Development of Zn-Co alloy coatings by pulsed current from chloride bath

Abstract

Zinc-M (where M = Ni, Co and Fe) alloy is of great interest owing to their better mechanical and corrosion properties compared with pure zinc coatings. Corrosion resistance of Zn-Co alloy coatings can be improved considerably by pulse plating. The paper details the optimization of Zn-Co alloy bath using pulsed current and details the superiority of pulse plating over direct current plating. Electroplating of Zn-Co alloys over mild steel was carried out under different conditions of pulse parameters like duty cycle, frequency and peak current density. The production and properties of the deposits were found to be influenced by pulse parameters employed. Within the ranges studied, the bath follows anomalous codeposition with preferential deposition of less noble zinc. The influence of current density on %wt. of Co in the deposit and cathode current efficiencies was studied. It was observed that the deposit at average current density of 5.0 A.dm-2, 50% duty cycle and 100 Hz frequency showed excellent corrosion resistance with fine structure. The peak performance of pulse electrodeposit against corrosion was attributed to the change in the surface homogeneity as evidenced by scanning electron microscope (SEM) image. The drastic decrease of corrosion rate in pulse electrodeposit was attributed to the formation of semiconductor films on the surface as supported by impedance spectroscopy signals.