Stray Current Effects

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. 

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],... 

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... 

Stray-current effects are basically associated with primary power supply... 

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). 

Fig. 10 Stray current effects in underground pipelines (a) stray currents cause corrosion in neighboring pipelines (b) redesign minimize stray current effects [39].

The setup details of the two-sided EMM technique are schematically represented in Fig. 4.6. The job sample is held vertically in the machining chamber. The job is made the anode of the electrolytic cell. The tool consists of two cathode assemblies mounted over the vertically held job. Highly localized dissolution of metal from the unmasked region of the two sides of the work sample is achieved by scanning the tool assembly over the work sample. The electrolyte flows through the tool assembly and passes across the surface between the cathode tool and masked workpiece anode. An extremely small lEG is maintained between the work sample and cathode, which provides uniform localized metal removal due to stable current flow distribution with negligible stray current effect. The cathode tool... 

In maskless EMM, there will not be any photoresist mask on the surface of the workpiece sample. Metal dissolution takes place from the desired area of the workpiece in a very selective and controlled way. Highly localized selective metal dissolution from the surface of the workpiece can generate a designed pattern or shape in a 2D or 3D scale. Anodic dissolution in maskless EMM is controlled by the current density, which depends on various predominant machining parameters and anodic reaction pattern. An lEG between the workpiece surface and tool is maintained at a very low value such that stray current effect is minimized. Passivating electrolyte is suitable for maskless EMM due to its ability to form transpassive oxide films and evolve oxygen in the stray current zone. Here, current efficiency varies as a function of the distance from the machining area hence, metal removal is more selective and localized across the zone of the workpiece surface where it faces the tool. 

J. Munda, M. Malapati, B. Bhattacharyya, Control of micro-spark and stray-current effect during EMM process, J. Mater. Process. Technol. 194 (2007) 151-158. 

Gas and water distribution Internal and external corrosion of piping systems (including stray current effects)... 

Stray currents are currents flowing in the electrolyte from external sources, not directly associated with the cathodic protection system. Any metallic structure, for example, a pipeline, buried in soil represents a low-resistance current path and is therefore fundamentally vulnerable to the effects of stray currents. Stray current tends to enter a buried structure in a certain location and leave it in another. It is where the current leaves the structure that severe corrosion can be expected. Corrosion damage induced by stray current effects has also been referred to as electrolysis or interference. For the study and understanding of stray current effects it is important to bear in mind that current flow in a system will not only be restricted to the lowest-resistance path but will be distributed between paths of varying resistance, as predicted by elementary circuit theory. Naturally, the current levels will tend to be highest in the paths of least resistance. 

Alternating current. There is an increasing trend for pipelines and overhead powerlines to use the same right-of-way. Alternating stray current effects arise from the proximity of buried structures to high-voltage overhead power transmission lines. There are two dominant mechanisms by which these stray currents can be produced in buried pipelines electromagnetic induction and transmission line faults. 

Controlling stray current corrosion. In implementing countermeasures against stray current effects, the nature of the stray currents has to be considered. For mitigating dc interference, the following fundamental steps can be taken ... 

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