Stray current sources

Eo = Open circuit potential without the stray current source operating... 

AE = Change in open circuit potential caused by stray current source... 

Stray currents are a real corrosion hazard to metal underground infrastructures, causing significant corrosion losses. Thus the electrolytic corrosion hazard should be predicted at the structure design phase, and the route should be properly chosen (far from stray current sources). The problem of electrolytic corrosion also can be limited by ex-... 

The stray current sources described in Chap. 7 can result in very rapid corrosion which is usually much more severe than the corrosion caused by other environmental factors (Fig. 13.34). Another type of stray current which is variable in natme may be observed during periods of "magnetic storm" activity. Long structures such as pipelines or cables are most apt to be affected. During magnetic storms, the intensity of the earth s magnetic field can vary. When these variations occur, potentials are induced in the pipe or cable in much the same manner as potentials are induced in an electric generator. 

It has been demonstrated that BE modeling can accurately predict experimental results. BE methods also can be used to evaluate the effect of a single parameter on system performance. In this way basic understanding of electrochemical corrosion and parameter interactions can be obtained. Several parametric studies have, for example, been published on damage levels in the propeller area, seawater conductivity, and paint resistance effects, as well as on the influence of stray current source on system performance [18]. 

Removal of the stray current source or reduction in its output current... 

Figure 11.19 illustrates the fluctuations in pipe-to-soil potentials for a pipeline in close proximity to an electrified rail transit system. Typically a number of trains would have passed this monitoring point during the data collection period in Fig. 11.19. Positive potential excursions associated with current discharge at the measuring point and negative potential transients related to current pickup are evident. At greater distances from the stray current source the potential profile is significantly more stable, as indicated by the second potential trace in Fig. 11.19.

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. 

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. 

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. 

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. 

Stray Electrical Currents and Induced Radio Frequency Currents. For information on stray currents see API 2003 [3j. For information on both hazards see the author s review Sources of Ignition in [157]. Electrostatic Discharge (ESD) Damage to Electronic Equipment. Marine Tankers and Barges (see [5] ISGOTT ). 

Electrical sources static electricity, electrical current, lightning, stray currents (radiofrequency electromagnetic radiation, overhead high voltage transmission lines, galvanic and cathodic protection stray currents)... 

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. 

Stray-current Corrosion corrosion caused by stray currents flowing from another source of e.m.f. (usually d.c.). 

Instruments for exact measurements usually have a sinusoidal current source and an electronic balance detector. The circuit is made as symmetrical as possible to avoid stray coupling. 

The hazards which stray currents present to electric blasting caps can be greatly diminished by isolating all electric power lines from ground except at the power source and providing a separate, common bus, bonded to the frames of all electrical equipment. 

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)... 

Apart from corrosion due to differential aeration, corrosion of underground metal structures and pipelines may also arise from stray currents. How this comes about can be seen in the accompanying diagram (Fig. 12.32). The presence of a current-carrying cable in conducting soil results in stray currents passing through the soil. These stray currents may set up a potential difference between two portions of a pipeline, which then develops electron-source (cathodic) and -sink (anodic) areas. Thus, pipelines tend to corrode when they pass near electric lines. 

At points where the current enters the structure, the site will become cathodic in nature because of changes in potential, while the area where the current leaves the metal will become anodic. Electric railways, cathodic protection, electrical welding machines, and grounded DC electrical sources are subject to stray current corrosion. (Craig)5... 

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