## 4 Discussion  
### Corrosion of Cr-Oxide-Forming Alloys in LBE  
For up to 1000 h at 600 \({}^{\circ}\)C, all tested alloys exhibited a similar evolution of duplex oxide layer structures, regardless of their dissimilar compositions and microstructures. This observation agrees with the results of previous low-temperature studies and reaffirms the similarity of corrosion processes at 600 \({}^{\circ}\)C for up to 1000 h: all the materials formed protective single oxide layers at low temperatures while the oxide layer became thicker and changed from a single layer to a duplex layer of magnetite (external) and Fe-Cr spinel (internal) with increasing temperature and test time [16, 27, 43]. Iron diffused from the metal substrate to the oxide layer, supporting the external magnetite layer growth and producing nanoscale vacancies. Oxygen diffused into specimens via nanochannels and formed the inner Fe-Cr spinel while LBE penetrated the outer magnetite oxide layers, which is very similar what was reported at temperatures below 550 \({}^{\circ}\)C [43]. After 2000 h corrosion tests at 600 \({}^{\circ}\)C, the duplex oxide layer structure was destroyed in all the specimens, with the oxide layer no longer being protective. This destruction can be rationalized as follows. As the oxide layer grows in thickness, it may be spalled  
Figure 12: Stable duplex oxide layers on SS316L after 1000 h at 600 \({}^{\circ}\)C.  
Figure 13: EDS analysis of LBE-penetrated region on SS316L in Cr-depleted and inner Cr-rich reformed oxide regions after 2000 h at 600 \({}^{\circ}\)C.  
due to excessive lateral compressive stress that leads to tensile stress in the direction perpendicular to the oxide layer [44; 45]. Moreover, the stress inside the oxide can increase with its changing composition, since the transition from the metal lattice to the cation lattice of the oxide is accompanied by a volumetric change [44; 46; 47]. A similar stress build-up has been reported for the Cr/Fe composition change in the oxide layer of Fe-Cr steel [48]. Compressive stress in the transverse direction can induce an upward normal stress [49]. In addition, the cracking of the oxide layer can also be induced by the difference of thermal expansion coefficients of its components, which was ruled out in this experiment by maintaining a constant temperature. Compared to the case of 1000 h, the oxide layer thickness increases after 2000 h, resulting in weaker layer integrity. After the duplex oxide layers are spalled, they can be re-formed on the bare metal surface exposed to LBE. The thin internal Cr-rich oxide layer grows concomitantly with the Cr-depletion zone due to the selective oxidation of Cr. In some T91 and HT9 specimens, LBE penetrated the Cr-depleted zone. If the above re-passivation fails, as observed for some specimens, the Cr-depleted layer may suffer from LBE penetration due to the significant dissolution of alloying elements after a long exposure to LBE at high temperature. For SS316L, LBE eventually penetrates the metal substrate. Roughly speaking, the maximum thickness of the duplex layer not accompanied by integrity loss has been experimentally determined as 44 \(\pm\) 6 \(\upmu\)m for HT9 and T91, and as 25 \(\pm\) 6 \(\upmu\)m for SS316L. To sum up, the corrosion process of steels in high-temperature stagnant LBE advances through several steps. Firstly, a single oxide layer of Fe-Cr spinel forms on the interface between LBE and substrate metal, which is protective against LBE penetration. As exposure time elapses, the oxide structure grows further and become a duplex structure where magnetite builds up on the interface between LBE above the Fe-Cr spinel layer. The growth of the oxide layer exerts a higher compressive stress to the duplex layer, which leads to the oxide being spalled. This removal allows LBE to penetrate into the outer layer of magnetite and then even into the spinel layer. Due to the selective oxidation given by Cr diffusion in the metal substrate, a Cr-depleted layer forms near the inner interface between the spinel and substrate.