Presence of Stray Currents

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 

Industry/Appllcatlon Potential Problem Sites for MIC Organisms Responsible 

Pipelines—oil, gas, water, wastewater Internal corrosion primarily at the bottom (6 00) position Dead ends and stagnant areas Low points in long-distance pipes Waste pipes—internal corrosion at the liquid/air interface Buried pipelines—on the exterior of the pipe, especially in wet clay environments under disbonded coating Aerobic and anaerobic acid producers, SRB, manganese and iron-oxidizing bacteria, sulfur-oxidizing bacteria 

Fire protection systems Dead ends and stagnant areas Anaerobic bacteria, including SRB 

oil platforms, and other aquatic structures Just below the low-tide line splash zone SRB beneath barnacles, mussels, and other areas sequestered from oxygen 

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Application of this method or Eq. (3-25 ) in the presence of stray currents is conceivable but would be very prone to error. It is particularly valid for good coating. Potential measurement is then only significant if stray currents are absent for a period, e.g., when the source of the stray current is not operating. In other cases only local direct measurements with the help of probes or test measurements at critical points can be considered. The potential test probes described in Section 3.3.3.2 have proved true in this respect. 

Anodic polarization can occur in the presence of stray currents. Oxygen is evolved on the passive steel according to ... 

Above the passivity range, that is for potentials above about +600 mV, the steel is brought to conditions known as transpassivity. oxygen may be produced on its surface according to the anodic reaction of oxygen evolution 2H2O —> O2 + 4H + 4e, which produces acidity. Steel reaches these conditions only in the presence of an external polarization (for example in the presence of stray currents). Since the anodic reaction is oxygen evolution, dissolution of iron and consequent corrosion of the steel does not take place (i. e. the passive film is not destroyed). Nevertheless, if these conditions persist until the quantity of acidity produced is sufficient to neutralize the aLkaHnity in the concrete in contact with the steel, the passive film will be destroyed and corrosion will initiate. This aspect wiU be dealt with in Chapter 9. 

Still in the experimental phase of application in concrete [11]. Other measurements that can help in detecting the presence of stray current in reinforced concrete are based on the potential difference present between different parts of the structure, due to the ohmic drop produced by the stray current... 

B. Bazzoni, L Lazzari, Potential monitoring in presence of stray currents . Corrosion Management, 1995, 6, 9 12. 

L. Bertolini, P. Pedeferri, B. Bazzoni, L. Lazzari, Measurements of ohmic drop free potential in the presence of stray current , NACE International Corrosion 98 Conference, paper 98560, March 1998. 

Potential Measurements in the Presence of Stray currents Stray currents flow in the electrolyte from external... 

Potential measurements of a given object are generally used for the evaluation of stray current interaction. A frequently changing value of this potential with time points to the presence of stray currents. Their flow can also be detected by measurement of the voltage drop on the ground surface. Stray currents are usually random in character, and that is why for measurement stochastic signal principles should be used. 

Determination of the corrosion hazard of industrial structures in the field of stray current interaction, due to difficulties, is the subject of a relatively small number of publications, for example, Bazzoni and Lazarri (1996). The correlation method described by Juchniewicz and Sokdlski (1985, 1986) allows, on the basis of field measurements, determination of the potential hazard to structures caused by electrolytic corrosion and the effectiveness of electrochemical protection from stay currents. The principle is based on simultaneous field measurements of two quantities connected by the presence of stray currents, and analysis of the spectrum of their mutual correlation. During field measurements, the following quantities are most frequently measured ... 

Investigating the presence of stray currents to prevent or explain corrosion problems is not a new field in corrosion engineering. In fact, as mentioned in App. A, such activities were carried out by probably the first corrosion engineers in North America when the American Committee on Electrolysis was established at the turn of the twentieth century to combat the serious effects of railcar stray currents to underground metal structures (Figs. 7.4 and 7.5). 

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. 

Stray electrical currents can also influence the readings. These were discussed in Section 2.3. the effect of stray current on half cell potentials can be used as a diagnostic tool where stray current corrosion is suspected in the presence of DC fields. If a half cell is mounted on or in the concrete linked to a logging voltmeter, then any fluctuations in potentials may be linked to t he operation of nearby DC equipment, especially if the equipment can be deliberately turned on and off and the potentials fluctuate accordingly. 

Occasional presence of stray electrical currents from external... 

The design of CP systems lies in the domain of experienced speciabsts. Only the basic steps involved in designing a sacrificial anode system are outlined. Prior to any detailed design work a number of fundamental factors such as the protection criteria, the type and integrity of the coating system, the risk of stray current corrosion, and the presence of neighboring structures that could be affected by the CP system have to be defined. 

Both chemical and physical environmental variables affect corrosion. The chemical influence of the environment depends on its composition and the presence of impurities such as heavy metal ions. Physical variables are tenq>erature, degree of movement and agitation, and pressure. Another physical variable that can cause corrosion of aluminum is the presence of stray electrical currents (alternating or direct). 

Because the corrosion resistance of lead and lead alloys is associated with the formation of the protective corrosion film, removal of the film in any way causes rapid attack. Thus the velocity of a solution passing over a surface can lead to significantly increased attack, particularly if the solution contains suspended particulate material. Lead is also attacked rapidly in the presence of high velocity deionised water. The lack of dissolved minerals in such water prevents the formation of an insoluble protective film. In most solutions, lead and lead alloys are resistant to galvanic corrosion because of the formation of a nonconductive corrosion film. In contact with more noble metals, however, lead can undergo galvanic attack which is accelerated by stray electrical currents. 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

In Fig. II. 1.12, cyclic voltammograms incorporating both IR drop and capacitance effects are shown. Effects for the ideal case of a potential independent working electrode capacitance give rise to an additional non-Faradaic current (Fig. II. 1.12b) that has the effect of adding a current, /capacitance = Cw x v, to both the forward and backward Faradaic current responses. Tbe capacitance, Cw, is composed of several components, e.g. double layer, diffuse layer, and stray capacitance, with the latter becoming relatively more important for small electrodes [61]. On the other hand, the presence of uncompensated resistance causes a deviation of the applied potential from the ideal value by the term R x /, where R denotes the uncompensated resistance and I the current. In Fig. II. 1.12, the shift of the peak potential, and indeed the entire curve due to the resistance, can clearly be seen. If the value of Ru is known (or can be estimated from the shape of the electrochemically reversible... 

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