Analysis of localized oxides and oxide films formed on alloy 625 and 617 in supercritical water and superheated steam environments at 750°C

摘要

Oxidation tests using coupon specimens of alloy 625 and alloy 617, and four-point bending tests for a duration of 100 hrs were carried out in supercritical water (SCW) environment at a pressure of 24 MPa and in superheated steam (SHS) environments at 750°C. The noticeable difference between the oxidation in two environments appeared in the oxidation behavior of primary precipitates like Nb rich MC carbide for alloy 625. In the SCW environment, a large number of pitting was observed. Those were thought to be made by electrochemical reactions, while in SHS most of the carbides were oxidized as Nb oxides with some extent of volume expansion. In the case of alloy 617, the pitting formation was also observed in both environments. It seemed to have a dependence on the oxidation behavior of the primary carbides similar to that of alloy 625. The effect of the applied stress was not apparent under current experimental conditions because permanent deformation was observed in all the specimens and stress relaxation subsequently occurred during the test. In addition, no cracks were observed on both alloys after four-point bending tests. In the outer layer of the oxide film formed in the SCW, the Cr content exhibit to have smaller in both alloys, and the oxide thicknesses were 0.53 to 0.88 times thinner than those in the SHS.