Influence of bicarbonate ions on stability of copper oxides and copper pitting corrosion

Abstract

The influence of bicarbonate ions on the stability of copper oxides and pitting corrosion has been studied by the combined use of long term exposure, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and atomic absorption spectroscopy (AAS). Contrary to earlier suggestions that the increasing concentration of bicarbonate ions inhibits copper pitting corrosion owing to the formation of a protective basic copper carbonate film, this study reveals that the lessening of this form of corrosion is a result of bicarbonate stabilisation of the copper oxide films formed before the carbonate salt. Analysis by SEM and XRD showed that the swfaces of specimens coated with copper oxides remained in their original conditions when bicarbonate ion concentration ≥61 mg L⁻¹ or a high ratio of bicarbonate to chloride ions was present. In contrast, pitting corrosion occurred when bicarbonate ion concentration ≤31 mg L⁻¹ was present. The dissolved copper concentrations obtained by AAS decreased with increasing concentration of bicarbonate ions in the copper-aqueous system.