Rebar corrosion is a significant deterioration problem in concrete that critically impacts the durability and structural integrity of reinforced concrete (RC) structures. The passivation and corrosion mechanisms substantially influence the service lifespan of steel rebar in the surrounding electrolytic environment. The passivation mechanism is a complex process dependent on the composition, microstructure, and surface condition of the steel rebar. The present work discusses the passivation and corrosion behavior of commonly used quenched self-tempered or thermos-mechanically treated bars (TMT), stainless steel (SS), and mild steel (MS) reinforcing bars in both chloride-free and chloride-contaminated simulated concrete pore solutions. Electrochemical techniques such as open circuit potential (OCP), linear polarization resistance (LPR), and cyclic polarization(CP) tests were carried out to evaluate passivation and corrosion behaviour of steel rebar specimens in chloride-free and chloride-contaminated simulated concrete environments. In addition, Mott–Schottky assessments were carried out to study the behavior and stability of passive oxide films formed on the rebar specimens. Results indicate that in chloride-free and chloride-contaminated simulated cementitious environments, the stainless-steel specimens exhibited stable and protective passive films characterized by low donor density, space charge thickness, and current density. Nevertheless, MS and TMT exhibited relatively higher current densities, which is due to the effect of lower capacitance. The n–type behavior was observed in TMT and MS specimens, which could be due to the inherent differences in the composition and microstructure.