Vacancy condensation and void formation in duplex oxide scales on alloys

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Abstract
The growth of duplex scales on iron-chromium alloys oxidised at 600°C in CO2-1 % CO gas is accompanied by the formation in the inner oxide layer of regularly-spaced lamellar voids parallel to the metal-oxide interface. It is proposed that these voids form at the metal-oxide interface by periodic condensation of vacancies which have been injected into the metal by the oxidation process. From lamellar void spacings, the vacancy fraction in the metal at condensation is estimated to be 6 × 10−3 and the energy to precipitate the first vacancy, E v, is ∼2·7 × 10−12 J. From consideration of surface energy changes accompanying lamellar void formation, an average value of E v= 1·3 × 10−19 J is estimated. The effects of chromium content in the metal and specimen geometry on lamellar void spacings are consistent with the proposed mechanism. The presence of filamentary microcrystals of oxide in the lamellar voids suggests that the inner oxide grows by gas phase transport of the oxidant to the metal surface.