The Importance of Short-Circuit and Related Transport Processes in High-Temperature Oxidation

Stringer, J.

doi:10.1007/978-1-4615-8723-1_28

Cited by 11 publications

(4 citation statements)

References 55 publications

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“…Spalling of the scale from the metal is a particularly undesirable mode of stress relief which may occur in scales subject to compressive stresses since it can be accompanied by exposure of large areas of metal to the environment. In practice, the electrostatic forces at the metal-scale interface are reasonably large and often exceed the yield strength of the oxide (Stringer 1972). Values of 10' to lo8 Nm-* are possible (Sandomirsky and Smilga 1959).…”

Section: Loss Of Adhesion Between the Scale And Metalmentioning

confidence: 99%

“…It has been shown (Evans and Hutchinson 1984) that flat interfaces resist spalling better than curved interfaces and premature spalling might be expected from regions of the substrate with small radii of curvature, as indeed is often observed. Some of the factors influencing adhesion-cohesion of scales to metal substrates have also been discussed by Stringer (1972) and Holmes and Pascoe (1972). It was argued that interfacial irregularities may be beneficial or detrimental.…”

Section: Loss Of Adhesion Between the Scale And Metalmentioning

confidence: 99%

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The protective action of oxide scales in gaseous environments at high temperature

Stott¹

1987

Rep. Prog. Phys.

128

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Some of the main principles which determine the protective nature of oxide scales on metals in oxidising environments at high temperature are considered and discussed with particular respect to NiO scales on nickel and Cr203 and A1203 scales on high-temperature alloys. Emphasis is given to the transport processes in the scales and the importance of short-circuit diffusion paths. The development of stresses during oxidation are discussed and related to the effectiveness of the scales in resisting mechanical breakdown. Relief of these stresses can occur by deformation of the oxide or metal substrate but, in certain circumstances, may result in fracture and/or loss of adhesion of the scale followed by spalling from the metal surface, sometimes with catastrophic consequences.Nickel is the base metal of many high-temperature alloys. Its oxidation behaviour has been widely studied because it forms a single oxide, NiO, which classically has been reputed to grow by diffusion of metal ions through a metal-deficient lattice. However, there have been significant developments in the understanding of this behaviour in recent years. Lattice diffusion of nickel ions through the oxide lattice is now not thought to be the main transport process except at very high temperatures. Rather, enhanced penetration of such ions along oxide grain boundaries probably predominates. The evidence to support this hypothesis is discussed and related to the observed oxidation characteristics.Most high-temperature alloys contain sufficient chromium and/or aluminium to form external Cr203 or A1203 scales for protection since such scales thicken at a reasonably slow rate. However, they are very susceptible to cracking and spalling, particularly during thermal cycling. Growth of the scales does not occur following lattice diffusion of reactants only but mainly following transport along short-circuit paths such as grain boundaries. Both the metal species and oxygen may be involved. The transport processes are believed to result in high stress development, sometimes leading to scale failure. Possible reasons for these effects are discussed in terms of the experimental observations. Particular emphasis is placed on the influence of reactiveelement additions to the alloy since these can have a marked, beneficial effect on the development and mechanical integrity of the Cr203 and A1203 scales. Such additions are now extensively incorporated into high-temperature alloys and coatings, resulting in much improved performance in service.

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Section: Loss Of Adhesion Between the Scale And Metalmentioning

confidence: 99%

Section: Loss Of Adhesion Between the Scale And Metalmentioning

confidence: 99%

The protective action of oxide scales in gaseous environments at high temperature

Stott¹

1987

Rep. Prog. Phys.

128

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“…A common factor appears to be the low solubility of the reactive element in the native oxide (Caillet et al, 1983) and Ecer et al (1982) have suggested that its segregation to grain boundaries blocks cation diffusion. There is a good deal of evidence that grain boundary transport of cations dominates oxidation processes of a variety of metals and alloys at temperature of practical importance (Stringer, 1973;Hughes et al, 1984). A detailed study has been made of the oxidation of nickel as a model system where grain boundary diffusion of metal is the dominant transport process below 1000°C (Atkinson et al, 1982).…”

Section: Introductionmentioning

confidence: 99%

Electron microscopy of oxide scales modified to resist high‐temperature oxidation

Chadwick

Taylor

1985

Journal of Microscopy

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The high temperature oxidation of metals and alloys is slowed to a useful extent by the presence of certain elements or their oxides in the metal or on its surface. Particles of the reactive element oxide are found in part of the oxide scale and, by an unknown mechanism, retard the diffusion of metal through oxide grain boundaries. Changes in texture are also observed, which might affect geometry of individual grain boundaries. In this study the rotational geometry of grain boundaries in nickel oxide scales has been characterized both in the presence of particles of cerium dioxide and in their absence. Proximity of the relationship of neighbouring grains to a coincidence orientation could have an important influence on diffusion rate. Ways of testing this are compared and a new method for analysing orientation data from electron diffraction is presented, which allows statistically sound conclusions to be drawn. The frequency of occurrence of coincidence orientations in the presence of CeO, is little greater than random, so one possible mechanism for the 'reactive element effect' may be rejected. I N T R O D U C T I O NThe oxidation of metals and alloys at high temperature has important economic consequences. To prolong the life of artefacts such as furnaces, jet engines and fuel elements for gascooled nuclear reactors, relatively high levels (>20%) of expensive chromium have to be used to give a protective Cr203 scale (Wallwork, 1976) and even then, loss of the scale by spalling can limit operating temperatures to well below the limits imposed by mechanical strength alone. One palliative measure is the introduction of a small proportion of certain reactive elements or their oxides into the metal by alloying or dispersion, or on its surface as a coating. This improves performance by reducing both spalling and the oxidation rate itself (Whittle 8t Stringer, 1980). The lanthanides, titanium, hafnium and thorium are all effective to varying extents. Similar results can be obtained by ion implantation. A common factor appears to be the low solubility of the reactive element in the native oxide (Caillet et al., 1983) and Ecer et al. (1982) have suggested that its segregation to grain boundaries blocks cation diffusion. There is a good deal of evidence that grain boundary transport of cations dominates oxidation processes of a variety of metals and alloys at temperature of practical importance (Stringer, 1973;Hughes et al., 1984). 0 1985 The Royal Microscopical Society 22 1

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“…Irving (8) using Whipple's analysis (9) for diffusion in a polycrystalline aggregate attempted to demonstrate that cubic-type kinetics could occur if the dominant diffusion paths were grain boundaries, but Gibbs (10) has emphasized the shortcomings of this approach. There is good evidence, however, that mass transport in growing oxides can occur predominantly along short-circuit paths [e.g., (11,12)] and any variation with time in the density of such paths will lead to departures from parabolic kinetics. On this basis a number of authors (13)(14)(15)(16)18) have developed a "decay" hypothesis which accounts for subparabolic kinetics by a decrease in the density of paths.…”

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confidence: 99%

Perturbation of Parabolic Kinetics Resulting from the Accumulation of Stress in Protective Oxide Layers

Evans

Norfolk

Swan

1978

J. Electrochem. Soc.

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A frequent observation in metal oxidation is the development of subparabolic kinetics, variously described as cubic or quartic. Although a number of detailed mechanisms have been proposed to account for this effect, none seem generally applicable. This paper presents a model of the oxidation process which is divorced from such restrictions. It is argued that deviations from parabolic behavior occur as a result of the concurrent development of stresses within the oxide. It is shown that the presence of stress fields can influence significantly the rate of transport of vacancy defects within the oxide such that tensile stresses produce positive deviations and compressive stresses, negative deviations from parabolic behavior. The model is applied in detail to Zircaloy‐2 oxidation at 773°K. It is predicted that the kinetics should be insensitve to the oxygen potential of the environment and this has been confirmed by previous experimental work. In addition, the absolute value of the oxidation rate is closely predicted using measured values of diffusion coefficients and the observed gradual departure from parabolic kinetics with increasing oxide thickness is accounted for.

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The Importance of Short-Circuit and Related Transport Processes in High-Temperature Oxidation

Cited by 11 publications

References 55 publications

The protective action of oxide scales in gaseous environments at high temperature

The protective action of oxide scales in gaseous environments at high temperature

Electron microscopy of oxide scales modified to resist high‐temperature oxidation

Perturbation of Parabolic Kinetics Resulting from the Accumulation of Stress in Protective Oxide Layers

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