Stray-current corrosion sources

Stray-current Corrosion corrosion caused by stray currents flowing from another source of e.m.f. (usually d.c.). 

At points where the current enters the structure, the site will become cathodic in nature because of changes in potential, while the area where the current leaves the metal will become anodic. Electric railways, cathodic protection, electrical welding machines, and grounded DC electrical sources are subject to stray current corrosion. (Craig)5... 

In sizing the mitigation bond, a trial-and-error solution may be possible in relatively simple cases in which a single source of stray current is involved. The size of the resistance bond can be determined by installing temporary variable resistances and by determining when stray current corrosion has been mitigated. The procedure is similar to that described for static stray currents, except that a mitigation curve, such as is shown in Fig. 17 must be obtained. 

Stray Current Corrosion Corrosion can be accelerated through ground currents from dc sources. Electrified railroads, mining operations, and other similar industries that utilize large amounts of dc currents sometimes allow a significant portion of current to use a ground path return to their power sources. These currents... 

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. 

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. 

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 causes and common means of detecting and mitigating stray current interference effects that result from direct current sources are reviewed in this Sect. [1, 40-44]. Alternating current, while creating a potential safety ha2ard, may contribute to corrosion of ferrous structure [42]. Extensive research is in progress in this regard. 

Generally there is far less evidence of stray current induced corrosion from external sources into atmospherically exposed reinforced concrete structures compared to buried structures generally and far less evidence of stray current induced corrosion in buried or submerged concrete compared to buried or submerged steel, as concrete has much higher electrical resistance. However, it can occur and can be tested for in the same manner as for other metalwork. There is discussion of interference testing for stray currents in NACE RP0169-2002 and in BSEN 13509 2000. 

Intentional Anodes and Cathodic Protection. If a bond from if to C in Fig. 12.1 is not feasible, an intentional anode of scrap iron may be buried in the direction of the rails and attached by a copper conductor to point B. Stray currents then cause corrosion only of the intentional anode, which is easily replaced at low cost. If a source of dc current is inserted between the intentional anode and the pipe such that current flows in the soil in a direction opposite to that of the stray current, the arrangement is equivalent to cathodically protecting the pipe. Cathodic protection is installed whenever the intentional anode is not sufficient to overcome all corrosion caused by stray currents. 

Stray currents are electrical currents on a pipeline caused by external sources. Examples of stray currents include d-c or a-c powered transit systems, electric welding operations, currents from mining operations, and interference from cathodic protection systems on other structures. Stray current can cause corrosion where it discharges from the surface of the pipeline. For example, one ampere of d-c stray current causes the loss of 20 lb (9.1 kg) of iron per yesir. A-C stray current has about 1% of the effect of d-c strays on iron pip>e, but a-c stra3rs have a detrimental effect on aluminum [25]. Note that a-c stray current is not necessarily the same as induced a-c sometimes found on pipelines near a-c power transmission hnes. 

The control of corrosion caused by stray current involves either reduction of the amount of current at the source or safe removal of the current from the corroding structure. The removal of the current from the structure involves the creation of a metallic path for the current to flow, rather than the electrolytic (soil) path. In order to create this metallic path, first the exact point of current discharge must be located through tests, and the quantity of current involved must be determined. The metallic path is provided after determining the quemtity of current that must be drained, tmd the characteristics that the drainage circuit must have. The resistance of the wire, its ampacity, and the exact point to which the current must be drained are examples of the chtuacteristics that must be determined... 

Where stray currents from a man-made source of direct current are a potential problem, metallically bonding the structure of concern to the source of the dc is often used to mitigate the corrosion that would otherwise occur. This is a common method of control where pipelines are subject to the adverse effects from stray currents generated by light rail transit systems, d-c welding machines, and impressed current cathodic protection systems installed on other nearby structures. 

After the plant is in place, local corrosion problems have to be taken into account. These corrosion problems do not exist everywhere, and built-in corrosion control would be prohibitively expensive. The primary corrosion control at this stage is protection against the effects of stray DC currents. Preventing or minimizing the pickup of stray ciurent, draining the stray currents back to their source through low resistance bonds, and applying cathodic protection, can accomplish this. Other corrosion problems of the in-place telecommunication cable plant are discussed below. 

When corrosion surveys have established the location and source of corrosion problems on a telecommunication cable plant, the next step is to establish mitigation measures. These measures can include placement of insulating joints to limit current pickup by the cable shield in cable entrance facilities (CEFs), some building entrances, vertical riser cables on telephone poles, and, under special conditions, in manholes. Most of these measures consist of placing low-resistance drainage bonds to return stray DC currents to their source without causing corrosion. If the stray current... 

Service life can also be affected by galvanic contact with a dissimilar metal. The less resistant material tends to be dissolved and may experience general corrosion, pitting/crevice corrosion, or SCC. Hydrogen may be liberated at the more resistant metal, making hydrogen embrittlement an issue if the material is susceptible. Stray currents, e.g., from a DC power source, may have the same effect as dissimilar metal contact. 

Currents flowing into electrolytic environments (ground, water) from inadequately insulated working electric circuits are called stray currents. From the corrosion point of view, industrial equipment powered by dc currents is the most hazardous source of stray currents for metal underground structures. Alternate stray currents of 50 Hz frequency pose a much smaller hazard. Corrosion losses caused by them are estimated at several percent in relation to the corrosion loss caused by dc stray currents of the same magnitude (Baeckmann et al., 1997). 

Stray currents are a real corrosion hazard to metal underground infrastructures, causing significant corrosion losses. Thus the electrolytic corrosion hazard should be predicted at the structure design phase, and the route should be properly chosen (far from stray current sources). The problem of electrolytic corrosion also can be limited by ex-... 

The drainage operation is based on controlled removal of stray currents from the underground structure to the source of their formation. The harmful phenomenon of currents outflow from the external surface of structures to the ground is eliminated in this way. Choice and localization of drainage devices should be preceded by determination of the electrolytic corrosion hazard (on the basis of specialist measurements) and analysis of the situation in the field. More information on drainage is given by Chaker and Lindemuth (1994) and Pignatelli (1985). 

The corrosion resulting from stray currents coming from external sources is similar to that from galvanic cells that generate their own current. However, the amplitude of stray currents may be much... 

In all the cells described so far, the source of the energy which makes a cell active has been within the cell. However, in stray current cells the energy comes from an electrical current external to the corrosion site per se. The source of energy for such cells may be a distant generator, a direct-current transmission line, a cathodic protection rectifier on some other line, a street car system, or an electric railway (Fig. 7.29). 

The stray current sources described in Chap. 7 can result in very rapid corrosion which is usually much more severe than the corrosion caused by other environmental factors (Fig. 13.34). Another type of stray current which is variable in natme may be observed during periods of "magnetic storm" activity. Long structures such as pipelines or cables are most apt to be affected. During magnetic storms, the intensity of the earth s magnetic field can vary. When these variations occur, potentials are induced in the pipe or cable in much the same manner as potentials are induced in an electric generator. 

It has been demonstrated that BE modeling can accurately predict experimental results. BE methods also can be used to evaluate the effect of a single parameter on system performance. In this way basic understanding of electrochemical corrosion and parameter interactions can be obtained. Several parametric studies have, for example, been published on damage levels in the propeller area, seawater conductivity, and paint resistance effects, as well as on the influence of stray current source on system performance [18]. 

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