Stray current interference causes

The causes and common means of detecting and mitigating stray current interference effects that result from direct current sources are reviewed in this Sect. [1, 40-44]. Alternating current, while creating a potential safety ha2ard, may contribute to corrosion of ferrous structure [42]. Extensive research is in progress in this regard. 

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. 

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. 

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). 

Care must be taken when designing cathodic protection systems in built-up areas as electrical interference from transmission lines or electrical rail systems may cause stray current corrosion at points in the structure close to the tramway or transmissions line [41]. 

Limitations due to convection. At longer times the buildup of density gradients and stray vibrations will cause convective disruption of the diffusion layer, and usually result in currents larger than those predicted by the Cottrell equation. The time for the onset of convective interference depends on the orientation of the electrode, the existence of a protective mantle around the electrode, and other factors (1, 2). In water and other fluid solvents, diffusion-based measurements for times longer than 300 s are difficult, and even measurements longer than 20 s may show some convective effects. 

---