Title: A new type of silica-induced moundless pitting corrosion in copper observed in Japan

Abstract

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. We herein present and discuss the morphological aspects and water quality features of moundless pitting corrosion after field surveys and field tests in Noboribetsu City, Hokkaido Prefecture. In addition, a laboratory experiment is conducted using synthetic freshwater to reproduce the moundless pitting corrosion. The various corroded copper tube specimens are then examined by a range of analytical techniques, including X-ray diffraction (XRD), energy dispersive spectroscory (EDS), Fourier transform infrated (FT-IR) spectroscopy, optical microscopy. XRD and FT-IR analyses revealed that the verdigris around the pit was amorphous and a copper-containing silicate mineral chrysocolla. Upon examination of the water quality of various regions of the Noboribetsu City to determine the prerequisites for pit formation, it was found that the silica level was higher than the average Japanese level. In addition, the levels of sulfate ions in pitting detected area were also higher than the Japanese average, while those of bicarbonate ions were lower than the Japanese average. Furthermore, we succeeded in reproducing moundless pits in Noboribetsu City using a 4-year field test. Following this simulation, we found that the mouths of some pits were closed in the early stages, and that these pits were covered with thin films that contained mainly silica. It was therefore concluded that the formation of moundless pits is largely dependent on the water quality, and silica is considered indispensable for their generation.