SESSION II: CORROSION MITIGATION MEASURES: COATING, NEW
3. EFFECT OF OXYGEN CONCENTRATIONS IN STATIC LBE ON
The typical cross-section morphologies of T91 and 15-15Ti steel after exposure in LBE containing 10-6 wt%, 10-7 wt% and 10-8 wt% oxygen at 500 ºC for up to 2000 h as shown in Fig. 3 and Fig. 4, respectively.
At 10-6 wt% oxygen concentration, an oxide scale with a three-layer structure was formed on the surface of T91. The outermost layer was Fe3O4, where severe LBE penetration occurred as a result of loose structure. The middle layer was composed of (Fe, Cr)3O4 with spinel structure, and the structure was relatively dense, which could effectively block LBE continue to penetrate.
The innermost layer was an internal oxidation zone (IOZ), which was structured like a needle-shaped or elongated channel and extended into the interior of the T91 substrate. For 15-15Ti, however, an oxide scale with a two-layer structure was formed on the surface, which is different from T91. The outer layer is Fe3O4, and the inner layer is (Fe, Cr)3O4. And LBE permeation into the oxide film could also be observed.
At 10-7 wt% and 10-8 wt% oxygen concentration, T91 underwent oxidation corrosion, but the oxide scale has changed from a three-layer structure to a two-layer structure, and no Fe3O4 layer existed. This is because the formed Fe3O4 is unstable at low oxygen content in LBE, easily decomposed, and dissolved in LBE, resulting in the disappearance of Fe3O4. For 15-15Ti, however, dissolution corrosion occurred under the conditions of 10-7 wt% and 10-8 wt% oxygen concentration, and LBE clearly penetrated the 15-15Ti matrix.
FIG. 3. Cross-section morphology of T91 steel after exposure in LBE containing 10-6, 10-7 and 10-8 wt% oxygen at 500 ºC for up to 2000 h (a) 10-6 wt% oxygen; (b) 10-7 wt% oxygen; (c) 10-8 wt% oxygen [17]
FIG. 4. Cross-section morphology of 15-15Ti steel after exposure in LBE containing 10-6, 10-7 and 10-8 wt% oxygen at 500 ºC for up to 2000 h (a)10-6 wt% oxygen; (b) 10-7 wt% oxygen; (c) 10-8 wt% oxygen [17]
The above results show that ferritic/martensitic steel T91 and austenitic steel 15-15Ti exhibit different corrosion behaviors in liquid LBE. The main reason for this is high content of Ni in 15-15Ti. Because the solubility of Ni element in LBE is much larger than that of Fe and Cr, when 15-15Ti is in contact with LBE, the Ni element in the steel preferentially dissolves, and when the oxygen concentration is low, preferential dissolution corrosion will occur.
The elemental diffusion model suggested by Müller and Martinelli [11,12] could explain the formation mechanism of the three-layer oxide scale on the surface of T91 steel at 10-6 wt%
oxygen concentration. As shown in Fig. 5, first, Fe cations diffuse outward and combine with O anions in LBE to form Fe3O4, which resulting in a large amount of Fe vacancies in the region near the surface of T91. These vacancies then aggregate at the steel/oxide interface to form micro-voids. In these micro-voids, Fe and Cr cations react with the diffused O anions to form Fe-Cr spinel (Fe2+O[Fe3+, Cr3+]2O3), filling the micro-voids. Once this thin spinel layer is formed, Fe cations will pass through it to diffuse from the substrate to the surface and combine with the O anions to form Fe3O4 on the surface; at the same time, the O anions will pass through Fe3O4 and Fe-Cr spinel layer to diffuse into the substrate. As a result, Fe-Cr spinel will form in the micro-voids of the Fe-Cr spinel/IOZ interface. And O anions will continue to diffuse at the grain boundary of the surface region of the substrate to react with Fe and Cr cations, forming Cr-rich oxides. And finally an IOZ layer will form. Therefore, according to this model, Fe3O4
grows outward on the steel surface, while Fe-Cr spinel grows inward, and the inter-diffusion of Fe and O plays an important role in the growth of the oxide film.
FIG. 5. Schematic diagram of the formation mechanism of three-layer oxide film on T91 steel
Fig. 6 shows the evolution of the oxide scale thickness of T91 steel with time under different oxygen concentration conditions. It can be seen from Fig. 6a that the thickness of (Fe, Cr)3O4
increased with increasing exposure time. In addition, at the same time point, the thickness of (Fe, Cr)3O4 is different under different oxygen concentration conditions. After exposure for
500 h, the thickness of (Fe, Cr)3O4 is the smallest when the oxygen concentration of LBE is 10
-8 wt%. However, the thickness of (Fe, Cr)3O4 at 10-7 wt% oxygen concentration is slightly greater than that at 10-6 wt% oxygen concentration. The possible reason is that the outer surface oxide film of Fe3O4 on T91 temporarily hinders the growth of (Fe, Cr)3O4 at 10-6 wt% oxygen concentration. At 1000 h and 2000 h, the thickness of (Fe, Cr)3O4 increased with increasing oxygen concentrations. It can also be seen from Fig. 6b that the total thickness of the oxide scale increased with increasing exposure time and oxygen concentrations. The above data indicates that oxygen content in LBE is a key factor in determining the corrosion behavior of steel materials, and the dominant factor affecting the types and properties of corrosion interface products.
FIG. 6. Comparison results for the thicknesses of the oxide layers formed in LBE with different oxygen concentrations: (a) the thickness of the magnetite + spinel layers; (b) the total thickness [17]
Since dissolution corrosion occurred for 15-15Ti under the conditions of 10-7 wt% and 10-8 wt%
oxygen concentration, the evolution relationship of corrosion depth with time under these two oxygen concentration conditions was compared. As shown in Fig. 7, the penetration depth of LBE increased with increasing exposure time under the same oxygen concentration, which basically conformed to the straight-line law. It can also be found that the penetration depth of LBE at 10-8 wt% oxygen concentration was slightly larger than that at 10-7 wt% oxygen concentration. So, the oxygen concentration is the dominant factor affecting the type and properties of the corrosion interface products.
FIG. 7. Evolution of LBE penetration depth with time in 15-15Ti steel at 10-7 wt% and 10-8 wt% oxygen concentration [17]