| Lead and lead alloys, such as lead bismuth eutectic (LBE), have gained worldwide recognition as potential candidates for coolant and target material in accelerator-driven systems (ADS) due to their excellent chemical, physical, and nuclear properties; however, they are corrosive to stainless steels, which are candidate materials for piping and molten metal containers in ADS applications. The Russians have used LBE as a coolant in their nuclear submarines for more than several decades and their experience has given way to the hypothesis that oxides protect steels from LBE corrosion, thus renewing interest in further understanding the oxidation kinetics and mechanisms of candidate stainless steel alloys.; The proposed work will contribute to the ongoing research effort by elucidating the oxidation kinetics and mechanisms, and characterizing the oxides of HT-9, a candidate stainless steel for ADS application. HT-9 (DIN X20CrMoWV12 or Fe-12Cr-MoVW) is a 12 wt% Cr martensitic/ferritic stainless steel alloy developed by Sandvik (Sweden). The approach is to perform thermogravimetric analysis (TGA) to evaluate the oxidation kinetics and assess oxidation rates. Flat bar samples polished to one micron were subjected to non-isothermal and isothermal oxidation scans. Non-isothermal scans were done in dry air from room temperature to 950°C with ramping rates of 2°C/minute and 5°C/minute to obtain preliminary oxidation behavior information. Isothermal scans were done in dry air from 600°C to 800°C for 48 hours, from 800°C to 900°C for 24 hours, and at 950°C for 6 hours. The structure and composition of the oxide film developed during the isothermal scans were characterized using scanning electron microscopy with energy dispersive spectroscopy capabilities (SEM/EDS) and X-ray diffraction (XRD).; Oxidation of HT-9 exhibited a complex behavior that does not follow a simple oxidation model and changes as a function of exposure time. At low temperatures (T ≥ 825°C), the weight change (DeltaW) was minimal and a protective, single-layer, Cr-rich spinel and/or corundum oxide film formed. At high temperatures (T ≤ 875°C), the DeltaW was faster and the oxidation behavior predominantly followed linear kinetics. The developed oxide film is a double layer structure, consisting of an outer Fe-rich corundum oxide layer and an inner Fe/Cr-rich spinel oxide layer. This oxide film grows by outward diffusion of metallic ions from the substrate. An internal oxidation zone with intergranular cracking was also observed at temperatures of 863°C and above. Breakaway oxidation occurred within the 850°C to 863°C range, in which the oxidation behavior shifts from parabolic kinetics (protective) to linear kinetics (non-protective) as initially foreseen in preliminary non-isothermal oxidation scans. |
