Q.: Why and how does a copper water line encased in concrete corrode if it is touching a piece of reinforcing steel? I have looked at tables of corrosion potential and they show copper to be more noble than steel. This says to me that the steel should corrode instead of the copper. What gives? Does it have anything to do with the water flowing through the pipe? Are there any studies or reports covering this phenomenon?
A.: The subject is more complicated than the usual table of electromotive series potentials might lead you to believe. Such tables are usually based on potentials measured in specified salt solutions of given concentrations at one temperature. If you change the kind or concentration of solution, or change the temperature or the composition of the alloy, everything may change. The relative positions of the two metals may widen or narrow or they may even reverse. Thus if Metal A and Metal B are in contact with one another and with some specific electrolyte, Metal A may corrode. But if you change the concentration of the electrolyte, the composition of the electrolyte, or the temperature, or if you use a different alloy of Metal A or Metal B or both, it is possible that Metal B will corrode instead of Metal A.
Some books you might like to look into are:
Corrosion of Metals in Concrete, Publication SP-49, American Concrete Institute, Box 19150, Detroit, Michigan 48219, 142 pages, $24.50 ($17.95 to ACI members). Published 1975. A collection of 10 state-of-the-art reports on corrosion mechanisms, field procedures and research.
Chloride Corrosion of Steel in Concrete, Special Technical Publication 629, American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103, 193 pages, $21.25 (less 20 percent to ASTM members). Published 1977. A collection of 17 papers on research and field practice in controlling corrosion of steel in concrete.
Corrosion of Reinforcing Steel in Concrete, Special Technical Publication 713, American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103, 204 pages, $22.50 (less 20 percent to ASTM members). Published 1980. A collection of 11 articles on the mechanisms of corrosion of steel in concrete and methods of evaluation and control.