Protection of Steel in Aggressive soil conditions

http://www.applechemie.com

Copper is essentially immune to corrosion. It behaves like a noble metal in most underground environments because of the naturally protective film that forms on the metal's surface. If this film, which often consists of reddish-brown cuprous oxide (Cu 20), is destroyed and cannot be repaired, copper will corrode. Fortunately, the protective film on copper remains intact or is readily repaired under most soil conditions.

Three definitive studies on the underground corrosion of copper have been conducted. 2-4 Very likely, this apparent lack of interest in the soil-side corrosion of copper is the result of the metal's known corrosion resistance in underground environments.

Analysis of the results obtained in these three independent investigations clearly establishes that the underground corrosion of copper is unusually complicated. In general, corrosion cannot be directly related to any one or even several factors, as many interrelated soil conditions are involved. The data presented in the three studies and the results of others 5,6 permit some tentative conclusions regarding the corrosion of copper in underground environments. These include:

·         Corrosion is often associated with a combination of elevated sulfate or chloride content in the soil in conjunction with poor drainage, a soil having considerable capacity to hold moisture, and a moderate to heavy annual rainfall (usually more than 76 cm [30 in.]).

·         Elevated concentrations of sulfate or chloride or both in the soil are probably the primary factor in the underground corrosion process, but considerable moisture is required to support the electrochemical action. The sulfates and chlorides may not adversely affect the copper if the rainfall is slight and drainage is adequate.

·         Very low soil resistivity (i.e., less than 100-500 ohm-cm) generally indicates a soil that could be aggressive.

·         Soils containing large quantities of organic matter (particularly soils containing organic acids) can be corrosive

·         Moist cinder fills are generally corrosive, either because of the sulfides present or because of the galvanic action created by the carbon particles in the cinders

·         Construction rubble that contains considerable lime should not be particularly corrosive, provided that localized concentration cells, especially oxygen differential cells, are not created during backfilling

·         Soils such as clay, sand, gravel, loam, and chalk seldom possess the combination of properties that are associated with corrosion

·         Organic soils supporting active anaerobic bacteria (i.e., sulfate-reducing species) can produce sulfides, which are aggressive to copper

·         Soils containing inorganic acids can be unusually aggressive to copper

Soils containing appreciable amounts of ammonia compounds are usually corrosive to copper. The appearance of the copper surface and identification of the corrosion products generally provide insight regarding the cause of deterioration. For example, green copper corrosion products are often basic copper carbonate, basic copper sulfate, or copper oxychlorides. The existence of appreciable amounts of copper oxychlorides in the corrosion products would be expected for a copper water tube that had been buried in a low-resistivity tidal marsh environment. A heavy black layer on the copper would suggest the presence of copper sulfide and the possibility of anaerobic bacteria activity as the cause of corrosion. The presence of ammonia, in combination with sulfate compounds on the copper surface, indicates that the cause of corrosion could be lawn fertilizer.