Synergistic effect of triton X100 and 3-hydroxybenzaldehyde on Zn-Mn electrodeposition from acidic chloride bath

Abstract

The effect of two non-complexing additives, namely Triton X100 and 3-hydroxybenzaldehyde (3-HBA) on Zn-Mn alloy deposition was investigated. The study was carried out using a simple KCl-based bath. To this end, cyclic voltammetry associated to the switching potential method, bulk electrolyses and scanning electron microscopy were used. Electrochemical study, from additives-free bath, showed that the potential of beginning Zn-Mn alloy deposition (E = −1.52 V vs. Ag/AgCl) stands between those of Zn (E = −1.03 V) and Mn (E = −1.65 V) alloying elements. The coatings obtained from this bath are Mn-poor alloys with dendritic structure, and the current efficiency is low (60%). It was found that the conjunction of triton X100 ad 3-HBA suppresses completely the individual electrodeposition of Zn. The synergistic effect of additives favors Zn and Mn co-deposition, but Mn-rich alloys are deposited only from E = −1.8 V vs. Ag/AgCl. The coatings obtained at lower cathodic potentials are adherent, smooth and bright. The current efficiency is enhanced and reaches 85%. Voltammogram data, from a comparative study using polyethylene glycol (PEG8000) instead of triton X100, display prominently the vital role that plays the hydrophobic head of triton X100 molecule in enhancing Mn incorporation into Zn matrix. Moreover, the switching potential technique demonstrated that at cathodic potentials higher than −1.8 V, the hydrogen evolution reaction is catalyzed when Mn-rich are deposited such that it gives rise to a higher pH increase at the cathode surface. This pH increase leads to an intense precipitation of hydroxides that blocks the alloy deposition process.