Soils stray currents

In soils, stray current corrosion can be caused by close proximity to other buried metal systems that are being protected by an impressed current cathodic protection system. These stray currents can leak onto a buried aluminum structure at one point, then off at another (where corrosion occurs), taking a low-resistance path between the driven buried aiwde and the nearby structure being protected. (Totmnon bonding of all buried metal systems in close proximity is the usual way to avoid such attack (Ref 39 and 40). 

The Berlin City electrical engineer M. Kallmann reported in 1899 on a system for controlling stray currents of electric railways [64]. As early as 1894, the Board of Trade in London issued a safety regulation for the British electric railways which specified a potential differential of not more than 1.5 V where the pipeline was positive to the rails, but 4.5 V with the rails positive. Extensive research was undertaken on reducing the risk of stray current in the soil by metallic connections from pipes to rails. However, as one writer noted, a procedure on these lines should definitely be discouraged as it carries the seed of its own destruction [64]. 

Where there are stray currents, the switching method described in Section 3.3.1 cannot be used. Stray current protection stations are usually installed where the pipeline has the most positive pipe/soil potential. When the stray current drainage is cut off, a too-positive stray current exit potential that is not 7/ -free is quickly established. In distant areas a too-negative stray current entry potential that is not 71 -free will be measured. The determination of the 71 -free pipe/soil potential is only possible in stray current areas when the origin of the stray current is not oper-... 

If an adjustable T-R is connected as forced stray current drainage between pipeline and rails and its output voltage is fixed at a definite level, the protection current and the pipe/soil potential can undergo considerable fluctuation. 

If the projected pipeline is situated in an area with dc railway lines, rail/soil potential measurements should be carried out at crossing points and where the lines run parallel a short distance apart, particularly in the neighborhood of substations, in order to ascertain the influence of stray currents. Potential differences at the soil surface can give information on the magnitude of stray current effects in the vicinity of dc railway lines. It is recommended that with existing pipelines the measurements be recorded synchronously (see Section 15.5) and taken into account during design. 

Aluminum-sheathed cables should not be connected to other cables because aluminum has the most negative rest potential of all applicable cable sheathing materials. Every defect in the protective sheath is therefore anodically endangered (see Fig. 2-5). The very high surface ratio SJS leads to rapid destruction of the aluminum sheathing according to Eq. (2-44). Aluminum can also suffer cathodic corrosion (see Fig. 2-11). The cathodic protection of aluminum is therefore a problem. Care must be taken that the protection criterion of Eq. (2-48) with the data in Section 2.4 is fulfilled (see also Table 13-1). Aluminum-sheathed cables are used only in exceptional cases. They should not be laid in stray current areas or in soils with a high concentration of salt. 

Measurement of the cable sheathing/soil potential can be used to assess the corrosion danger from stray current interference (see Section 15.5.1). Since the measured values vary widely and the stray currents cannot be switched off, IR-free potential measurements are only possible with great effort. In order to keep the IR term of the potential measurement low, the reference electrode must be placed as close as possible to the measured object. With measurements in cable ducts (e.g., underneath tramway tracks), the reference electrodes can be introduced in an open duct. 

Direct current installations that are grounded in several places cause stray currents in the soil which can interfere with other installations (see Section 9.2). All dc railways are sources of stray currents. Protection methods that can be applied in the same way to cables are described in Chapter 15. 

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]. 

Figure 15-2a shows the stray current interference by a bipolar high-voltage dc power line [7]. When the system breaks down, large voltage cones occur in the soil at the grounding installation. A few kilometers away, the current density in the soil is relatively low. 

Fig. 15-5 Stray current interference in the region of dc railway (a) Polarization of the railway lines, (b) voltage between the soil in the vicinity of the rails against a remote ground. Polarization of the pipeline (c) without stray current drainage, (d) with stray current drainage without a resistor, (e) with stray current drainage via a resistor R. Current in the pipeline (f) without stray current drainage, (g) with stray current drainage.

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. 

Then stray current enters the pipeline and the pipe/soil potential becomes more negative. The recording in Fig. 15-8b shows the behavior with a direct stray current drainage to the rails. With > 0, a current flows off the pipeline via the stray current return conductor back to the rails so that there is no anodic polarization of the pipeline. With Uf g < 0, a current flows over the connection in the pipeline and anodically polarizes it. Direct stray current drainage is therefore not possible in this case. Figure 15-8c shows the result of a rectified stray current drainage to the rails. Now the pipeline is always cathodically polarized. Cathodic protection is, however, also not fully attained. 

Very positive tramway rails can produce such a negative pipe/soil potential by high stray current entry that it becomes more negative than the imposed nominal potential. The protection current device must be designed so that in this case the dc output is controlled at zero. If no further protection stations are installed on the pipeline, the protection current device must be arranged so that an imposed minimum protection current is not undercut. 

Fig. 22-3 shows the total number of perforations per kilometer in a 180-km DN 500 long-distance gas pipeline with a wall thickness of 9 mm which was laid in 1928 in a corrosive red-marl soil. There was no influence from stray currents. 

Ohmic interference the production of electrical potentials in conductors by electrical contact, by arcing or by a local voltage cone, caused by fault currents or stray currents in the soil (see Chapter 15). 

Pick-up of stray current (a.c. or d.c.) (Section 10.5). Decreased use of d.c. in many areas has led to less possibilities of pick-up of direct current from utilities, mines, etc. The importance of grounded a.c. systems has been discounted, but Waters has shown that alternating currents can accelerate corrosion. Furthermore the rectifying effects of oxide films, clay minerals and other soil factors are not understood. 

If a continuous metallic structure is immersed in an electrolyte, e.g. placed in the sea or sea-bed or buried in the soil, stray direct currents from nearby electric installations of which parts are not insulated from the soil may flow to and from the structure. At points where the stray current enters the immersed structure the potential will be lowered and electrical protection (cathodic protection) or partial electrical protection will occur. At points where the stray current leaves the immersed structure the potential will become more positive and corrosion may occur with serious consequences. 

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