When assessing corrosion growth rates, the properties of the electrolyte are one of the most critical parameters. For underground pipelines, this electrolyte is the soil. Soil has a variety of corrosion properties such as porosity, composition, and water retention. One of the most critical properties is the soil's resistivity, the electrolyte's ability to conduct electric current. The soil's resistivity is not constant; it is highly seasonal and varies based on weather patterns, local conditions, and contamination.
This work presents a database for collecting soil resistivity measurements and a methodology to assemble high-resolution seasonal maps. In working closely with government agencies that use this data for agriculture, this work demonstrates a process to re-use agricultural conductivity datasets for estimating soil resistivity. This process is validated against field resistivity measurements collected by a North American pipeline operator.
The electrical conductivity of the electrolyte is one of the key parameters in the electromechanics of corrosion. Highly conductive electrolytes will permit more current and increase corrosion rates. Conversely, resistive electrolytes will enable less current to flow until the necessary conditions for corrosion are no longer satisfied or slowed.
Coatings act as a high resistance layer which impedes current flow. In the presence of coating, current may still flow but be restricted to negligible levels. In the presence of a coating holiday, the exposed steel interfaces directly with the soil, and the electrical current is impeded solely by the soil resistivity. Both examples of this are visible in Figure 1, where R represents total resistivity. This work omits any further discussion of coating resistivity, and comprehensive studies and standards have been developed around assessing coating resistivity [1][2][3]. On a local scale, omitting attenuation effects, Figure 1 models the circuit diagram for leakage current and resistivity.
The implications of variations in soil resistivity are significant; applying Ohm’s Law and holding voltage constant, the relationship between resistivity and current is inversely related; a 1 unit increase in ohms results in a 1 unit decrease in current (measured in amps). Furthermore, It has been empirically shown that low resistivity soils tend to result in increased pitting, increased depth-wise metal loss, and greater corrosion rates [4]. The general schema for soil resistance to corrosivity mapping is detailed in Table 1.[1]
