Effects of pH Value of the Electrolyte and Glycine Additive on Formation and Properties of Electrodeposited Zn-Fe Coatings
Environmentally friendly and cyanide-free sulfate bath under continuous current and the corrosion behavior of electrodeposits of zinc-iron alloys were studied by means of electrochemical tests in a solution of 3.5% NaCl in presence and absence of glycine. The effects of pH on the quality of Zn-Fe coatings were investigated in order to improve uniformity and corrosion protection performance of the coating films. The deposit morphology was analyzed using scanning electron microscopy (SEM), and X-ray diffraction (XRD) was used to determine the preferred crystallographic orientations of the deposits. It was found that the uniformity and corrosion resistance of Zn-Fe coating films were strongly associated with pH of the coating electrolyte. To obtain the effect of pH on the film quality and corrosion performances of the films, the corrosion test was performed with potentiodynamic anodic polarization method. It was also observed that uniformity and corrosion resistivity of the coating films were decreased towards pH = 5 and then improved with increasing pH value of the electrolyte. The presence of glycine in the plating bath decreases the corrosion resistance of Zn-Fe coatings.
Deposition of Zn–Co by constant and pulsed current
The initial stages of both Zn–Co and Zn electrodeposition were investigated by electrochemical quartz crystal microgravimetry (EQCM). The initial electrode mass growth, determined under both pulse and constant current conditions, was much higher than predicted by Faraday's law. This was explained in terms of the precipitation of scarcely soluble compounds of zinc on an electrode surface. The EQCM data confirm that the hydroxide suppression mechanism explains the anomalous Zn and Co codeposition. A nonuniform adsorption of brightener (benzalaceton) on the profiled surface was concluded on the basis of plating distribution investigations. The additive adsorbs to a greater extent on the surface projections.
Electrodeposition of zinc + cobalt alloys: initiations and development of anomalous co-deposition
Zinc + cobalt co-deposition was analyzed using potentiodynamic stripping of deposits. Results indicate the initial formation, on vitreous carbon, of a cobalt sub-monolayer. This formation explains the onset of the anomalous deposition process: zinc(II) adsorbs on the initial cobalt, and favours zinc deposition. The presence of zinc(II) exerts an inhibitory effect that hinders cobalt deposition. The formation of either pure zinc or zinc-rich deposits depends on the proportion of zinc(II) in the neighbourhood of the substrate. For the highest Zn(II)/Co(II) ratios, zinc(II) prevents the cobalt co-deposition and the deposit consists of pure zinc in a wide range of overpotentials and deposition times. A fast depletion of zinc(II) near the electrode is required for the co-deposition to begin. For zinc(II)-rich baths small amounts of Sb(III) increase the cobalt content in the deposit.