PSI - Issue 59

Anatolii Klymenko et al. / Procedia Structural Integrity 59 (2024) 214–221 Anatolii Klymenko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction As it is known, the use of heavy liquid metals in nuclear reactors as coolants, represented by the Pb or Pb-Bi eutectic, is associated with the problem of their corrosiveness with respect to austenitic stainless steel AISI 316L, which is the main structural material of the vessels. This is due to the content of highly soluble Ni and Cr, which undergo selective leaching with the formation of near-surface layered ferrite on the steel, as highlighted by Tsisar et al. (2014, 2016), Schroer et al. (2014), Cionea et al. (2016), Wang et al. (2022) and Wang et al. (2023). Oxidation of stainless steel AISI 316L under controlled oxygen conditions (2 m/s, 10 -7 wt.%) at 450 °C and 550 °C is accompanied by the formation of a thin (≤0.5 μm) oxide film based on Cr with simultane ous selective leaching of Ni and Cr, which leads to the zonal formation of layered ferrite with local corrosion damage of a maximum depth of 210 μm, penetrated by Bi and Pb, was reported by Tsisar et al. (2014) and Cionea et al. (2016) . At 650 °C, after 10 0 h of exposure under conditions of a controlled oxygen content of 10 -2 wt.%, a multioxide layer 17.5 µm thick is formed on the steel. In this case, the outer layer is formed on the basis of Fe 3 O 4 oxide, and the inner dense layer is formed on the basis of Fe – Cr oxide. At an oxygen content of 10 – 5 wt. % for 100 h of exposure, the corrosion depth reached 50 µm (Wang et al., 2022). At 700 °C, a change in the surface morphology is characteristic of stainless steel AISI 316L with the formation of a granular poro us structure with an oxide layer thickness of ~35 μm (230 h), identified as magnetite with the simultaneous formation of FeCr 2 O 4 with a Cr content mainly in the inner part of the oxide layer. For temperatures above 800 °C, a thin and lamellar porous surface morphology is characteristic with an oxide layer thickness of ~30 μm (360 h) with the formation of Fe 3 O 4 and an amorphous, less stable Fe-O layer as a result of repeated thermal cycles, as well as increased exposure to oxygen, as reported by Cionea et al. (2016). The inhomogeneity of the process of oxidation of steel into lead is also noted with an increase in temperature above 500 °C due to a change in the nature of diffusion of Cr and Fe into the oxide film and through the film and oxygen from the alloy into the steel (Yeliseyeva et al., 2008). Benamati et al. (2002) and Toshinsky et al. (2020) noted that consideration should be given to the dissolution of steel components in liquid metal with the penetration of the melt into the matrix at a temperature of ~550 °C due to the approach of the melt oxidation potential to the thermodynamic potential of Fe 3 O 4 oxide with increasing temperature. Moreover, the formation of non-continuous oxide layers on the surface of stainless steel AISI 316L, which contributes to the development of local corrosion processes, is characteristic of static test conditions in lead melt (Rozumová et al., 2021). In addition, under conditions of interaction with melts of heavy metals, the influence of the medium on the ability of steel to plastic deformation at elevated temperatures was established, and a decrease in the plasticity of materials at operating temperatures leads to a decrease in their resistance to embrittlement, as shown by Yas’kiv et al. (2016). Thus, in the lead melt, the initiation and propagation of cracks occurs from the contact surface with lead to the core and is accompanied by a change in the fracture mechanism from brittle to ductile. In addition, a decrease in the plasticity of materials under conditions of interaction with heavy metal melts at operating temperatures leads to a decrease in their resistance to embrittlement. The purpose of this work was to research the corrosion resistance of stainless steel AISI 316L in a liquid-metal medium under continuou s exposure to a constant temperature of 450 °C and 650 °C and to study the influence of the kinetic factor on the process of formation of corrosion products in a lead melt in order to form and accumulate experimental data. 2. Materials and methods We studied samples of austenitic stainless steel AISI 316L of the following chemical composition: 65.63% Fe, 17.20% Cr, 11.97% Ni, 1.29% Mn, 0.81% Si, 2.52% Mo, 0.58% Ti. The lead melt components composition was 99.99% Pb. The surface was cleaned with sandpaper with a medium grain size of 180WPF, and metal dust was cleaned with filter paper, followed by washing both off-running and distilled water and degreasing with an ethyl alcohol solution. To determine the corrosion rate, both the prepared samples and the samples after testing were weighed on an analytical balance VLR-200 with measurement accuracy up to 0.00005 g. A corrosion rate was calculated by weight loss method: =8.76 ∑ ⁄ (1)