Determination of Oxidation Mechanisms of Ferritic-Martensitic Alloys in Supercritical Water.

Abstract

Objective of this thesis is to understand the oxidation mechanisms of ferritic-martensitic alloys in supercritical water (SCW). The approaches included; i) determine the effects of SCW temperature on the oxidation rate, ii) determine the diffusion processes in oxide in terms of diffusion species and mechanisms and iii) understand the structure of the various oxide phases and their formation. Microstructure of oxide formed on four F-M alloys; T91, HCM12A, HT-9 and 9Cr-ODS, exposed in 400 – 600°C SCW consisted of two oxide layers and a transition layer. The outer oxide consisted of dense columnar grains of magnetite (Fe3O4). The inner oxide consisted of small equiaxed grains of Fe-Cr spinel oxide Fe3-xCrxO4, where x ~ 0.7-1 depends on alloy type. The transition layer consisted of grain boundary oxides of chromia and chromite, and fine spinel oxide grains precipitated inside laths. SCW temperature has an influence on oxidation. The oxidation rate increased exponentially as a function of temperature. The oxidation rate followed trend of Cr content in alloy in which HCM12A and HT-9 with high Cr exhibited lower oxidation rate than T91. The 9Cr-ODS exhibited relatively good oxidation resistance compare to T91 with a similar Cr content. This reflects an effect of Y2O3 nano particles. Interpretation of the oxidation rate reveals that; i) the power relation of the oxidation shows that the oxidation occurred by a diffusion controlled mechanism, ii) the activation energy of oxidation suggests the occurrence of short circuit diffusion along grain boundaries and defects in the oxide, and iii) the rate limiting mechanism is Fe diffusion. Original alloy surface is at the outer – inner oxide interface. Formation of the outer oxide occurs by outward diffusion of Fe ions and transport of Fe ions dominates by vacancy diffusion. The inner oxide occurs by inward diffusion of oxygen, and micropores occur as a result of outward diffusion of Fe ions to the outer oxide. Transport of Fe ions through the inner layer occurs by cation vacancies in most of the layer, and interstitial diffusion dominates near the transition layer – inner oxide interface. Oxygen transport occurred primarily by the short-circuit diffusion.