Pulse Electroplating of Zinc/Nickel Alloy from a Deep Eutectic Solvent (DES)

Abstract

Electrodeposition of metal coatings such as nickel, zinc, tin, chromium and cadmium for the purposes of corrosion protection has been a core interest of the electrochemical community for more than one hundred years. Cadmium has been a favourite for military and aerospace applications because of the combination of good corrosion protection and lubricity. However, cadmium salts are very toxic and hazardous and so the use of cadmium coatings is now proscribed in all but the most demanding of applications. Alternatives to cadmium coatings have included aluminium and more recently zinc nickel alloys. The latter is more accessible via aqueous methods but requires rigorous control and monitoring of bath conditions in order to maintain the composition and quality of the coating. An alternative to aqueous electrolytes is a derivative of ionic liquids that feature eutectic mixtures of quaternary ammonium salt and hydrogen bond donor. These have become known as deep eutectic solvents (DSE). Here we have used a DES to produce coherent and adherent coatings of zinc nickel alloy in an electrochemical study and in pre-commercialisation scale-up trials. In a digital controlled pulse-plating method, alternative forward (deposition) and reverse (stripping) currents, are applied to adjust the deposit composition and morphology, as has been demonstrated in film plating from aqueous electrolytes. This report focuses on pulse-plating γ-Zn11Ni2 alloy from a eutectic mixture of choline chloride (ChCl) and ethylene glycol (EG), also known by the commercial name of Ethaline. The effect of pulse shape and frequency on deposit morphology and crystal structure were analysed and compared with coatings derived from constant current (DC) plating methods, through Hull cell tests, XRD, and SEM/EDX measurement. The deposit composition is shown here to be dependent on the average current density, the surface morphology is not sensitive to the pulse frequency, while dendritic growth is shown to be suppressed by pulsed current. We show that the crystal grain size can be controlled according to the pulse shape and frequency. By choosing a certain shape and frequency, the pulse-plating in the DES can provide improved coating qualities and improved process control. Additionally, we have extended the laboratory investigation to pre-commercial scale-up trials with a commercial electroplating company (EC Williams, UK). This project is funded by an Innovate UK action and is entitled Cadmium Replacement Using Pulse-Plating and Ionic Liquids (CRUPPAIL). Barrel-plating trials using pulse and reveres pulse methods on small components such as screws have given even adherent coatings, Figure. The combination of pulse methods and DES electrolyte give a levelling effect, Figure (a), and good throwing power in confined geometries, Figure (b). Figure: Scanning electron micrographs of Zinc Nickel alloy (85:15%) deposited using DES and pulse-plating on mild steel substrates, (a) sheet, (b) screws. Figure 1