Pipelines stray-current corrosion

The major stray-current corrosion problems now result in cathodic protection systems. Current from an impressed-current cathodic protection system will pass through the metal of a neighboring pipeline at some distance before it returns to the protected surface. Increased anodic corrosion is frequently localized on the pipe at the zone where the current leaves the pipe back to the protected steel tank. 

Street railways have now in large part been replaced by other forms of transportation, but the problems of stray-current corrosion originating from metropolitan railway transit systems continue [6]. Also, cathodically protected structures requiring high currents, when located in the neighborhood of an unprotected pipeline, can produce damage similar to that by the railway illustrated in Fig. 12.1. 

Michael J. Szeliga, Stray current corrosion, in Peabody s Control of Pipeline Corrosion, 2nd edition, R. L. Bianchetti, editor, NACE International, Houston, Texas, 2001, pp. 211-236. 

Biological CorrosiMi on steel, Cu - alloys, Zn - alloys in seawater. Stray-Current Corrosion on a pipeline near a railroad. 

Malo JM, Salinas V, Uruchurtu J. Stray current corrosion causes gasoline pipeline failure. Materials Performance, 1994 33 63. 

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.

Any underground metallic structure would corrode at the point of exit of Fe" " " ions. To prevent this undesirable stray current corrosion a metallic bond, such as a bond cable between the pipeline and the negative bus of the DC substation, is installed as shown in Fig. 5.46. The current is then drained off by the metallic bond and all the surface of the secondary pipes becomes completely cathodic. The situation here is rather over-simplified, as there may be hundreds of substations serving the system depending on the traffic load and the load may vary during the 24 hours period. In certain instances, a bond connected may not be useful as the direction of flow of current in the bond may reverse and the current may flow to the pipelines rather than to the negative return. In order to handle this problem, rectifier discs may be inserted in the circuit so as to prevent the reversal of the current flow. 

Figure 5.46 Stray current corrosion caused by DC transit system. (From Peabody, A.W. (1967). Control of Pipeline Corrosion, NACE. Reproduced by kind permission of NACE, Int., USA)...

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. 

An electric railway or tramway system with an adjacent buried pipeline or cable which may cross the running rails at intervals is illustrated in Fig. 10.35 in which the arrows indicate the general flow of stray currents when one vehicle is in service. Rapid variations of current and potentials will occur as the tram or train moves along the rails. Corrosion will occur at points near the sub-station or near negative feeders where the stray current leaves the buried structure to return to the negative busbar at the sub-station. 

Apart from corrosion due to differential aeration, corrosion of underground metal structures and pipelines may also arise from stray currents. How this comes about can be seen in the accompanying diagram (Fig. 12.32). The presence of a current-carrying cable in conducting soil results in stray currents passing through the soil. These stray currents may set up a potential difference between two portions of a pipeline, which then develops electron-source (cathodic) and -sink (anodic) areas. Thus, pipelines tend to corrode when they pass near electric lines. 

The corrosivity of soils also depends upon the oxidation-reduction potential as classified by Booth et al.15 The classification scheme of the corrosivity of soils is given in Table 4.4b. Macrogalvanic cells are formed in underground pipelines due to foreign structure the combination of new and old pipe dissimilar metals (stainless steel and carbon steel) differential aeration dissimilar soils and stray currents. All these lead to localized corrosion of underground pipelines. 

Cathodic protection has many problems and limitations apart from huge capital investment and maintenance costs. One of the more serious problems associated with cathodic protection is the possible effects of stray currents on the corrosion of adjacent metal structures. For example, a CP system that is efficiently protecting pipeline A might increase the corrosion of neighboring pipeline B (Fig. 10a). 

Fig. 10 Stray current effects in underground pipelines (a) stray currents cause corrosion in neighboring pipelines (b) redesign minimize stray current effects [39].

Stray current flowing along a pipeline very often will not cause damage inside the pipe, because of the high conductivity of the electric path compared with the electrolytic path. The damage occurs when the current reenters the electrolyte and will be localized on the outside surface of the metal. If the pipe has insulated joints and the stray current enters the internal fluid, localized corrosion on the internal side of the pipe will occur. The best solution to avoid this mode of corrosion is the electrical... 

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