| Corrosion of materials in lead-bismuth eutectic (LBE) is the important limiting factor to keep the integrity of internal thin-walled components such as fuel cladding during the period of service, which is one of the key engineering problems to be resolved in the development of lead-based reactors. Martensitic steel T91 and Austenite steel 15-15Ti were commonly chosen as the fuel cladding materials for lead-based reactors. In this work, the oxygen concentration as a key factor is proposed to study the corrosion interface behaviors of 15-15Ti/LBE and T91/LBE. Based on the operation condition of China lead-based research reactor (CLEAR-I), corrosion tests of these two materials were carried out in stagnant and flowing LBE to study the corrosion behavior and mechanism. The main results and conclusions are as follows:(1) The influence and mechanism of oxygen concentrations on corrosion behavior of materials in stagnant LBEIn order to investigate the influence of oxygen concentrations on corrosion behavior of T91 and 15-15Ti in LBE, corrosion tests of these two materials were carried out in stagnant LBE containing 10-6,10-7 and 10-8 wt% oxygen at 500℃ for up to 2000 h, respectively. The oxide scale was not observed on the original specimen surfaces by the experimental analysis for the cross-section morphology and surface phase. The composition, morphology and structure of corrosion products at the interface of matrix and LBE after exposure were analyzed, and the results show as follows:As for T91, the oxide-scale structure changes from a three-layer magnetite/spinel/internal oxidation zone (IOZ) scale under the oxygen concentration of 10-6 wt%, to formation of a two-layer spinel/IOZ scale under the oxygen concentrations of 10-7 and 10-8 wt%. As for 15-15Ti, oxidation occurred and a two-layer magnetite/spinel scale was formed under the oxygen concentration of 10-6 wt%. Under the oxygen concentrations of 10-7 wt% and10-8 wt%, the selective dissolution of Ni into LBE occurred at the interface of 15-15Ti matrix and LBE, accompanied by the penetration of LBE into the matrix.The quantitative analysis of the oxide scale thickness under the oxygen concentration of 10-6 wt% shows that the growth rate of oxide scale on the 15-15Ti surface is lower than that on the T91 surface. The thermodynamic analysis of the oxide scale stability shows that the Cr concentration in the Fe-Cr spinel formed on the 15-15Ti surface is higher, resulting in the Fe-Cr spinel layer has a more stable and more compact structure. Due to a higher Ni content in 15-15Ti, the selective dissolution of Ni into LBE is more easy to occur. Hence, severe corrosion dissolution ocurred when 15-15Ti was exposed to the LBE under low oxygen concentrations. With the decrease of oxygen concentration, the oxidation rate decreases while the dissolution rate increases. Hence, the thickness of the whole oxide scale decreases with the decrease of oxygen concentration. As for the 15-15Ti, the oxygen concentration in LBE is lower, the material tends to dissolution, and the penetration depth of LBE into the matrix is greater.(2) The corrosion behavior and mechanism of materials in flowing LBEIn order to evaluate the corrosion performance of the two materials under the service condition of CLEAR-I, the corrosion tests were carried out in flowing (1m/s) oxygen-controlled LB E (1-3×10-6 wt%O) at 500℃ for 5000 h by the forced convection experiment loop. The oxide scale was not observed on the original specimen surfaces by the experimental analysis for the cross-section morphology and surface phase. The composition, morphology and structure of corrosion products at the interface of matrix and LBE after exposure were analyzed, and the results show as follows:a three-layer oxide scale, consisting of Fe3O4, (Fe,Cr)3O4 and IOZ, is formed at the interface of T91 and LBE, while a single layer (Fe,Cr)3O4 is formed at the interface of 15-15Ti and LBE. This is because a small quantity of alloying elements near the steel surface dissolved into LBE at first when the two materials were exposed to the LBE, subsequently, the O in LBE diffused into the matrix and reacted with Fe and Cr, resuluting in the formation of a very thin spinel layer. And then, the mutual diffusion of Fe and O at the LBE/matrix interface causes the formation of the oxide scale:the Fe-Cr spinel grows inwards at the interface of the matrix and Fe-Cr spinel and Fe3O4 grows outwards at the interface of the LBE and Fe-Cr spinel. Because the diffusion rate of the Fe element in the 15-15Ti matrix through the (Fe,Cr)3O4 layer is lower, it is not easy to form the Fe3O4 layer on the 15-15Ti surface, and only a small amount of Fe3O4 is formed on the local surface area after 5000 h. However, the formation rate of Fe3O4 is larger than the dissolution rate at the interface between (Fe,Cr)3O4 and LBE because the (Fe,Cr)3O4 layer on the T91 surface is more porous. Hence, the oxide scale structure on the T91 surface is the same as that in the static LBE. The growth kinetics curves of the oxide scale formed on both two materials surfaces follow a parabolic rule (△x2=kpt). As for the T91, the rate constants (kp) of Fe3O4, (Fe,Cr)3O4 and IOZ layers are 0.052,0.040 and 0.0057μm2/h, respectively. In the case of 15-15Ti, the kp of (Fe,Cr)3O4 is 8×10-5μm2/h. The calculation results of the dissolution rate of the material components show that the dissolution rate of 15-15Ti is 500×t-0.5 mg/m2h, and is approximately 1.29 times as T91. However, the component dissolution of T91 mainly comes from the direct dissolution after the material compositions through the oxide scale, while the component dissolution of 15-15Ti is the dual role of the exfoliation and/or dissolution of Fe-Cr spinel and the direct dissolution of the material compositions.From the viewpoint of materials protection, dissolution can cause serious damage on material, hence, under the condition of 500℃, the oxygen concentration of 10-6 wt% is able to make the protective oxide scale be formed on the 15-15Ti and T91 surfaces. From the viewpoint of heat transfer, the thicker oxide scale is a disadvantage factor affecting the thermal conductivity of T91. Above research results can provide a certain amount of data and reference for the operation optimization of the future reactor and the cladding materials selection. |
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