Soils, corrosion resistivity measurements

Practical measurements providing data on corrosion risk or cathodic protection are predominantly electrical in nature. In principle they concern the determination of the three principal parameters of electrical technology voltage, current, and resistance. Also the measurement of the potential of metals in soil or in electrolytes is a high-resistance measurement of the voltage between the object and reference electrode and thus does not draw any current (see Table 3-1). 

Combination electrical methods Tomashov and Mikhailovsky describe a method developed in the Soviet Union. This test is essentially a combination of resistivity measurement and polarisation rates on iron electrodes in soil in situ. The usefulness and value of this procedure has not as yet been determined by practical application by corrosion engineers. The development of this combination test does, however, represent an attempt to integrate some of the complex factors controlling corrosion rates in soil. Much more research on these factors and methods of measurement should in the future enable the corrosion engineer to evaluate soil properties with respect to application of corrosion-alleviating operations. 

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

The term aggressive is often used to imply some approximately quantitative estimate of the likelihood of corrosion and depends on measuring factors such as soil water (resistivity), pH, redox potential, salt concentrations and bacterial populations in order to establish criteria for the prediction of corrosion rates . Similar measurements for predicting corrosion... 

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 ... 

An electrical resistance (ER) probe is designed to measure corrosion in soil, without the necessity to be dug up for examination. This is an advantage over the coupons. The ER of the probe, made of the same metal as the structure, will change its value because of loss of metal, due to conversion to corrosion products on the metal surface. The ER probe also gives indirect information about the soil corrosivity and its changes with time. 

A sample of soil can be collected tind taken back to the laboratory for pH analysis. There, the soil can be mixed with distilled water (1 1 by volume), shaken well, and then the pH measured in a sample of the filtered extract solution. In the case of soils rich in sulfide, the pH value could be more acidic because of the oxidation of sulfides to sulfates as the soil dries out. The treatment of the soil prior to pH measurement is still an open issue. Drying of soil samples and addition of distilled water are two of the treatments used. However, the soil resistivity value is not sufficient to unambiguously indicate the soil corrosivity. For example, a neutral soil (pH 6.6-7.3) could be very corrosive due to the presence of chloride ions. Table 7 presents an approximate relationship between pH of soil water extract and soil corrosivity. 

These standard methods and practices provide the necessary information for electrochemical potentiostatic and potentiodynamic anodic measurements, calculation of corrosion rate from electrochemical measurements, and conducting potentiodynamic polarization resistance measurements. Recently, Electrochemical Impedance Spectroscopy (EIS) htts been introduced for corrosion measurements of steel structures corroding in soils. These tests can be... 

As mentioned before, soil is a physically, chemically, and biologically complex system. Factors that affect corrosion in soil, in addition to specific ions, are resistivity of soil, oxygen content, and acidity. Field measurements of soil resistivity are covered in ASTM G 57, Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method, which is the most widely used test, and using the proper meter produces accurate and reproducible results. Conducting field measurements of soil pH is covered in ASTM G 51, Test Method for pH of SoU for Use in Corrosion Testing. The corrosion resistance of lead and its alloys depends mainly upon the presence of silicate, carbonate, and to a lesser extent sulfates, in contributing to the passive film formation. 

As discussed previously, some soil environments and ground waters can be corrosive to buried metal structures. Resistivity measurements, which can be made both in the field and in the laboratory, indicate how corrosion currents will flow through soils or ground waters. High concentrations of chlorides and sulfates contribute to a reduction in resistivity and an increase in the corrosion activity of a material. In the presence of oxygen, chloride ions can be extremely corrosive to steel. Similarly, high levels of sulfates can cause a reduction in soil or groundwater resistivity and corrosion of steel and concrete. 

Soil resistivity measurement is the first important step in the design of a cathodic protection system as the current requirement would differ from one soil resistivity to another for the system. It may, however, be pointed out that there is no single method available to determine precisely the degree of corrosivity caused by soils. SoU resistivity only provides a rough guide to the corrosivity of the soils. There are several methods... 

Direct shear test of soils under consolidated drained conditions pH of soil for use in corrosion testing Field measurement of soil resistivity using the Wenner four-electrode method Optimum S03 in portland cement... 

Potential measurements of protected structure, especially in soil, are comprised with an error resulting from the voltage drop between the structure and the reference electrode. This error depends on the resistance of layers of corrosion products and insulation, and the resistance of the electrolytic environment. Equation (1) shows the amount of ohmic potential drop in the measurement of the potential of a... 

Many other issues are involved in the application of cathodic protection. For example, consider the case of cathodic protection of underground structures in which the corrosivity of soil is likely to play a major role, as does the degree of aeration and the resistivity. Bacterial effects also can change the corrosion potential. AU these factors influence the corrosion process so that along a pipeline there can be varying cathodic control requirements that have to be estimated from potential measurements, experience, and so forth. 

Kuhn [19] ilrst postulated in 1933 that the potential needed to stop corrosion is probably in the neighborhood of —0.85 V vs. CU/CUSO4. The results obtained from extensive studies on cathodic protection [20-27] helped National Association of Corrosion Engineers (NACE) to establish criteria for cathodic protection [28]. NACE RP-01-69 specifies A negative (cathodic) potential of at least 850 mV vs. Cu/CUSO4 should be appfied to protect the structure [28,29], However, in the presence of sulfides, bacteria, elevated temperatures, acid environments, and dissimilar metals, the criteria of—850 mV may not be sufficient [5,30—33]. According to NACE, one should also account for the IR drop at the metal-soil interface, which is included in most practical measurements and is an uncertain value depending on the electrolyte (soil) resistance. 

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