The dissolution of austenitic steel in liquid lead-based alloys can induce a phase transformation characterized by a sharp dissolution front separating ferrite and austenite grains, a process commonly referred to as ferritization. Although widely reported, the mechanism driving this transformation remains under debate. This study re-examines ferritization as a discontinuous reaction via a migrating liquid film and proposes a thermodynamically consistent model for the initiation and propagation of the dissolution front. The proposed mechanism is supported by experiments at 500–550°C, literature evidence, and diffusion calculations. Under low oxygen conditions, Cr transport through liquid Pb channels is identified as the rate-limiting step, setting the theoretical corrosion rate in stagnant environments. High-speed erosion-corrosion tests show enhanced corrosion rates, driven by erosion-limited channel lengths that locally boost mass transport. In contrast, under moderate oxygen concentrations relevant for lead-cooled fast reactor (LFR) operation, the rate-limiting step shifts to metal transport across a nanometer-scale amorphous oxide layer at the reaction front. Other Ni-containing austenitic steels, including alumina-forming austenitic (AFA) alloys and Ni-based high-entropy alloys (HEAs) can also be susceptible to discontinuous reactions under direct contact with liquid Pb-based alloys, lacking the self-healing oxide protection as observed in alumina-forming ferritic steels. This limitation may present a concern for the long-term use of bare austenitic steel in liquid Pb environments.

Three new multi-phase alumina-forming steels with compositions Fe-(10–14.5)Cr-(10–12)Ni-3.5Al (wt.%) were exposed to stagnant lead at 550 and 650 °C for up to 1000 h The experimental alloys formed stable and protective alumina (Al2O3) layers at both temperatures, crucial for preventing lead penetration and material degradation. In contrast, 316 L and T91 steels, candidate materials for nuclear applications, showed significant oxidation and lead penetration, particularly at the higher temperature. The designed alloys retained their mechanical properties after exposure, with one of them even increasing yield strength due to phase transformations. The findings highlight the potential of these new alloys with no reactive elements and no thermomechanical treatments, to operate in environments with high-temperature liquid lead, such as Gen IV nuclear reactors or high-temperature concentrated solar power plants.

Liquid metal embrittlement was observed in an alumina-forming martensitic (AFM) steel tested in both liquid Pb and lead-bismuth eutectic (LBE) in the temperature range 350-550 °C and 150-550 °C, respectively, using slow strain rate testing (SSRT) in a low oxygen environment ( ~10-11 wt.% O dissolved in Pb). A significant decrease in the total elongation to failure (TEF) could be observed in both environments, with LBE yielding the lowest measured TEF of 0.9% strain at 150 °C. The elongation to failure followed the classic pattern of a ductility dip, gradually recovering with increasing testing temperature so that fully ductile behaviour was restored at 550°C. There may be a potential to improve the performance of the AFM alloy by optimizing the microstructure through adjustments to the austenitizing, quenching and tempering conditions. 

Liquid metal embrittlement was observed in an alumina-forming martensitic (AFM) steel tested in both liquid Pb and lead–bismuth eutectic (LBE) in the temperature range 350–550 °C and 150–550 °C, respectively, using slow strain rate testing (SSRT) in a low oxygen environment (∼10-11 wt% O dissolved in Pb). A significant decrease in the total elongation to failure (TEF) could be observed in both environments, with LBE yielding the lowest measured TEF of 0.9 % strain at 150 °C. The elongation to failure followed the classic pattern of a ductility dip, gradually recovering with increasing testing temperature so that fully ductile behaviour was restored at 550 °C. There may be a potential to improve the performance of the AFM alloy by optimizing the microstructure through adjustments to the austenitizing, quenching and tempering conditions.

Liquid metal embrittlement (LME) in three newly developed alumina-forming austenitic (AFA) alloys, two 50 kg batches and one 5-ton heat, was studied in the temperature range 350–600 °C in liquid Pb and 140–600 °C in LBE using slow strain rate testing (SSRT) in a low-oxygen environment. No significant decrease in the engineering strain was observed in either environment. However, the presence of secondary cracks along the length of the specimen and brittle intergranular areas on the fracture surfaces indicates that the AFA alloys do show a minor degree of embrittlement above 570 °C. This appears to be related to grain boundary wetting by Pb/LBE. At temperatures below 570 °C, this wetting effect does not seem to be strong enough to induce LME in the alloys, and their ability to form a sufficiently protective oxide means that they remain unaffected by LME. The results indicate that the AFA alloy group can perform sufficiently well in liquid Pb/LBE environments, and long-term testing should be carried out to determine their viability as candidate materials for use in Pb- and LBE-based cooling systems.

This work focuses on mechanical properties, susceptibility to liquid metal embrittlement (LME), and erosion-corrosion of alumina-forming steels using a Slow Strain rate testing rig (SSRT) and an Erosion Corrosion-rig (ECO) developed at KTH. The environments investigatedare liquid lead and lead-bismuth eutectic (LBE) intended for use in high-temperature energy applications such as generation IV nuclear power or fast nuclear reactors. The lead and LBEare intended to serve as a heat-transfer medium in the reactor. These higher temperature sand harsher environments put new demands on the construction materials used. The work has been mainly focused on mechanical testing using slow strain rate testing (SSRT) to evaluate susceptibility to LME. However, since other properties, such as oxidation, are intimately intertwined with the LME phenomenon, liquid metal corrosion and erosion are also part of this work. The tested materials include a ferritic FeCrAl steel designated EF100, three alumina-forming austenitic (AFA) steels and an alumina-forming martensitic (AFM) steel. The temperature range of the tests in liquid Pb was 340-600 °C and in LBE 140-600 °C with varying oxygen activities. Microstructure analyses were performed to underst and theunderlying mechanisms responsible for LME. The ferritic EF100 showed excellent performance in liquid Pb, exhibiting no signs of being affected by LME. However, in liquid LBE, it was severely affected by LME. The effects of Bi were investigated by stepwise additions of Bi to pure Pb, and signs of LME were observed already at 3-5 wt.% Bi. The AFM alloy suffered from severe LME in both liquid Pb and LBE, starting at the melting point of the liquid metal. The AFA alloys showed no signs of LME in either liquid Pb or LBE in the temperature range of 350-550 °C and 140-550 °C, respectively. However, above 570 °C, signs of LME were observed in all three alloys. Erosion-corrosion was found to have the largest impact on steels containing Ni (e.g., 316L and AFA 3), while the steels with a higher hardness and that were able to form a protective oxide scale remained largely unaffected (Kanthal AF, APMT, EF100, Alkrothal 14, coated 316L PC/DG, and AFM).

Denna avhandling fokuserar på aluminiumoxidbildande ståls mekaniska egenskaper och deras påverkan av flytande metallförsprödning (LME) i smält bly och bly-vismut eutectiska (LBE) miljöer. Dessa smälta metaller är tänkta att kunnas användning som värmetransportertmedium inom högtemperatur- energiapplikationer så som generation IV kärnkraftsreaktorer. Dessa höga temperaturer och miljöer ställer nya krav på dekonstruktionsmaterial som ska användas. Avhandlingsarbetet har framför allt gått ut på att utvärdera de olika ståltypernas mekaniska egenskaper genom ”Slow strain rate testing” föratt utvärdera effekter av LME- påverkan. Eftersom även andra materialegenskaper spelar också stor roll för hur de de olika ståltyperna beter sig, är både oxidation och erosionskorrosion en del av detta arbete. De undersökta stålen inkluderade ett ferritiskt stål-EF100, tre olika typer av aluminiumoxidbildande autentiska stål-AFA, samt ett martensitiskt stål benämnt AFM. Dessa stål undersöktes inom temperaturintervallet 350-600 °C för blysamt 150-600 °C för LBE. Stålens mikrostrukturer analyserades för att öka förståelsen för de mekanismer som styr LME. Det ferritiska EF100-stålet visade inga tecken av att påverkas av LME i flytande Pb men var dock påverkat av LME in LBE. AFM-stålet betedde sig på ett liknande sätt som AF100-stålet men var påverkat av LME i både flytande Pb och LBE. AFA stålen påverkades varken av LME i Pb eller LBE men visade tecken på LME vid entemperatur av 570 °C. Omfattningen av erosions-korrosion undersöktes också vilken visade sig hastor påverkan hos stål innehållande nickel - Ni (316L och AFA3). Stål med en högre hårdhet och som bilade skyddande ytoxider påverkades väldigt lite (Kanthal AF, APMT, EF100,Alkrothal 14, aluminiumoxid belagd 316L, och AFM).

Liquid metal embrittlement (LME) of a Fe-10Cr-4Al ferritic steel was studied at 375 degrees C in liquid Pb-Bi alloys. Slow strain rate testing (SSRT) in low oxygen conditions was used to evaluate the ductility as a function of Bi concentration. It was found that susceptibility to LME increased strongly with the Bi concentration. The steel showed a reduction in its total strain to failure, which started at 3-5 wt.% Bi. The alloying elements (Fe, Cr, and Al) have a higher solubility in Bi than pure lead (Pb), so they are expected to dissolve more readily when Bi is added to the Pb. This is believed to be part of the explanation for the observed increase of LME. Lead with up to 3 wt.% Bi induced no LME in the studied corrosion resistance FeCrAl alloy.

The susceptibility of Fe-10Cr-4Al steel to liquid metal embrittlement (LME) in low oxygen liquid lead and lead-bismuth eutectic (LBE) environments has been investigated using a newly developed slow strain rate testing (SSRT) technique that can be employed at elevated temperatures. This study showed that the Fe-10Cr-4Al steel suffered embrittlement when exposed to LBE over a wide temperature range. The embrittlement, here measured as a reduction in fracture strain, was observed at the melting temperature of LBE and reached a maximum at approximately 375 degrees C. At temperatures above 425 degrees C, the material's ductility regained its original levels. The exposures in liquid lead showed no indications of embrittlement, but a ductile behavior over the entire tem-perature range studied (340-480 degrees C).