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<p>Nickel–cobalt (Ni–Co) alloys are valued for their magnetic, catalytic, and corrosion-resistant properties. In this study, Ni–Co alloys were electrodeposited onto copper substrates from a choline chloride–ethylene glycol (1:2) deep eutectic solvent (DES) containing iodine, with boric acid (BA). UV–visible spectroscopy revealed a gradual color change from deep blue-green (Co-rich) to pale yellow (Ni-rich), indicating variation in metal–ligand speciation and complex formation. Electrodeposition was carried out potentiostatically at 83 ± 2 °C under a constant current. Cyclic voltammetry confirmed the reduction potentials of Ni<sup>2+</sup> and Co<sup>2+</sup> ions in the DES, verifying their co-deposition behavior. Surface morphology and microstructure analyses (FE-SEM and XRD) showed a transition from hexagonal close-packed (HCP) to face-centered cubic (FCC) structures with increasing nickel content. Electrochemical impedance spectroscopy (EIS) demonstrated that the 80Ni–20Co coating exhibited the highest charge-transfer resistance (R<sub>ct</sub> = 1462.5 Ω&#xa0;cm<sup>2</sup>), indicating superior corrosion protection due to stable passive film formation, while the 50Co–50Ni alloy displayed the lowest double-layer capacitance (C<sub>dl</sub> = 1.394 × 10<sup>-3</sup> F&#xa0;cm<sup>-2</sup>) and highest hardness (78.8 HV), signifying a dense and coherent microstructure. These results reveal how BA and alloy composition synergistically influence the microstructure, corrosion resistance, and mechanical properties of Ni–Co coatings from DES-based electrolytes.</p> Graphical abstract <p></p>

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Tailoring microstructure and corrosion resistance of Ni–Co alloys: the critical role of boric acid

  • 期刊论文

  • 发表日期：

    2025年12月20日

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  • Narmin Mahmood,
  • Hassan H. Abdallah

摘要

Nickel–cobalt (Ni–Co) alloys are valued for their magnetic, catalytic, and corrosion-resistant properties. In this study, Ni–Co alloys were electrodeposited onto copper substrates from a choline chloride–ethylene glycol (1:2) deep eutectic solvent (DES) containing iodine, with boric acid (BA). UV–visible spectroscopy revealed a gradual color change from deep blue-green (Co-rich) to pale yellow (Ni-rich), indicating variation in metal–ligand speciation and complex formation. Electrodeposition was carried out potentiostatically at 83 ± 2 °C under a constant current. Cyclic voltammetry confirmed the reduction potentials of Ni2+ and Co2+ ions in the DES, verifying their co-deposition behavior. Surface morphology and microstructure analyses (FE-SEM and XRD) showed a transition from hexagonal close-packed (HCP) to face-centered cubic (FCC) structures with increasing nickel content. Electrochemical impedance spectroscopy (EIS) demonstrated that the 80Ni–20Co coating exhibited the highest charge-transfer resistance (Rct = 1462.5 Ω cm2), indicating superior corrosion protection due to stable passive film formation, while the 50Co–50Ni alloy displayed the lowest double-layer capacitance (Cdl = 1.394 × 10-3 F cm-2) and highest hardness (78.8 HV), signifying a dense and coherent microstructure. These results reveal how BA and alloy composition synergistically influence the microstructure, corrosion resistance, and mechanical properties of Ni–Co coatings from DES-based electrolytes.

Graphical abstract