Levels of cathodic protection
Cathodic protection involves polarisation of a metal in the active direction, towards more reducing potentials.
Cathodic polarisation has the effect of slowing or stopping metal oxidation, in other words, preventing corrosion.
Cathodic protection moves the potential of a metal surface in a cathodic direction to reduce the thermodynamic tendency for corrosion. When steel receives enough current to shift the potential to a certain level, the corrosion is essentially stopped.
Potential values for corrosion porotection is depending on environmental conditions. However, as a guiding practice, full cathodic protection of steel is usually obtained by a potential more negative than:
-800 mV vs Ag/AgCl in aerated seawater
+250 mV vs Zn in aerated seawater
-850 mV vs Cu/CuSO4 in soil
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Cathodic protection measurement
Cathodic protection potential measurement is a method used to determine the adequacy of a cathodic protection system, applied to protect a certain structure. The adequacy, or the corrosion protection effect, is determined by comparing the measured potential with certain criteria.
For adequate measurement, you have to ensure proper selection and installation of equipment used in field measurements.
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Cathodic over-protection risks
The potential of steel under cathodic protection should not be lowered too much or, in other words, the cathodic current density should not be too high. In this case, some hydrogen can be formed, which can lead to risk of embrittlement of certain steels. The pH can become very alkaline, leading to coating damage.
Criteria linked to the risks of over-protection appears in the technical standards and recommendations.