Influence of Applied Potential, Water Content and Forced Convection on the Electrodeposition of Ni Films on Steel from Choline Chloride Based Deep Eutectic Solvents

Electrochemically deposited Ni coatings are widely used in the aviation, automotive, telecommunication and electronic industry because of their unique mechanical, protective and decorative properties. These films are commonly deposited from aqueous electrolytes due to the simplicity and cost-effectiveness of the processes. However, the use of water-based baths is related to several issues, i.e. water reduction and hydrogen gas formation that lead to the formation of pores and cracks which result into more brittle coatings. A promising alternative to avoid these problems is to use non-aqueous electrolytes such as Deep Eutectic Solvents (DESs).

The feasibility of electroplating Ni and its alloys from choline chloride based DESs has been reported by several research groups [1–3]. In this context, recent studies have been focusing on the use of water as an additive. In most of the cases, just a small addition of water to those non-aqueous electrolytes is beneficial due to a decrease of viscosity and an increase of conductivity [3,4]. However, the electrocatalytic reduction of water during electrodeposition from DESs has also been shown to result into complex chemical and electrochemical processes that have a strong influence in the structure and morphology of the electrodeposited metallic phase [5].

Moreover, industrial electroplating processes are always carried out under forced convection to increase process efficiency and reduce deposition time. The interplay between electrodeposition kinetics and the mass transport of reactants and byproducts of the Ni electrodeposition from DESs has not been evaluated in detail yet. Similarly, the influence of applied potential on the structure and morphology of Ni coatings electrodeposited from DESs, as well as the effect on the electrochemical reduction processes are so far not fully understood.

In this presentation, we report on the inter-related effects of water content, applied potential and forced convection in Ni electrodeposition on steel from 1 choline chloride (ChCl): 2 urea (U) DESs. Electrochemical methods such as cyclic voltammetry (CV) and chronoamperommetry (CA) were combined with surface analysis techniques (field emission scanning microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy) in order to understand the occurring phenomena under both stagnant and forced convection conditions. The influence of mass transport on Ni electrodeposition process was studied by linear sweep voltammetry in combination with a rotating disk electrode (LSV-RDE).

The obtained results allowed a better understanding of the complex chemical and electrochemical processes occurring at the electrode surface depending on the mass transport conditions, applied potential and water content, which may result into the formation of NiO-OH and incorporation of DES decomposition by-products in the growing films.