Molten LBE has a high solubility of nickel, iron, and chromium, which are the most important alloy elements

in austenitic stainless steels. Thus, nickel super alloys and austenitic stainless steels cannot be used as the structural materials for LBE-cooled systems, especially at temperatures >500 ° C. Ferritic steels have been con­sidered more appropriate for LBE application.

Exposure of 9Cr-ODS steels to an LBE environ­ment at 530 °C was carried out in the DELTA Loop of the Los Alamos National Laboratory. The molten
alloy flow velocity in the loop is 1.2 m s-1, and oxygen sensors were used to measure and maintain an oxygen concentration of about 1 x 10~6wt%. Samples were exposed for 200, 400, and 600 h, in order to study the early stages of oxide formation and growth. A cross­sectional view and the distribution of elements are shown in Figure 32.54 In a short time, the 9Cr-ODS steel formed a protective duplex oxide layer consisting

Time to rupture (h)

of an outer magnetite (Fe3O4) layer and an inner Fe-Cr spinel ((Fe, Cr)3O4) layer, which is sometimes accom­panied by an O-enriched and Fe-depleted diffusion zone at the oxide-bulk interface. Over time, the outer magnetite layer is removed and the underlying spinel layer serves to mitigate more catastrophic corrosion degradation such as dissolution and liquid metal attack along the grain boundaries. Very thin oxides are not particularly protective in regard to loss of metal, as manifested by the thick diffusion zones associated with them. Furukawa pointed out that at tempera­tures above 600 °C, the thickness of the oxide layer diminishes with increasing temperature. This behavior can be ascribed to a change in the stable form of iron oxide from magnetite to wustite at 570 °C. Beyond this temperature, dissolution attack was observed on some portions of 9Cr-ODS steel, and the oxide layer’s adhesion to the material began to weaken.55

It has been reported that the addition of aluminum to steel effectively prevents LBE corrosion. Figure 33 shows the appearance of ODS steel specimens after a corrosion test in LBE for 1 x 104h at 650 ° C.43 The 18wt% Cr-ODS steel without the addition of Al dissolved markedly into LBE, while those ODS specimens containing 4wt% Al almost completely maintained their shape even in Al-added 14Cr — and 16Cr-ODS steels, indicating a very high resistance to LBE corrosion. It is noteworthy that this corrosion resistance was independent of Cr concentration from 13 to 19 wt% in Al-added ODS steels. From the distribution of elements across the cladding surface, we deduce that LBE corrosion can be prevented by the formation of an Al enriched film.56 It was demonstrated that Al-added 16Cr-ODS steel (16Cr- 2W-4Al-0.1Ti-0.35Y2O3) has superior corrosion resis­tance at 650 °C for 5000 h.