Cavity formation and metallurgical changes induced by growth of oxide scale

Even under conditions of protective oxidation, significant structural changes can develop in the metal substrate. One observation frequently reported is that of subsurface cavitation, but it is not likely that the same physical processes always apply. In some cases, vacancy injection by the removal of cations and subsequent Kirkendall void formation does seem to occur, but it is shown that such injection requires vacancies to have a low binding energy to an oxide/metal interface that has widely spaced sink sites. Other examples of subsurface cavities can be associated with particle dissociation, creep damage, and gas bubble formation. Some metallurgical changes are considered, which can be induced either by the selective removal of cations or by the injection of large amounts of oxygen. The basis of theoretical development is also discussed. Finally, consideration is given to the role of secondary reaction products, specifically carbon and hydrogen, which can cause precipitation when they enter the metal substrate.

MST/952