Our company provides soil corrosivity testing services for assisting the design and installation of pipelines and buried steel structures. Knowledge of soil corrosivity is critical for the design of cathodic protection measures and coatings, or predicting the effective lifetime of a a buried storage tank. Factors such as soil composition, moisture content, pore water chemistry, pH and redox potential (see below) all effect the soil resistivity, which is the principal diagnostic factor.
Soil resistivity testing procedure with a 4-electrode Wenner array
In a standard soil resistivity test four equidistant electrodes are set in a fixed configuration array. A low frequency current is applied across the outer electrodes and the resultant voltage is measured between the inner electrodes. The reading is converted by standard equations into a value representing the average resistivity of the ground between the electrodes. The depth penetration of each reading is directly proportional to the electrode spacing of the array.
Using a standard array based on BS 1377, specific sections of a proposed new pipeline route are targeted to measure the resistivity of each lithology encountered, and investigate special situations such as fault zones and infilled channels. Surveys are customised to take readings down to pipeline depth at each test location. Deeper readings are required near water courses and at cross-overs where the pipeline needs to be embedded to a greater depth. The resultant resistivity data is converted into corrosivity factors and integrated into the design of effective protection measures.
Soil redox testing on the side of a road cutting
An important component of soil corrosivity assessment involves the measurement of soil redox potential. This test measures the ability of a soil to ionise, by the transference of electrons either to or from a reference electrode. The measurement data indicates if the soil is aerobic or anaerobic, and whether iron oxides or nitrates in the soil have been reduced or occur in an oxidized form.
The in-situ soil redox measuring system consists of a high impedance voltmeter, plus a platinum test electrode and a silver reference electrode (BS ISO 11271) that are inserted in the ground below root level. The magnitude and polarity of the potential between the platinum and reference electrodes is measured. Results are corrected for soil temperature and converted into standard form as a hydrogen electrode equivalent, termed the redox potential (Eh) which is used in the overall corrosivity assessment.
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