Corrosion stray currents

Stray-current Corrosion. Stray currents in the past have resulted from DC-powered trolled systems, which have become obsolete. An electric welding machine on board a ship with a grounded DC line located on shore will cause accelerated attack of the ship s hull as the stray currents generated at the welding electrodes pass out of the ship s hull through the water back to the shore. Houses in close proximity can dramatically corrode at the waterline. The pipes in one house can be completely corroded, while those in the neighboring house may be intact. 

CORROSION, STRAY CURRENT - A form of attack caused by electrical currents going through unintentional path. 

General corrosion is the most common form of corrosion. This can be uniform (even), quasi-uniform, or uneven. General corrosion accounts for the greatest loss of metal or material. Electrochemical general corrosion in aqueous media can include galvanic or bimetallic corrosion, atmospheric corrosion, stray current dissolution, and biological corrosion (Table 1.1). 

External corrosion of water systems may be caused by general corrosion, stray current corrosion MIC, and/or galvanic corrosion. Corrosion mitigation techniques include the application of protective coatings, wrapping pipe in a plastic cover, and the application of CP. The areas of major external corrosion impact are generally those where localized attack may take place, such as in the proximity of other systems like galvanic corrosion or in areas where stray currents may occur. 

Both DC and AC stray currents on a water line can cause corrosion. Stray current studies (18) show that the corrosion rate because of the dc current is generally greater than the corrosion rate because of ac current. General external... 

The first chapter constitutes an introduction to corrosion and various forms of corrosion such as general or uniform or quasi-uniform corrosion, galvanic corrosion, stray current corrosion, localized corrosion, such as pitting and crevice corrosion, metallurgically influenced and microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking. 

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. 

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]. 

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). 

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

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. 

Measurement of the cable sheathing/soil potential can be used to assess the corrosion danger from stray current interference (see Section 15.5.1). Since the measured values vary widely and the stray currents cannot be switched off, IR-free potential measurements are only possible with great effort. In order to keep the IR term of the potential measurement low, the reference electrode must be placed as close as possible to the measured object. With measurements in cable ducts (e.g., underneath tramway tracks), the reference electrodes can be introduced in an open duct. 

Corrosion by Anodic Interference (Cell Formation, Stray Currents)... 

Fig. 22-3 (1) Total number of wall penetrations by corrosion per kilometer of a DN 500 pipeline as a function of service life. (2) Total number of wall penetrations per kilometer of a pipeline with severe stray current exit.

Fig. 22-3 shows the total number of perforations per kilometer in a 180-km DN 500 long-distance gas pipeline with a wall thickness of 9 mm which was laid in 1928 in a corrosive red-marl soil. There was no influence from stray currents. 

Since stray current corrosion damage can occur after only a few years, the economy of stray current protection measures is obviously not questionable [12], In Fig. 22-3 the effect of stray currents is shown by curve 2 [14]. Without there being firm evidence, it is apparent that the shape of the corrosion damage curve in steel-reinforced concrete (see Sections 10.3.6 and 4.3) is similar to that for stray current corrosion [15]. 

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