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 SO42and Cl-, and low ratio of HCO3-/SO42-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 SO42forms 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.

A new type of silica-induced “moundless” pitting corrosion in copper observed in Japan

“…It is also noteworthy that the bicarbonate ion concentration is lower than the Japanese average level of 53 ppm for all the three Noboribetsu water systems. It has been claimed that bicarbonate possesses a pH buffering effect, and could therefore decelerate pitting propagation by minimising the pH shift of the water retained in the pit [ 8 , 16 ]. Thus, pitting propagation might proceed more rapidly in water with a comparatively low bicarbonate level.…”

“…From the results of the field and laboratory tests, the constitution of the moundless pit was estimated, as shown in the right-hand panel of Figure 16 . The left-hand panel of Figure 16 gives the constitutions of the conventional type I and II pits for comparison [ 3 , 16 ]. The presence of verdigris covering the pit top is essential for pit growth in type I or type II pits.…”

“…In general, Japanese river waters are classified as soft water, since they contain comparatively low levels of Ca and Mg, and as a result, the majority of pitting damage detected in copper tubes in Japan was reported to be type II pitting. However, despite their different classifications, the morphological features of type I and II pitting are comparable, since both constitute a semispherical pit grown beneath a mound of verdigris [ 15 , 16 ].…”

“…In general, Japanese river waters are classified as soft water, since they contain comparatively low levels of Ca and Mg, and as a result, the majority of pitting damage detected in copper tubes in Japan was reported to be type II pitting. However, despite their different classifications, the morphological features of type I and II pitting are comparable, since both constitute a semispherical pit grown beneath a mound of verdigris [15,16]. Interestingly, since the 1990s, pitting corrosion that is not classified as either type I or type II has been discovered in some areas of Japan, and such pitting corrosion has recently been discovered throughout Japan.…”

Corrosion Mechanisms for Copper and Silver Objects in Near-Surface Environments

“…Redox equilibria are not critical in determining corrosion processes in some cases, and more complex diagrams are required. For example, copper corrosion associated with simultaneous formation of cuprite and nantokite (CuCI) under a coating of cuprite is characteristic of both bronze disease (Scott 1990a) and type 2 pitting (Lucey 1967;Sato 1982). In other cases, corrosion products are divalent copper minerals (generally paratacamite in salt water and malachite or brochantite (CU4S04(OH)6) in fresh water).…”