Electrodeposition of Zn, Co and ZnCo from acid sulfate solutions onto steel was investigated in this first part of a study of the effects of SiC or Al2O3 particles on these processes and the formation of ZnCo -SiC and ZnCo - Al2O3 electrocomposites. Zn electrodeposition shows a well-defined pre-bulk region, where the hydrogen evolution reaction (HER) and Zn underpotential deposition (upd) compete. Zn bulk electrodeposition begins with primary nucleation and diffusion-controlled growth, strongly dependent on conditions favoring previous Zn upd against HER. It is assumed that this first bulk process takes place over the upd Zn. Zn bulk electrodeposition is followed by secondary nucleation and growth. Co electrodeposition begins with a slow Co-(aq)(2+) reduction in parallel with HER, followed by a faster Co(aq)(2+) reduction. Zn-(aq)(2+) strongly hinders the initial Co(aq)(2+) reduction. The ZnCo and Zn electrodeposition curves are initially similar, retaining features of pre-bulk and bulk Zn electrodeposition.