Corrosion Inhibition of Nickel Achieved by Phosphate Addition into Chloride-Rich Media toward Seawater Electrolysis

Ni-based materials are promising candidates for anodic electrodes and electrolysis cell materials in seawater electrolysis systems for hydrogen production. However, their practical application is hindered by severe corrosion in chloride-rich electrolytes. Incorporating phosphate into the electrolyte has emerged as an effective strategy to enhance corrosion resistance, though the underlying mechanisms remain poorly understood. In this study, the corrosion inhibition mechanism of phosphate on Ni was systematically investigated. Polarization measurements in 0.5 M borate–0.5 M KCl electrolytes at pH 9.2 with varying phosphate concentrations revealed that Ni undergoes pitting corrosion with Cl–, which is significantly mitigated by phosphate addition. This enhancement is attributed to two factors: 1) structural modification of the passive film, and 2) suppression of pit propagation. STEM/EDS analysis shows phosphate incorporation into the Ni passive film, altering its structure from crystalline to amorphous, which correlates with the enhanced protective ability of the passive film. Furthermore, potentiostatic polarization reveals that phosphate addition inhibits pit propagation even after its initiation. This inhibition is likely due to the pH buffering effect of phosphate. These findings offer new mechanistic insights into phosphate-assisted corrosion resistance enhancement and provide a foundation for the design of corrosion-resistant nickel-based materials for highly concentrated chloride environments.