Oxidation Behaviour of Ni-Base Superalloys in Supercritical Water: A Review

In the quest for creating a sustainable future, coal-fired boilers and nuclear reactors are increasingly moving towards higher steam temperatures and pressures that can help in reducing emissions and increase operational efficiencies. This trend has now transitioned to adopting Advanced Ultra-Supercritical (AUSC) water technology that involves steam temperatures and pressures as high as 760 celcius and 32 MPa. This transition poses serious material challenges, and one such is the demanding steam oxidation resistance expected of the candidate Ni-base alloys. Steam oxidation is a severe problem that can cause significant downtime in power generation. Several studies have focused on testing the steam oxidation resistance of candidate Ni-base alloys in supercritical water environments under varying test conditions. They differ from each other in terms of various operational parameters, including the time, temperature, pressure, alloy surface condition, dissolved oxygen (DO) content, steam flow/stagnancy conditions, and presence of additional oxidants like acids, salt, etc. The presence of so many such variables and the complex interplay between them makes it challenging to compare the oxidation performance of various Ni-base alloys and understand the effect of each operational parameter towards oxidation. In addition, widely differing opinions are being held about the role of several operating parameters in causing steam oxidation. This review work is an attempt to consolidate all such relevant works, critically analyze the results reported, generalize the trends observed, identify the variations among reported works and throw light on the shortcomings.