Sources of Stray Currents

Sources of dc stray currents are commonly electric railways, grounded electric dc power lines, electric welding machines, cathodic protection systems, and electroplating plants. Sources of ac stray currents are usually grounded ac power lines or currents induced in a pipeline by parallel power lines. An example of dc stray current from an electric street railway system in which the steel rails are used for current return to the generating station is shown in Fig. 12.1. Because of poor 

Street railways have now in large part been replaced by other forms of transportation, but the problems of stray-current corrosion originating from metropolitan railway transit systems continue [6]. Also, cathodically protected structures requiring high currents, when located in the neighborhood of an unprotected pipeline, can produce damage similar to that by the railway illustrated in Fig. 12.1. 

TABLE 12.1. Weight Loss of Metals by Stray-Current Corrosion 

If insulating joints are installed in the above-mentioned pipe in order to reduce stray-current pickup, corrosion is now focused on the water side of the joint where any current that persists leaves the pipe to enter the water. Or, if a high-resistance joint exists between two sections of a buried pipe, corrosion may be more pronounced on the side where current enters the soil (Fig. 12.3). 

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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. 

The other terminal of the voltmeter is connected to another ground stake which may be used to probe the earth at different points around the blasting site. Voltages detected in this manner-should he considered potential sources of stray current and tested in die manner described here under "Stray Electric Currents" (Ref 27, pp 173—74)... 

In sizing the mitigation bond, a trial-and-error solution may be possible in relatively simple cases in which a single source of stray current is involved. The size of the resistance bond can be determined by installing temporary variable resistances and by determining when stray current corrosion has been mitigated. The procedure is similar to that described for static stray currents, except that a mitigation curve, such as is shown in Fig. 17 must be obtained. 

Currents flowing into electrolytic environments (ground, water) from inadequately insulated working electric circuits are called stray currents. From the corrosion point of view, industrial equipment powered by dc currents is the most hazardous source of stray currents for metal underground structures. Alternate stray currents of 50 Hz frequency pose a much smaller hazard. Corrosion losses caused by them are estimated at several percent in relation to the corrosion loss caused by dc stray currents of the same magnitude (Baeckmann et al., 1997). 

Welding machines, electrolytic plants, galvanizing plants, and battery plants are sources of stray currents where accidental leakage occurs from the working circuit to the ground, metal structures, or earthing system. The intensity of stray currents from industrial equipment may even reach several hundred amperes. 

Local sources of stray currents should be determined and evaluated for their effect on the designed utility (underground and submerged). 

Application of this method or Eq. (3-25 ) in the presence of stray currents is conceivable but would be very prone to error. It is particularly valid for good coating. Potential measurement is then only significant if stray currents are absent for a period, e.g., when the source of the stray current is not operating. In other cases only local direct measurements with the help of probes or test measurements at critical points can be considered. The potential test probes described in Section 3.3.3.2 have proved true in this respect. 

The effect of stray currents arising from a d.c. source or from cathodic protection of an adjacent structure are considered in Sections 11.5 and 11.6. 

Nevertheless, special consideration should be given to any measured small positive changes in structure/soil potential on a nearby buried pipe or cable if there is reason to believe that the secondary structure is already corroding because of local soil conditions, or as a result of stray currents from another source. 

Generally there is far less evidence of stray current induced corrosion from external sources into atmospherically exposed reinforced concrete structures compared to buried structures generally and far less evidence of stray current induced corrosion in buried or submerged concrete compared to buried or submerged steel, as concrete has much higher electrical resistance. However, it can occur and can be tested for in the same manner as for other metalwork. There is discussion of interference testing for stray currents in NACE RP0169-2002 and in BSEN 13509 2000. 

There are two major sources of stray capacitance. First, the capacitance of the po-tentiostat and leads. By using high-quality cable of minimum length, for example, by mounting the current-to-voltage converter directly over the electrochemical cell, and by avoiding the use of switches as far as possible, stray capacitance from the electrochemical system can be minimized. Second, the microelectrode itself. For example, if there is a small imperfection in the seal between the insulator and the electrode material, then solution leakage will cause the RC cell time constant to increase massively and the Faradaic response may... 

Intentional Anodes and Cathodic Protection. If a bond from if to C in Fig. 12.1 is not feasible, an intentional anode of scrap iron may be buried in the direction of the rails and attached by a copper conductor to point B. Stray currents then cause corrosion only of the intentional anode, which is easily replaced at low cost. If a source of dc current is inserted between the intentional anode and the pipe such that current flows in the soil in a direction opposite to that of the stray current, the arrangement is equivalent to cathodically protecting the pipe. Cathodic protection is installed whenever the intentional anode is not sufficient to overcome all corrosion caused by stray currents. 

Stray currents are electrical currents on a pipeline caused by external sources. Examples of stray currents include d-c or a-c powered transit systems, electric welding operations, currents from mining operations, and interference from cathodic protection systems on other structures. Stray current can cause corrosion where it discharges from the surface of the pipeline. For example, one ampere of d-c stray current causes the loss of 20 lb (9.1 kg) of iron per yesir. A-C stray current has about 1% of the effect of d-c strays on iron pip>e, but a-c stra3rs have a detrimental effect on aluminum [25]. Note that a-c stray current is not necessarily the same as induced a-c sometimes found on pipelines near a-c power transmission hnes. 

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. 

The degree of interaction of stray currents is determined from the correlation dependence of the structure potential and the voltage between the structure and the source of theses currents. Examples of dependencies are presented in Fig. 8-27. 

Additional conclusions on the character of stray currents interaction in the place of measurement can be drawn from the shape of the correlation spectrum of measured quantities. Mostly, this takes the shape of an ellipse. All of its deformations (bends, diffusion, etc.) show, e.g., additional sources... 

The drainage operation is based on controlled removal of stray currents from the underground structure to the source of their formation. The harmful phenomenon of currents outflow from the external surface of structures to the ground is eliminated in this way. Choice and localization of drainage devices should be preceded by determination of the electrolytic corrosion hazard (on the basis of specialist measurements) and analysis of the situation in the field. More information on drainage is given by Chaker and Lindemuth (1994) and Pignatelli (1985). 

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