Stray current cells

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 corrosion effects of stray current can be easily demonstrated with a simple laboratory setup such as shown in Fig. 7.30( j). After only a few minutes of passing a small DC current in the cell containing a dilute saline solution, the formation of hydrogen bubbles is readily visible on the steel nail connected to the negative post of the DC power supply [Fig. 7.30(b)], while the nail connected to the positive post shows signs of rapid corrosion a few minutes later [Fig. 7.30(c)]. 

Fioure 7.30 Experiments to illustrate the effects of stray current showing (a) the experimental setup containing a DC power supply plus two steel nails partially immersed in a dilute saline solution and connected to the positive (left) and negative (right) posts of the DC supply the effects of imposing a DC current on the nails (b) after five and (c) ten minutes. 

Impressed current cells in which the soil is the electrolyte are of two kinds accidental and deliberate. An example of a deliberate cell is an impressed current system which supplies cathodic protection to increase the life of a structure by forcing an anodic current to anodes in a remote location. Accidental systems, however, may exist under a variety of circumstances. Any direct current flowing in the soil from any source whatsoever can, if it finds a pipeline or other metal object in its path, collect on the pipe in one area and discharge from it in another. The area where it collects becomes a cathode while the area where it discharges is an anode, and thus corrosion occurs. 

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Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Ohmic voltage drops of the protection current were exclusively involved in the processes for 7/ -free potential measurements described above. Besides this, other foreign currents can cause potential drops and falsify the potential measurement (e.g., cell currents, equalizing currents and stray currents). 

Enhancement of Anodic Corrosion by Cell Formation or Stray Currents from dc Installations... 

Corrosion by Anodic Interference (Cell Formation, Stray 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. 

Isolated lithium near the anode becomes a local cell because of stray current. As the stray current is high when the cell discharge current is high, lithium recombination occurs easily at a high discharge current [46],... 

A titanium system, properly designed with stray current dumpers (Fig. 23.8), will perform the necessary tasks of the cell header and is comparatively easy to repair in... 

In order to handle hot chlorine gas from the cells a header system has been developed (Fig. 23.16) whereby stray current dumpers take care of the brine condensates that would otherwise result in damage. 

The conventional method for determining the cell constant of a conductance cell involves the use of solutions of known specific resistance. The a-queous KC1 solutions of Jones and Bradshaw 32) are the currently accepted standards. These workers carefully measured three solutions of given weight concentrations corresponding to molarities of about 1,0.1 and 0.01. There are two disadvantages to this approach. First, a solution of an exactly specified concentration must be prepared. Second, it does not permit measurement of the cell constant over a range of concentrations in order to test for stray current leakages which would cause systematic variations in the calculated constant. 

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. 

Thus, if dissimilar pipes are butt-welded with the electrolyte flowing through them, the most severe corrosion will occur adjacent to the weld on the active metal. The current of the galvanic cell takes the path of least resistance and this affects corrosion in that current does not readily flow around corners. In soft water, the critical distance between copper and iron may be 5 mm in seawater it may be several decimeters. The critical distance is greater the larger the potential difference between anode and cathode. Then, the geometry of the circuit affects galvanic corrosion and this is observed in the case of stray current corrosion.7 (Baboian)5... 

Stray current problems and the general appraisal of a CP system often require knowledge of the potential variations in the electrolyte. To do this, either the potential between the two half-cells placed in the electrolyte is measured or the potential of the structure is measured relative to the same half-cell placed in the electrolyte. When measurements are made between half-cells, the meter must have a high resistance. Alternatively, a bridge system can be used, particularly in liquid electrolytes, in which the half-cell is placed in contact with two different areas of structure. 

A trial-and-error method can be used to determine the correct bond resistance and current for a solution to static stray current problem. A reference cell is placed at the point of maximum stray current exchange to monitor the potential-to-soil of the interfered structure. With the current source operating, a series of bonds are placed between the two pipelines. When the potential-to-soil with the current source operating and the bond installed equals the potential-to-soil with the current source deactivated (with the bond disconnected), the correct bond resistance is determined. The current flow through the bond and the resistance of the bond can be measured. 

This chapter has discussed the mechanism of what happens at the steel surface. The chemical reactions, formation of oxides, pitting, stray currents, bacterial corrosion, anodes, cathodes and reference electrode potentials (half cells) have been reviewed. A more detailed account of the electrochemistry of corrosion and corrosion of steel in concrete is given in Appendix B. Chapter 3 will discuss the processes that lead to the corrosion and the consequences in terms of damage to structures. We will then move on to the measurement of the problem and how to deal with it. 

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... 

Electrolytic cells have historically been mounted off the ground at full basement height. Some of the arguments for elevating the cells include the need to electrically isolate cells from spills in the basement, to protect workers from stray current, to adjust the level of the cells, to inspect for leaks and perform repairs, and to limit pumping costs by employing gravity flow. With the advent of polymer concrete cells and of synthetic cell liners, the frequency of leaks and the need for electrical isolation have been drastically reduced. 

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. 

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