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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3

= ⁄( )

(2) where ∑ – average corrosion rate by weight loss, ( 2 ⁄ ℎ) ; – corrosion rate in units of penetration, mm/year; 8.76 – conversion coefficient; – metal density, 3 ⁄ ; – sample mass loss, ; – surface area of the test sample, 2 ; – test duration, h . The chemical removal of molten lead from the samples was carried out with a solution containing acetic acid, hydrogen peroxide and ethyl alcohol in a ratio of 1:1:1 (Huang et al., 2021). The metallography and durometric analysis were carried out by Neophot 32 optical microscope and by LECO M-400 hardness meter M-400 at 0.5 N load respectively. The structure study and determination of elemental composition by X-ray spectral microanalysis (XSMA) was carried out using an electronic scanning microscope JEOL JSM-35 CF (Japan) and Tescan MIRA 3 LMU with energy dispersive spectrometer (EDS) Oxford X-max 80 mm 2 (INCA Energy-350 from Oxford Instruments, UK). The microstructure study was carried out on specimens after electrolytic etching in chromic anhydride CrO 3 at 15 V with holding- time 30…40 s at a solution temperature of 20 °C. Corrosion tests of the samples were performed at 450 °C at an exposure of 24/7 and duration 1440 h with an intermediate samples study after 240, 480, 720, 1000 and 1440 h in a lead melt in ceramic crucibles placed in autoclaves to limit the oxygen access. With a view to limit oxygen access, tests of steel samples were carried out in stainless steel hermetic autoclaves. The oxygen concentration in the lead melt at 450 °C was 1.92 · 10 -4 wt.% and at 650 °C was 6.06 · 10 -3 wt.% (Gromov et al., 1997). 3. Results and discussion 3.1. The structure of the base metal and corrosion rate The structure of the base metal with traces of rolled material consists of twinned austenite grains of irregular geometric shape, which are difficult to pickle (Fig. 1a). The grain size is 5...30 μm. One can see line inclusions of sulfides along the rolling lines, as well as chaotically located nitrides of titanium and other dispersed non-metallic inclusions (NI). The Vickers microhardness (HV) in the initial state is 1570±40 MPa.

Fig. 1. Photographs of the microstructure of samples of AISI 316L steel in the initial state (a) and after exposure in liquid in lead melt for 240 h (b), 720 h (c) and 1440 h (d) at 450 °C.

The metallographic analysis of steel samples after testing showed that at 450 °C the austenitic granular structure practically does not change with increasing testing time and is characterized by twins of irregular geometric shape, the presence of rolled traces with longitudinal linear inclusions of titanium sulfides and nitrides, as well as randomly located NI (Fig. 1a-d ). After 240 h of testing, the microhardness was 1710±100 MPa and 4360±655 MPa, respectively, of the base metal and corrosion products. With an increase in the duration of tests up to 720 h (Fig. 1c), the precipitation of carbide and nitride NI along the grain boundaries on the base metal increased. In addition, NI on the rolled strips are stitched and rounded. The resulting corrosion products were mostly dense and partially destroyed. A characteristic feature was the formation of a dense continuous light layer of a modified structure at the base metal/corrosion products interface. The microhardness was 2220±95 MPa, and 2630±530 MPa, respectively, of the base metal and corrosion products. After 1440 h of testing (Fig. 1d), irregularly shaped grains were observed on the base metal. Linear NI were also observed during rolling and titanium nitrides and carbonitrides randomly located in the metal structure. With an