2009 Volume 58 Issue 12 Pages 434-440
Copper tubes were immersed for one year in fifteen kinds of synthetic freshwater to reproduce a moundless type pitting corrosion. Most or part of the area of some specimen surfaces changed its color into dark brown for about one week immersion. Such a dark brown area formed in freshwater for about one week immersion is called “initial film” of a copper. The formation of the initial film was accelerated in the test water containing a chloride ion. After one-year immersion tests, moundless type pitting corrosion occurred on the specimen immersed in the test water containing 40 mg SiO2/L, 50 mg SO42−/L, 10 mg Cl−/L and 10 mg HCO3−/L. These pits were located on the boundary of the initial film. The initial film was formed intentionally by one-week immersion in the solution containing silica and chloride ion. Cathodic reductions of the initial film revealed that the thickness of the initial film was about 400 nm of cuprous oxide. The galvanic corrosion test was performed using galvanic couples of a copper tube with the initial film and an as-received copper tube. The galvanic current flew from the copper tube with initial film to the as-received copper tube. This result indicates that the macro-cell formation of the area with an initial film and the area without an initial film causes the moundless type pits on the boundary of the initial film.
