Cathodic protection monitoring methods

An alternative means of avoiding the hazard from fire is to bury the vessels or to employ the increasingly popular method of mounding. In either case, acknowledgment of the reduced hazard is indicated by the reduced separation distances (see Table 20.4). Since both burial and mounding preclude the possibility to monitor continuously the external condition of the vessels, very high-quality corrosion protection needs to be applied, often supplemented by cathodic protection, depending on soil conditions. 

Willis, A. D., New Monitoring Methods of Cathodic Protection Systems , Anti-Corrosion, 27, 6, April (1980)... 

The best way to overcome the limitations of cathodic protection related to changes in the environment is to monitor the potential and adjust the cathodic currents accordingly. This cannot be readily done with sacrificial anodes, and the method of impressed-current cathodic protection is sometimes preferred, in spite of its higher cost. 

A large percentage of mains (57%) and services (46%) are metallic systems (steel/cast iron/copper), and corrosion is a major problem. For distribution pipe, external corrosion is the primary problem and internal corrosion has also been observed in some instances. The methods of corrosion monitoring on cathodically protected piping are similar to the methods used in the case of transmission pipelines... 

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

The effectiveness of cathodic protection can be accomphshed by monitoring the pipeline potential using the close interval potential survey (CIPS) method or by using direct current voltage gradient (DCVG) or IR coupon techniques. Also, physical and electrochemical methods can be used to estimate the corrosion rates of a cathodicaUy protected system. 

Combined methods are also used to monitor cathodic protection efficiency. In the intensive measurement described by Wessling et at. [68], both CIPS and DCVG are used. The worker walks the pipeline route and records the distance and the switch on and switch off potential changes vs. the portable reference electrode at small intervals. The measurements are supplemented with ON/OFF potential gradients in one or two directions perpendicular to the pipeline. The measured (real) values serve as the basis for calculating the insulation defects (% IR). Other techniques such as the intensive holiday detection method can be used to detect the coating defects in the pipelines. Implementation of the above methods has led to a considerable decrease in the number of breakdowns of underground pipelines. 

Cathodic protection (CP) is described in Section 10.4. The simplest and most important method for monitoring structures protected by CP is potential measurement. This is used to check that the potential is below a certain limit at which the corrosion rate is assumed to become significant (see Section 10.4). As described earlier (Chapter 3) the potential is measured with a voltmeter with high internal resistance. One terminal on the voltmeter is connected to the reference electrode, which is held close to the actual part of surface of the structure to be monitored. The other terminal on the meter is connected to flic structure. 

For offshore pipelines, there are four possibilities for monitoring the efficiency of the cathodic protection [ 11,31,33-36]. These methods are represented schematically in Fig. 16-14. 

McKubre MCH, Syrett BC (1986) Harmonic Impedance Spectroscopy for the Determination of Corrosion rates in Cathodically Protected Systems, Corrosion Monitoring in Irtdtrs-trial Plants Using Nondestructive Testing and Electrocherrrical Methods, ASTM STP 908, G.C. Moran and P. Labine (Eds.), American Society for Testing and Materials, Philadelphia... 

Corrosion test methods can be divided into electrochemical and non-electrochemical methods. Among the electrochemical techniques that have been used successfully for corrosion prediction are potentiodynamic polarization scans, electrochemical impedance, corrosion current monitoring, controlled potential tests for cathodic and anodic protection, and the rotating cylinder electrode for studies of velocity effects [3i,32]. Though not literally a test, potential-pH (Pourbaix) diagrams have been used as road maps to help understand the results of other tests. 

From the presented review, it is apparent that at present we do not have at our disposal a reliable and effective electrochemical technique enabling rapid and accurate determination of the corrosion current of metals in cathodic polarization conditions, although achievement of the assumed aim seems much closer. At present, there is a significant demand for such types of methods, which could be the basis of modem on-line monitoring systems of corrosion of polarized structures, and would introduce an expected technological breakthrough in the control of the effectiveness of electrochemical protection. 

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