Corrosion resistant alumina-forming alloys for lead-cooled reactors

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Generation IV nuclear power technologies provide attractive solutions to the common issues related to conventional nuclear power plants currently in operation worldwide. Through a significant reduction of the long-term radiotoxicity of nuclear waste, a more efficient use of nuclear fuel resources, and implementation of inherent safety features, Generation IV will make nuclear power sustainable and thus increase the public acceptance of nuclear power. Due to its attractive safety features, the lead-cooled fast reactor (LFR) is one of the most studied Generation IV reactor concepts currently. It is well known that liquid lead is corrosive to steels at elevated temperatures, thus limiting the operation temperature of the LFR. The use of alumina-forming FeCrAl alloys has been proposed to mitigate oxidation and corrosion issues. Commercial FeCrAl alloys have Cr-concentrations typically about 20 wt. % and are thus prone to α-α’ phase separation and embrittlement at temperatures up to about 500 °C. Reducing the Cr-concentration to levels around 10 wt. % would theoretically resolve the said issue. However, the oxidation and corrosion resistance may be impaired. In the scientific literature, compositional limits indicating the formation of protective alumina layers at various temperatures have been presented. Long-term corrosion studies are however scarce. Moreover, in-depth studies on the compositional limits regarding α-α’ phase separation are lacking. In this thesis, the long-term (up to 10,000 h) corrosion resistance and phase stability of alumina-forming alloys are studied at temperatures up to 550 °C. In addition, the influence of reactive elements (RE), e.g. Ti, Zr, and Y, on the liquid lead corrosion resistance of Fe10CrAl alloys is evaluated. By balancing the reactive element and the carbon content, with respect to carbide formation, it is demonstrated in this thesis that it is possible to form protective alumina layers on Fe10Cr4Al alloys from 450 °C, despite the low Al and Cr concentrations. It was found that the RE/carbon ratio needed to form protective alumina layers on Fe10Cr4Al alloys must be larger than unity to mitigate the detrimental effect of Cr-carbide formation.  The underlying phenomena are discussed, and a mechanism is suggested based on the outcome of the long-term oxidation studies. The phase stability of Fe10CrAl alloys was studied through thermal aging experiments in the temperature interval of 450 to 550 °C. In addition, the results were well reproducible using a developed Kinetic Monte Carlo (KMC) model of the FeCrAl system. Furthermore, the model indicated that the Cr-concentration should be limited to about 11 wt. % in a FeCr4Al alloy to mitigate α-α’ phase separation at all temperatures of interest for an LFR. The liquid lead corrosion resistance of alumina-forming austenitic stainless steels was shown to be superior compared to regular stainless steels, albeit the effect of ferrite stabilizing elements needs to be further addressed. The results included in this thesis provide a valuable input on the key issues related to the development of corrosion resistant alumina-forming alloys of interest for liquid lead applications. Moreover, the superior oxidation properties of the studied alumina-forming alloys make them of interest for use in other energy applications, where corrosion issues limits the operation temperature and thus the efficiency.