Influence of Chloride Ions on Corrosion Mechanism of Pipeline Steel in Bicarbonate Alkaline Environments

The corrosion evolution of pipeline steel was investigated in NaCl + 0.75 M NaHCO₃ solutions with different chloride concentrations using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis, X-ray photoelectron spectroscopy (XPS), and surface characterization techniques. The results demonstrated that chloride concentration was a critical factor governing the transition of corrosion behavior from a relatively stable oxide-film-controlled state to active dissolution. At low Cl^- concentrations, a stable current-density region was observed, indicating the formation of a protective oxide film. With increasing Cl^- concentration, the stable current region gradually disappeared, accompanied by a continuous decrease in polarization resistance and oxide-film resistance, revealing the progressive deterioration of corrosion resistance. Mott-Schottky analysis showed that the oxide film formed in 0.5 wt.% NaCl + 0.75 M NaHCO₃ solution (pH ≈ 8.3) exhibited n-type semiconductor characteristics, with a donor density of 8.69 × 10²¹ cm^-3. XPS analysis revealed that the oxide film mainly consisted of Fe²⁺-related oxides. The experimental results indicate that the corrosion evolution was associated with the competitive interaction between HCO3- and Cl^-, where bicarbonate favored oxide-film stabilization at low chloride concentrations, whereas chloride enrichment promoted oxide-film deterioration and accelerated active dissolution. This study establishes the relationship between chloride concentration variation, oxide-film degradation, and corrosion evolution of pipeline steel in high-bicarbonate alkaline environments, providing experimental insights for corrosion assessment and control under chloride-enriched service conditions.