Optimization of Heat Treatment Parameters in CA-50 Steel Rebars: Impact on Microstructure and Corrosion Resistance

This study evaluates the influence of heat treatment parameters on the microstructure and corrosion resistance of CA-50 low-carbon steel rebars (0.20–0.25 wt.% C) processed by the Thermex route. A full 23 factorial design combined with response surface methodology was employed to investigate the effects of residence time (15–35 min), heating rate (5–15 °C/min), and soaking temperature (730–850 °C). Corrosion behavior was assessed by linear potentiodynamic polarization and electrochemical impedance spectroscopy in 3.5 wt.% NaCl solution. The corrosion potential (Ecorr) varied between −520.6 and −618.1 mV, with optimal values close to −535 mV obtained at low heating rates and short residence times. Polarization resistance (Rp) ranged from 70.4 kΩ to 166.6 MΩ, with the highest value observed for treatment at 790 °C, 10 °C/min, and 25 min, representing an increase of more than fivefold compared to the reference condition. Statistical analysis revealed that residence time and heating rate significantly affect Ecorr (R2 = 96.8%), while Rp is governed exclusively by residence time (p = 0.004). Microstructural analysis correlated refined and homogeneous ferritic–pearlitic structures with improved corrosion resistance, whereas grain coarsening led to severe electrochemical degradation.