Measurements of Pitting Potential of Moundless Type Pit in Condensed Tap Water

摘要

The pitting potential of moundless type pit for copper tubes was measured in the condensed tap water. For comparative study, two types of tap water were prepared as test solution; one type of water is "pitting area's water" which causes the moundless type pit and another type is "pitting-free area's water" which never causes the moundless type pits. Both types of water were condensed by heating and evaporating until the electric conductivity showed about 50 mS/m. Water-glass scales were observed on the surface of the specimen after the 3 month immersion tests in pitting area's condensed tap water. Pits occurred in the potentiostatic corrosion test at 150, 200 and 300 mV (Ag/AgCl) for 200 hours in pitting area's condensed tap water. The appearances of pits occurred in the potentiostatic corrosion tests are similar t" the real moundless type pits because the entrances of pits were open. The pitting potential of mondless type pit of 150 mV (Ag/AgCl) was determined by the potentiostatic corrosion test in condensed tap water for 200 hours.