Field test piping of copper tubes has been installed in pitting and pitting-free area in Noboribetsu city to reproduce moundless type pitting corrosion. Inner surfaces of the copper tubes sampled from field test piping after 9 years have been investigated by optical microscopic observation and EPMA analysis. Inner surfaces of the copper tubes in pitting-free area were corroded like pockmark, while many pits occurred on the copper tubes in pitting area and have morphological characteristics similar to the moundless type pits. The pitting corrosion like moundless type pits were observed on the copper tubes in pitting area over 7 years after the installing. Depth of pits occurred on the 7 to 9-year old copper tubes is about 100 μm. It is possible that the slow pace of pitting initiation and propagation in this field test in pitting area is attributed to the cover the inner surface of the copper tube with the scale composed of Si and Al or Si and Fe.
