A cathodic protection installation must be monitored and controlled regularly to be sure that the protection is effective.
Level of protection
The corrosion rate of a metal, expressed for example in mm/year, is proportional to the anodic current density, in A/m² (selon la loi de Faraday). (according to the Faraday’s law). For carbon steel, this approximates to 1mm/year = 7,8 kg/m².an = 850 mA/m². When a cathodic protection installation is designed, it is necessary to ensure that at any point on the structure, the corrosion rate will be sufficiently reduced by the reduction of potential. The criterion generally selected is the one leading to a residual corrosion rate lower than 10 μm/an.
The criteria we need to consider for a an effective cathodic protection installation, when it is operating, are founded on the corrosion potential of steel with respect to a reference electrode. The method to measure this potential depends on the type of structure and on the environment which surrounds it. It should be noted that the value of the ohmic drop (derived from the magnitude of the current produced by the resistance) existing between the measurement electrode and the surface for which we wish to measure the potential can lead to significant measurement errors. In low-conductive environments like soils, it is necessary to eliminate or reduce as much as we can this ohmic drop either by placing the measurement electrode in the immediate vicinity of the surface of the metal, or, most often, by measuring the potential immediately after the interruption of the cathodic protection current, in the absence of external electric disturbances.
Influence of the ohmic drop on the measurement of the electrode potential of a pipe
(Even though the position 1 for the reference electrode is better than the position 2, the measurement is distorted by the ohmic drop)
From experience, in the usual aqueous environments, cathodic protection is effective if the real potential of steel (without the ohmic drop) is more negative than –850 mV with respect to a copper/saturated copper sulphate electrode or –800 mV with respect to a silver/silver chloride/sea water electrode. In some cases, for example under anaerobic corrosion conditions (mud in sea-bed for example), the potential must be still lowered by 100 mV (that is to say respectively –950 mV or –900 mV).
Other more or less empirical criteria are sometimes taken into account. The most widely used is the one called “depolarization”: the potential of steel is measured as quickly as possible after the cathodic protection current is switched off ( thus avoiding the ohmic drop), then a few hours later. The protection is considered effective if the corrosion potential of the protected structure is at least 100 mV higher.
Cathodic over-protection risks
The potential of steel under cathodic protection should not be lowered too much (or, which is equivalent, the cathodic current density should not be too high). Indeed, in this case, some hydrogen can be formed (risk of embrittlement of certain steels) and the pH can become very alkaline, (coating damage). Some criteria linked to the risks of over-protection appear in the technical standards and recommendations.