Electrochemical studies of sol-enhanced Zn–Ni–Al2O3 composite and Zn–Ni alloy coatings

• Zn–Ni–Al2O3 composite coatings were produced by sol-enhanced electroplating.
• Al2O3 nano-particles increased number of active nucleation sites and nucleation rate.
• The incorporation of Al2O3 in the Zn–Ni coating refined the crystal grain size.
• Zn–Ni–Al2O3 coatings were more uniform and compact compared to Zn–Ni coatings.

Zn–Ni–Al2O3 nano-composite coatings were electrodeposited on mild steel using a novel sol enhanced electroplating method. The effect of alumina sol on the electrodeposition process, and coating properties was investigated using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The results indicated that the electro-crystallization processes of Zn–Ni and Zn–Ni–Al2O3 were governed by a three-dimensional nucleation process controlled by diffusion. Evaluation of nucleation mode in the presence and absence of alumina sol showed that the progressive nucleation was predominant for the Zn–Ni alloy deposit. However, for the Zn–Ni composite coating, the nucleation mode was closer to instantaneous nucleation. Nucleation parameters such as density of active nucleation sites and nucleation rate were increased in the presence of alumina nano-particles in the bath. Zn–Ni–Al2O3 nano composite coatings produced more uniform and compact deposits, with fine grained microstructure when compared to Zn–Ni coatings. XRD results showed that the phase structure of both alloy and composite coatings was single Ni5Zn21-γ phase, and that the incorporation of alumina in the Zn–Ni coating refined the crystal grain size.