Microstructure, Corrosion, and Hardness Properties of Ni Coatings Electrodeposited from a Deep Eutectic Solvent-Based Electrolyte

Deep eutectic solvents (DESs) are environmentally friendly ionic liquids that have recently been employed for the electrodeposition of metals. In this paper, Ni was electrodeposited at different current densities of 1-30 mA cm^-2 from a choline chloride/ethylene glycol DES-based electrolyte and an efficient current density range of 1-8 mA cm^-2 was identified to produce Ni coatings on AISI 1045 medium carbon steel with reasonable substrate corrosion protection properties while enhancing surface hardness. The coatings produced at current densities equal to or lower than 4 mA cm^-2 show better polarization behavior as well as the corrosion rate of 10-35 times lower than that of the substrate alone, while the surface microindentation hardness increases to 477 ± 7 Hv_10gf (1.5 times that of the substrate). The enhanced microindentation hardness can be related to the nanostructured Ni crystallites and the ultrafine Ni cluster size. SEM micrographs including cross sections following the potentiodynamic polarization test, revealed the absence of corrosion pits in the Ni coating electroplated at 8 mA cm^-2 likely due to the coalescence of the corrosion pits led to more general Ni corrosion while keeping the substrate intact.