Cathodic protection monitoring

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. 

The coupons have been used to monitor cathodic protection systems in Europe since 1960 [70]. Most of these coupons were installed since 1975. Coupons are used in the form of steel electrodes of a strictly determined shape and surface area, protected by cathodic protection together with the structure. The schematic of the coupon installation on a pipeline is shown in Fig. 15.10. 

Monitoring cathodic protection is emphasized by ASME B31-11, which recommends testing at two month intervals or less of all cathodic protection rectifiers and connected protective devices. Erosion-corrosion of slurry pipelines may be reduced using special corrosion inhibitors, by avoiding sharp corners and short elbows, and providing some lining such as HDPE, rubber, or polyurethane. Monitoring of the pipeline on a yearly basis and at intervals that do not exceed 15 months is recommended by B31.il. Areas prone to more rapid localized erosion-corrosion should be monitored more frequently as dictated by experience. Corrective measures should be established on the basis of bis torical leaks. 

The reference electrode generally used for monitoring cathodic protection in soils is the copper sulfate electrode. This consists of an electrode of electrolytic copper in a solution saturated with copper sulfate crystals. However, the most universal reference electrodes are of the silver chloride type. These consist of a silver wire, plate, or mesh coated with silver chloride so that both silver and silver chloride are in contact with a chloride ion rich electrolyte. The potential of the Ag/AgCl reference electrode will depend upon this chloride ion concentration. 

Monitoring cathodic protection systems and corrosion damage to pipelines (and other structures) under their influence is a highly specialized subject. This material is therefore presented separately from the general corrosion monitoring chapter, although readers should be able to identify some overlap in basic concepts. 

Fig. 3-1 Computer-aided data storage system for monitoring the cathodic protection of a long-distance pipeline.

According to Ref. 32, the functioning of impressed current cathodic protection stations should be monitored every 2 months, and the stray current protection station every 1 month. If protection installations are provided with measuring instruments for current and potential, this supervision can be carried out by operating staff, so that the readings are recorded and sent to the technical department for... 

The cathodic protection of pipelines is best monitored by an intensive measurement technique according to Section 3.7, by an off potential survey eveiy 3 years and by remote monitoring of pipe/soil potentials. After installation of parallel pipelines, it can be ascertained by intensive measurements whether new damage of the pipe coating has occurred. These measurements provide evidence of possible external actions that can cause mechanical damage. 

For commissioning and monitoring of cathodic protection stations, the advice in Refs. 1 and 2 is relevant. For potential measurement, the explanations in Section 3.3 are valid. 

Determination of tank/soil potentials is only possible at the outer edge of the tank. For monitoring the local cathodic protection, the distance between test points should not exceed 2 m [3]. The measurement of the tank/soil potential by the current... 

For adjustment and for monitoring the cathodic protection, test points are necessary along the cable (see Section 10.3.2). It is convenient to install these test points at the cable sleeves. This gives distances between test points of about 0.5 km. The installation of test points to measure conduit current is also convenient for locating accidental contacts. 

Monitoring the cathodic protection of filter tanks extends to the control of protection current levels with the use of ammeters and/or current-operated light diodes. The internal condition of tanks should be inspected visually every 2 years. 

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. 

Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

Cathodic protection and associated instruments have developed in-line with the changing monitoring demands of both the onshore and offshore industries. In particular, for potential and current density measurements, far greater quantities of data are sought and are required to be processed into an easily assimilated form. Thus cathodic protection instrumentation has benefited from an increased association with microprocessor-based data handling and storage systems. 

The measurement of current densities in the vicinity of a cathodically protected structure is a comparatively new principle used chiefly to monitor the effectiveness of offshore protection systems. These measurements are undertaken by twin half-cell devices either installed for stationary use or moved about the structure by diver or remote controlled vehicle. 

Morgan, J. H. Monitoring and Control of Cathodic Protection of Offshore Structures , Materials Performance, 14 No. 5, 37. (1975)... 

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

McKenzie, S. G., Techniques for Monitoring Corrosion of Steel In Concrete , Seminar Corrosion In Concrete-Monitoring, Surveying and Control by Cathodic. Protection , Global Corrosion Consultants, Telford (1986)... 

When the bottom plates are welded together, the coating is partially destroyed. Research and field work showed that protection can be achieved using volatile corrosion inhibitors under the tank [688]. This works alone or in combination with cathodic protection. Double tank bottoms for leakage monitoring are often specified for new tanks. However, the same problem of coating destruction occurs. Volatile corrosion inhibitors are an excellent solution from both a technical and an economic standpoint. This type of corrosion inhibitor... 

The continued effectiveness of a cathodic protection system shall be monitored in accordance with para. GR-5.3.1.1(c). 

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. 

Cathodic protection if internal inspection is conducted and tank is less than 10 years old and is monitored monthly. 

---