Stray DC current

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. 

In structures affected by electrical fields, DC stray current in the concrete can enter the reinforcement in some areas (i. e. it passes from the concrete to the steel) and return to the concrete in a remote site. The passive layer can be destroyed in those areas where the current leaves the steel (Chapter 9). 

Consequences of DC stray current in reinforced concrete change according to the properties of the concrete (alkahne, carbonated or contaminated by chlorides), to the duration of the current circulation and to the current density. It is therefore necessary to distinguish concrete structures not contaminated by chlorides and not carbonated from those contaminated by chlorides in quantities insufficient to initiate corrosion and, finally, from those that already have corroding rebars because of chlorides or carbonation. 

DC stray currents may have more serious consequences in chloride-contaminated concrete. On passive reinforcement in concrete containing chloride in a quantity below the critical content and thus in itself insufficient to initiate localized corrosion, the driving voltage AE required for current to flow through the reinforcement is lower than in chloride-free concrete and decreases as the chloride content increases (Figure 9.7). This is a consequence of less perfect passivity, and in particular a lower pitting potential. 

Although there is no experience, interaction between AC and DC stray currents cannot be excluded, since AC can influence the anodic behaviour of steel [8]. Therefore attention should be dedicated to possible synergistic effects of AC and DC stray currents that might, under specific circumstances, be able to stimulate the corrosion rate of depassivated steel or promote corrosion on passive steel. 

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... 

The telecommunication plant is made of a number of metal and non-metal components, exposed to various corrosive environments. Man-made effects, such as DC stray currents, can further accelerate corrosion of the plant. 

The metals of construction and some plant components are tested before manufacturing for corrosion resistance in the enviromnents to which they will be exposed. Laboratory tests and field exposures are used for this purpose. The plants are also tested after exposure to the environment to establish the local corrosion effects, especially those caused by DC stray currents. 

Additionally, stray-current corrosion is common on seagoing vessels, such as boats. For example, powered battery chargers generate dc stray currents, which may flow indefinitely if proper precautions are ignored or overlooked. This type of corrosion can manifest as pitting in confined ares, metal surface discoloration, rust formation on steel parts or weakening of batteries. 

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). 

Sources of DC stray currents and their conductive paths Development of corrosive fumes in specific conditions Nature of the effect - beneficial or detrimental, etc. 

Enhancement of Anodic Corrosion by Cell Formation or Stray Currents from dc Installations... 

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. 

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. 

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 Currents from dc Railways 15.2.1 Regulations for dc Railways... 

Stray Currents from High-Voltage dc Power Lines... 

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. 15-2 Stray current interference from high-voltage dc transmission installations (a) bipolar system, (b) monopolar system.

Frequently, measures for dc installations are not sufficient to limit stray currents. This applies particularly to dc railways. In many cases, additional protective measures for the affected installations are advisable [1] or even necessary [11]. 

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.

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. 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.

In harbors, central dc supply equipment is still occasionally used as welding equipment on wharves, travelling dc cranes, dc supply for ships in dock, etc. In Ref. 1 proposals for avoiding stray currents are detailed in which separate dc supply equipment is stipulated. In ac distribution networks in which the subsequent dc network is grounded in only one place, there is no possibility that the dc currents will occur as stray currents. 

Considerable stray currents can, of course, be caused by dc-driven cranes that load and unload ships where the rails act as the return conductor for the current. The rails run parallel to the harbor basin, quay walls of steel-reinforced concrete or steel piling walls. These can take up a large part of the stray current and conduct it further because of their small longitudinal resistance. Noticeable stray current inter-... 

Stray current effects with reinforced concrete are not likely from the usual causes, but are possible with roadways and bridges over which dc railways pass. In... 

API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents (Washington, DC American Petroleum Institute, 1991). 

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