Electrochemical Fabrication of Ni–Co Alloy over a Wide pH Range Using Sodium Citrate as a Complexing Agent

In this study, nickel–cobalt (Ni–Co) coatings were fabricated via electrodeposition using a 22 central composite factorial design with two central and two axial points, totaling ten experiments. The effects of pH and current density on the coatings’ chemical composition and properties were evaluated. Coatings were characterized by microstructure, morphology, magnetic properties, and corrosion resistance. The results showed that pH significantly influenced chemical composition, while current density had no notable effect. Acidic pH produced cobalt-rich coatings (43–81 at.%), with uniform morphology, higher saturation magnetization, and lower corrosion resistance. Maximum cobalt content (81 at.%) resulted in a mixed face-centered cubic (fcc) + hexagonal close-packed (hcp) phase. Alkaline pH yielded nickel-rich coatings (89–95 at.%), featuring nodular morphology, lower magnetization, higher corrosion resistance, and, exclusively, the fcc phase. The highest polarization resistance (66.1 kΩ) occurred at pH 8.83 and 60 mA/cm2, while resistance decreased with increasing cobalt content. The pH effect on deposition was linked to the formation of citrate complexes: ammonia and citrate complexes promoted nickel deposition under alkaline conditions, while stable cobalt complexes dominated in an acidic pH. These findings highlight the potential to tailor Ni–Co coatings for applications such as corrosion-resistant coatings (nickel-rich) or magnetic devices (cobalt-rich).