Study on the corrosion behavior of laser surface remelted and laser cladding of ferritic/martensitic steels after exposure to lead-bismuth eutectic at 700 °C

Abstract

This study explores two vital structural materials, T91 (Fe-9Cr) and SIMP (Fe-11Cr) steels, in the context of lead-cooled fast reactors and accelerator-driven sub-critical systems (ADS). Lead-bismuth eutectic (LBE) functions as a key coolant and spallation target material due to its impressive thermal conductivity, neutron yield, and chemical properties. Unfortunately, materials in contact with LBE are prone to severe corrosion at elevated temperatures (T>500 °C), compromising their integrity. To bolster corrosion resistance, we utilized laser remelting and laser cladding to apply FeCrAl/TiN coatings on the steel surfaces. Our study scrutinizes the corrosion behavior of steel in LBE saturated with oxygen at 700 °C and investigates the underlying causes. Following 240 h of exposure to corrosion, T91 and SIMP steels subjected to laser remelting displayed substantial oxide scale formation. Lead-bismuth atoms infiltrated the outer oxide layer (Fe3O4), diminishing adhesion between the inner and outer oxide layers, leading to the detachment of the outer oxide layer. The inner oxide layers, composed of Fe-Cr spinel, were approximately 123 μm thick for T91 steel and 77 μm for SIMP steel, underscoring SIMP steel's superior corrosion resistance. For T91 steel treated with laser cladding FeCrAl/TiN coating, a characteristic duplex oxide layer with a total thickness of around 83 μm was formed, with noticeable deposition of Pb-Bi atoms at the interface between the outer and inner oxide layers. Conversely, only a protective alumina layer safeguarded SIMP steel from LBE corrosion. This outcome emphasizes the efficacy of laser cladding FeCrAl/TiN coating in providing superior protection for SIMP steel over T91 steel. Our research significantly contributes to the development of anti-corrosion coatings for high-temperature LBE environments.