Abstract: 3.5% NaCl solutions at various pH levels were prepared. A comprehensive approach combining measurements of open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curve, and long-term immersion tests was employed to investigate the influence of pH on the electrochemical corrosion behavior of 42CrMo steel, supplemented by surface morphology characterization and weight-loss analysis. The results indicate that the corrosion of 42CrMo steel in the test solutions was primarily governed by an activation-controlled dissolution. Acidic conditions (low pH) disrupted the oxide film on the steel surface, leading to a significant decrease in impedance and an acceleration of corrosion rate. With the prolongation of immersion time, the corrosion resistance of the steel declined, the most aggressive attack occurred at pH1, where a corrosion rate of 5.83 mm/a was recorded after 15 d. In contrast, in pH3 and pH5 solution, the corrosion rates remained low (0.11 and 0.12 mm/a, respectively) and exhibited minimal variation over time.