Investigation of the effect of chloride ions on the corrosion of DH36 steel in acidic solution

This study systematically investigates the corrosion behavior of DH36 steel under the combined influence of chloride (Cl⁻) and hydrogen (H⁺) ions, simulating marine splash zone environments. Electrochemical techniques—including potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS)—coupled with microstructural analyses (SEM and EDS) were employed to evaluate corrosion mechanisms in NaCl solutions (1–7.5 wt.%) and in acidic solutions containing 3.5 wt.% NaCl with HCl concentrations ranging from 0 to 0.6 mol/L. Results reveal that with increasing Cl− concentration, the corrosion potential decreases from -519 mV to -588 mV and the corrosion current density increases from 0.0137 mA/cm² to 0.0279 mA/cm², indicating accelerated corrosion rates. Notably, at >5 wt.% NaCl, pit density surges to 11,440 pits/mm² (7.5 wt.%), while pit size expands by ∼80%. In acidic Cl⁻-rich environments (HCl ≥ 0.4 mol/L), corrosion intensifies, and corrosion products become porous, facilitating longitudinal grooves and deep pits. EIS data confirm decreased polarization resistance (e.g., R p drops from 4,211 Ω·cm² to 2,297 Ω·cm² in 7.5 wt.% NaCl), and Warburg impedance emerges, signifying diffusion-controlled corrosion. The dominant mechanism involves electrochemical reactions where Cl⁻ and H⁺ act in concert to aggressively promote Fe dissolution via soluble complexes (e.g., [FeCl(OH)]⁻ ad ), while acidic conditions inhibit passivation. These findings highlight the critical vulnerability of DH36 steel in aggressive marine settings, providing essential insights for enhancing corrosion resistance in offshore structures. • Synergy between H⁺ and Cl⁻ ions drastically accelerates DH36 steel corrosion. • A critical Cl⁻ threshold (>5 wt.%) exists, beyond which pitting severity surges. • Acidic conditions inhibit passivation and promote porous corrosion products. • Corrosion mechanism shifts to diffusion-control in aggressive H⁺/Cl⁻ environments. • Electrochemical and morphological analyses reveal dominant pitting mechanisms.