Steel Pipe Native Potentials In Soils Affect CP Criteria

By F.M. Song and H. Yu, Southwest Research Institute®, San Antonio, TX | March 2012, Vol. 239 No. 3
Buyer's Guide

In field practice, an off-potential is usually measured by interrupting all possible external current sources within a sufficiently small time interval. In this work, it may be regarded as a polarized potential, although these two potentials differ from each other. A polarized potential can only be measured local at the exposed structure-electrolyte interface. By contrast, the off-potential is usually measured on-ground and represents an average covering a section of the pipe to be measured.

The goal of this article is to provide an understanding and a rule-of-thumb relationship between steel native/rest potentials and soil resistivity from which the relationship between off-potential criteria in different soil resistivity ranges and the magnitude of cathodic polarization can be better understood.

Chart Development
Let us discuss the development of the steel native/rest potential vs. soil resistivity chart.

Table 1 shows a soil resistivity classification.(6,7) The low or medium soil resistivity
is considered to be below 10K ohm.cm; high or very high soil resistivity between 10K
and 100K ohm-cm; ultra high or super high soil resistivity above 100k ohm-cm. The terminology used for the resistivity classification in Table 1 is different from that given in the ISO CP standard.(1)

songtable1.png

In addition, the ranges of soil resistivity given in Table 1 are more detailed and data are unavailable to support developing a correlation between soil resistivity and corrosivity. Table 2 shows a relationship between soil resistivity and corrosivity.(8) In general, the higher the soil resistivity, the less corrosive the soil is to steel (CP effect not considered). When the soil resistivity is below 10K ohm.cm, the soil corrosivity to steel is classified as varying from very corrosive (0-500 ohm.cm), to corrosive (500-1K ohm.cm), to moderately corrosive (1K-2K ohm.cm), and to mildly corrosive (2K-10K ohm.cm). When soil resistivity is greater than 10K ohm.cm, the soil corrosivity to steel is classified as being progressively less corrosive.

songtable2.png

Table 3 shows soil corrosiveness vs. steel native potential.(8) The soil resistivity (the right most column of Table 3) can be derived by a comparison of soil corrosivity in Tables 2 and 3. The higher the soil resistivity, the more aerated the soil or the more likely passivated the steel by the soil, and thus, the less negative the steel native potential.

songtable3.png

Although it is likely that the criteria used to classify soil corrosiveness in Tables 2-3 are different, the criteria may be similar. A conservative soil corrosivity ranking would assume that the “very corrosive and corrosive” categories in Table 2 (soil resistivity less than 1K ohm.cm) correspond with “severe” in Table 3 (native potential more negative than -600 mV), “moderately and mildly corrosive” in Table 2 (soil resistivity of 1K-10K ohm.cm) correspond with “moderate” in Table 3 (native potential between -500 and -600 mV), “progressively less corrosive” in Table 2 (soil resistivity greater than 10K ohm.cm) correspond with “slight” (native potential between -400 and -500 mV) and “noncorrosive” (native potential less negative than -400 mV) in Table 3.