The corrosion of copper in ethylene glycol-water mixtures containing chloride ions

The anodic oxidation of copper at 25°C in 50% (w/w) ethylene glycol-water and in aqueous solutions has been studied by linear sweep voltammetry. The effect of chloride concentration at pH 0 and 3 has been explored. The results in both solvents follow a similar pattern. At pH 0 and in the absence of chloride, only one anodic peak is observed corresponding to the dissolution of copper metal as copper(II) ions. At intermediate chloride concentrations (0.01–0.03 M), two additional peaks are detected which have been attributed to the following reactions: $$\begin{gathered} Cu + Cl^ - \to CuCl + e^ - \hfill \\ CuCl \to Cu^{2 + } + Cl^ - + e^ - \hfill \\ \end{gathered} $$ When the chloride concentration is increased further, the three peaks gradually collapse back into one, corresponding to the dissolution of copper as a copper(I) chloro-complex. An additional peak appears at pH 3 which has been ascribed to the formation of copper(I) oxide. The results have been interpreted usingE-pCl diagrams determined for the copper-chloride system in both 50% ethylene glycol-water and aqueous solutions. Further information has been obtained from rotating disc measurements and from microscopy. The relevance of these results to corrosion in automotive cooling systems is discussed.