Surface micro-morphology of 304 SS before corrosion (a), after corrosion in molten NaNO₃–NaCl–NaF salt (b) and molten NaNO₃–NaCl–NaF vapor (c). (Local enlarged region of A1 (b-1), A2 (c-1) and A3 (c-2)).

The corrosion resistance of 316 stainless steel (316SS) in molten KCl-MgCl₂ salts was studied through static immersion corrosion at high temperatures. Below 600 °C, the corrosion rate of 316SS increased slowly with increasing temperature. When the salt temperature rises to 700 °C, the corrosion rate of 316SS increases dramatically. The corrosion of 316SS is mainly due to the selective dissolution of Cr and Fe at high temperatures. The impurities in molten KCl-MgCl₂ salts could accelerate the dissolution of Cr and Fe atoms in the grain boundary of 316SS, and purification treatment can reduce the corrosivity of KCl-MgCl₂ salts. Under the experimental conditions, the diffusion rate of Cr/Fe in 316SS changed more with temperature than the reaction rate of salt impurities with Cr/Fe.

A thin-walled copper (Cu)–tin (Sn) alloy cylinder was treated after spinning at 200–400°C for 0.5 h. The characteristics of the alloy microstructure under different temperatures were analyzed through electron back-scattered diffraction. The results were as follows. The grain size at 200–300°C decreases as the heat treatment temperature rises, but the grain size at 400°C increases. At 200–300°C, the microstructure primarily consists of deformed grains. It is found that the main reason for the formation of high-angle grain boundaries (HAGBs) is static recrystallization. For the grain boundary orientation differential, the low-angle sub-grain boundary gradually grows into the HAGB, and multiple annealing twin Σ9 boundaries appear. Grain orientation is generally random at any temperature range. The mechanical property test indicated that, at the upper critical recrystallization temperature of 300°C, the elongation of the Cu–Sn alloy gradually increases, and its yield strength and ultimate tensile strength rapidly decrease.