Copper soil corrosion

Several extensive series of soil-corrosion tests have been carried out by the National Bureau of Standards in the United States, and the results have been summarised by Romanoflf. In one series two types of copper and ten copper alloys were exposed in fourteen different soils for periods up to 14 years. The results for the copper specimens are summarised in Table 4.12. 

Table 4.12 Soil-corrosion tests on copper by National Bureau of Standards and British Non-ferrous Metals Research Association...

An unresolved question remains concerning the presence of significant quantities of sulfur in the materials from Etowah Mound, Not only do the fibers contain sulfur, but copper sulfate corrosion layers are also present on the associated copper metal from these burials (21). Because sulfur is present in the soil only in trace quantities, some other sulfur source must be found. The source of the sulfur may be a treatment of the textile or may be a product of the decomposing body nearby, Further examination other textiles from these burials should provide answers to the questions of the source of sulfur. 

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

Copper is generally considered to have good resistance to corrosion in soils. Corrosion concerns are mainly related to highly acidic soils and the presence of carbonaceous contaminants such as cinder. Sulfides, often produced by SRBs, also greatly increase the risk of corrosion damage. 

Serra E.T., de-Araujo M.M., Mannheimer W.A., On the influence of alternating current on the corrosion of aluminium and copper in contact with the soil. Corrosion 79, paper No. 55. 

Atmospheric exposure, fresh and salt waters, and many types of soil can cause uniform corrosion of copper aHoys. The relative ranking of aHoys for resistance to general corrosion depends strongly on environment and is relatively independent of temper. Atmospheric corrosion, the least damaging of the various forms of corrosion, is generaHy predictable from weight loss data obtained from exposure to various environments (31) (see Corrosion and CORROSION CONTKOL). 

Stainless steels in soil can only be attacked by pitting corrosion if the pitting potential is exceeded (see Fig. 2-16). Contact with nonalloyed steel affords considerable cathodic protection at f/jj < 0.2 V. Copper materials are also very resistant and only suffer corrosion in very acid or polluted soils. Details of the behavior of these materials can be found in Refs. 3 and 14. 

The resistivity of the soil in any particular location will be a function of moisture content, soil temperature and presence of dissolved salts. At a site where climatic conditions vary considerably throughout the year, earth electrodes should be buried at a depth where such changes will not affect the resistivity. Grounding rods are generally made of copper bonded onto a steel core. The copper provides a good connection to earth and offers a high corrosion resistance, while the steel core gives the mechanical... 

Light, sandy, well-drained soil of high electrical resistivity is low in corrosivity and coated steel or bare stainless steels can be employed. It is unlikely that the whole pipe run would be in the same type of soil. In heavier or damp soils, or where the quality of back filling cannot be guaranteed, there are two major corrosion risks. Steel, copper alloys and most stainless steels are susceptible to sulfide attack brought about by the action of sulfate-reducing bacteria in the soil. SRB are ubiquitous but thrive particularly well in the anaerobic conditions which persist in compacted soil, especially clay. The mechanism of corrosion where SRB are involved is described in Section... 

Oxidation-reduction potential Because of the interest in bacterial corrosion under anaerobic conditions, the oxidation-reduction situation in the soil was suggested as an indication of expected corrosion rates. The work of Starkey and Wight , McVey , and others led to the development and testing of the so-called redox probe. The probe with platinum electrodes and copper sulphate reference cells has been described as difficult to clean. Hence, results are difficult to reproduce. At the present time this procedure does not seem adapted to use in field tests. Of more importance is the fact that the data obtained by the redox method simply indicate anaerobic situations in the soil. Such data would be effective in predicting anaerobic corrosion by sulphate-reducing bacteria, but would fail to give any information regarding other types of corrosion. 

The corrosion rates of wrought iron and mild steel when immersed in seawater or buried in soil are not significantly different when the copper contents are similar. 

The British Non-Ferrous Metals Research Association carried out two series of tests, the results of which have been given by Gilbert and Gilbert and Porter these are summarised in Table 4.12. In the first series tough pitch copper tubes were exposed at seven sites for periods of up to 10 years. The two most corrosive soils were a wet acid peat (pH 4-2) and a moist acid clay (pH 4-6). In these two soils there was no evidence that the rate of corrosion was decreasing with duration of exposure. In the second series phosphorus-deoxidised copper tube and sheet was exposed at five sites for five years. Severe corrosion occurred only in cinders (pH 7 1). In these tests sulphides were found in the corrosion products on some specimens and the presence of sulphate-reducing bacteria at some sites was proved. It is not clear, however, to what extent the activity of these bacteria is a factor accelerating corrosion of copper. 

Cinders and acid peaty soils are obviously among the soils most corrosive toward copper. There is, however, no direct relationship between the rate of corrosion and any single feature of the soil composition or constitution". For instance, in the American tests corrosion in several soils with either low pH or high conductivity was not particularly severe, while the British tests show that high chloride or sulphate contents are not necessarily harmful. 

In tests carried out by the National Bureau of Standards in the USA specimens of copper alloys, lead, zinc and zinc alloys were buried at a number of different sites for periods varying from 11 to 14 years. The soils tested covered a pH range from 2-6 to 9-4 and resistivities ranged from 62 to 17 800 fi cm. The weight losses and maximum depths of pitting were recorded, and the results indicated that the most severe corrosion occurred in soils of poor aeration having high acid and soluble-salt contents. 

Most corrosion processes in copper and copper alloys generally start at the surface layer of the metal or alloy. When exposed to the atmosphere at ambient temperature, the surface reacts with oxygen, water, carbon dioxide, and air pollutants in buried objects the surface layer reacts with the components of the soil and with soil pollutants. In either case it gradually acquires a more or less thick patina under which the metallic core of an object may remain substantially unchanged. At particular sites, however, the corrosion processes may penetrate beyond the surface, and buried objects in particular may become severely corroded. At times, only extremely small remains of the original metal or alloy may be left underneath the corrosion layers. Very small amounts of active ions in the soil, such as chloride and nitrate under moist conditions, for example, may result, first in the corrosion of the surface layer and eventually, of the entire object. The process usually starts when surface atoms of the metal react with, say, chloride ions in the groundwater and form compounds of copper and chlorine, mainly cuprous chloride, cupric chloride, and/or hydrated cupric chloride. 

Excavated copper alloys are also susceptible to accelerated corrosion even a long time after exhumation due to the presence of chlorides coming from the soil that have attacked the alloy and formed a layer of cuprous chloride (nantokite) deep inside the corrosion layers, just within reach of the metallic core. Cuprous chloride is the stage of a series of reactions in the presence of oxygen and moisture that make it very unstable. The overall gross reaction may be written as [249] ... 

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