Steel Pipe Native Potentials In Soils Affect CP Criteria
Soil resistivity, corrosivity and steel native potentials in soils are interrelated. A chart was developed in this work to quantitatively map the rule-of-thumb relationship between steel native or rest potentials and soil resistivity.
This chart was validated with available literature data from independent sources and can be used to provide a rough estimate of the magnitude of polarization applied on buried steel pipes for a given soil resistivity and an off-potential measured. Polarization provides a more direct measure (than an off-potential) on the magnitude of corrosion rate reduction. The chart shows that meeting the -850 mV, -750 mV and -650 mV (vs. Cu/CuSO4) off-potential criteria for soil resistivity ranges of, respectively, <10K, 10K-100K and >100K ohm.cm would - in general - yield at least 100 mV cathodic polarization.
Cathodic protection (CP) has been used for many decades for the control of external corrosion of buried or submerged steel piping systems. CP criteria can be used to guide the level of CP that needs to be provided on the pipe surface. The International Standard Organization (ISO) and European (EN) CP standards (ISO 15589-11 and EN 129542 ) recommend three off-potentials -850 mV, -750 mV and -650 mV vs. Cu/CuSO4 (CSE) - for the corrosion control for soil resistivity ranges of <10K, 10K-100K and >100K ohm.cm, respectively.
Unfortunately, few references, if any, are given in these standards to justify why these criterion values (and not others) should be used with respect to different soil resistivity ranges. Nevertheless, if the steel native or rest potentials can be known, the level of cathodic polarization applied on the steel pipe surface for a given off-potential can be evaluated and provides a more direct measure (than an off-potential) on the magnitude of corrosion rate reduction. In other standards, such as the NACE Standard Practice (SP) 0169-2007 and Australian CP Standard (SAA AS 2832.1), -850 mV on-potential criterion with CP current applied is recommended to use. To clarify the different terminologies used in this article, Figure 1 is provided which results from modification of a similar chart reported elsewhere.(5)
Figure 1 shows schematically the two methods used in the field to measure the polarization on buried piping. The meanings of on- and off-potentials, polarization (decay or growth) and ohmic voltage drop (IR), native and rest potentials are labeled and shown clearly. Figure 1(a) shows the method of polarization growth, and Figure 1(b) shows the method of polarization decay. The potentials shown in each of the figures include the native potential (Ecorr), the on- and off-potentials, the “decayed-off” potential (potential measured during depolarization), and the “rest potential” (potential when depolarization becomes steady), or the polarization growth or decay and the IR drop.
The difference between on- and off-potentials measured under the same conditions may be generally considered as the IR voltage drop, with the on-potential being generally more negative than the off-potential. Open circuit potential (OCP) is measured with no external current applied to a metal surface. It is generally referred to as a steel native potential or a free corrosion potential, although it can also be an instant off-potential, a decayed off-potential, or a rest potential. The values of these potentials can be measured correctly only when there is absence of interference by stray currents or long-line currents.
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