Stray current induced corrosion

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder 

Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30800-8 

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The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

P. Pedeferri, "Stray-current induced corrosion in reinforced-concrete structures , in Progress in the Under- [7]... 

P. Pedeferri, Stray Current Induced Corrosion in Reinforced Concrete Structures Resistance of Rebars in Carbon, galvanized and Stainless Steels (in Italian), La Metallurgia (10 ... 

Since the maximum voltage that can be generated with zinc anodes is extremely unlikely to generate hydrogen embritdement, galvanic systems have been used to protect prestressed concrete members. They are also used on fusion bonded epoxy coated steel reinforced piles as the effects of electrical discontinuity between bars is unlikely to lead to significant stray current induced corrosion as the currents and potentials are low. 

Figure 7.19 Illustration of how an isolated piece of steel can be subjected to stray current induced corrosion, as the current enters at the end nearest the anode cathodically protecting it, and leaves at the end nearest the cathodically protected steel, causing corrosion.

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. 

There is also a NACE Standard Recommended Practice RP0187. The 1987 edition has been re-issued in 2006 while undergoing revision at the time of writing. It mentions stray current induced corrosion and corrosion monitoring systems which the other standards do not and has short paragraphs covering the same topics as ACT 222.3R-03. 

Stray current case study—dc rail transit systems. Stray current-induced corrosion damage has been associated with North American dc rail... 

We have seen that stray current can hardly induce corrosion on passive steel in non-carbonated and chloride-free concrete. However, the potential adverse effects of stray current on concrete structures may become increasingly important with the increased use of underground concrete construction. Stray-current effects are rarely recognised as such. The importance increases further due to the increase of the required service lives (i. e. 100 y or more). 

All of the correlations seen above refer to situations of steel reinforcement in the free corrosion condition, that is, in the absence of factors that modify the potential of the system. They are in particular not appHcable to structures in concrete containing corrosion inhibitors galvanized reinforcement (on stainless steel it is possible in the same way) structures subjected to electrical fields produced by stray current that induce current exchange between reinforcement and concrete (this case is dealt with in Section 9.4). 

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. 

Stray electric currents are those that follow paths other than the intended circuit, or they may be any extraneous currents in the earth. If currents of this kind enter a metal structure, they cause corrosion at areas where the currents leave to enter the soil or water. Usually, natural earth currents are not important from a corrosion standpoint, either because their magnitude is small or because their duration is short. Under some conditions, pipelines can incur considerable corrosion damage as a result of telluric currents—that is, currents induced in the steel pipeline by changes in the geomagnetic field of the earth [1]. 

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 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. 

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.

Stray currents are currents flowing in the electrolyte from external sources, not directly associated with the cathodic protection system. Any metallic structure, for example, a pipeline, buried in soil represents a low-resistance current path and is therefore fundamentally vulnerable to the effects of stray currents. Stray current tends to enter a buried structure in a certain location and leave it in another. It is where the current leaves the structure that severe corrosion can be expected. Corrosion damage induced by stray current effects has also been referred to as electrolysis or interference. For the study and understanding of stray current effects it is important to bear in mind that current flow in a system will not only be restricted to the lowest-resistance path but will be distributed between paths of varying resistance, as predicted by elementary circuit theory. Naturally, the current levels will tend to be highest in the paths of least resistance. 

Stray-current corrosion, or stray-current electrolysis, is different fixim natural corrosion because it is caused by an externally induced electrical current (alternating, ac, or direct current, dc) and is basically independent of such environmental factors as oxygen concentration or pH. Environmental factors can enhance other corrosion mechanisms invcdved in the total corrosion process, but the stray-current corrosion portion of the mechanism is unaffected. 

Corrosion may also occur without the presence of different electrode potentials if there is an applied electrical current due to the pickup of stray electrical currents from electrical conductors and equipment or the incidence of induced electrical currents. 

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