Electrodeposition of zinc-nickel coatings from sulfate-chloride electrolytes containing boric and citric acids

In this work, the process of electrodeposition of zinc-nickel coatings on brass and steel substrates from acidic and slightly acidic sulfate-chloride electrolytes was studied. Boric and citric acids were used as a buffer additive. The molar ratio of nickel and zinc ions in the Ni(II)/Zn(II) electrolyte was chosen with a large excess of nickel ions (> 10 times). The chemical composition of the electrolyte was determined by complexometric titration with Trilon B. The bulk composition and morphological properties of the coating were determined using a JEOL JSM-6460LV scanning electron microscope with computer control. The operating current density was selected using an electrochemical cell with a corner cathode - a Hull cell. The deposition of the alloy was carried out by the electrochemical method in a direct current galvanic bath. Nickel plates were used as anodes. It has been established that for electrolytes with boric acid additions, the limiting value of the current density is 0.8 A/dm2, and when citric acid is used as a buffer additive in the electrolyte, the limiting value increases to 2.5 A/dm2 without deteriorating the quality of the coating. Alloys deposited from an electrolyte with citric acid have, according to electron microscope studies, a finer grained structure, and visually they look smoother and shinier. Calculations have shown that deposition from an electrolyte with citric acid makes it possible to obtain a higher current efficiency compared to deposition from an electrolyte with boric acid. The rate of deposition of coatings from electrolytes with citric acid is much higher in the entire range of current densities, regardless of the nature of the substrate. Thus, the addition of citric acid to the electrolyte for the deposition of zinc-nickel coatings provides significant advantages over the addition of boric acid, which is traditionally used for these purposes.