Pitting Corrosion Resistance of LDED-Manufactured IN625-YSZ Coatings Exposed to Sulfide-Containing NaCl Solution

Pitting corrosion is a more insidious and dangerous failure mode than uniform corrosion for protective coatings in marine environments, especially when sulfide ions (S2−) are present. In this work, Inconel 625 (IN625) and IN625-5 wt.% yttria-stabilized zirconia (YSZ) composite coatings were fabricated on 20G steel by laser-directed energy deposition (LDED) using optimized parameters. The coatings exhibit dense microstructures, with porosities of 1.87% ± 0.10% and 1.67% ± 0.10%, respectively. The pitting resistance was systematically evaluated by cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution without and with 20 ppm Na2S. The CPP results show that the addition of YSZ decreases the pitting potential (Epit) and protection potential (Eprot), indicating reduced resistance to pit initiation, while the smaller hysteresis loop suggests an improved tendency for repassivation. S2− induces a notable drop in Eprot and impairs repassivation for both coatings, yet the IN625-YSZ coating retains a slightly higher Eprot than IN625. EIS analysis reveals that the IN625-YSZ coating in S2−-containing solution shows an increased Rct of 2.738 × 105 Ω·cm2 compared with its counterpart in 3.5 wt.% NaCl solution, while the corresponding RL decreases to 2.513 Ω·cm2, suggesting a weakened outer barrier layer despite partial interfacial blocking. Post-corrosion morphology shows that YSZ particles act as preferential pitting nucleation sites; in sulfide-free solution they produce numerous shallow pits, whereas in S2−-containing solution they lead to larger and deeper pits. The results may provide a reference for the future design and evaluation of LDED-manufactured composite coatings under sulfide-containing marine environments.