Physical and Chemical Characterization of Oxide Layers Formed on Fe-Cr-Al alloys in Liquid Pb45Bi55 and Its Alternatives at High Temperatures
- Author(s): Popovic, Miroslav
- Advisor(s): Hosemann, Peter
- et al.
Lead-bismuth eutectic (LBE, Pb45Bi55) is a promising candidate alloy for use as a fluid in heat storage and transport applications in nuclear and solar power generating systems, due to its superior thermal, chemical, physical and neutron-moderating properties. In order to further improve the thermal efficiency in these systems, it is a goal to push the operating (upper) temperature up to 800 oC or more. However, a major drawback in using liquid LBE at high temperatures is the increasing solubility of many chemical elements from solids in LBE (particularly nickel from austenitic materials). This can be mitigated by adding an appropriate amount of oxygen into liquid LBE and keeping control of the oxygen concentration to enable formation of protective oxide layer or layers on or near the structural material surface, whereby the oxidation of the elements depends on temperature, oxygen concentration and chemical potential of oxidation. Addition of aluminum to the iron-based ferritic materials has been found to be beneficial in preventing dissolution and excessive oxidation, by forming an Al-oxide layer that provides an effective diffusion barrier for metal ions and protects the material.
In this work, the structure of the oxide phases formed on candidate materials after exposure to oxygenated LBE at temperatures up to 800 C has been analyzed and characterized by various spectroscopic and microscopic methods. Three Fe-Cr-Al ferritic alloys, containing various percentages of Al and Cr, have been exposed to LBE in an in-house designed experimental setup in controlled oxygen atmosphere, in various test duration. The main goal was to determine the influence of four factors on the formation of oxide layers: Al content in Fe-Cr-Al alloys, temperature, oxygen content in LBE, and duration of exposure. The structural analysis of the layers has been performed by transmission electron microscopy, energy-dispersive X-ray spectroscopy, Raman spectrometry and X-ray photoelectron spectroscopy. It has been found that the oxygen content in LBE around 1×10-6 wt% is optimal for the formation of stable, continual and well-adhered oxide layers. Higher Al content in steels stimulates formation of mostly Al2O3, while lower Al content leads to the enrichment in Fe- and Cr-oxides. At higher oxygen content (5×10-6 wt% O) and lower Al content, at 800 0C, formed oxide scale is a complex structure of more than one layer. Lowering of the oxygen concentration to 1×10-6 wt% in LBE at same temperature leads to the reduction in diversity of oxides (further domination of Al-oxide), while the concentration of 1×10-7 wt% O was found to be too low for compact and well-defined scales formation. The same effect of enrichment in Al2O3 in oxide scales is obtained by lowering the temperature.
Oxide scales degradation is another topic that has been dealt with in the study. At high temperatures and higher oxygen concentration in LBE, the diffusion of oxygen through the oxide layers into the bulk increases and may lead to the internal oxidation of Al. This can generate strain in the oxide and in near-oxide bulk due to the mismatch between the crystal structures of oxides and of the bulk, leading subsequently to the oxide layers breaking and spallation. The other cause of strain can be the mismatch of thermal expansion coefficients of oxides and of the bulk, in cooling at the end of heat transfer systems operation. Strain in the oxide scales might lead to their breaking and failure. According to the results obtained by the X-ray microdiffraction analysis, strain in the lattice mainly comes from plastic deformation, and more dominant mechanism of strain induction is by the internal oxidation. Besides that, the influence of an additional thermal disturbance has been studied by inducing a mid-term thermal cycle in corrosion tests in LBE at various test temperatures. It has been found that thermal cycling leads to the increase in oxide scales failure with the increase in operating temperature and with increasing Al content in samples.
The last goal of this work was to investigate if the addition of a third element X (a minor additive) to LBE can improve the protection of structural materials by possible formation of intermetallic layers, besides the oxides, i.e. to design and evaluate optimal Pb-Bi-X ternary alloys as an alternative to LBE. Theoretical considerations indicated Pb-Bi-Sb and Pb-Bi-Ge as the most promising alternatives, while the experimental evaluation gave some evidence of the intermetallic formation only on high-Al tested sample in Pb-Bi-Sb. However, no significant advantage over LBE has been found in behavior of these two ternary alloys towards the Fe-Cr-Al materials.