Water installations are often constructed from copper tubes. 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. Some influencing factors are well known (sulfates, chlorides and hydrogen carbonate), in detail some knowledge about the mechanism is missing. Beginning with a detailed failure analysis some possible factors have been identified. Especially the appearance of silicate films at the metal surface has been detected. Laboratory investigations have been performed in an electrolyte which simulates a drinking water. The experiments have been performed to study the mechanism of pitting corrosion. 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. Polarization curves do show that the cathodic oxygen reduction is increased by the appearance of silicate films. Polarization curves and galvanostatic investigations do show that the mechanism of copper dissolution is changed. The formation of Cu2+ -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. Therefore the mechanism of pitting corrosion of copper in drinking water with a sufficient amount of silicate is quite clear. First a silicate film is formed at the fresh copper surface. A local anode is established under the silicate film. A passivation of copper is hindered due to the existing silicate film. The local cathode surrounds the anode. The electrochemical reactions are increased due to the hindered oxidation of the copper surface by silicate layers. Results of the experiments will be presented as well as the explanation of the model.