Electrochemical preparation of supersaturated solid solution ZnFe alloy coating on mild steel from ChCl-EG deep eutectic solvents for corrosion protection

The ZnFe alloy coating on mild steel was produced through electrodeposition from a choline chloride-ethylene glycol (ChCl-EG) deep eutectic solvent (DES) containing ZnCl₂ and FeCl₂ at a temperature of 343 K. Although the reduction potential of Zn(II) is more negative than of Fe(II), but the presence of Fe(II) ions facilitates the under-potential deposition of Zn. A dense and uniform ZnFe alloy coating devoid of hydrogen brittleness can be achieved at low current density (3–5 mA cm⁻²) with a modest C_Zn(II)/C_Fe(II) ratio, specifically between 3:1 and 5:1. Increasing the C_Zn(II)/C_Fe(II) ratio in the bath while increasing the current density leads to a decrease in the iron content of the ZnFe alloy coating. The crystal structure and corrosion resistance of these coatings on mild steel are contingent upon deposition condition and their composition. At moderately high current density (≥5 mA cm⁻²), ZnFe alloy with an iron content between 8.5 at.% to 14.0 at.% exhibits a supersaturated solid solution phase (η-phase), whereas an increase in Fe content to 18.2 at.% leads to a mixed phase comprising η, FeZn₁₅ and Fe₄Zn₉ phases. Additionally, as current density decreases, similar iron contents (14.2 ∼ 14.7 at.%) yield mixtures including η, Γ₁,and FeZn₁₅ phase within the resultant alloys. Notably, a ZnFe alloy containing approximately 14.0 at.% Fe on mild steel demonstrates optimal corrosion resistance in 3.5 wt% NaCl solution due to its dense microstructure coupled with appropriate iron content levels. These results offer a promising approach for fabricating dense ZnFe alloy coating owing to its simplicity, non-polluting nature, efficiency, and absence of hydrogen evolution.