INTERFACIAL SCALING REACTIONS AND THE REACTIVE ELEMENT EFFECT

Recent advances in the understanding of the structure and dynamics of solid/solid interfaces has led to some new answers for traditional questions about the role of the metal/scale interface in the growth of reaction product scales on pure metals. In particular, the action of interfacial dislocations in the creation and annihilation of the point defects supporting the diffusional growth of scales is of interest, as is their role as growth sites, for scales grown either by anion or cation diffusion. New knowledge about the metal/scale interface has permitted the consideration of blocking the interfacial reaction step as a means to retard the scaling kinetics and to alter the fundamental scaling mode. Such considerations seem consistent with the well-known phenomena ascribed to the reactive element effect (REE), which traditionally has been discussed for alloys forming chromia and alumina scales but has recently been extended to other metals. An insight into the REE mechanism is provided through the consideration of pinning of interfacial dislocations by the segregation of large reactive element ions, an action which poisons the usual interfacial reaction step.