Stray current, 1.20

Avoid passage of electric current between metal and its environment, e.g. buried or submerged pipelines, tank bottoms and structures, eleclrie traction, welding plants, power undertakings, and cathodic protection schemes. 

The current jump depends on the magnimde of the potential difference, the electrical conductivity of the liquid in the pipe, soil, or surrounding medium, the geometric configuration of the pipe or structure and insulator, the temperature, and any surface films. 

A major increase in the length of the separator (e.g. a short length of non-metallic pipe) has no great effect on control of the external current jump (e.g. in soil or other conductive media). 

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

The leakage current can be reduced by increasing the resistance between the source and earth, by rail bonding, and rialto-negative ties, by increasing the conductivity of the conductor (rail, lead), by proper scheduling of substation operation, or by welding across each rail section. 

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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. 

Beyond the simple resistance of a material of construction to dissolution in a given chemical, many other properties enter into consideration when makiug an appropriate or optimum MOC selection for a given environmental exposure. These factors include the influence of velocity, impurities or contaminants, pH, stress, crevices, bimetallic couples, levels of nuclear, UV, or IB radiation, microorganisms, temperature heat flux, stray currents, properties associatea with original production of the material and its subsequent fabrication as an item of equipment, as well as other physical ana mechanical properties of the MOC, the Proverbial Siebert Changes in the Phase of the Moon, and so forth. 

API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents, 6th ed., September 1998... 

Deliberate stray current drainage was installed at a subrectifier in Germany as early as 1895 during the electrification of the Aachen tramway. The effective protection extended over a relatively small field since the comparatively large resistance of the pipe Joints did not permit a greater extension of protection. 

The Berlin City electrical engineer M. Kallmann reported in 1899 on a system for controlling stray currents of electric railways [64]. As early as 1894, the Board of Trade in London issued a safety regulation for the British electric railways which specified a potential differential of not more than 1.5 V where the pipeline was positive to the rails, but 4.5 V with the rails positive. Extensive research was undertaken on reducing the risk of stray current in the soil by metallic connections from pipes to rails. However, as one writer noted, a procedure on these lines should definitely be discouraged as it carries the seed of its own destruction [64]. 

The Journal fur Gasbeleuchtung mentions electrolytic corrosion damage caused by direct current cables in Berlin in 1892, and a few years later damage by tramway currents was reported in 14 German towns. As early as 1894 the electrolytic processes of stray current corrosion were explained in detail in this Journal by G.Rasch [65]. 

Fig. 1-16 First polarized relay for stray current drainage in Germany at immigrath in 1953.

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

The data for J and Af/ in Eq. (3-26) change simultaneously with changes in the protection current or by the action of stray currents. Inserting them in Eq. (3-25) gives ... 

Ohmic voltage drops of the protection current were exclusively involved in the processes for 7/ -free potential measurements described above. Besides this, other foreign currents can cause potential drops and falsify the potential measurement (e.g., cell currents, equalizing currents and stray currents). 

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. 

Potential Measurement under the Influence of Stray Currents... 

Where stray currents are involved, several measurements have to be taken that are continually changing with time, simultaneously with each other. A double recorder is most suitable for this. Linear recorders with direct indication of the measurements cannot be used for potential measurements because the torque of the mechanism is too small to overcome the friction of the pen on the paper. Amplified recorders or potentiometer recorders are used to record potentials. In amplified recorders, as in amplified voltmeters, the measured signal is converted into a load-independent impressed current and transmitted to the measuring mechanism, which consists of a torque motor with a preamplifier. The amplifier results in an... 

Where there are stray currents, the switching method described in Section 3.3.1 cannot be used. Stray current protection stations are usually installed where the pipeline has the most positive pipe/soil potential. When the stray current drainage is cut off, a too-positive stray current exit potential that is not 7/ -free is quickly established. In distant areas a too-negative stray current entry potential that is not 71 -free will be measured. The determination of the 71 -free pipe/soil potential is only possible in stray current areas when the origin of the stray current is not oper-... 

In Fig. 3-25 the locational dependence of t/, and is shown together. For practical applications and because of possible disturbance by foreign fields (e.g., stray currents) and t/g are less amenable to evaluation than f/g, which can always be determined by a point of inflection between two extreme values [50]. Furthermore, it should be indicated by Fig. 2-7 that there is a possibility of raising the sensitivity by anodic polarization which naturally is only applicable with small objects. In such cases care must be particularly taken that the counter electrode is sufficiently far away so that its voltage cone does not influence the reference electrodes. 

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

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. 

Anodic polarization exiting stray currents, contact with foreign cathodic... 

Cathodic polarization entering stray currents, cathodic protection. 

Anodic polarization can occur in the presence of stray currents. Oxygen is evolved on the passive steel according to ... 

Finally there are large plate anodes up to 1 m square with cable connections as hangers. Such anodes serve to drain stray currents in ships in fitting out or repairs (see Section 15.6). 

If an adjustable T-R is connected as forced stray current drainage between pipeline and rails and its output voltage is fixed at a definite level, the protection current and the pipe/soil potential can undergo considerable fluctuation. 

The adjustment of a protection station or of a complete protection system where there is stray current interference is made much easier by potential control. Potential control can be indispensable for electrochemical protection if the protection potential range is very small (see Sections 2.4 and 21.4). This saves anode material and reduces running costs. 

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