Formation of Thin Layers of Cu_2O on Freshly Etched Copper in Aerated, Moderately Acid Chloride Solutions

摘要

When rotating disc electrodes of freshly etched copper (pure or electroplated with or without embedded TiO_2 particles) were immersed in aerated chloride solutions at pH 4.0 or lower a characteristic behaviour was seen. For a relatively short time pH and the corrosion potential decreased. Then the potential increased over a long time, up to some 15 h. pH also started to increase, but a pH-stat maintained a constant value during the rest of the experiment. Examination by Auger spectroscopy of an electrode taken out after the short time exposure showed a thin layer (< 18 A) of Cu_2O, contaminated by chloride. On an electrode exposed for a long time the layer had grown to ca. 18 A, and no chloride was detected. These observations seem to agree with a description by Sato, according to which the potential of a corroding metal should decrease when in contact with an n-type conducting oxide, but increase when in contact with a p-type conducting oxide. Strehblow et al. claim that pure Cu_2O is a p-type conductor, while contamination by chloride makes it an n-type conductor. However, an alternative interpretation appears to be more correct for the present case. Another interesting point was an s-shaped course of the potential increase at long times together with a negligibly small change in corrosion rate. Suitable derivations show that an Avrami type process took place, apparently the formation of the pure oxide layer. However, analysis and interpretation of the anodic reaction indicate that about half of the corrosion current was spent on oxide formation, but the thickness of the oxide layer at long times represented only about one percent of the thus calculated charge. Thus formation and dissolution of oxide took place at nearly equal rates.