A study of scale cracking and its effects on oxidation and hot corrosion. Final report, November 1, 1987--December 31, 1990

Abstract

This report briefly summarizes progress during the first two and one half years of a three-year program. One part of the program is a study of the hot corrosion of materials; the other part is concerned with the measurement of stresses in oxide scales and the effects of such stresses on the oxidation behavior of alloys. The hot corrosion effort has been focussed on SiC, where corrosion rates have been measured in an oxidizing environment as a function of the activity of vapor phase potassium salts (up to {approx} 300 ppm). Potassium dissolves in the normally protective SiO{sub 2} film, causing an increase in the oxidation rate of up to 700 - 800 times that in {open_quotes}clean{close_quotes} environments. In the study of stresses in oxide scales, we have achieved a major success in having directly measured in-situ strains at the oxidation temperature by x-ray diffraction. To our knowledge, such direct measurements have not been reported before. To date, the technique has been applied to Ni/NiO, Cr/Cr{sub 2}O{sub 3} and FeCrAlY/Al{sub 2}O{sub 3} systems. We feel such data will stimulate new lines of research by: (a) providing a basis for testing new theoretical models of processes that generate stresses in growingmore » oxide films, and (b) providing a foundation for the practical issue of devising improved alloys where the design can now be directly related to the chief degradation process, ie, design to minimize stresses and scale cracking. Theoretical studies during the past year have attacked the problem of understanding crack initiation and propagation at the metal/scale interface. The elastic-plastic solution for the local stresses about a crack in an anisotropic solid have been obtained and are being applied to the bi-material problem of the oxide/metal interface.« less