A morphological study was carried out on the pitting corrosion of copper In soft tap water at room temperature. Pitting corrosion was initiated through localized breakdown of a relatively thin protective cuprous oxide film. This film increased in thickness and became a scale that separated from the metal surface by cuprous oxide crystals. The oxide crystals ruptured the scale and extruded from it forming mounds at sites that corresponded to grain boundaries. The initiation of pitting attack was observed along the grain boundaries, and the pits developed into hemispherical shapes. After the pits were already established, basic copper sulfate corrosion products were formed on top of the scale through a precipitation reaction that involved basic copper chloride crystals grown underneath the scale.
A new mechanism is proposed that can explain pitting corrosion of copper in soft and hard waters. The presence of chloride ions is a prerequisite in the breakdown of the film and the initiation of pitting attack. Furthermore, the pit morphology depends on the deformation of the grains, and the composition of corrosion products inside the pits is the same whether the water is soft or hard. The concentration of the chemical species present in the water determines the composition of the corrosion products covering the pits.
