*4.4. Precipitation of Carbides*

The last difference between the material state after CHT and that after SZTs is an accelerated precipitation rate of nano-sized carbides. According to recent studies [28,50], the precipitation of the Cr-rich M23C6 carbides during tempering is considered to induce Cr-depleted zones around them, and thereby retards the formation of a protective passive

film on the steels' surfaces. On the other hand, the precipitation of M3C carbides with similar Cr content as the matrix has a less detrimental effect on the growth of protective films [28].

For the Vanadis 6 steel, it has been demonstrated recently that the SZT accelerates the precipitation rate of transient cementitic carbides at low tempering temperatures, but these treatments suppress the precipitation of stable M7C3 phase during tempering in the secondary hardening temperature range [19,25].

Figures 8 and 9 show that the corrosion potential, *Ecorr*, decreases with increasing tempering temperature for CHT steel, as well as for the steel which was subjected to the SZT at −140 ◦C. It should be mentioned here that the potentiodynamic measurements of specimens subjected to other regimes of SZT (−75, −196, and −269 ◦C) provided similar qualitative results. Additionally, it is shown (Table 4) that the corrosion current, *Icorr*, increases rapidly with the tempering, which is clearly reflected in the corrosion rate of different SZT specimens (Figure 11). The mentioned changes in corrosion behaviour characteristics can be ascribed to the precipitation of different carbides and the corresponding changes in the matrix. Only cementitic particles were found in the experimental steel after tempering within the low-tempering temperature range [19,25]. The precipitation of M3C does not evoke the Cr depletion of the matrix as the M3C contain only very low chromium amount. The only factor that increases the corrosion may be the higher number of activated sites by forming large amounts of M3C/matrix boundaries. Increased tempering temperature leads to precipitation of M7C3 particles in the case of CHT specimens, which reduces the number of solutionised Cr atoms in the microstructure and thereby considerably deteriorates the corrosion characteristics. Instead, the precipitation of M7C3 carbides was not evidenced after SZTs, and the only consequence of the tempering treatment is the increase in the number of M3C particles and their coarsening [19]. Hence, the corrosion characteristics of SZT Vanadis 6 steel are less negatively influenced by high-temperature tempering.

Based on the obtained results, the possible corrosion mechanism of the Vanadis 6 steel in 3.5% NaCl water solution could be delineated. As mentioned above, the steel contains ECs (vanadium-rich, MC), SCs (chromium rich, M7C3), and certain but very limited amounts of SGCs (Figure 16a). During the corrosion tests, both the ECs/matrix and SCs/matrix interface types are extensively attacked by the corrosion environment, and the carbides are extracted from the surface, which enhances further corrosion (Figure 16b). Additionally, the matrix is considerably attacked by corrosion in this case, as Figure 12a illustrates, and the specimen surface manifests significantly enhanced roughness.

Conversely, the examined steel contains considerably enhanced population density of SGCs after an application of SZTs. The SGCs/matrix interfaces are attacked less extensively by the corrosion environment (Figures 12c and 16c). Moreover, the area percentage of carbides increases at the same time by the application of SZTs. The carbides manifest more noble behaviour than the matrix (Figure 14), and these particles are less covered by the corrosion products (Figure 13). The resulting effect is that the corrosion rate of the SZT specimens is lowered (Figure 11), implying that the application of SZT generally improves the corrosion resistance of the Vanadis 6 steel in 3.5% NaCl water solution.
