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Cathodic protection (CP) coupons and soil corrosion probes can be effective tools in applying CP criteria in challenging operating conditions. In situations where CP potentials cannot be reliably measured, electrical resistance soil corrosion probes can be used as an alternative method of measurement.
Cathodic protection (CP) coupons are used for measuring polarized ("off") or depolarized (100-mV criterion) potentials on pipelines and other structures. They are used where these potentials cannot be measured easily. This is usually in situations such as cross-bonded systems, structures influenced by interference from CP systems not controlled by the main protected system operator, or if it is not practical to simultaneously switch off numerous sources of CP current connected to the pipeline.
Soil corrosion probes are used to measure the corrosion rate of the metal, and are connected to the structure so that they receive the same level of CP as the structure. Thus, even if the measured potential reading on the structure is not at the minimum required level, the corrosion rate measurement (mpy or µm) can be used as an alternative criterion to assess CP effectiveness. Low levels of polarization in high-resistivity soils and erroneous potential readings caused by poor reference cell/soil contact are examples of situations where soil corrosion probes can be useful.
The new International Organization for Standardization (ISO) standard for CP criteria, ISO 15589-1:2003,1 lists different CP potential criteria for various soil resistivities. Coupons and probes can be used to implement this standard-coupons can be used to measure polarized potentials, while soil corrosion probes can be used in cases where the required polarization cannot be achieved, but where it can be shown that there is effective protection on the structure through corrosion rate measurements.
Cathodic Protection Coupons
The concept behind the use of CP coupons is that a coupon provides an estimation of the potentials on the adjacent structure by approximating coating holidays in the vicinity of the coupon test station. Thus, for structures of known (and reasonably good) coating quality, the coupon "off" potential, measured by interruption of the CP current to the coupon, provides an estimate of the polarized potential of the structure.
NACE International has issued a recommended practice (RP0104-2004)2 detailing the use of CP coupons for CP potential measurements. NACE test...