STRAY CURRENT TECHNIQUE

Additionally, stray-current corrosion is common on seagoing vessels, such as boats. For example, powered battery chargers generate dc stray currents, which may flow indefinitely if proper precautions are ignored or overlooked. This type of corrosion can manifest as pitting in confined ares, metal surface discoloration, rust formation on steel parts or weakening of batteries. 

The potential attenuation (fluctuation of potential) in an impressed-current system has a detrimental effect on conx)sion prevention of a metallic structure containing defects. For instance, a coated pipeline buried in soil is subjected to experience current leakage due to coating defects known as holidays. 

In this case, the following concrete assurr rtions are considered 

the potential along the y-direction (perpendicular to the disk flat surface) is defined as 

In pursuing the mathematical details peculiar to Uhlig s model, when the current is defined as Jx = Ai, while the cross sectional area of the pipe is A = 2Trrx, the current gradient in the x-direction is 

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Let s use Fontana s cases [5] for protecting a pipeline against corrosion near a buried sted tank using the stray-current technique. Firstly, Figure 8.8a shows the detrimental effect of stray current Geakage current), which has the following path (dashed lines) in the cathodic circuit... 

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. 

J. A. Gonzales, The determination of the corrosion rate of steel embedded in concrete by the polarization and AC impedance methods , in Corrosion Effect of Stray Currents and the Techniques for Evaluating Corrosion of Rebars in Concrete, ASTM STP 906, V. Chaker (Ed.), Philadelphia, 43-63, 1986. 

External corrosion of water systems may be caused by general corrosion, stray current corrosion MIC, and/or galvanic corrosion. Corrosion mitigation techniques include the application of protective coatings, wrapping pipe in a plastic cover, and the application of CP. The areas of major external corrosion impact are generally those where localized attack may take place, such as in the proximity of other systems like galvanic corrosion or in areas where stray currents may occur. 

The setup details of the two-sided EMM technique are schematically represented in Fig. 4.6. The job sample is held vertically in the machining chamber. The job is made the anode of the electrolytic cell. The tool consists of two cathode assemblies mounted over the vertically held job. Highly localized dissolution of metal from the unmasked region of the two sides of the work sample is achieved by scanning the tool assembly over the work sample. The electrolyte flows through the tool assembly and passes across the surface between the cathode tool and masked workpiece anode. An extremely small lEG is maintained between the work sample and cathode, which provides uniform localized metal removal due to stable current flow distribution with negligible stray current effect. The cathode tool... 

This measuring technique is applied when there are relatively high IR values due to cell currents with intermeshed objects or contact with steel-concrete structures, as well as the influence of stray currents, which cannot be switched off. The principle corresponds to the information in Fig. 2-3 on the use of measuring probes. 

One technique that has heen applied for many years uses two sets of measurements where one of the electrodes is positioned over the pipeline and the second electrode is placed several meters to the side. The data from this approach provide a comparison of the local potential to the more distant potential made to the side of the pipeline, as illustrated in Fig. 4. Regions where the "Over Pipe potential are more positive than the 25 ft From Pipe potential are cathodic to other sections of the pipe Une. Potential effects of stray currents, galvanic currents, md cathodic protection interference are identified from this information [2i]. 

Much of the modern direct indication equipment measures admittance or impedance by calculating the complex ratio of complex current and voltage, using sophisticated computing techniques. Apparatus of this kind is often equipped to accept inputs through y. 4. or 5 terminals, so that errors due to series and, or parallel stray impedances can be eliminated. 

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