In recent years, numerous leak incidents caused by moundless pitting corrosion were experienced for once-through type cold or hot water supply system in houses all over Japan. The moundless pitting corrosion and its protection for copper tubes are summarized. The "moundless corrosion is different from the Type II pitting corrosion, and it is characterized by absence of the mount of corrosion product such as Cu_4SO_4(OH)_6, Cu_2(OH)_2CO_3 arround the pits. When the moundless pitting corrosion is developed, the rest of the surface is covered with a glass-like amorphous scale containing Cu_2O. Silicon was detected in the glass-like amorphous scale. The moundless pitting corrosion has been found in the case of SiO_2 > = 20ppm and Mattoson ratio (SO_4~(2-) / HCO_3~-) > 0.5. Experimentally, the moundless pitting corrosion is prevented when small amount of ferrous or ferric ions from steel pipe line have leached. A new copper tube in which tin is coated on the inner side have been developed. The new tube has shown excellent corrosion resistance performance for more than ten years.
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