The influence of multiple complexing agents on the optimized performance of Zn-Ni pulse electrodeposition

The pulse electrodeposition of Zn-Ni coatings in alkaline cyanide-free baths was investigated at fixed average current with identical charge input. Under direct current electrodeposition, gains in corrosion resistance achieved solely by adjusting complexing agents are often limited. We therefore coupled pulse parameters with a composite complexant system. Tetraethylenepentamine (TEPA) was used as the primary complexing agent, and sodium potassium tartrate and sodium citrate were introduced as auxiliary complexing agents. Microstructural and electrochemical results show that the pulse on-off pattern is well matched to the interfacial kinetics under composite complexation. During the on-time, the high peak current alleviates slow interfacial decomplexation in TEPA-dominated baths and promotes renucleation. Tartrate and citrate help maintain near-surface Zn/Ni balance and mitigate current localization. During the off-time, they facilitate ion replenishment and lower hydrogen coverage, suppressing defect accumulation. As a result, compact γ-Ni 5 Zn 21 -dominant deposits with enhanced preferred orientation are obtained. Relative to direct current electrodeposition in the TEPA-only bath, pulse electrodeposition at the optimized duty cycle decreases the corrosion current density from 6.07 to 0.745 μA·cm −2 . With tartrate-citrate addition, it further decreases to 0.264 μA·cm −2 , while the charge-transfer resistance increases to 4.46 × 10 4 Ω·cm 2 . These results show that composite complexation enhances the corrosion-protection benefit of pulse control in cyanide-free alkaline Zn-Ni electrodeposition.