This article discusses the most common soil resistivity testing method and provides some guidelines for properly collecting sufficient data for the cathodic protection system designer.
One of the most important design parameters when considering the application of cathodic protection for buried structures is the resistivity of the soil. Soil resistivity testing is an important consideration for assessing the corrosivity of the environment to buried structures. It also has a tremendous impact on the selection of anode type, quantity, and configuration. Thus, it is critical that the CP designer have accurate data on the soil conditions at both the structure and at any proposed anode system locations. The lack of sufficient soil resistivity data can render a cathodic protection system (CP system) design ineffective and can result in costly remediation efforts during commissioning.
Soil resistivity is the principal diagnostic factor used to evaluate soil corrosivity. When performing soil resistivity testing, there are numerous factors that can be assessed, including soil composition, moisture content, pH, chloride and sulfate ion concentrations, and redox potential. These are all common components of a lab or in-situ soil testing program and all have an impact on soil resistivity. While a comprehensive soil testing program may be warranted, especially when performing failure analysis, for most environments the soil resistivity testing data provides an outstanding basis for assessing soil corrosivity. Below is a typical chart correlating soil resistivity with soil corrosivity.
|Soil Resistivity (ohm-cm)||Corrosivity Rating|
|10,000 to 20,000||Mildly corrosive|
|5,000 to 10,000||Moderately corrosive|
|3,000 to 5,000||Corrosive|
|1,000 to 3,000||Highly corrosive|
SOURCE: Corrosion Basics: An Introduction, NACE Press Book, 2nd edition by Pierre Roberge