Influence of Aging Time on Microstructure and Corrosion Behavior of a Cu-Bearing 17Cr–1Si–0.5Nb Ferritic Heat-Resistant Stainless Steel

17Cr–1Si–0.5Nb–1.2Cu ferritic heat-resistant stainless steel was aged at 750 °C from 10 min to 30 h to simulate time aging and study the microstructural evolution and its effect on corrosion behavior by using optical microscopy, scanning electron microscopy, transmission electron microscopy, potentiodynamic polarization, electrochemical impedance spectroscopy, and the Mott–Schottky approach. Four types of precipitates were discovered, including ε-Cu, NbC, Fe3Nb3C, and Fe2Nb-type Laves phase. The nano-sized ε-Cu phase forms first, and its fraction follows the parabolic law change and is the largest. Compared to NbC and Fe3Nb3C particles, the coarsening of the Laves phase is the most pronounced. The aging process is divided into three parts: early-aged (0–5 h), peak-aged (5 h), and over-aged (5–30 h). However, the corrosion resistance is reduced in the early-aged stage of 0–2 h. Further extending the aging time to 30 h, the corrosion resistance is gradually improved. This change may be related to the competitive relationship between the beneficial effects of the Cu-rich phase and the harmful effects of Nb-containing particles. The dissolved Cu on the surface becomes more effective for the suppression of the anodic dissolution by the formation of ionic compounds of chlorine, thereby reducing the deterioration of corrosion resistance caused by Nb-rich precipitation.