Abstract
Many copper water lines for municipal drinking water in Santa Fe, New Mexico USA, have developed pinhole leaks. The pitting matches the description of Type I pitting of copper, which has historically been attributed to water chemistry and to contaminants on the copper tubing surface. However, more recent studies attribute copper pitting to microbial induced corrosion (MIC). In order to test for microbes, the copper tubing was fixed in hexamethyldisilazane (HMDS), then the tops of the corrosion mounds were broken open, and the interior of the corrosion pits were examined with scanning electron microscopy (SEM). The analysis found that microbes resembling actinobacteria were deep inside the pits and wedged between the crystallographic planes of the corroded copper grains. The presence of actinobacteria confirms the possibility that the cause of this pitting corrosion was MIC. This observation provides better understanding and new methods for preventing the pitting of copper tubing in municipal water.
Highlights
This study was initiated upon receipt of copper tubing from several different residential cold water lines that had developed pinhole leaks after fifteen years of transporting municipal water
When the black oxide was chipped off, the shiny, underlying copper surface was seen. (The X-ray diffraction (XRD) results for the green and black oxide layers are shown .) The hot-water copper surfaces that corroded by microbial induced corrosion (MIC) according to Labuda [14] were completely different from the cold-water copper surfaces observed in this investigation
scanning electron microscopy (SEM) investigation has shown the presence of microbes inside corrosion pits in copper tubing
Summary
This study was initiated upon receipt of copper tubing from several different residential cold water lines that had developed pinhole leaks after fifteen years of transporting municipal water. There are currently hundreds of papers written on the subject of the pitting corrosion of copper tubing in municipal drinking water. What is in common for Types 1, 2, and 3 pitting is that the copper pits are covered with a copper oxide membrane, and above the membrane is a porous mound or cap. Beyond these similarities, their appearances diverge greatly. The pit cavities may be concentrated in either chlorides [1,6] or sulfides for Type 3 [5]
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