Protection from Stray Current

Still in the experimental phase of application in concrete [11]. Other measurements that can help in detecting the presence of stray current in reinforced concrete are based on the potential difference present between different parts of the structure, due to the ohmic drop produced by the stray current 

We have seen that stray current can hardly induce corrosion on passive steel in non-carbonated and chloride-free concrete. However, the potential adverse effects of stray current on concrete structures may become increasingly important with the increased use of underground concrete construction. Stray-current effects are rarely recognised as such. The importance increases further due to the increase of the required service lives (i. e. 100 y or more). 

The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

Electrical discontinuity in reinforcement also contributes to mitigate the circulation of current In fact, it forces the absorbed current to enter and leave the reinforcement, each time losing part of the driving voltage to overcome cathodic or anodic overvoltages. 

Pedeferri, Stray-current induced corrosion in reinforced-concrete structures , in Progress in the Under- [7] 

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To evaluate the cathodic protection—with the exception of very high-resistance soils—from experience, an average value of the on potential of f/cu-cuso4 = -1 -5 V is to be used. With this value, no danger from stray currents should be experienced [6]. 

Electrolytic cells have historically been mounted off the ground at full basement height. Some of the arguments for elevating the cells include the need to electrically isolate cells from spills in the basement, to protect workers from stray current, to adjust the level of the cells, to inspect for leaks and perform repairs, and to limit pumping costs by employing gravity flow. With the advent of polymer concrete cells and of synthetic cell liners, the frequency of leaks and the need for electrical isolation have been drastically reduced. 

Where stray currents from a man-made source of direct current are a potential problem, metallically bonding the structure of concern to the source of the dc is often used to mitigate the corrosion that would otherwise occur. This is a common method of control where pipelines are subject to the adverse effects from stray currents generated by light rail transit systems, d-c welding machines, and impressed current cathodic protection systems installed on other nearby structures. 

Determination of the corrosion hazard of industrial structures in the field of stray current interaction, due to difficulties, is the subject of a relatively small number of publications, for example, Bazzoni and Lazarri (1996). The correlation method described by Juchniewicz and Sokdlski (1985, 1986) allows, on the basis of field measurements, determination of the potential hazard to structures caused by electrolytic corrosion and the effectiveness of electrochemical protection from stay currents. The principle is based on simultaneous field measurements of two quantities connected by the presence of stray currents, and analysis of the spectrum of their mutual correlation. During field measurements, the following quantities are most frequently measured ... 

Corrosion due to stray current—the metal is attacked at the point where the current leaves. Typically, this kind of damage can be observed in buried stmctures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

Stray currents from foreign sources are to be regarded in the same way as galvanic currents. The explanations for Eq. (4-11) are relevant. Protective measures afed cribed in Chapters 9 and 15. 

Factors that are important for the limitation of protected areas are the pipe network structure, degree of mesh, number of service pipes, type of pipe connections, quality of the pipe coating and availability of protection current as well as stray current effects. A protected area in a distribution network is shown in Fig. 10-11 with separate parts of the network (NT I to NT IV). Previous experience has shown that protected areas of 1 to 2 km with lengths of pipeline from 10 to 20 km are advantageous [30],... 

If the protection current becomes too high due to this connection in cathodi-cally protected tank installations, then insulating joints are usually installed in the pipeline from the filling nozzle. Care must be taken that the continuity bond is not broken. If there is a danger of stray currents with dc railways due to a permanent connection between track and filling equipment, the continuity bond should be applied only during the filling process. 

Figure 15-8 shows synchronous recordings of the voltage between the pipeline and the rails, of the pipe/soil potential f/cu cuso4 drained current in the region of a tramway transformer substation with and without various protective measures. Figure 15-8a records values without protective measures. If the rails are negative with respect to the pipeline (f/R s > 0), the pipe/soil potential becomes more positive. Stray current exit exists. From time to time, however, < 0. 

With forced stray current drainage, the current is returned from the pipeline to the rails by means of a grid-fed rectifier. The transformer-rectifier is connected into the stray current return conductor, the negative pole is connected with the installation to be protected and the positive pole is connected to the rails or the negative side of the bus bar in the transformer substation. 

In Fig. 15-9 two potentiostatically controlled protection rectifiers and an additional diode are included to drain peak currents. At pipeline crossings with an external rail network (e.g., in regions outside the urban area), the forced stray current drainage should be installed as close as possible to the rails that display negative potentials for the longest operation time. The currents absorbed from the positive rails continue to flow also in the region outside the rail crossings. Here the use of potentiostatically controlled rectifiers is recommended these should be connected not only to the rails but also to impressed current anodes. 

Fig. 15-8 Synchronous current, voltage and potential recording with stray current interference from dc railways (a) Without protective measures, (b) direct stray current drainage to the rails, (c) rectified stray current drainage to the rails, (d) forced stray current drainage with uncontrolled protection rectifier, (e) forced stray current drainage with galvanostatically controlled protection rectifier (constant current), (f) forced stray current drainage with potentiostatically controlled protection rectifier (constant potential), (g) forced stray current drainage with potentiostatically controlled protection rectifier and superimposed constant current.

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

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