Study of Moundless Pitting Corrosion of Copper Tubes and Prevention of its Occurrence

摘要

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 different from the Typell pitting corrosion is characterized by absence of the mount of corrosion product such as CuSO{sub}4(OH){sub}6, Cu(OH){sub}2CO{sub}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{sub}2O. Si was detected in the glass-like amorphous scale. The moundless pitting corrosion has been found when SiO{sub}2> = 20pprn and Mattsson ratio (: (SO{sub}4){sup}(2-)/(HCO{sub}3){sup}-) >0.5. Experimentally, the moundless pitting corrosion has been prevented when small amount of ferrous or ferric ions from steel pipe line have leached. A new copper tube in which tin is coated at the inner side have been developed. The new tube has shown excellent corrosion resistance performance for more than ten years.