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This work deals with the electrochemical nucleation of cobalt, onto a glassy carbon electrode, from a deep eutectic solvent formed by choline chloride and urea in a 1:2 molar ratio. The nucleation mechanism was studied through electrochemical techniques. From the analysis of the experimental potentiostatic current transients, the kinetic parameters such as the nucleation frequency, A, and the number density of active sites, N0, were determined. In this work, A was also expressed as a function of the wetting angle, θ, (A(θ)) where the local distribution of θ was calculated through a stochastic simulation of the misorientation angle distribution derived from a created random texture. The A(θ) obtained from such a stochastic approach was in good agreement with the experimental data thus leading to determination of the Gibbs free energy of nucleation ΔG(nc) and the critical nucleus size nc, aside from other kinetic parameters such as the transfer coefficient α, the exchange current density j0, and the surface tension σ. Furthermore, a new method of calculating the nucleation energy from a voltammetric trace recorded during cobalt nucleation onto glassy carbon was proposed and validated. This method provides a novel tool to assess the experimental magnitude and to measure the nucleation energy.

The present paper shows experimental evidence of electrochemically synthetizing cobalt with hexagonal and face centered cubic structures from a choline chloride-based deep eutectic solvent. The nucleation and diffusion-controlled growth mechanisms of cobalt onto a copper electrode were studied through electrochemical means in order to determine several kinetic parameters of interest such as the nucleation rate, A, and the number density of active sites, N 0 . Current density transients were analyzed using a model comprising the simultaneous presence of cobalt 3D nucleation with diffusion-controlled growth and residual water reduction on the growing surfaces of the metallic nuclei. The massive electrodeposition of cobalt was carried out on copper substrates using potentiostatic and galvanostatic techniques. The analysis of results obtained from the surface deposit microstructure and morphology using scanning electron microscopy images and magnetic measurements, confirmed the existence of the hexagonal and cubic cobalt structures. It was found that fiber textures parallel to the electrodeposition axis are formed in the case of the cobalt cubic structure. Furthermore, mainly the current density and kinetics of crystallites nucleation are considered the key parameters to favor the crystal structure change, thus formation of the texture of cobalt from DES.

The nucleation and growth of chromium onto glassy carbon electrode from Cr(III) ions dissolved in a deep eutectic solvent, composed by a mixture of choline chloride and ethylene glycol were studied. From both potentiostatic and potentiodynamic methods, it was evidenced that Cr electrodeposition, from Cr(III) ions, takes place through two stages: firstly, from Cr(III) to Cr(II), forming soluble species, followed by reduction from Cr(II) to Cr(0). It was found that the Cr electrodeposition kinetics and mechanism are different depending on whether it is occurring from Cr(III) or from Cr(II) ions. From analysis of the potentiostatic experimental current density transients, new models were proposed to explain the chromium nucleation and growth in these initial conditions. These models involve contributions to the overall current due to: an adsorption process, diffusion of Cr(III) ions to the electrode surfaces to form Cr(II) ions, Cr 3D nucleation and diffusion-controlled growth and residual water reaction over the growing surfaces of the Cr nuclei. The proposed models help determining the charge percentage due to each individual contribution to the total process. Scanning electron microscopy and energy dispersive spectroscopy were used to determine the morphology and the elemental composition of the deposit respectively.

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The corrosion inhibition efficiency, g, of sodium (6R,7R)-3-[(acetoxy)methyl]-8-oxo-7-[(2-thienylacetyl)amino]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, cephalothin, was evaluated in the system API 5L X52/1 M HCl, using electrochemical impedance spectroscopy and SEM analysis. The results showed that the inhibition mechanism involves blockage of the steel surface by the inhibitor molecules through a Langmuir-type adsorption process. The studies to determine the durability where the inhibitor remained effective showed that, at stagnant conditions, the g was maintained over 92% for up to 600 immersion hours at 50 ppm, reaching the best corrosion efficiency when temperature was 25°C. Ó 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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