A new type of silica-induced “moundless” pitting corrosion in copper observed in Japan

A new type of pitting corrosion in copper, namely “moundless” corrosion, has recently been reported in Japan. This type of pitting corrosion has some unique morphological features that differ from ordinary types of pitting corrosion, such as type I or type II. Firstly, this type of pitting corrosion has no mound of corrosion products that cover the mouth of the pit. In addition, a glassy verdigris exists around the pit. Furthermore, the pit measures <1 mm in diameter, but is extremely deep. In our study, we reproduced moundless pits by field testing in an area where moundless pits have often occurred. Moundless pits were also generated in synthetic freshwater through the dissolution of some selected components in a laboratory test. This synthetic freshwater contained 40 ppm SiO2, 50 ppm SO42−, 10 ppm Cl−, and 10 ppm HCO3−. Surface analysis of the moundless pit revealed that the mouths of the pits were covered with thin films that were mainly composed of silica.