2008 Volume 57 Issue 4 Pages 172-179
It has been about fifteen years since a new type pitting corrosion of copper tubes which has been called ‘moundless type pit’ was discovered in Japan. This type of pitting corrosion has some unique morphological features which differ from ordinary types of pitting corrosion such as type I or type II. First, this type of pitting corrosion has no mound of corrosion products which covers on the entrance of a pit. Second, there is a glassy verdigris around the pit. Third, the size of the pit is less than 1 mm in diameter but very deep. We reproduced moundless type pits by a field test in the area where moundless type pits have often occurred. Moundless type pits were also generated in synthetic freshwater dissolving some chemicals in a laboratory test. This synthetic freshwater contains 40 ppm SiO2, 50 ppm SO42–, 10 ppm Cl– and 10 ppm HCO3–. Surface analysis of the moundless type pits revealed that the mouth of the pits was covered with a thin film which was mainly composed of silica. The initiation and propagation mechanisms of moundless pitting are discussed on the basis of the experimental results.
