Liquid Metal Corrosion Effects in MYRRHA Candidate 316L Austenitic Stainless Steel
Author: Klok, O.
Subject: Liquid Metal Corrosion Effects in MYRRHA Candidate 316L Austenitic Stainless Steel
Promotor: De Graeve, I.
SCK CEN Mentor: Lambrinou, K.
Lead-bismuth eutectic (LBE) is foreseen as coolant and spallation target in the MYRRHA accelerator-driven system. Compatibility of structural materials with liquid LBE is a key issue in the development of MYRRHA. One of the effects that can occur during the interaction of materials with liquid LBE is liquid metal corrosion (LMC). The present state-of-the-art shows that LMC is a complicated physicochemical phenomenon. In order to develop a mitigation strategy, investigation and understanding of the LMC mechanisms is necessary. So far, a significant progress in understanding of LMC was achieved. However, one cannot say that this phenomenon is fully understood. Based on that, this PhD project was launched within the MYRRHA materials qualification program and its goal is to contribute to the understanding of the effect of various parameters on the corrosion behavior of the MYRRHA candidate 316L austenitic stainless steel in contact with liquid LBE. The main focus was placed on the dissolution corrosion. The parameters studied in this PhD project were either material-related or LBE environment-related. The studied material-related parameters included the degree of plastic deformation, deformation twin density, chemical inhomogeneities (typically observed as “chemical banding” in industrial-size steel heats), grain size and the steel surface state. The investigation of the LBE-related factors is mainly focused on the effect of LBE oxygen concentration on the initiation and advancement of dissolution corrosion at specific exposure conditions (temperature and duration of exposure, respectively). All corrosion tests were performed in static LBE. Depending on the questions that had to be answered at each stage of this research, an extensive microstructural characterization of the tested steel specimens was carried out by employing various characterization techniques. Based on the obtained results, the impact of various parameters on the LMC behavior of 316L steel was identified and corrosion mechanisms were proposed. The current LMC mitigation strategy implies the fact that LBE oxygen concentration should be high enough to form a protective oxide on the steel surface, which prevents LBE dissolution attack. However, in this PhD work it was found, that a high LBE oxygen concentration might accelerate dissolution corrosion in comparison with a low one, once it is initiated. In the proposed mechanism, the severity of dissolution attack at high oxygen concentration was associated with the enhanced dissolution of Fe due to formation of the Fe-oxides, which creates favorable conditions for further propagation of corrosion without formation of ferrite. The absence of ferrite enables faster transport of dissolved elements in LBE, which results in higher dissolution rate in comparison with the low oxygen concentration. At the same time, the results of this work showed that an increase of LBE oxygen concentration is capable of delaying the initiation of dissolution corrosion. The corresponding mechanism, which explains the effect of LBE oxygen concentration on the initiation of dissolution corrosion, was proposed. The results of this work also showed that the steel surface state has an impact on the initiation of both steel dissolution and oxidation. According to the existing literature, steel plastic deformation might have various effects on the LMC behavior of austenitic steels. In this work, it was confirmed that plastic deformation accelerates 316L steel dissolution corrosion. The latter was associated with steel microstructural defects that form during plastic deformation, in particular, deformation twins. Decrease of the steel grain size was also found to accelerate steel dissolution corrosion. In addition, it was found that chemical inhomogeneities with respect to the Ni-content, which occur during steel manufacturing, have an impact on the development of dissolution corrosion only in the case of plastically deformed steel. Contrary to the general perception, Ni-poor bands were observed to corrode faster, as compared to Ni-rich ones. Taking into account the evidence obtained by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), a dissolution corrosion mechanism explaining the effect of chemical banding on LMC was suggested. Thus, in this work some of the missing links in understanding of the LMC behavior of 316L steel were identified and investigated. Progress in understanding the effects of various parameters on LMC was achieved. However, further investigation is still required to understand fully the LMC behavior of 316L steel. It is believed that the results of this PhD thesis provided a basis for the further investigation of the effect of various parameters on LMC behavior, which might lead to the development of parametrical correlations for the reactor design.