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Corrosion Behaviour of Ni-Cr Alloys in Oxygen at High Temperature

Paper Number: 68
Author/s: Y.L. Hsu, Y. Xie, J. Zhang
Organisation: School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia

Abstract

To meet an increased energy demand, it is necessary to increase steam turbine operating temperature of coal-fired power plant from conventional practice of 540°C to ultra/advanced ultra-supercritical practice over 700°C. Because of this temperature increase, traditional ferritic/martensitic and austenitic steels cannot survive. Instead, nickel- base alloys have to be used because of their superior creep strength and steam oxidation resistance at high temperatures.

In this work, oxidation behaviour of binary Ni-Cr alloys with Cr concentration from 5 to 30 wt% was investigated in 20%O2-80%Ar gas for up to 310 h reaction at 650, 700 and 800 °C. All samples were electro-polished to avoid the accelerated alloy Cr diffusion due to the effect of surface cold work. The results showed that for Cr concentration lower than 30 wt%, all alloys formed a multi-layered oxide scale, together with an internal oxidation zone. Increasing Cr concentration to 25 wt% led to the formation of a chromia band at the reaction front and a significant reduction in internal oxidation zone. When Cr concentration reached 30 wt%, a thin protective chromia scale was observed at all temperatures. The effects of chromium and temperature on oxide formation were discussed based on reaction kinetics and diffusion analysis during high temperature reaction process.