G.C. Wood

Oxidation of alloys

AbstractKey principles of alloy oxidation are discussed, with emphasis on Cr2O3 and Al2O3 formation on nickel-, cobalt, and iron-base alloys. The various special cases of alloy oxidation, which are quantifiable to varying degrees, are presented schematically. The important competition between surface scale development and internal oxidation is emphasized and extended to explain transient oxidation. The ability to measure and model the distribution of alloying elements in steady-state scale and substrate is described. The priority now is to understand further alloying element and defect segregation and transport in grain boundaries, and also other short-circuit paths including pores, in Cr2O3 and Al2O3 scales.Interaction between alloy depletion, void formation, and phase-boundary oxidant transport in single- and multi-phase alloy substrates requires further elucidation. Brief consideration of ternary and quaternary alloy oxidation illustrates the, ability partially to explain complex alloy behaviour. The r...

Review of mechanisms of erosion-corrosion of alloys at elevated temperatures

Abstract Erosion-corrosion of materials at elevated temperatures is a major problem in various industrial processes which range from some of the more advanced coal-conversion processes, to the turbine blades of jet engines. The extent of wastage in such environments is dependent on a wide variety of parameters which include properties of the impacting particles, target material and the corrosive environment. At this stage various laboratory studies have been carried out and patterns have emerged of the effects of the main erosion-corrosion variables on alloy wastage. One of the most consistent results from the laboratory studies has been on the effect of temperature. The wastage has been seen to increase to a critical temperature and subsequently decrease with further increases in temperature. This curve has been shown to shift to higher temperatures with increasing oxidation resistance of the alloy, and to higher temperatures and wastage rates with increasing particle impact energy. Other variables which may affect this curve are particle angularity, nature of the corrosive environment, impact angle and composition of the oxide scale formed during the erosion process. This paper reviews some recent laboratory erosion-corrosion results. The general trends in alloy wastage as functions of the main erosion-corrosion parameters are shown.

The friction and wear behaviour of nickel-base alloys in air at room temperature

Abstract Measurements are presented of the friction and wear during sliding of specimens of Nimonic 75, C263, Nimonic 108 and Incoloy 901 on like specimens in air nominally at room temperature. The worn specimens have been examined using microhardness measurements, optical and scanning electron microscopy, X-ray diffraction and electron diffraction. These techniques suggest mechanisms for the room-temperature wear of these alloys associated with their strength properties. In particular, changes in the coefficient of friction and the wear rate during sliding can be correlated with work hardening, and possibly some degree of age hardening, of the load-bearing areas, due to the severe mechanical and thermal stresses developed. There is no evidence that oxide films formed on the contact areas during sliding have a significant effect on the tribological behaviour of these alloys. Such films are merely removed from the surface as wear debris.

The effect of pre-oxidation of chromia and alumina forming alloys on erosion in laboratory simulated fluidized-bed conditions

The effects of pre-oxidation of alloys have been investigated in laboratory simulated fluidized bed erosion-corrosion environments. Two alloys, Incoloy 800H and Fecralloy, which formed chromia and alumina respectively, under steady state oxidation conditions, were tested. The research consisted of preoxidizing the alloys in a range of oxidation environments. including air, oxygen and H-2/H2O. The scales were then examined and analysed to determine the composition and adhesion to the metal. The erosion-corrosion tests were carried out at 500-degrees-C, using 200-mu-m silica as erodent, at velocities of 1.9 m s-1, and for exposure times of up to 168 h. The specimens were analysed subsequently by analytical scanning electron microscopy and X-ray diffraction. The pre-oxidation results showed that. for both alloys, the scales formed in H-2/H2O were more adherent and slower growing than those formed in air or oxygen. The erosion-corrosion tests on pre-oxidized specimens indicated that the scales formed in H-2/H2O were the most resistant to particle impacts. The results also showed that pre-oxidized Incoloy 800H eroded more rapidly than pre-oxidized Fecralloy. As the erosion-corrosion rates of the two non pre-oxidized alloys were dissimilar to each other, it was not possible to establish whether there was a significant difference between the erosion resistance of the pre-formed scales. under these conditions. Possible reasons for the improved performance of the scales formed in low oxygen pressures are discussed. The differences in erosion-corrosion rates of the alloys are also addressed, with emphasis on alloy deformation characteristics and transient oxidation rates.

The erosion-corrosion of alloys under oxidizing-sulphidizing conditions at high temperature

A whirling-arm erosion-corrosion rig has been designed and constructed to allow studies under gaseous conditions of high-sulphur, low-oxygen activities at high temperatures; these environments are pertinent in processes such as gasification of coal and catalytic cracking of oil. The system can operate at particle impact velocities up to 30 m s(-1), particle impact fluxes up to 1 g cm(-2) s(-1) and temperatures up to 800 degrees C. The particles are carried to the specimen chamber in a stream of nitrogen and mixed with hydrogen, hydrogen sulphide and, if necessary, water vapour to attain the required sulphur and oxygen activities. In this paper, results are presented for two commercial austenitic high-temperature alloys, 310 stainless steel and Alloy 800HT, during impact erosion by 25 mu m alumina particles at velocities of 10-25 m s(-1) and particle fluxes in the ranges 0.06-0.16 g cm(-2) s(-1) and 0.38-0.95 g cm(-2) s(-1) at 500 degrees C. The gaseous environment resulted in the development of sulphide scales on the alloys during exposures in the absence of the erodent particles. The erosion-corrosion damage was determined in terms of mean thickness-change measurements obtained every 5 h and overall metal-recession rates obtained by cross-sectional examination at the end of the 35 h or 70 h exposure periods. The results are discussed in terms of the synergistic interactions of growth of metal sulphides and removal of such phases by the impacting particles.

Computer simulation of erosion-corrosion interactions at elevated temperatures

Abstract Although there have been some attempts to model erosion by solid particles at elevated temperatures, there have been few efforts to develop a model which can generate images of the surface morphologies in the various erosion-corrosion regimes. Many classifications of erosion-corrosion regimes have been identified and there is evidence that there are at least three such regimes. The physical significance of the surface in such regimes may be difficult to visualize, particularly when the situation is neither “erosion-dominated” (erosion of the substrate) nor “corrosion-dominated” (erosion plays a minor role compared with corrosion). The object of this research has been to develop a physical model to simulate the transitions between erosion--corrosion regimes at elevated temperatures. Properties of the particle (shape, size, velocity, hardness, flux), the target (corrosion resistance, hardness, impact angle) and the environment (gas composition and temperature) are considered in the model. The results are then used to construct a computer-generated image of the eroding surface. This paper describes the physical basis of the model and shows how the transitions between the regimes can be achieved by variation of the erosion and corrosion parameters. Typical results are shown and compared with existing erosion-corrosion data. The future development of the research is outlined in terms of the application to other environments in which erosion-corrosion occurs.

High temperature erosion of pre-oxidized and as received alloys: Effects of impact angle, temperature and hot hardness

The effects of pre-oxidation of Incoloy 800H and Inconel 738 as a possible protective measure in reducing erosion-corrosion of the underlying alloy at temperatures up to 700°C have been investigated under conditions of moderate particle velocities and high particle loadings. This research also included assessment of the variation of erosion-corrosion with impact angle and alloy hot hardness. The results showed that the pre-formed scale exerted negligible effect on the erosion-corrosion of the substrate under these conditions. There were no significant differences between the wastage rates of the pre-oxidized and the as received alloys as the temperature was increased. The variation of erosion rate with impact angle followed the classical ductile mode at high temperatures. The reasons why pre- oxidation is ineffective under these conditions are discussed in terms of existing knowledge of erosion-corrosion in similar conditions.

Computer simulation of the effect of pre-oxidation in erosion-corrosion environments

The combined effects of erosion and corrosion can lead to a number of material wastage scenarios in industrial environments. In some cases, for example at high temperatures in fluidized bed environments, the wear may be corrosion dominated where chipping of an oxide scale is the predominant process, while in more aggressive environments such as gas turbine conditions, formation of a protective scale may never occur. Thus, pre-oxidation to form a protective scale or coating of a substrate may be effective in reducing material loss in corrosion-dominated environments since particle energies may be low enough to prevent significant deformation of the underlying surfaces. A computer model has been developed to simulate the erosion of a preformed scale or a coating on a metal substrate. This model uses computer graphics to demonstrate the sequential stages of erosion-corrosion. By incorporating established erosion and corrosion algorithms to calculate the wastage rate, the surface topography and extent of wear can be shown after exposure to erosion-corrosion conditions of varying severity. Thus, one can extrapolate from this technique whether erosion of oxide or metal is likely to be dominant wastage mechanism. This model was developed to describe the erosion-corrosion of a preformed oxide scale on a metal and comparisons are made between the erosion results for pre-oxidized surfaces and the graphics after computer simulation of erosion-corrosion. However, since the factors which determine adhesion of a preformed scale may be similar in many respects to those of a coating, the model may equally describe the erosion-corrosion behaviour of a coated metal. Hence, the paper shows the stages of the model development and compares the experimental results with those of the model.