The dissolution of copper in ethylene glycol-water mixtures

The main objective of the work described in this thesis was to determine the kinetics and mechanism of the dissolution of copper in ethylene glycol-water mixtures containing chloride ions. Additionally, the study aimed to investigate the thermodynamics of the copper-chloride system in ethylene glycol-water mixtures. E-pCl diagrams were established experimentally for copper in aqueous and 50% ethylene glycol-water mixtures. Over the pCl range investigated (1-7), the stable species were Cu, Cu2+, CuCl and CuCl2'. The two diagrams were rather similar, and the diagram for the aqueous system agreed well with diagrams constructed from published data. The mechanism of the oxidation of copper in aqueous and 50% ethylene glycolwater solutions was elucidated by linear sweep voltammetry, rotating ring-disk measurement, photoelectrochemistry, and impedance spectroscopy. The effects of chloride ion concentration, proton concentration, and the speed of rotation of the disk on the reaction were examined. The results from the linear potential sweep voltammetry and the rotating ring-disk electrode studies were most informative. In the absence of chloride ion, the oxidation of copper (0) to copper (II) dominates. As the chloride ion concentration is increased (0.02 mol dm'3 < [Cl'] < 0.2 mol dm'3), several reactions are possible depending on the potential and chloride ion concentration. At high chloride ion concentrations, only soluble chlorocopper(I) species are formed. The results were somewhat similar for aqueous solutions and solutions containing 50% ethylene glycol, and in both cases could be interpreted using the E-pCl diagrams determined earlier. Because copper (I) oxide is believed to be formed at some chloride ion concentrations, and is known to be photoactive, attempts were made to identify it by photoelectrochemical methods. Although it was possible to detect copper (I) oxide by this method at high pH’s, the technique could not be used at low pH’s. The rates of the reaction of copper with copper(II) as the oxidant were measured by chronopotentiometry. The effects of ethylene glycol and temperature in addition to the above parameters were tested. The possibility of measuring the dissolution rate by impedance spectroscopy was also investigated. It was found that the impedance spectrum for a dissolution reaction is only equal to the sum of the impedance spectra of the constituent half reaction when both half reactions are diffusion controlled. The relevance of this work to the corrosion of car cooling systems is discussed