reaction temperatures of 900 and 1000°C

The oxidation of titanium in carbon dioxide has been assessed at reaction temperatures of 900 and 1000°C by gravimetric, metallographic and microhardness techniques. The overall kinetics were parabolic at 900°C and paralinear at 1000°C. The results in general showed considerable scatter, but it was established by a statistical analysis of data at 1000°C that the scatter was independent of specimen preparation method. Oxygen dissolution in metal substrates during oxidation was very extensive and it has been shown that the observed saturation after exposure for long times at 1000°C was associated with the parabolic/linear overall kinetic transition. Linear rates of scaling were observed in character with the porous and layered nature of the scales.     

Studies of the oxidation of titanium in carbon dioxide—I. Characteristics within the reaction temperature range 675–800°C

The oxidation of titanium in carbon dioxide has been examined in the temperature range 675–800°C, involving assessment of kinetics, and metallographic and microhardness studies on oxidized substrates and scales. Overall linear kinetics were exhibited—in the temperature range 675–750°C two consecutive linear stages were observed, whereas at 775 and 800°C this linear/linear pattern was less well defined, tending to be replaced by simple linear kinetics.Microhardness studies indicated the presence of oxygen gradients within the substrates. The microhardness contours merged after long exposure periods indicating oxygen saturation, the time for saturation coinciding with the linear/linear transition time.A linear/linear scaling pattern was also observed, the time for the scaling transition coinciding with both that for the total weight gain transition and for attainment of oxygen saturation. The macroscopic features of scales could be correlated with these transition phenomena.     

Stress relief and plastic flow in nickel oxide at elevated temperatures

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 , 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.     

A simplified surgical approach to the incomplete form of testicular feminization

A simplified surgical approach to the intersex child with the incomplete form of testicular feminization is presented. The procedure is particularly useful in patients where the gonadal tissue is located in the scrotolabial folds and eliminates the necessity for exploratory laparotomy. Representative drawings of the procedure are shown.     

Microstructural Features of Scales and Their Likely Influence on Creep Behaviour

The microstructural features of scales as formed during the high temperature corrosion of metals can vary considerably. Apart from the obvious differences between scales derived from different metals, even with a given metal the microstructure of the scale is affected by factors such as metal purity and the conditions of exposure, namely the duration and temperature of oxidation and the partial pressure of the oxidant in the atmosphere. In particular variations in grain size and morphology, and in the degree and morphology, and in the degree and nature of porosity can occur; in addition in some systems precipitation of a second phase of variable morphology can take place. Some of these differing structural features in scales are discussed and illustrated. It is suggested that such variations in microstructure will considerably affect the mechanical properties of the scales in which they occur. In general few data concerning mechanical properties, in particular plastic deformation characteristics of scales have been published; furthermore little attempt has been made to assess the possible role of such variations in structure. Rather more information is, however, available from the field of ceramics, and this knowledge is critically reviewed. The usefulness and limitations of such information in relation to the interpretation of the creep behaviour of cobalt monoxide scales is illustrated.