A numerical analysis of oxide spallation

Abstract

During the cooling of protective oxide layers on high-temperature alloys, the oxide is usually placed under compression. This paper considers buckling and wedging processes of decohesion of the oxide in this situation. It is shown that the wedging mechanism, in which a tensile wedge crack grows at the oxide–metal interface, is favoured for thick oxides and an intrinsically strong oxide–metal interface. The results of numerical analyses of such wedging failure are presented for the particular case of a chromia film on an austenitic steel. Both elastic and creep deformation processes were considered. It is shown that creep relaxation within the steel markedly reduces the rate of growth of the interfacial crack compared with purely elastic behaviour. Agreement with experimental oxide spallation data is reasonable for this creep case.

Role of Nanostructure in Electrochemical Corrosion and High Temperature Oxidation: A Review

“…21-Schematic representation of cracking, decohesion blistering and spallation in oxide scales under compression. [211,212] 6. In case of Ni-based superalloys like K38G, KF17, and LDZ125, decreasing the grain size to nanocrystalline regime facilitates the formation of a continuous alumina oxide layer as opposed to their coarsegrained counterparts which form a mixture of Ni, Al, and Cr oxides.…”

“…[211,212] However, in the case of nanocrystalline alloys, additional parameters can affect the interfacial stresses. The grain growth in nanocrystalline matrix at higher temperatures will affect the stresses at the oxide-alloy interface on three accounts: (a) scale plasticity and gradient in the oxide point-defect density will vary as the thickness of the oxide scale increases during the initial stages of oxidation, (b) dynamic grain growth in the matrix during oxidation will affect the incoherency at the oxide-alloy interface which is likely to affect the nucleation of interfacial cracks and decohesion of oxide scale, and (c) Even after a uniform protective oxide film is formed, the increase in matrix grain size will affect the diffusion of protective alloying element which is required for the self-healing of the cracks developed in the oxide film after prolonged oxidation.…”

Development of a Comprehensive Oxide Scale Failure Diagram

“…1) relates the failure strain for the different scale failure mechanisms to scale thickness and dates to the original approach of U.R. Evans in 1948 [12], which is used in most failure models in the literature [5,7,9,11,13,14]. In this approach, the elastically stored energy in the oxide scale resulting from stresses and strains in the oxide/metal system is compared to the energy which is needed to create two new surfaces in the form of a separation of the oxide from the metal substrate or of a through scale crack.…”

“…While the use of thermodynamic stability diagrams has become a standard step in the assessment of high temperature corrosion resistance of metals and alloys, and powerful software exists for this purpose [2,3], there is still a need for an equivalent tool describing the limits of mechanical integrity of protective scales. There have been several proposals for scale failure diagrams [4][5][6][7] among which the EPRI report FP 686 [4] has received particular industrial attention due to its relevance to the spalling of steam grown oxides on heat exchanger steels in fossil power plants [8]. Besides these diagrams, a significant number of publications exist modelling the different failure mechanisms and reporting about measurements of critical failure stresses and strains.…”

Cyclic Oxidation of P91 by Thermogravimetry and Investigation of Integrity of Scale by “Transient-Mass-Gain” Method

“…This is the fourth stage, wherein the alloy undergoes an accelerated loss in wall thickness. This is a life-limiting situation, because if it is allowed to continue for a long time the load bearing capacity of the component would be impaired, ultimately leading to failure [12].…”