Abstract
Pitting potential and repassivation potential were measured for copper tubes in 60°C of hot-water by using a test equipment simulating a central hot-water supply. The feed tap water with SO42-/HCO3->1.0 was classified as soft water. Prior to the measurements, copper tubes were subjected to 545d of hot-water circulation at 60°C so as to form copper-orthosilicate films on their inner surfaces. Pitting potential could not be determined from an anodic polarization curve because it did not give any current rise due to pitting. Accordingly, the pitting potential of 210mV (vs. Ag-AgCl) was determined by 100d of potentiostatic corrosion tests at a variety of potentials, wherein pitting corrosion was evaluated by visual examination by means of an optical microscope. The repassivation potential of 190mV was determined by 75d of potentiostatic tests. Prior to the tests, copper tubes with copper-orthosilicate films were held at 60°C, at the potential 250mV for 65d so as to introduce pits having depths of approximately 1/2 of wall thickness. Pit propagation was evaluated by means of the extreme value statistics. The structure and morphology of Type II pits observed in the field were reproduced exactly in the present simulated corrosion tests.
