Effect of pH-dependent bath speciation on cobalt electrodeposition from sulfate-gluconate solutions
(Faculty of Non–Ferrous Metals, AGH University of Krakow, Al. Mickiewicza 30, 30-059 Krakow, Poland)
Abstract: pH-dependent multiple equilibria in cobalt sulfate-gluconate baths were calculated using stability constants adopted from literature. Changes of the bath speciation were then discussed in terms of spectrophotometric experiments and buffering properties of the solutions (pH 3-10). Cyclic voltammetry indicated changes in electrochemical behavior of cobalt species caused by different ionic compositions of the electrolytes. Tafel slopes were calculated and discussed in relation to electroreduction of cobalt species. Chronoamperometric studies showed 3D instantaneous nucleation of cobalt followed by diffusion-controlled growth, but it was disturbed at higher pH due to the release of cation from gluconate complexes as a limiting step. Diffusion coefficients of cobalt species were found. Changes in the pH were also reflected by modifications of morphology (SEM), development of preferred orientation planes (XRD, texture coefficients) and current efficiency, but not the thickness of the coatings deposited at constant potential of -1.0 V (vs Ag/AgCl). Anodic stripping analysis showed changes in anodic responses originated from the existence of preferentially oriented planes in cobalt layers.
Key words: cobalt; electrodeposition; nucleation; speciation; structure