Electrical resistivity, soils

Sample tests in the field. These include coupons, stressed samples, electrical-resistance probes exposed to the plant corroding medium, or samples exposed to the atmosphere, to soils, or to fresh, brackish, or sahne waters. 

The data in Table 4-1 show the considerable influence of the electrical resistivity of soil. This is particularly so in categories 2, 7, and 10. From a profile of the soil resistance along the course of a pipeline with welded connections or with electrically conducting thrust couplings, one can readily recognize anodic areas, and... 

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... 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

Graphite anodes when used in soils are invariably placed in a carbonaceous backfill. This helps to compensate for the lower electrical resistivity of graphite when compared with silicon iron. In such an environment, no build-up of a film of high resistance between the anode and backfill occurs, unlike silicon-iron anodes where the resistance can increase with... 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

Thermal enhancement of volatilization.19 21 Volatility of contaminants increases greatly with temperature, so several techniques have been developed to raise soil temperature, including the injection of hot air or steam, electrical resistance heating, and radio frequency heating. 

Electrical resistance blocks Measures resistance resulting from a gradient between the sensor and the soil higher resistance indicates lower soil moisture Consists of electrodes embedded in a gypsum, nylon, or fiberglass porous material... 

Recalcitrant organic contaminants in the unsaturated zone, those that are difficult to remediate by vapor extraction, biodegradation, or other biological-chemical processes, can be removed via vitrification and electrical heating. This process involves heating the soil by electrical resistance or applied electrical fields to vaporize semi-volatile and volatile compounds, thermally to break down larger organic molecules, and (if heated sufficiently) to vitrify the mineral matter in the soil. 

Geotech Development Corporation offers a proprietary Cold Top ex situ vitrification process for the treatment of contaminated soil. The system melts the soil using an electric resistance furnace that can operate at temperatures of up to 5200°F. The vendor claims that wastes are transformed into an essentially monolithic, vitrified mass. The process is termed cold top vitrification because soil is added to the top of the melt to act as an insulator and to minimize the loss of volatile metals into the off-gas treatment system. The technology has been evaluated in a pilot-scale facility and is commercially available. 

Resistivity. The resistivity method measures the electrical resistivity of the geohydrologic section, which includes soil, rock, and ground water. Interpretation of these measurements provide information on layering and depths of subsurface horizons as well as lateral changes in the... 

Buried pipelines are subject to external corrosion from ground water and highly conductive soils. The corrosiveness of soils is often estimated based on soil resistivity measurement. The measurement is made with the Wenner four-pin method, which is used in conjunction with a Vibroground(1 and a Miller U 10-pin conductor set to determine the average electrical resistivities. A general relationship between corrosion and soil resistivity is as follows ... 

In addition to the desired repellency effects, other undesirable fabric properties are often found with repellent finishes. These include problems with static electricity, poor soil removal in aqueous laundering, stiffer fabric hand, greying (soil redeposition) during aqueous laundering and increased flammability. Some fabric properties that are often improved by repellent finishes include better durable press properties, more rapid drying and ironing, and increased resistance to acids, bases and other chemicals. Table 6.1 shows typical textile applications for repellent fabrics and their requirements. 

It is also a measure of the moisture stress in plants or soil, measured in megapascals. A more negative value indicates greater moisture stress. Soils with no moisture stress have a water potential of 0 to -1 mPa. Two methods of measuring soil water potential are the heat-dissipation method or the electrical-resistance method. 

A.M. Ahmed and W.N. Sulaiman, Evaluation of Groundwater and Soil Pollution in a Landfill Area Using Electrical Resistivity Imaging Survey, Environ. Manag. 28(5), 655-663, Nov. (2001). 

The salinity may be determined directly as a total content of water-soluble salts, or as a sum of contents of particular ions. The measurement of the specific electric resistance of the soil is a very rough, but simple and rapid method. It is used for soil surveys on a wide scale. For achieving more precise results, it is possible to use a method standardized by measuring the resistance in saturated soil paste (the soil paste is prepared by mixing a soil sample with distilled water to obtain a moderately liquefied paste) and the results are corrected with respect to a temperature of 16°C. Tables are used to convert the measured electrical resistance values to the soil concentration. For more precise data, it is possible to separate the solution from the saturated soil paste and to measure the specific electric conductivity. The solution separated from the soil paste is called the saturated soil extract. 

Under an in situ condition, a platinum electrode is inserted into the soil at a predetermined depth. Usually platinum electrodes are left in place for subsequent measurements. At the time of measurements, the reference electrode is inserted at a short distance into the wet soil. It is not necessary that the reference electrode be placed at the same depth as the platinum electrode, as long as there is enough soil moisture to ensure a good electrical contact. However, the electrical resistance of the measuring circuit can be affected by the distance between the platinum electrode and the reference electrode. This may not be a serious problem in saturated soils. 

Examples of one-dimensional and two-dimensional models to predict the transport of heavy metals under constant DC current are explained in Chapter 25. This model ignores hydraulic advection, electrophoresis, diffusion, and electroosmosis processes and considers only electromigration. Electrical potential distribution is assumed to be a function of the electrical resistance of the soil and depends on the instantaneous local concentration and mobility of all the ions existing in the pore water of the soil. Local chemical equilibrium is assumed to calculate the concentration of chemical species. Validation of these and other developed models based on laboratory and field test results is critical to gain confidence in the accuracy of the model predictions. 

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