An electrochemical study on the interaction between copper ions and the eco-friendly corrosion inhibitor decanoic acid in a 50% (v/v) ethanol/water mixture

In this paper, the electrochemical properties of the ternary system ethanol (E) -water (W) -decanoic acid (HC10) in the absence and presence of Cu2+ ions were investigated. The ternary system was considered, as it is often used as a medium in which copper materials are immersed to treat their surfaces with the green inhibitor HC10. This compound is used to prevent corrosion of copper-based materials of cultural heritage interest. The three-component system was made, mainly, of 50%(v/v) E/W and HC10 varying from 1 to 16 mM, while CuSO4 was employed as source of Cu2+ ions. The measurements were performed using cyclic voltammetry (CV) and a platinum microdisk electrode. CVs, performed in the 50%(v/v) E/W- HC10 solutions, provided a reduction process, involving hydrogen ions released from the weak acid; from the half-wave potential and the diffusion limiting current, the dissociation constant of HC10 was evaluated. The reduction of Cu2+ to Cu0 in the E/W mixture was investigated in detail without and with different amounts of HC10, as well as in the presence of Cl− ions. It was found that Cu2+ interacted slowly with HC10, under the acidic conditions ensuing by the dissociation of the acid itself, and provided soluble copper(II)-carboxylate species. These were reduced at potentials more negative than that due to the reduction of the free Cu2+ ions. The presence of Cl− ions did not provide any sensible change in the CV, apart from the split of the wave, for Cu2+ reduction, due to the formation of the stable CuCl2− specie. The layer of copper formed onto the electrode surface, ensuing the reduction of the various free Cu(II) or Cu(II)-carboxylate species present in the solutions, was passivated by the excess or unreacted HC10 in the medium. The decrease of the ethanol content in the E/W mixture below about 40%(v/v) caused the formation of a sparingly soluble compound, identified as Cu(C10)2 by FTIR analysis.