Abstract

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 \(\text{HCO}_3^-\) 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.

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