Pitting Corrosion of Copper Tubes for Drinking Water Applications Due to Silicate Films

摘要

Copper is a well-established material for water transport. Sometimes pitting corrosion could appear. In the last few years some serious failures in cold water tubes in one special region in Germany have been observed. The appearance of silicate films at the metal surface has been detected. Measurements with the Scanning Kelvin Probe and Scanning Vibrating Electrode do show the existence of local anodes under artificially prepared silicate films on copper. At the phase boundary between the silicate film and the untreated copper surface, the free corrosion potential shows the most negative value. Interestingly this anode is surrounded by the cathode formed by the free copper surface and also by a cathodic area represented by the complete silicate covered copper surface. Galvanostatic investigations do show that the mechanism of copper dissolution is changed. The formation of Cu~(2+) -ions is hindered, only Cu~+ ions can be formed under thick silicate films. Moreover the existence of the silicate film hinders the copper to form a protective oxide film. The copper dissolution is higher than without a silicate film. Without a silicate film at the surface, the copper dissolution decreases with exposition time. This is the first time that the mechanism of pitting corrosion of copper in cold water due to the formation of silicate films has been proofed.