Cathodic protection field measurements

In order to obtain the actual field attenuation characteristics, a pipeline can be placed under temporary cathodic protection and measurements made... 

Structure-to-electrolyte potential measurements are analyzed to determine whether a structure is cathodically protected these measurements are made by the use of cathodic protection criteria. Unfortunately, no one simple criterion has been accepted by all cathodic protection engineers that can be practicably measured in the field under all circumstances. Guidelines for selecting the proper criterion under various circumstances will be provided below. Guidance concerning the criteria of cathodic protection for external corrosion control on underground structures is found in two recommended practices (RPs) published by the National Association of Corrosion Engineers (NACE). These are RP-01-69 and RP-02-85. A summary of the criteria for steel and cast iron structures follows [8]. 

Heterogeneous surface areas consist of anodic regions at corrosion cells (see Section 2.2.4.2) and objects to be protected which have damaged coating. Local concentrations of the current density develop in the area of a defect and can be determined by measurements of field strength. These occur at the anode in a corrosion cell in the case of free corrosion or at a holiday in a coated object in the case of impressed current polarization (e.g., cathodic protection). Such methods are of general interest in ascertaining the corrosion behavior of metallic construction units... 

Finally, calomel electrodes (and more especially hydrogen electrodes) are not suitable for field measurements because they are not sufficiently robust. The calomel electrodes are however essential for calibrating the field reference electrodes. Saturated KCI calomel electrodes are the most suitable because there is then no doubt about the reference potential of the calibrating electrode. Lack of adequate calibration is a common cause of cathodic protection system mismanagement. 

The deviation from the correct potential value by inclusion of the field gradient in the measurement is often called the IR error and it must be minimised in assessing the state of polarisation of a cathodically protected structure. The recommended protection potentials all assume no significant IR error in the measurement. 

Fig. 10.9 Diagram illustrating the source of the IR error in potential measurements on a cathodically protected structure. BA is the absolute electrode potential of the structure CD is the absolute electrode potential of the anode and CB is the field gradient in the environment due to cathodic protection current flux. A reference electrode placed at E will produce an IR error of EFin the potential measurement of the structure potential. If placed at G the error will be reduced to GH. At B there would be no error, but the point is too close to the structure to permit insertion of a reference electrode. If the current is interrupted the field immediately becomes as shown by the dotted line, and no IR is included...

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

Often it is necessary in designing a cathodic-protection system to know the conductivity of a protective coating (e.g. bitumen enamel) on a structure. This measurement is usually carried out by finding the resistance between an electrode of known area placed in contact with the coating and the structure itself. The electrode placed on the structure can be either of thin metal foil or, preferably, of material such as flannel soaked in weak acidic solution. The resistance between the pad and the metal is measured by means of either a resistivity meter, as previously described, or a battery with a voltmeter and an ammeter or microammeter. Generally speaking, in field work where such measurements have to be made, a resistivity meter is preferable. 

Commercial sensors suitable for automated measurements are available and can be used, for example, for field measurements of hydrogen uptake by cathodically protected steel buried in soil sediments. Other types can be mounted in process plants for monitoring purposes. 

The evaluation of field of current density is essential in problems of galvanic corrosion. In many cases the direct measurement of current density is not feasible, while the electric potential can be obtained from experimental measurements. This is particularly true in case of cathodic protection systems in general, where many surveying techniques (for example DCVG and CIS for underground structures) rely in potential measurements at different points at the electrolyte in order to identify the current distribution along the metallic structures. 

Successful application of cathodic protection depends upon the selection, design, installation, and maintenance of the system. Before designing the cathodic protection systems, adequate field data must be collected, analyzed, and evaluated. Nature and conditions of the soil are reflected by field measurements like soil resistance, hydrogen ion activity (pH), and the redox potential. To understand the nature of the pipeline, potential measurements, coating resistance, and meaningful design current requirement tests must be conducted. 

In order to get more information about the state of cathodically protected surfaces in the sea, current density measurements have also sometimes been carried out in addition to potential measurements. Current density determination is usually done by measuring the electric field and eonverting this to current density. The field is determined by measuring the potential difference AP between two reference electrodes separated by a eertain distanee L. If the current density in the direction through both electrodes is constant over the distance L, the current density is given by... 

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