Pitting of copper under laboratory and field conditions

Abstract

An investigation of the pitting in soft tap water of copper has revealed that fresh specimens tested under accelerated laboratory conditions behave similarly to copper tubes that have failed during service. In both cases, the pits are hemispherical in shape and are surrounded by an intergranularly attacked metal surface. They contain cuprous oxide and chloride crystals and are covered with an adherent oxide scale. Bluish green mounds, composed of basic copper sulphate crystals, are observed on top of the scale of the laboratory tested specimens. On the failed tubes, the deposits were composed mainly of sulphate, phosphate, and chloride. SEM examination of specimens after polarisation experiments revealed that the scale is initially separated from the bare metal surface by a layer of cuprous oxide crystals. Pits initiate irregularly along grain boundaries and later develop into a hemispherical shape. The development in shape is associated with precipitation of cuprous chloride inside the pits and on the metal surface below the scale. This is followed by precipitation of basic sulphate crystals and readhesion of the scale to the underlying metal surface.