Copper is essentially immune to corrosion in most underground environments because of the protective films that naturally form on the metal's surface. In general, there is no major concern regarding the corrosion of copper underground unless it is exposed to certain aggressive substances. These include moist environments containing (1) appreciable quantities of chlorides, sulfates, sulfides, or ammonium compounds or both; (2) organic or inorganic acids or both; or (3) active anaerobic bacteria or all three. It is also known that copper should not be directly embedded in cinders that generally contain sulfides or carbon or both. Fortunately, when corrosive conditions exist, a variety of cost effective techniques can be used to mitigate unacceptable corrosion of buried copper.
In spite of these well known general guidelines regarding the underground behavior of copper, situations do occur where the metal is placed in contact with aggressive thermal insulations. When these insulations are hygroscopic, have low resistivities and acidic pH values, and contain sulfur compounds along with carbon, it is understandable that unacceptable corrosion of the copper will occur.
The conditions that caused natural carbonaceous granular “thermal insulation” to deteriorate the underground copper tube domestic hot water system for a low-rise high density housing development are presented. Basically, the copper water tubes corroded because the “thermal insulation” absorbed large quantities of moisture, had a low resistivity (less than 1200 Ω∙cm), and an acidic pH (as low as 4.8). Corrosion was facilitated by the presence of appreciable quantities of sulfur (8000 ppm) and possibly carbon in the “insulation.”
Corrosion of the adversely affected copper water tube system was mitigated by cathodic protection. In order to minimize shielding effects created by the complexity of the underground system, deep anode beds were incorporated into the cathodic protection design. The unaffected underground copper tube domestic cold water and steel gas distribution systems were bonded to the cathodically protected copper hot water tubes in order to prevent the possibility of stray current corrosion.
In addition, other insulation systems have been shown to be deleterious to copper. For example, certain chemically treated cellulosic insulation materials can contain constituents that become aggressive to copper when either moist or under high relative humidity conditions.