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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increase in the test time from 240 to 1440 h, an increase in the process of intergranular corrosion from the surface layers (Fig. 1b, 2a) into the depth of the base metal of the samples (Fig. 1d, 2c) was observed. At 650 °C after 240 h (Fig. 2a), a large number of hardening NI were observed along the grain boundaries. The microhardness was 1705±55 MPa and 7050±420 MPa, respectively, of the base metal and corrosion products. After 720 h (Fig. 2b), an increase in the grain size and the amount of hardening dispersed NI both inside and along the grain boundaries was observed in the grain structure. Corrosion products were already formed in the form of local fragments. The microhardness was 2600±20 MPa and 3800±60 MPa, respectively, of the base metal and corrosion products. After 1440 h (Fig. 2c), the structure of the base metal had the characteristic features of tempering. Linear NI during rolling were larger scattered NI located mainly along the grain boundaries. The grains, having lost their geometric shape, became more rounded, which indicates in favour of the occurrence of partial collective recrystallization.

Fig. 2. Photographs of the microstructure of samples of AISI 316L steel after exposure in liquid in lead melt for 240 h (a), 720 h (b) and 1440 h (c) at 650 °C.

The behaviour of stainless steel AISI 316L in molten lead for test durations of 240, 720, 1000 and 1440 h are the same bo th at 450 °C and at 650 °C. Also, with an increase in temperature, a change in the intensity of the local corrosion process was characteristic from insignificant surface single local pitting at 450 °C to deep cavities located over the entire surface at 650 °C due to the appearance of dislocation concentrators, catalysts of the corrosion process, due to etching of the grain boundaries of the matrix structures of austenitic steel at 550 °C ( Fedirko et al., 2019, Klymenko et al., 2022). According to the results of corrosion tests, it was found that in the dynamics of changes in the corrosion rate of stainless steel AISI 316L in lead melt both at 450 °C and at 650 °C, there is a similar trend of decreasing the value of the indicator with an increase in the duration of testing and an increase in the value with increasing temperature (see Table 1). So, the corrosion rate in lead melt decreases from 0.275 mm/year (240 h) to 0.0610 mm/year (1440 h) and from 1.376 mm/year (240 h) to 1.095 mm/year (1440 h), respectivel y, at 450 °C and 650 °C, due to the oxide films formation on the sample's surface.

Table 1. Corrosion rate of the investigated steel. Temperature T, °C

Corrosion rate, mm/year

Test duration t, h

0.275/1.376 0.166/1.500 0.084/0.978 0.038/1.021 0.061/1.095

240 480 720

450/650

1000 1440

3.2. Local chemical analysis of corrosion products Corrosion damage to the surface layers as a result of prolonged exposure of the lead melt to the sample's steel surface is also associated with the formation of porous layers of oxidation/corrosion products. All samples were characterized by the formation of a layer of metallic lead with oxidation/corrosion products of a layered structure,