Case Studies on Pitting Corrosion Failures of Copper Tubes in Hot Water

Abstract

Pitting failures of copper tubes in hot water were investigated in detail. Sample tubes from various tubing lines were collected at three buildings in Tokyo. Two cases of the three were similar and their results are summarized as follows: (1) Water was characterized by high levels of SO₄^2- and Cl^-, and low ratio of HCO₃^-/SO₄^2- in comparison with the averages of Japanese river water; (2) Pits were covered with mounds of greenish blue corrosion products containing Cu₄SO₄(OH)₆; (3) Pits contained reddish brown products of Cu₂O. Chloride was concentrated at the bottom of the pits, where crystalline CuCl was detected in some cases; (4) The pH value within the pits was 3 to 4. These characteristic features suggest that the pitting corrosion of above cases is classified Type II as many other cases experienced in Japan. It is considered that Cl^- in water first forms CuCl as a corrosion product which may hydrolyze to develope an acid and Cu₂O in the pit, and that SO₄^2- forms the mound of Cu₄SO₄(OH)₆ on the pit, which hinders the acid solution to be washed away from the pit. The low level of HCO₃^- (low buffer capacity) is in favor of the ready decay of pH value in the pit.
Pitting corrosion of the third case showed some different features, particularly with respect to water quality. Since this type of pitting has rarely been reported in Japan, it is difficult at this stage to explain its mechanism.