The inner surfaces of the copper tubes after the four-year field test for moundless type pitting corrosion have been investigated by the optical microscope, SEM, EDS and EPMA. The upper side of the inner surface of tube has many greenish-blue corrosion products. Optical micrographs revealed that there are many moundless pits in the centers of these corrosion products. These moundless pits were classified into two types. One type is "open type" which has an open entrance of the pit, another is "closed type" which is totally covered with a thin film on the entrance of the pit. The thin film on the closed type pit is fragile, so cracks are found on the films of some closed type pits. This observation suggests that the open type pit is formed as a result of breaking the thin film of closed type pit. EPMA analysis revealed that the film was mostly composed of Si. On the contrary, the lower side of the inner surface of tube was totally covered with greenish-yellow corrosion products. EDS analysis revealed the presence of iron in these corrosion products. Pitting corrosion has not been observed on the inner surface of the lower side covered with the greenish-yellow corrosion products. These facts indicate that the corrosion products film containing iron contributes to the prevention of the moundless type pitting corrosion of the copper tube.
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