Corrosion behavior of nickel base heat resisting alloys for nuclear steelmaking system in high-temperature steam.

Abstract

The corrosion behavior of nickel base heat resisting alloys is investigated in steam at 800°C and 40atm, simulating the superheated steam of the nuclear steelmaking system. The alloys tested are five new alloys developed for the nuclear steelmaking system and one commercial alloy Inconel 617.A protective surface oxide scale, consisting of Cr2O3 mainly and of MnCr2O4, forms on the alloys except Inconel 617. Internal oxides, consisting of Al and Ti oxides, also form in the alloys containing Al. For Inconel 617, nodular oxides, consisting of NiO in the outer region and Cr internal oxides in the inner region, form in addition to Cr2O3 and MnCr2O4 after 1000h. No decarburization occurs in each alloy.The effect of alloy compositions on the corrosion behavior is discussed. The rate of Cr depletion resulting from the formation of Cr2O3-rich scale is found to be influenced by the presence of MnCr2O4 layer outside the Cr2O3 scale, and is described by a parabolic rate law for the alloys with thin MnCr2O4 layer and by a cubic rate law for the alloys with thick MnCr2O4 layer. Al and Ti, which are reactive more than Cr, are found to improve a resistance to spalling of the Cr2O3-rich scale, through a key-on eject by the internal oxides and by the enrichment of Ti oxides at the Cr2O3 scale/alloy interface.