Anion-Specific Failure Mechanisms of Polydopamine Coatings on N80 Steel in HCl and H2SO4: A Comparative Study in Strongly Acidic Media

Polydopamine (PDA) coatings have great potential for corrosion protection in harsh acidic industrial environments. However, their performance and failure mechanisms in strongly acidic media at identical H⁺ concentrations but with different anions remain unclear. This work systematically compares the corrosion inhibition behavior of PDA coatings on N80 steel in hydrochloric acid and sulfuric acid media using static weight-loss measurements, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), UV-Vis spectrophotometry, and surface characterization. The results demonstrate significant anion-dependent behavior. At 25 °C, the corrosion inhibition efficiency reached 96.55% in 0.05 mol L −1 H 2 SO 4 with a coating peeling rate of only 1.22%. In contrast, the efficiency was only 59.72% in 0.10 mol L⁻ 1 HCl with a peeling rate of 3.38%. The charge-transfer resistance was higher and the coating structure was more stable in H 2 SO 4 . Increasing the acid concentration or temperature reduced the protective performance of the coating, with faster degradation observed in HCl. The Mechanistic analysis indicates that small, highly mobile Cl⁻ ions penetrate the coating and disrupt intermolecular interactions, whereas SO 4 2 ⁻ interacts weakly with protonated PDA, thereby preserving coating integrity. These results provide a theoretical basis for designing PDA-based anticorrosion coatings for high-chloride acidic industrial environments.