Cathodic stray currents

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. 

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). 

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

Cathodic polarization entering stray currents, cathodic protection. 

According to Ref. 32, the functioning of impressed current cathodic protection stations should be monitored every 2 months, and the stray current protection station every 1 month. If protection installations are provided with measuring instruments for current and potential, this supervision can be carried out by operating staff, so that the readings are recorded and sent to the technical department for... 

Aluminum-sheathed cables should not be connected to other cables because aluminum has the most negative rest potential of all applicable cable sheathing materials. Every defect in the protective sheath is therefore anodically endangered (see Fig. 2-5). The very high surface ratio SJS leads to rapid destruction of the aluminum sheathing according to Eq. (2-44). Aluminum can also suffer cathodic corrosion (see Fig. 2-11). The cathodic protection of aluminum is therefore a problem. Care must be taken that the protection criterion of Eq. (2-48) with the data in Section 2.4 is fulfilled (see also Table 13-1). Aluminum-sheathed cables are used only in exceptional cases. They should not be laid in stray current areas or in soils with a high concentration of salt. 

To evaluate the cathodic protection—with the exception of very high-resistance soils—from experience, an average value of the on potential of f/cu-cuso4 = -1 -5 V is to be used. With this value, no danger from stray currents should be experienced [6]. 

Fig. 14-10 Forced drainage of stray currents and partial cathodic protection of a 110-kV pressurized cable with a low-resistance connection to the station grounds.

Then stray current enters the pipeline and the pipe/soil potential becomes more negative. The recording in Fig. 15-8b shows the behavior with a direct stray current drainage to the rails. With > 0, a current flows off the pipeline via the stray current return conductor back to the rails so that there is no anodic polarization of the pipeline. With Uf g < 0, a current flows over the connection in the pipeline and anodically polarizes it. Direct stray current drainage is therefore not possible in this case. Figure 15-8c shows the result of a rectified stray current drainage to the rails. Now the pipeline is always cathodically polarized. Cathodic protection is, however, also not fully attained. 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

Further chapters cover in detail the characteristics and applications of galvanic anodes and of cathodic protection rectifiers, including specialized instruments for stray current protection and impressed current anodes. The fields of application discussed are buried pipelines storage tanks tank farms telephone, power and gas-pressurized cables ships harbor installations and the internal protection of water tanks and industrial plants. A separate chapter deals with the problems of high-tension effects on pipelines and cables. A study of costs and economic factors concludes the discussion. The appendix contains those tables and mathematical derivations which appeared appropriate for practical purposes and for rounding off the subject. 

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. 

To prevent underground corrosion, lead is frequently protected with coatings of tar, bitumen, resin, etc., which are only effective if they completely insulate the metal from corrosive agents and stray currents. No coating is fully effective, but some give good protection ". The most successful method used is cathodic protection which for impressed currents, if correctly applied, can protect indefinitely (see Chapter 10). It is effective at a potential of E° = —0-8 V or about 0-1 V more negative than... 

Stray current schemes are relatively rare in occurrence in the UK as few localities now have widespread d.c. transport systems. Such systems are extensively used in overseas countries where d.c. transport systems are in use, i.e. Australia and South Africa. Where stray current can be employed it is normally the most economical method of applying cathodic protection since the power required is supplied gratis by the transport system. 

If a continuous metallic structure is immersed in an electrolyte, e.g. placed in the sea or sea-bed or buried in the soil, stray direct currents from nearby electric installations of which parts are not insulated from the soil may flow to and from the structure. At points where the stray current enters the immersed structure the potential will be lowered and electrical protection (cathodic protection) or partial electrical protection will occur. At points where the stray current leaves the immersed structure the potential will become more positive and corrosion may occur with serious consequences. 

Stray-current electrolysis occurring as a result of the application of cathodic protection to a nearby immersed or buried structure is known as cathodic-protection interaction and is described in Section 10.6. 

Cable sheaths may be covered with paper and hessian wrappings impregnated with bituminous compounds or with extruded or taped plastics outer sheaths. At pinholes or discontinuities in protective coatings the sheath will be particularly liable to electrolytic corrosion in stray-current areas, and it is desirable to supplement this form of protection by drainage bonds or direct cathodic protection. 

Stray currents are produced in the electrolyte during the operation of cathodic-protection systems and part of the protection current may traverse nearby immersed structures which are not being cathodically protected. The resultant corrosion produced on the unprotected structure is referred to as corrosion interaction or corrosion interference. 

The severity of corrosion interaction will depend on the density of the stray current discharged at any point on the secondary structure. This may be assessed by measuring the changes in structure/soil potential due to the application of the protection current. Potential tests should be concentrated on the portions of pipe or cable which are close to the structure to be cathodic-ally protected, where the potential change is likely to be more positive. 

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