Soils stray current corrosion

In soils, stray current corrosion can be caused by close proximity to other buried metal systems that are being protected by an impressed current cathodic protection system. These stray currents can leak onto a buried aluminum structure at one point, then off at another (where corrosion occurs), taking a low-resistance path between the driven buried aiwde and the nearby structure being protected. (Totmnon bonding of all buried metal systems in close proximity is the usual way to avoid such attack (Ref 39 and 40). 

The conductivity of the soil is important as it is evident from the electrochemical mechanism of corrosion that this can be rate-controlling a high conductivity will be conducive of a high corrosion rate. In addition, the conductivity of the soil is important for stray-current corrosion (see Section 10.5), and for cathodic protection (Chapter 10). 

Fig. 10.33 Stray current corrosion, (a) A pipeline or cable may provide a lower resistance path than the soil, (b) Welding operations on a ship may give rise to stray currents if the earth bonding is insufficient, (c) Leakage currents may be induced by an overhead train cable.

Test stations Special devices installed above in cathodic protection systems. They are used to measure pipe-to-soil potential, line current and current flow of a bond, to monitor potential measurements and also to measure stray current corrosion. 

Stray current corrosion differs from other forms in that the source of the current causing the corrosion is external to the affected equipment. This cause of metal deterioration is frequently misdiagnosed. Stray-current corrosion can cause local metal loss in huried or submerged metal structures, but it occurs much less frequently in underwater transporting equipment than in underground structures. Stray-current corrosion is almost always associated with direct current. At the anodic areas, metal goes into solution and the electrolyte tends to become acidic. It is most commonly encountered in soils containing water. 

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. 

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. 

Pick-up of stray current (a.c. or d.c.) (Section 10.5). Decreased use of d.c. in many areas has led to less possibilities of pick-up of direct current from utilities, mines, etc. The importance of grounded a.c. systems has been discounted, but Waters has shown that alternating currents can accelerate corrosion. Furthermore the rectifying effects of oxide films, clay minerals and other soil factors are not understood. 

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. 

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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