Stress generation and vacancy annihilation during scale growth limited by cation-vacancy diffusion

Abstract

The growth of cation-diffusing scales on pure metals is described in terms of intrinsic dislocations for an epitaxial metal-scale interface. This model is consistent with the experimental observations of high local deformation and intimate contact, and epitaxial relations between the innermost grains of the scale and the underlying metal. The annihilation of metal vacancies at the metal-scale interface occurs by the climb into the metal of some fraction of the intrinsic misfit interface dislocations. Epitaxy is maintained by an adjustment of the spacing of the remaining interface dislocations, a process which generates tensile stress in the metal and compression in the scale. Above a critical interfacial strain, the glide of dislocations in the metal, in combination with dislocation glide in the scale, recreates or resupplies the interface dislocations. These processes provide plastic deformation in both phases near the interface and permit the retention of epitaxy during metal recession. The model may explain the origin of stresses arising during the growth of cation-diffusing scales on an extensive flat surface and the influence of surface orientation and surface preparation on the oxidation kinetics, etc. The epitaxial growth of NiO scale on pure Ni is described as a typical example.