Corrosion Behaviour of Alumina-Forming Austenitic (AFA) Steels in Molten Lead for Lead-Cooled Fast Reactor Applications

The continuous growth in energy demand and the need to reduce the environmental impact of traditional fossil fuels make the development of new technologies essential. In this context, Lead-cooled Fast Reactors (LFRs), a Generation IV nuclear technology using liquid lead as coolant, represent a promising solution. The use of liquid lead allows the instauration of a fast neutron spectrum, enabling the closure of the fuel cycle, reducing radioactive waste, and improving fuel utilization efficiency. In addition, safety is improved in comparison with standard Light Water Reactors (LWRs) thanks to the high boiling point of liquid lead, allowing operation close to the atmospheric pressure. However, liquid lead presents compatibility issues with structural conventional austenitic steels at the high operating temperatures of these systems, exhibiting significant corrosive effects. For this reason, extensive research and development of new advanced materials are currently underway to address the challenges related to the high-temperature aggressiveness of liquid lead. This thesis work, carried out within the framework of the Horizon Europe INNUMAT project, was conducted at the ENEA Brasimone Research Centre (Camugnano, Bologna) and at the University of Modena and Reggio Emilia. The materials investigated in this study include several types of AFA (Alumina-Forming-Austenitic) steels, which are primarily characterized primarily by the addition of aluminium to promote the formation of a protective alumina surface layer at high temperatures, thereby shielding the material from corrosive lead attack. In addition, 1515-Ti AIM1 austenitic steel was studied, an advanced Ti-stabilized austenitic steel subjected to cold working to enhance its mechanical strength. The objective of this work is to evaluate the effects of liquid lead corrosion on these materials at high temperature and to investigate the influence of chemical composition and surface preparation on corrosion resistance. To this end, preliminary corrosion tests with 1000 hours of duration were performed in liquid lead at 550 and 600°C with an oxygen concentration in the range of 10-8-10-7wt.%. Subsequently, both virgin and exposed samples were analysed using various characterization techniques, including SEM/EDS, XRD, GI-XRD, and optical microscopy (OM), to assess the effect of molten lead exposure and to compare the performance of AFA steel with that of 1515-Ti steel. Although the tests in this work have been conducted so far over relatively short periods (longer-term exposure up to 5,000 hours are still ongoing within the INNUMAT project), the preliminary results obtained after 1,000 hours highlight the superior corrosion resistance of AFA steels compared to FeCrNi steels, as well as an influence of surface preparation on corrosion phenomena.