Preliminary Study of Oxidation Mechanisms of High Cr Steels in Supercritical Water

Abstract

Two ferritic steels with 14 and 25 wt% Cr were exposed to Supercritical Water (SCW) for 500 hours. Oxidation products were examined by analytical electron microscopy. The 14Cr steel was uniformly covered by a 10 mm-thick, Fe-rich oxide which grew outwards from the original steel/SCW interface. An equally thick dual oxide layer was found underneath the original interface. Two modes of oxidation were observed on the 25Cr steel. The majority of the surface was covered with a submicron, homogeneous Cr-rich protective oxide layer. Additionally, external Fe-rich oxide nodules formed along grain boundaries and polishing lines with mirror-image Cr-rich oxide nodules penetrating into the alloy. Nodule initiation was connected with the pre-exposure steel homogenization treatment, which could have formed undesirable phases along grain boundaries and resulted in local Cr depletion; however the oxidation propagated beyond any possible depleted zone.