The comparative study of citrate-sulphate bath with and without ammonium ion and its effect on electrodeposition and selected properties of ternary Zn-Fe-Mo alloy coatings for corrosion protection

Abstract

This study evaluates Zn-Fe-Mo alloy coatings, electrodeposited from citrate-sulphate baths with ammonium ions, focusing on how current density correlates with coating properties for protecting carbon steel from corrosion. Electroplating in a bath containing (NH₄)₂SO₄ salt takes place under less negative reduction potentials which resulted in higher content of Fe (up to 12 wt.%) and Mo (up to 5 wt.%). Regardless of the presence or absence of ammonium ions, Fe and Mo contents decreased with rising deposition current density. Deposits from both types of baths were grayish and powdery on the surface at lower current densities (10–15 mA cm⁻²) while at the higher ones (20–25 mA cm⁻²) they were smooth and fine. It was accompanied by a reduction of surface roughness from 0.8 to 0.04 µm. Coatings deposited at 20 mA cm⁻² were built of columnar beads formed by 20–40 nm crystallites. Deposits were highly textured in the presence of (NH₄)₂SO₄ while those obtained in Na₂SO₄ containing bath were not. Besides pure Zn and Fe phases, possible iron-zinc intermetallic with stoichiometry close to Fe₂₂Zn₇₈, Fe₁₁Zn₄₀ or FeZn_10.98 were found. Increasing the electrodeposition current density from 10 to 25 mA cm⁻² caused a rise in the corrosion resistance for both types of coatings. More specifically, ammonium ions favour the fabrication of more noble deposits with the lowest corrosion current densities and the highest charge transfer resistances after 24 h exposure in 0.5 M solution of NaCl.