Pitting of copper under moderately oxidizing conditions.

摘要

A mathematical model of a corrosion pit on copper under moderately oxidizing conditions has been developed. The concentration profiles for the aqueous species in the pit were determined through equilibrium and transport rate calculations. These concentration profiles were used to determine the fraction of the oxidized copper which is transported as aqueous species and the fraction precipitated as copper oxide at various locations in the pit. The fraction of the oxidized copper which is transported out of the cavity seems to be decisive for the propagation of a corrosion pit. If the fraction of the oxidized copper which forms copper oxide in the cavity is large enough the passage from the anodic site at the bottom of the pit to the cathodic sites outside the pit would become blocked and the pit would be unable to propagate. The same argument is applied to the formation of an occluded cell at a pore in an otherwise covering oxide layer on copper and the ratio between the amounts of copper oxide and the aqueous species at the metal surface seems to be decisive for the formation of an occluded cell. An occluded cell, where the liquid in contact with the metallic copper is partly shielded from the bulk water by corrosion products, seems to be a prerequisite for pitting of copper under the conditions considered here. Calculation of the conditions required for pitting was performed for 25 degrees C and for 100 degrees C for a water of pH 9. The corrosion current obtained from the mathematical model of the pit chemistry was used to estimate the maximum depth of a corrosion pit in copper as a function of time. At 25 degrees C at potentials only slightly higher than that at which pitting may initiate the propagation rate is very low. A calculation for 100 degrees C shows that corrosion pits with a much higher propagation rate may develop. The potential has a strong influence on the predicted maximum propagation rate. (au). (Atomindex citation 24:032221)