<p>This research focused on studying the nucleation and growth of Zn-Co alloy electrodeposited onto AISI/SAE 1018 steel from its precursor ions dissolved in reline, a deep eutectic solvent formed by the eutectic mixture of choline chloride and urea, and its corrosion inhibition capability on AISI/SAE 1018 steel in saline conditions. It is reported for the first time the mechanism and kinetics involved during Zn-Co alloy electrochemical nucleation and growth onto the mild steel surfaces. Zn-Co alloy is formed throughout multiple three-dimensional nucleation with diffusion-controlled growth; however, concomitant electrochemical reactions also occurred, namely, the electrochemical double-layer (adsorption-charging) process, residual water reduction on the steel surfaces, and on the growing surfaces of the Zn-Co alloy nuclei. The physical–mathematical model proposed to analyze the experimental current density transient allowed determining the kinetic parameters for these reactions and to deconvolute each of these individual contributions. From this analysis, it was possible to determine that the Zn-Co deposit inhibited water reduction on the steel surface while catalyzing it on its own surface. SEM and EDS allow determining the presence of the Zn-Co alloy on the steel surfaces after potentiostatic electrochemical deposition and its morphology features. The coating was homogeneous, adherent, and covered the entire electrode surface. EIS tests revealed that Zn-Co coatings protected steel from corrosion in an aqueous saline medium by increasing its charge transfer resistance, offering up to 98.7% corrosion inhibition, depending on Zn-Co alloy deposition time.</p>