Soil corrosivity measurements

Soil corrosivity measurements Simple to conduct May act as a screening mechanism for more expensive methods Cities may have similar levels of corrosivity across their region, making use difficult Do not always correlate with corrosion reality... 

With buried pipelines, the degree of corrosion danger from cell formation and the effectiveness of cathodic protection can be determined by pipe/soil potential measurements along the pipeline (see Sections 3.6.2 and 3.7). This is not possible with well casings since the only point available for a measuring point is at the well head. Therefore, other methods are required to identify any corrosion risk or the effectiveness of corrosion protection. 

Perhaps the most widely known measurement technique is that adopted by the West German Gas Industry and developed by Steinrath for buried pipework. This assigns a value (See Table 2.20) to each parameter measured the summation of these values determines the corrosivity of the soil. The parameters measured are shown in Table 2.20. Although this technique was developed for the pipeline industry it can be used with some success for general soil corrosivity assessment. 

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 ... 

Escalante, E Measuring the Corrosion of Metals in Soil, Corrosion Testing and Evaluation, R. Baboian and S. W. Dean, Ed., ASTM International, West Conshohocken, PA, 1990. 

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. 

An indicator of soil corrosivity is the value of soil oxidation-reduction (ORP) or redox potential. It is calculated from the potential difference measured with a probe that contains an inert platinirm (Pt) electrode and a saturated calomel electrode (Hg/HgjClj/KCl, +0.241 V versus SHE) as a reference electrode. The value of this soil redox piotential depends on the dissolved oxygen content in the piore water and provides some information on the conditions under which sulfate-reducing bacteria could grow. The use of redox potentials to predict soil corrosivity is presented in Table 4. 

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... 

ASTM Method for Field Measurement of Soil Allows soil corrosion results to be... 

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... 

The American Water Works Association (AWWA) has developed a numerical soil corrosivity scale that is applicable to cast iron alloys. A severity ranking is generated by assigning points for different variables, presented in Table 2.28. When the total points of a soil in the AWWA scale are 10 (or higher), corrosion protective measures (such as cathodic protection) have been recommended for cast iron alloys. It should be appreciated that this rating scale remains a relatively simplistic, subjective procedure for specific alloys. Therefore, it should be viewed as a broad indicator and should not be expected to accurately predict specific cases of corrosion damage. 

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). 

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

Measurement of the cable sheathing/soil potential can be used to assess the corrosion danger from stray current interference (see Section 15.5.1). Since the measured values vary widely and the stray currents cannot be switched off, IR-free potential measurements are only possible with great effort. In order to keep the IR term of the potential measurement low, the reference electrode must be placed as close as possible to the measured object. With measurements in cable ducts (e.g., underneath tramway tracks), the reference electrodes can be introduced in an open duct. 

In the Verband Deutscher Elektrotechniker (VDE) regulations [1,4], no demands are made on the accuracy of the measured or calculated voltage drops in a rail network. An inaccuracy of 10% and, in difficult cases, up to 20%, should be permitted. A calculation of the annual mean values is required. If the necessary equipment is not available, a calculation is permitted over a shorter period (e.g., an average day). Voltage drops in the rail network only indicate the trend of the interference of buried installations. Assessment of the risk of corrosion of an installation can only be made by measuring the object/soil potential. A change in potential of 0.1 V can be taken as an indication of an inadmissible corrosion risk [5]. 

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