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Detection, Identification and Estimation of Stray Currents
4 Detection, Identification and Estimation of Stray Currents CHAPTER OUTLINE 4.1 Stray Current Corrosion Identification Without Resort to Special Measurement Instruments.................................................................................................................................... 85 4.2 Detection and Estimation of Stray Currents by Measurements of Metal - Soil Potential and of Voltage Drop on Underground Structures ................................................... 89 4.2.1 Instruments and Techniques of Potential Measurements .............................................. 89 4.2.2 Detection of Stray Currents Presence on Unprotected Underground Structures ........ 90 4.2.3 Measurements of Soil Resistivity ....................................................................................... 93 4.2.4 Estimation of Static Stray Current Values on Unprotected Underground Pipelines by Potential Difference Measurements ............................................................ 93 4.2.5 Direct Measurements of Static Stray Current Along Unprotected Pipelines ................ 94 4.2.6 Finding Dynamic Stray Current Direction on Unprotected Pipelines by Repeated Measurements.................................................................................................... 95 4.2.7 Detection of Stray Currents and Their Sources on Protected Pipelines ........................ 97 4.2.8 Detection of Well Casing Stray Currents ........................................................................ 100 4.2.9 Detection of Telluric Currents.......................................................................................... 103 References........................................................................................................................................... 104
Equipment and procedures of detection and identification of stray currents on coated and uncoated underground pipelines and other metallic structures depend to some extent on the character of stray current (static or dynamic) and on the protection state of the structure (unprotected or cathodically protected). Therefore, along with the character of the detected stray current, the state of the underground structure will be further indicated: unprotected and, for short, protected (implying that the structure is cathodically protected).
4.1 Stray Current Corrosion Identification Without Resort to Special Measurement Instruments It was already emphasized that corrosion rate of metals under the attack of stray currents can be by orders of magnitude higher than the rates of any other kinds of underground Electrocorrosion and Protection of Metals. https://doi.org/10.1016/B978-0-444-64021-5.00004-0 Copyright © 2019 Elsevier B.V. All rights reserved.
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86 ELECTROCORROSION AND PROTECTION OF METALS
or underwater corrosion of metals. So, corrosion damage of a metallic structure resulted from an abnormal metal corrosion rate can be taken as the first evidence of stray current presence. In fact, it was proposed to apply this “feature” of stray currents corrosion for detection stray current attack of underground structures with the help of special metallic probes [1]. The probes, in the form of wire or of thin film deposited on an inert material, were made of the same metal as the structure. Measurement sensitivity of the probes was 0.01 mm/y. The corrosion rate was determined by measuring electric resistance change of the probes. It was proved in preliminary tests that the corrosion rate of the structure metal and of the probe metal was similar. The cross-section of the corroding probes diminished, and consequently, their electric resistance increased during the test. The test was carried out at an underground structure disposed in the field of stray current attack, and the probes were placed along the structure, close to it. Three probes were tested along the structure, at different distances from the stray current source. The results of the test, where R/Ro is the ratio of the changing electric resistance to its initial value, are given in Figure 4-1. Corrosion rate measured on the probe that was closest to the stray current source attained 8.38 mm/y, whereas at the most remote probe it was equal only to 0.96 mm/y, by an order of magnitude lower. Thus, this method makes possible not only assessing the corrosion rate of the structure, but also finding stray current direction and the location of stray current source. Corrosion of carbon steel structures which is caused by stray current attack is often classified as general corrosion accelerated at the locations of the stray currents attack [2]. Perhaps, this is explained by an active state of iron in such neutral media as soil and water. In fact, at the anodic polarization of iron in these media its dissolution (electrolysis) may have not local, but a rather regular character, in accordance with the Faraday low, as it was already mentioned above. 1.0
0.8
R/R0
0.6
0.4
0.2
8.38 mm/y
0.0
A 0
1.40 mm/y
0.96 mm/y
B 1
2
3 Time (h)
C 4
5
FIGURE 4-1 Response of thin film probes at tree distances [1].
6
Chapter 4 Detection, Identification and Estimation of Stray Currents
87
Corrosion damage may be localized close to the insulation flanges of a pipeline, at one side of the joint. Such a character of the damage localization points to stray current attack. Moreover, it indicates the stray current direction (Figure 4-2): stray current of anodic direction exerts corrosion damages on one side of the insulating joints (areas A1 and A2), whereas another side near the joint (areas C1 and C2), where the current has cathodic direction, does not undergo corrosion damage. When conductive liquids are flowing inside the pipeline, similar localization of the corrosion damages may occur near the insulating flanges also at the internal surface of the pipes and flanges. Electrocorrosion damage of underground structures often occurs in the form of pitting that can be related to irregular state of the metal surface. Tendency to corrosion damage in the form of pitting is most typical for coated structures, since in this case the current concentrates at small defects of the coating. An example of such kind of local corrosion is shown in Figure 4-3A and B [4]. It is pertinent to note in this connection that a single pit was the origin of the notorious Guadalajara catastrophe in April 1992 that was caused by stray current attack of unprotected ductile iron water pipeline [5]. This pipeline was attacked by stray current coming from the cathodically protected gasoline pipeline disposed under the water Stray current direction A1
Insulating joints C1
Anodic zone
A2
C2
Cathodic zone
FIGURE 4-2 Localization of stray current corrosion at the anodic areas of insulating joints.
(A)
Stray Current Corrosion
Underground Corrosion
(B)
FIGURE 4-3 Character of stray current corrosion on a coated pipe: (A) a pipe section with two pits and (B) magnified photograph of the left pit [4]. Courtesy of Dr. Mehrooz Zamanzadeh.
88 ELECTROCORROSION AND PROTECTION OF METALS
pipeline, close to it. Water flow through the mentioned pit of the water pipeline led to corrosion damage of the gasoline pipeline and to gasoline sewer explosion. Result of this catastrophe: about 1000 dead, 800 injured, and damage of V76 million. The pits resulted from stray current attack are often cone shaped with a cone vertex directed inside the metal. However, the local damages caused by stray current may have quite other shapes; moreover, similar corrosion damages may be related to other causes, such as bio-corrosion. Therefore, the shapes of the corrosion pits do not point unambiguously to the corrosion origin. Sometimes the character and the rate of a selective corrosion point to the influence of stray currents. For example, when graphitization of cast iron in water mains occurs in a relatively short period of time, it is most probably related to stray current effect [2,6]. This effect is explained by selective dissolution of ferritic phase along the grain boundaries of cast iron matrix at the stray current attack [2]. Another kind of accelerated selective corrosion, plug-type dezincification of brass CuZn40Pb2Sn in seawater was considered in [7]. A through-hull fitting of this material was mounted at the bilge of fishing vessel Random Harvest. The fitting failed after 16 months period of work. The dezincification rate of the fitting was 1.27 mm/y, about twice as much as dezincification rate of another, quite similar fitting also installed at the vessel bilge. Analysis of the fitting damage and its location leads to the conclusion that the most probable cause of such a high dezincification rate was related to the attack of DC stray current from dump wiring of vessel batteries. This corrosion damage led to the vessel flooding. As discussed in Chapter 2, corrosion of metals which are in neutral media in a passive state, such as stainless steel and aluminum, almost always has a form of pitting which occurs as a result of different kinds of corrosion factors, including stray current attack. However, when elevated rate of pitting penetration into the metal occurs, this can point to the influence of stray current. Abnormal consumption of aluminum sacrificial anodes for cathodic protection of offshore platforms may indicate stray current attack. Although this indication may be to some extent needless: the presence of the welding generator out of the platform unambiguously pinpoints the presence of stray currents [8], as it was noted in the previous chapter. Abnormal corrosion of zinc sacrificial anodes for protection of boats and brightness of the anode surface is also an indication of stray current corrosion attack [9]. The corroded surface of bronze propellers and other metallic parts of the boats also becomes sometimes shining. The brightness of the anodes and of metallic parts is explained by a high intensiveness of the metal dissolution (electrolysis) under the influence of external anodic current. It is probable that the anodic process of metal dissolution is followed by acidification of the water layer adjacent to metal surface which prevents corrosion products deposition on this surface. The absence or low quantities of corrosion products at the damaged areas of a metallic structure is considered in Ref. [10] as an indication of stray current attack. Anodic current dissolves the grain boundaries of lead. This specific influence makes the anodic current responsible for the intercrystalline corrosion of lead sheaths of
Chapter 4 Detection, Identification and Estimation of Stray Currents
89
telephone cable in soil [11]. Moreover, ions of chlorine tend to move toward the anodic areas, and their concentration at these areas is higher than the average chloride concentration in the soil. So, intercrystalline corrosion of lead and more than 5% concentration of lead chloride near the corroded areas (when general salinity of the soil is low) may furnish a proof of anodic stray current attack of lead sheaths [11]. Thus, stray current attack can be often revealed at the initial inspection stage of the damaged underground or underwater metallic structures. The results of such inspection are most trustworthy when they are supported by the data on the presence of some potential stray current sources disposed nearby the damaged structure. This common information may significantly facilitate and simplify the choice of ways of stray current mitigation and elaboration methods for protection from electrocorrosion. However, in most cases special methods and appropriate devices are necessary for reliable detection and identification of stray currents and their location, direction and distribution.
4.2 Detection and Estimation of Stray Currents by Measurements of Metal - Soil Potential and of Voltage Drop on Underground Structures 4.2.1
Instruments and Techniques of Potential Measurements
Copperecopper sulfate reference electrode (CSE) is commonly accepted for metal potential measurements at underground pipelines, cables and other metallic structures. The body of the electrode (Figure 4-4) is made of transparent polymer material or of glass. A plug of porous material serves as a body bottom. The electrode is filled with saturated solution of copper sulfate. Surplus crystals of CuSO4 are added to the solution To voltmeter
Copper rod Insulating plug Transparent body
Crystals of copper sulfate
Copper sulfate saturated solution Porous plug
FIGURE 4-4 Copperecopper sulfate reference electrode (CSE).
90 ELECTROCORROSION AND PROTECTION OF METALS
Pipe line
+
V
-
Voltmeter
Reference electrode
FIGURE 4-5 Scheme of potential measurement on underground pipeline.
for keeping saturation concentration of copper sulfate. A rod made of high purity electrolytic copper that passes through the insulating plug is dipped into the copper sulfate solution. The pores of the bottom plug filled with the solution play the part of an electrolytic bridge when the bottom is pressed against the ground in the process of measurements. The potential of CSE at temperature of 25 C is equal to 320 mV with respect to standard hydrogen electrode; the deviations of measurements do not surpass 5 mV. A scheme of potential measurement on underground pipeline is shown in Figure 4-5. The reference electrode is installed directly above the pipeline and recessed into the earth by several centimeters. Voltmeter that is used for the measurements must have a high input resistance, surpassing the total resistance of the reference electrode and other components of the measurement circuit by several orders of magnitude. Input resistance of modern digital voltmeters for such measurements is higher than 20 MU that makes possible not to consider the circuit resistance in the process of measurements. Most of the modern pH-meters are provided with an option of voltage measurements, so these devices can also be used as voltmeters with high input resistance. Negative terminal of voltmeter is usually connected to the reference electrode, and positive terminal is connected to pipeline offset or to some other accessible part of the underground pipeline.
4.2.2
Detection of Stray Currents Presence on Unprotected Underground Structures
The potential value of carbon steel in such electrolytes as soil and water depends on many factors, including the composition and the state of the metal surface, electrolyte composition, air access etc. This is the cause of carbon steel potential variations in these media over a wide range, from 0.2 to 0.8 V (CSE), as it is shown in Table 1-7. Corrosion factors accelerating the corrosion process of carbon steel (differential aeration, crevices, bacteria etc.) may even more enlarge the range of potential variations. So, individual values of the measured potentials do not allow judging the presence of stray currents. It was noted in Chapter 3 that stray currents coming to underground pipeline from such sources as DC railway systems and mining operations have a dynamic character that is manifested in their fluctuations. Fluctuations of stray currents lead to corresponding fluctuations of potentials. Maximal amplitudes values of the potential in both, negative and positive sides can by an order of magnitude surpass the potential measured
Chapter 4 Detection, Identification and Estimation of Stray Currents
91
at the absence of stray current attack [12,13]. Measured potential amplitudes in the earth depend on the distance of the reference electrode from the stray current source [12]. The more remote is the reference electrode from the current source, the lower are the measured potential amplitudes. Results of simultaneous measurements of the potential fluctuations when copper/ copper sulfate reference electrodes were located at a short and at a remote distance from the stray current source are shown in Figure 4-6 [12]. It is seen that the fluctuation amplitudes at a remote distance are by an order of magnitude lower than at a short distance, but the profiles of the fluctuations are similar. These potential fluctuations do not provide quantitative information on the stray current values, but they are evidence of the dynamic stray current presence in the soil regardless of the reference electrode distance from the current source. The areas where the amplitudes are the highest in the negative side may indicate that these are the areas of stray current pick up. The areas of maximal fluctuation amplitudes to the positive side may be suspected as most hazardous and require special inspection. However, it must be taken into account that these changes in the fluctuations may be related with other factors, such as soil resistivity etc.
(A) 10
(B)
8 6 4 2 0 –2 –4 –6 –8 –10 13:00
13:03
1 0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1 13:00
13:03
13:06
13:06
13:09
13:09
13:12
13:12
13:14
13:14
Time, h:min FIGURE 4-6 Earth gradients on an ungrounded embedded track system. The CSE reference electrodes were located at distances: (A) short (7.6 m) and (B) remote (53.3 m) from the stray current source [12].
92 ELECTROCORROSION AND PROTECTION OF METALS
The fluctuation may also indicate the sources of stray current. For example, attenuation of the fluctuation in the noon hours or in the night can indicate that the industrial machinery is the stray current origin [14]. It is possible often to reveal correlation between the electric cars schedule and the voltage fluctuations by regular registration of fluctuations 24 h a day. At the side where the current is picked up by the pipeline, it has predominantly cathodic direction and potential values attain maximal negative values. These values are diminishing along the underground structure and attain minimal negative or, in the considering range of the measured potentials, maximal positive values at the area of current flow from the metallic structure toward the substation. At this, anodic side the metallic structure undergoes the most severe corrosion attack. It was already noted in Chapter 3 that an alternative zone between the cathodic and anodic zones, where the fluctuating currents periodically change their direction, is also hazardous from the point of view of stray current corrosion attack. Detection of static stray current presence in the soil requires a great number of systematic potential measurements along the pipeline. The results of potential distribution may indicate the dangerous areas of the pipeline. Those are the areas of the most positive potential values. Presence in the soil of stray current and its direction can be detected by using a circuit that includes a voltmeter with high input resistance and two reference electrodes [13]. The scheme of the measurements is shown in Figure 4-7. Two similar reference electrodes are placed approximately 8 m apart, along a line which is perpendicular to the pipeline. If the potential of right electrode is positive with respect to the left one, the stray current in the soil is directed to the left, toward the pipeline. Data on stray current direction make possible the detection of cathodic and anodic areas on the pipeline. Different methods of potential measurements with two reference electrodes along underground pipelines are considered in [15]. Among them a method called side-drain measurements is described. In this method first electrode is placed directly above the pipeline and the second electrode is disposed at a right angle to the pipeline, at a distance surpassing the pipeline depth in about 2.5 times. The measurements are made on both sides of the pipeline. The results of these measurements provide information on the stray current direction and on the most dangerous areas on the pipeline. Voltmeter
-
V
+
Reference electrodes
Pipeline
Stray current direction
FIGURE 4-7 Detection of stray current presence and direction.
Chapter 4 Detection, Identification and Estimation of Stray Currents
93
However, these data still do not allow assessing the danger of the revealed stray currents which is determined by the stray current value. Data on soil resistivity at the area of stray current attack are necessary for assessing the values of stray currents.
4.2.3
Measurements of Soil Resistivity
Soil resistivity is one of the most important parameters determining the corrosion hazard to the underground metallic structures. Magnitudes and distribution character of stray currents in the soil highly depend on the soil resistivity value that may vary from tens to thousands Um. The most accepted field measurements of the soil resistivity are carried out by Wenner four electrode method in accordance with Standard ASTM G57e06 [16]. Schemes of soil resistivity measurements based on this method are given in many literature sources [17,18]. According to these schemes (Figure 4-8) two reference electrodes and two metal electrodes are dipped into the soil in a straight line above the underground pipeline at equal distances a. The recommended length of distance a can be chosen from inequality: 2h 3a 4h;
where h is the pipeline depth at the measured area. Direct current battery of voltage about 80 V provides current I in the circuit of the external metal electrodes. Two internal reference electrodes are measuring in this field the potential difference DV at a distance a in the electric field produced by the metal electrodes. Soil resistivity ⍴ can be found from the measurement results by following formula given in [17]: r¼
2paDV I
To prevent the interference of stray current with the results of the measurements it is recommended to carry out a second similar test with a reversed current direction.
4.2.4
Estimation of Static Stray Current Values on Unprotected Underground Pipelines by Potential Difference Measurements
The assessment of static stray current can be carried out by measurement of potential difference between two reference electrodes. Like in the side-drain method, one electrode is placed directly above the pipeline and the second electrode is placed at some A V Metal electrode
a
Reference electrodes
a
Metal electrode
Soil surface
a
FIGURE 4-8 Scheme of four electrode method for soil resistivity measurement.
94 ELECTROCORROSION AND PROTECTION OF METALS
distance y apart the pipeline, at a right angle to it. According to formula proposed in [17] the measured difference of potentials DV between the electrodes: rj y 2 þ h2 ln 2p h2
DV ¼
where ⍴ ¼ soil resistivity, h ¼ depth of buried pipeline at the measured area, y ¼ distance between the points of the potential measurement, and j ¼ stray current entering or leaving the pipe per its length unit. If to accept that y ¼ 10 h, the formula for estimation of the stray current value is simplified: DV ¼ 0:734rj
4.2.5
Direct Measurements of Static Stray Current Along Unprotected Pipelines
Magnitude and direction of static stray current flowing along a pipeline can be measured with the help of a very sensitive voltmeter that has a full-scale deflection 1e10 mV and accurately calibrated constant lead wires. The span L chosen for the voltage drop measurement between the contact points A and B (left part of Figure 4-9) depends on the pipeline diameter and its wall thickness. The greater is the diameter and the thicker is the wall of the pipe, the longer must be the span L, which may attain 100e200 m. In all cases electric resistance RAB of section AB must be no less than 0.001 U and the magnitude of this resistance must be accurately measured or calculated. The section of the pipe must be uniform, without any flange joints and fittings. Contact resistance between the pipe and the lead wires in points A and B must be excluded. If voltage difference DV ¼ VB VA ¼ 0, this indicates that stray current attack of the pipeline is absent. In the case of stray current I flow along the pipeline it is equal to ratio of voltage drop DV ¼ VB VA to section resistance RAB: I ¼ DV=RAB
If additional wires are necessary for the measurements, their resistance must be measured, and a consequent correction must be introduced into the ratio. Pipeline +
V
A
−
+ V − B
C
L
D
L
Direction of stray current flow FIGURE 4-9 Stray current measurement along underground pipeline.
Chapter 4 Detection, Identification and Estimation of Stray Currents
95
The considered method can be also used for detection areas of stray current discharge or picking up by the pipeline. For this purpose, two circuits that are identical to the above considered circuit are created at the ends of the inspecting area, as it is shown in Figure 4-9. Voltage drop measurements are carried out simultaneously at both circuits. If DVAB ¼ DVCD, neither discharge nor picking up of stray current occurs at the pipe section AD. If DVAB > DVCD, or DVAB < DVCD, this indicates, respectively, that current discharge or current pick up occurred at this pipeline section. The difference between ratios DVAB/RAB and DVCD/RCD provides information on the magnitude of discharged or picked up current.
4.2.6
Finding Dynamic Stray Current Direction on Unprotected Pipelines by Repeated Measurements
Current measured at CD
Dynamic stray currents are easier to identify than static currents owing to their fluctuations. However, the same fluctuations make their measurement and value estimation much more problematic, as compared with the static stray currents. Consequently, it is more difficult to detect on the pipeline areas of dynamic stray current discharge and pick up. Considered above method of voltage drop measurement at two calibrated pipeline spans can be used for this purpose, however, series of at least 20e40 simultaneous measurements at sections AB and CD are necessary [14]. Instant current values obtained from simultaneous voltage measurements at sections AB and CD are plotted on the graph (Figure 4-10). If the currents are statistically equal at sections AB and CD, the straight line drawn across the obtained values will pass at an angle of 45 . If the angle is greater than 45 degrees, as it is shown in Figure 4-10, the current value at section CD is greater than at section AB. This indicates that stray current pick up occurred at some areas at section BD. If the angle is less than 45 degrees, this means that current discharge took place at this section.
M Current measured at AB FIGURE 4-10 Location of areas of dynamic stray currents discharging or picking up [14].
96 ELECTROCORROSION AND PROTECTION OF METALS
Stray current source + V-2 -
+
V-1
-
Reference electrode Underground pipeline FIGURE 4-11 Setup for determination of stray current direction.
Intersection point M of the straight line with the ordinate, above axis AB, serves as an indication that some steady component of current is present at section CD that is not influenced by the current source. If a straight line cannot be drawn across the plotted points, this is an indication that two or more sources of stray currents are present around. When the chosen resistance magnitudes of sections AB and CD are equal each other, instant values of voltage drops can be plotted on the graph instead of calculated current values. As was noted above, owing to potential fluctuations, dynamic stray current on a pipeline can be easily detected by potential to soil measurements with a single reference electrode and a high resistance input voltmeter. However, these results do not provide any information on the direction of the detected currents. When the source of stray current is known, its influence on the pipeline can be analyzed with the help of graphs called beta curves. The procedure of beta curves plotting is the following. Along with installation of a reference electrode and voltmeter V-1 of high input resistance (Figure 4-11), the pipeline is connected through voltmeter V-2 with the current source. In the moments of connection instant values of voltage difference between the stray current source and the pipeline (DV2) and of pipeline potential to soil (DV1) are simultaneously measured. If in the moment of measurement, the current flows down from the current source to the pipeline, its direction will be fixed on voltmeter V-2 and potential swing to the negative side will occur on the scale of voltmeter V-1. As the current will have an opposite direction, the potential on the V-1 will swing to the positive side. Because of several tens of measurements linear dependences are obtained of pipeline potential shift DV1 on voltage change DV2. Such dependences are called beta curve [13,15,19]. When stray current is directed from the current source to the pipeline and picked up by the pipeline, the straight line has a form shown in Figure 4-12a [19]: voltage shift DV2 to the positive side leads to shift of the pipe-to-soil potential DV1 to the negative side. When the stray current has an opposite direction and flows down from the pipeline, the straight line has a form shown in Figure 4-12b. When voltage DV2 increases, the pipeline potential DV1 shifts to the positive side. In other words, in this case stray current discharges from the pipeline at the considered area and the pipeline undergoes corrosion attack by anodic stray currents.
Chapter 4 Detection, Identification and Estimation of Stray Currents
(A)
97
(B)
+4.0
V2, volts
+3.0 +2.0 +1.0
0 -1.4
-1.2
-1.0
-0.8
-0.6
-0.4 -0.4
-0.2 V1, volts
0
FIGURE 4-12 a, b Forms of beta curves: a e pick up area; b e discharge area [19].
The obtained beta curves indicate not only the direction of the stray current on the pipeline, but also the influence grade of the current source on the pipeline at different areas. The smaller is the straight-line angle to the horizontal axis, the stronger is the influence, because the higher is the potential shift DV1 on the pipeline. This angle b is defined by ratio: b ¼ DV 1 =DV 2
At the area where this slope of the straight line is the smallest the stray current is maximal, and this area is the most disposed to stray current corrosion. When the straight line is close to vertical, any influence of the stray current source on the pipeline does not take place. When a straight line cannot be drawn through the plotted points this means that there are some additional sources of stray current, which must be detected.
4.2.7
Detection of Stray Currents and Their Sources on Protected Pipelines
The problem of stray current detection on cathodically protected pipelines located close to each other is more complicated than the above considered problem of current detection on unprotected pipelines in consequence of stray current interference with the protection currents. The above considered methods of stray current detection by voltage drop measurements can be used for protected pipelines on condition that the cathodic protection current of all closely spaced pipelines will be interrupted. An interrupter must be installed for this purpose in the cathodic protection circuits of the considered pipeline. Such requirement is feasible, since these devices are commonly used in modern cathodic protection systems for performing protection potential control and monitoring along the pipelines. One of the most used systems of cathodic protection control called close-interval potential survey (CIPS or CIS) consists of numerous measurements of potentials
98 ELECTROCORROSION AND PROTECTION OF METALS
Pipeline-to -soil potential, V
metal-to-soil along the underground metallic structures. These measurements are carried out at regular intervals and assess the protection potential level on the structure, which normally must be no less negative than 0.85 V. The protection potential value may fail in some areas of the structure due to influence of different factors, referred to relative positions of pipelines and ground beds, to characteristics and operational conditions of cathodic protection systems, to soil features etc. One of the main causes of such a failure is the attack by stray currents. Any source of stray current may upset normal operation of cathodic protection. Cathodically protected foreign pipelines disposed close to the considered protected structure are one of common current sources, especially at crossing areas of these objects. An example of such influence is given in Figure 4-13. At the absence of stray currents the potential metal-to-soil along a cathodically protected pipeline 1 is equal to or more negative than 0.85 V. However, in the area of crossing with a foreign cathodically protected pipeline the potential drastically shifts to the positive side, to the values which are much more positive than 0.85 V. Localization of this hazardous area on the base of data shown in Figure 4-13 enables revealing the stray current source, if other suspected sources of stray currents do not exist in this region. When too negative potential value of the protected pipeline is revealed, this may point to interference of stray current pick up from a ground bed of a foreign cathodic protection system. In these cases, measures must be undertaken for search hazardous areas along the pipeline where the current leaves the pipeline and, consequently, where the potential values are more positive than 0.85 V. Reliable detection of stray current source can be executed by application of an interrupter introduced into the cathodic protection circuit of the pipeline which is suspected as a source of stray current. The interrupter is set to operate by unequal periods of switching “off” and “on,” for example, respectively, 40 and 20 s or 20 and 10 s. CSE reference electrode is installed above the crossing area for potential measurement at interrupter positions “on” and “off.” Potential reading at the position “on” is carried out
-1.2 -1.0 -0.8 -0.6 -0.4
-0.85 V Hazardous area of the interfered pipeline Foreign cathodically protected pipeline
1
2
Interfered cathodically protected pipeline
FIGURE 4-13 Metal-to-soil distribution of potential along a cathodically protected pipeline crossing with a foreign cathodically protected pipeline.
Chapter 4 Detection, Identification and Estimation of Stray Currents
99
just before switching off the circuit, and the second reading must be made immediately after the interruption. If the potential metal-to-soil along influenced pipeline returns to the necessary range, this indicates that pipeline (2) is a single source of stray current. It was noted in the previous chapter that the influence of protected pipelines on other underground structures depends to a large extent on the coating quality of interfering pipelines. The most dangerous current source is a protected uncoated underground structure. If pipeline (1) in Figure 4-13 is coated and pipeline (2) is uncoated, it is most probable that the last one will be a strong current source with respect to pipeline (1). The lower is the coating quality of a protected underground structure, the more this structure is dangerous as a stray current source. Vice versa, a cathodically protected pipeline with a high-quality coating will have a minimal effect on metallic structures located in close proximity. A detailed analysis of mutual influence of several crossing pipelines of different kinds is given in [20]. Presence of dynamic stray currents hinders the detection of static stray currents. As was mentioned above, the source of the dynamic current can be revealed by registration of the potential fluctuations during 24 h. Results of such registration are given as an example in article [20] by M.J. Szeliga. The measurements of potentials (Figure 4-14) were carried out on a cathodically protected pipeline located close to electric transit system. At night hours the potential of the pipeline was stable and corresponded to the required values of the protection potential (0.85 V to CSE). The most intensive potential fluctuations occurred in the morning and afternoon rush hours. In these periods of time the potential shifted to the positive side, to values which are more positive than 0.85 V with respect to CSE reference electrode. Static stray currents sources can be detected in this area in the night hours with the aid of interrupter, as it was described above.
PIPE TO COPPER SULPHATE POTENTIAL-VOLTS
–2.0
–1.0
0
+1.0
MORNING RUSH HOUR
AFTERNOON RUSH HOUR
+2.0 MIDNIGHT
6 AM
NOON
6 PM
MIDNIGHT
FIGURE 4-14 Influence of dynamic stray current on pipe-to-soil potential of a pipeline in different periods of time during 24 h [20].
100
ELECTROCORROSION AND PROTECTION OF METALS
Coordination of all operations, including temporary disconnection of closely spaced cathodic protection circuits, is necessary in the process of detection of stray current sources. Therefore, cooperation of all owners of cathodically protected underground structures disposed in the examined area must be available.
4.2.8
Detection of Well Casing Stray Currents
The annular gaps between the casing strings and the well are filled by cement for preventing corrosion damage of well casing. However, cement not always can be introduced over the whole casing height and some dip parts of the strings often remain in contact with a mixture of underground water and mud liquid used in drilling operations. This mixture may possess high aggressiveness. Macrocells are often formed between the areas filled with cement and areas contacting directly with the aggressive liquid. Therefore, cathodic protection and constant control of corrosion state of the external surface of casing strings is necessary. There are two features that differentiate the methods of corrosion control and monitoring of well casing from the above considered corrosion control methods of horizontal underground metallic structures: practically impossible access to the external surface of strain casing and relatively easy access to its internal surface. So, direct potential measurements of the external surface cannot be carried out, but a probe with contacting elements can be introduced inside the internal casing string, and it can attain any area along the string height. In this regard a method of the external surface control with a special measuring probe was developed. Descriptions of the measurements which are carried out with this probe are given in Ref. [21]. A schematic of such a probe is shown in Figure 4-15. It includes two groups of contacting elements, A and B. The elements are electrically shorted inside the groups; the groups are insulated from one another and connected to a high-sensitive voltmeter with a scale of mV. In the measurement operations the probe is introduced inside the casing string and all contacting elements are pressed against the internal string surface by springs providing a reliable electrical contact with the metal of the internal string surface. Voltage drops between the macrocells M and N, which are spaced on the external string surface by distance L, are measured. The recommended distance L is in the range of 8 m. The measurements can be carried out along the whole vertical casing string. In each of the measurements the voltmeter determines not only the voltage drop value, which is usually of the order of several mV, but also cathodic and anodic areas of the macrocells. These measurements allow mapping the distribution of cathodic and anodic areas along the string axis. Example of potential distribution, called voltage profile along an unprotected casing string [21] obtained at the absence (curve 1) and at the influence of stray current attack (curve 2) is shown in Figure 4-16. The curves start from the depth of about 190 m; up to this depth the string has a double well filled with a dense layer of cement and any
Voltmeter + Well casing M A
L
Contacts
B N Pipe of plastic Springs FIGURE 4-15 Measuring probe for corrosion state assessment of casing string external surface.
-600
Δ V, μ volts 1000
1800
Depth of double wall casing
0
100
Depth, m
n 200
300
m d c
k g
400
500
2
600
f
1
700
800
e
FIGURE 4-16 Profiles of voltage drop on unprotected casing string: (1) absence of stray currents; (2) attack by stray currents [21].
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ELECTROCORROSION AND PROTECTION OF METALS
potential drop does not exists at this section. A slope cd obtained at the absence of stray current indicates that anodic area of the casing string is located at a depth of about 300e370 m: at this string section the voltage drop has a negative value. The results of these measurements are not accurate enough, since they do not consider the influence of microcells and assume that the measured values do not change during the measuring operations and for some time after the measurements. However, these results provide information on the most dangerous steady-state macrocells. Well casing and connected to it facilities may be attacked by direct stray currents of the same origin as other, above considered underground metallic structures: high voltage transmission lines, welding devices, electric traction systems, grounding of electrical equipment, telluric currents. Areas of stray current pick up or discharge disposed close to the ground surface can be detected by the previously considered methods in which CSE reference electrodes and high input resistance voltmeters are used [22]. However, the major source of stray currents on the well casing are other cathodically protected casing strings, especially ground beds of their cathodic protection systems. The depth of oil and gas well casing attains thousands of meters that requires using deep ground beds disposed at a significant (by the order of hundreds meters) distance from the protected well casing. This increases the probability of stray current interference to the adjacent well casing which is not connected with the mentioned ground bed. Sometimes preliminary visual inspection of local corrosion damages may indicate that they originate from stray current attack. In a special Report from Alberta Oil Tool [23] data are conveyed on a specific view of pits resulting from arcs induced by stray currents: “Arcs originating from sucker rods leave a deep, irregular shaped pit with smooth sides, sharp edges and a small cone in the base of the pit. Arcs originating from the tubing leave deep pits with smooth sides and sharp edges that are random in dimension and irregular in shape. Stray current corrosion pits are usually singular and isolated in a row down one side of the sucker rod near the upsets.” The results of visual inspection are not reliable enough and may be used only for corrosion control of accessible equipment. The considered method with a special measuring probe is best suited for stray current detection on the well casing. This is seen in Figure 4-16 from the profile of curve 2, obtained at the same casing string attacked by stray current. Voltage drop in this case is four to six times greater than at the stray current absence. Cathodic area (efg) of current pick up and hazardous anodic area of current discharge (kmn) are clearly defined at curve 2. The method makes possible detection of the areas of stray current discharge and pick up at any depth of the well casing. As electrical resistance value of the L long tube is known, the axial current values can be calculated at every section of the string, where the voltage drop is measured. Since the directions of currents are also known, the values of currents along the string as well as the values of currents pick up and discharge at every section can be determined.
Chapter 4 Detection, Identification and Estimation of Stray Currents 103
For increasing the accuracy during measuring operation, the internal volume of the string must be filled with a liquid of high resistivity, such as demineralized water or some nonconductive oil. The considered method can be also used for stray current detection on a cathodically protected well casing.
4.2.9
Detection of Telluric Currents
Telluric currents are also considered as a kind of stray currents [3]. As distinct from the stray currents that come from electric traction systems, telluric currents can penetrate to the underground structures not only through the coating defects, but also owing to the induction effect. In the last case, the metal-to-soil potential fluctuations are not determined by current pick up or discharge from the metallic structure. Moreover, coating does not protect from the penetration of telluric currents. Potential fluctuations produced by telluric currents may be like the fluctuations of dynamic stray currents. In the periods of solar activity, the action of telluric current can be detected by finding the correlation between the magnetic field strength and the fluctuations of the metal-to-soil potential in the inspected metallic structure [24], provided that these two parameters are measured simultaneously. On the other hand, when inspection of the underground structure is carried out in an area known as a region of telluric current activity, it is preferable to carry out the measurements in the periods of a minimal solar activity. This makes possible avoiding the interference of telluric currents since these currents deteriorate results of the measurements. The absence of correlation in fluctuations of the metal-to-soil potential with the traffic schedule of electrified traction systems may indicate the presence of telluric currents. There are data [25] that in the presence of telluric currents the profile of potentialtime fluctuations on the coated underground pipelines remains similar at a length of tens and even of hundreds of kilometers. So, the information on the character of telluric currents on the coated pipeline can be obtained by measurement in a limited number of locations. Detection of telluric stray currents can be carried out by measuring near ground and far ground potentials [26]. Sometimes telluric current can be detected by the method of the voltage drop measurements along the pipeline. The most reliable information on the attack of cathodically protected pipeline by telluric currents can be obtained in close-interval potential survey [26,27]. Modern techniques make it possible to measure pipe-to-soil potential considering IR drop at the measured potential points [28]. When using these techniques is related with some difficulties, the information can be obtained by installation of measuring coupons in the most hazardous areas of the pipeline, as it is described in [29,30]. The coupons which reproduce defects in the pipeline coating are made of the same steel as the pipeline.
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ELECTROCORROSION AND PROTECTION OF METALS
They are located close to the pipeline and electrically connected with it by cables. So, the coupons are cathodically protected, like the pipeline, but the cables can be switched off in the moments of current measurement. CSE reference electrodes are located close to the surface of each of the coupons that makes possible measuring IR free potential values. Coupons that are not connected with the pipeline and provided with CSE electrodes are also installed close to the pipeline for reference. This method can be successfully used for potential control along buried pipelines under the conditions of attack by manmade stray currents as well as by telluric currents [30].
References [1] S.Y. Li, S. Jung, K.W. Park, S.M. Lee, Y.G. Kim, Mater. Chem. Phys. 103 (1) (2007) 9e13. [2] V.S. Sastri, E. Ghall, M. Elboujdanini, Corrosion Prevention and Protection, John Willey & Sons, Ltd, N.Y, 2007, 562 p. [3] P.R. Roberge, Handbook of Corrosion Engineering, Mc Crow-Hill, N.Y, 2000, 1128 p. [4] M. Zee, Catastrophic Failure of Aging Underground Pipelines Is Inevitable under Certain Corrosion Conditions, Exova, Pittsburgh, 2016. www.puc.state.pa.us/.../gassafe/.../Gas_Safety_Seminar_2016. [5] J.M. Malo, V. Salinas, J. Uruchurtu, Mater. Perform. 33 (8) (1994) 63. [6] In: J.R. Davis (Ed.), Corrosion: Understanding the Basics, ASM, Materials Park, Ohio, 2000, 563 p. [7] Report on the Investigation of the Flooding of the UK Charter Fishing Vessel Random Harvest, 1999. https://assets.publishing.service.gov.uk/.../random_harvest. [8] J. Britton, Mater. Perform. 30 (2) (1991) 30e33. [9] D. Pascoe, Corrosion, A Boatman’s Primer on the Essential, 2015, www.yachtsurvey.com/corrosion. htm. [10] W.B. Holtsbaum, Stray currents in underground corrosion, in: Corrosion, Environments and Industry, ASM Handbook, vol. 13C, Materials Park, Ohio, 2006, pp. 107e114. [11] U.R. Evans, The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications, Edward Arnold Ltd, London, 1960. [12] S. Greenberger, T. Elliott, Electric rail corrosion and control, in: corrosion, environments and industries, ASM Handbook 13C (2006) 548e558. [13] J.R. Walters, Stray-current corrosion, in: L.L. Schreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, third ed. Corrosion Control, vol. 2, 1994, pp. 10:122e10:129. [14] M.E. Parker, E.G. Peattie, in: Pipeline Corrosion and Cathodic Protection, Gulf Publishing Company, Houston, TX, 1999, 166 p. [15] AUCSC, Education and Training for Corrosion Control, Appalachian Underground Corrosion Short Course, West Virginia University, Morgantown, 2014. [16] Standard ASTM G57 e 06, Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-electrode Method, 2012. [17] H.H. Uhlig, R.W. Revie, Corrosion and Corrosion Control, John Willey & Sons, N.Y, 1985, 464 p. [18] E.I. Dizenko, V.F. Novosiolov, P.I. Tugunov, V.A. Yufin, Anticorrosion Protection of Piping and Vessels (In Russian), Nedra, Moscow, 1978, 199 p. [19] J.H. Fitzgerald III, Stray-current analysis, in: R.W. Revie (Ed.), Uhlig’s Corrosion Handbook, third ed., John Willey & Sons, Inc. ECS, Hoboken, New Jersey, 2011, pp. 1013e1020.
Chapter 4 Detection, Identification and Estimation of Stray Currents 105
[20] M.J. Szeliga, Stray current corrosion, in: R.L. Blanchetti (Ed.), Peabody’s “Control of Pipeline Corrosion”, second ed., NACE International, Houston, 2001, pp. 211e236. [21] W. Prinz, B. Leutner, Cathodic protection of well casing, in: W. v. Baeckmann, W. Schwenk, W. Prinz (Eds.), Handbook of Cathodic Protection: Theory and Practice of Electrochemical Protection Processes, third ed., Gulf Publishing Co, Houston, 1997, pp. 415e426. [22] W.B. Holtsbaum, Well casing external corrosion and cathodic protection, in: Corrosion, Environments and Industry, ASM Handbook, vol. 13C, Materials Park, Ohio, 2006, pp. 97e106. [23] A Special Report from Alberta Oil Tool, October 2001, 14 p. www.albertaoiltool.com/pdf/Rod_ Failure.pdf. [24] D.H. Boteler, L. Trichtchenko, Telluric influence, in: R.W. Revie (Ed.), Oil an Gas Pipelines, Integrity and Safety Handbook, J. Villey & Sons, Inc., 2015, pp. 275e288. [25] Stray currents in underground corrosion, in: W.B. Holtsbaum (Ed.), ASM Handbook, in: S.D. Cramer, B.S. Covino (Eds.), Corrosion: Environments and Industries, vol. 13C, 2006, pp. 107e114. [26] A. Kowalski, The close-interval potential survey (CIS/CIPS) method for detecting corrosion in underground pipelines, in: M.E. Orazem (Ed.), Underground Pipeline Corrosion, Elsevier, Amsterdam, N.Y, 2014, pp. 227e246. [27] D. Hevle, A. Kowalski, Close-interval survey techniques, in: S.D. Cramer, B.S. Covino (Eds.), ASM Handbook, Corrosion: Environments and Industries, vol. 13C, 2006, pp. 84e88. [28] W. v Baeckmann, W. Schwenk, Fundamentals and practice of electrical measurements, in: W. v. Baeckmann, W. Schwenk, W. Prinz (Eds.), Handbook of Cathodic Protection: Theory and Practice of Electrochemical Protection Processes, third ed., Gulf Publishing Co, Houston, 1997, pp. 79e138. [29] P.J. Nicholson, Mater. Perform. 46 (10) (2007) 24e28. [30] C.D. Stears, O.C. Moghissi, L. Bone III, Mater. Perform. 37 (2) (1998) 23e31.
Equipment and procedures of detection and identification of stray currents on coated and uncoated underground pipelines and other metallic structures depend to some extent on the character of stray current (static or dynamic) and on the protection state of the structure (unprotected or cathodically protected). Therefore, along with the character of the detected stray current, the state of the underground structure will be further indicated: unprotected and, for short, protected (implying that the structure is cathodically protected).
4.1 Stray Current Corrosion Identification Without Resort to Special Measurement Instruments It was already emphasized that corrosion rate of metals under the attack of stray currents can be by orders of magnitude higher than the rates of any other kinds of underground Electrocorrosion and Protection of Metals. https://doi.org/10.1016/B978-0-444-64021-5.00004-0 Copyright © 2019 Elsevier B.V. All rights reserved.
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86 ELECTROCORROSION AND PROTECTION OF METALS
or underwater corrosion of metals. So, corrosion damage of a metallic structure resulted from an abnormal metal corrosion rate can be taken as the first evidence of stray current presence. In fact, it was proposed to apply this “feature” of stray currents corrosion for detection stray current attack of underground structures with the help of special metallic probes [1]. The probes, in the form of wire or of thin film deposited on an inert material, were made of the same metal as the structure. Measurement sensitivity of the probes was 0.01 mm/y. The corrosion rate was determined by measuring electric resistance change of the probes. It was proved in preliminary tests that the corrosion rate of the structure metal and of the probe metal was similar. The cross-section of the corroding probes diminished, and consequently, their electric resistance increased during the test. The test was carried out at an underground structure disposed in the field of stray current attack, and the probes were placed along the structure, close to it. Three probes were tested along the structure, at different distances from the stray current source. The results of the test, where R/Ro is the ratio of the changing electric resistance to its initial value, are given in Figure 4-1. Corrosion rate measured on the probe that was closest to the stray current source attained 8.38 mm/y, whereas at the most remote probe it was equal only to 0.96 mm/y, by an order of magnitude lower. Thus, this method makes possible not only assessing the corrosion rate of the structure, but also finding stray current direction and the location of stray current source. Corrosion of carbon steel structures which is caused by stray current attack is often classified as general corrosion accelerated at the locations of the stray currents attack [2]. Perhaps, this is explained by an active state of iron in such neutral media as soil and water. In fact, at the anodic polarization of iron in these media its dissolution (electrolysis) may have not local, but a rather regular character, in accordance with the Faraday low, as it was already mentioned above. 1.0
0.8
R/R0
0.6
0.4
0.2
8.38 mm/y
0.0
A 0
1.40 mm/y
0.96 mm/y
B 1
2
3 Time (h)
C 4
5
FIGURE 4-1 Response of thin film probes at tree distances [1].
6
Chapter 4 Detection, Identification and Estimation of Stray Currents
87
Corrosion damage may be localized close to the insulation flanges of a pipeline, at one side of the joint. Such a character of the damage localization points to stray current attack. Moreover, it indicates the stray current direction (Figure 4-2): stray current of anodic direction exerts corrosion damages on one side of the insulating joints (areas A1 and A2), whereas another side near the joint (areas C1 and C2), where the current has cathodic direction, does not undergo corrosion damage. When conductive liquids are flowing inside the pipeline, similar localization of the corrosion damages may occur near the insulating flanges also at the internal surface of the pipes and flanges. Electrocorrosion damage of underground structures often occurs in the form of pitting that can be related to irregular state of the metal surface. Tendency to corrosion damage in the form of pitting is most typical for coated structures, since in this case the current concentrates at small defects of the coating. An example of such kind of local corrosion is shown in Figure 4-3A and B [4]. It is pertinent to note in this connection that a single pit was the origin of the notorious Guadalajara catastrophe in April 1992 that was caused by stray current attack of unprotected ductile iron water pipeline [5]. This pipeline was attacked by stray current coming from the cathodically protected gasoline pipeline disposed under the water Stray current direction A1
Insulating joints C1
Anodic zone
A2
C2
Cathodic zone
FIGURE 4-2 Localization of stray current corrosion at the anodic areas of insulating joints.
(A)
Stray Current Corrosion
Underground Corrosion
(B)
FIGURE 4-3 Character of stray current corrosion on a coated pipe: (A) a pipe section with two pits and (B) magnified photograph of the left pit [4]. Courtesy of Dr. Mehrooz Zamanzadeh.
88 ELECTROCORROSION AND PROTECTION OF METALS
pipeline, close to it. Water flow through the mentioned pit of the water pipeline led to corrosion damage of the gasoline pipeline and to gasoline sewer explosion. Result of this catastrophe: about 1000 dead, 800 injured, and damage of V76 million. The pits resulted from stray current attack are often cone shaped with a cone vertex directed inside the metal. However, the local damages caused by stray current may have quite other shapes; moreover, similar corrosion damages may be related to other causes, such as bio-corrosion. Therefore, the shapes of the corrosion pits do not point unambiguously to the corrosion origin. Sometimes the character and the rate of a selective corrosion point to the influence of stray currents. For example, when graphitization of cast iron in water mains occurs in a relatively short period of time, it is most probably related to stray current effect [2,6]. This effect is explained by selective dissolution of ferritic phase along the grain boundaries of cast iron matrix at the stray current attack [2]. Another kind of accelerated selective corrosion, plug-type dezincification of brass CuZn40Pb2Sn in seawater was considered in [7]. A through-hull fitting of this material was mounted at the bilge of fishing vessel Random Harvest. The fitting failed after 16 months period of work. The dezincification rate of the fitting was 1.27 mm/y, about twice as much as dezincification rate of another, quite similar fitting also installed at the vessel bilge. Analysis of the fitting damage and its location leads to the conclusion that the most probable cause of such a high dezincification rate was related to the attack of DC stray current from dump wiring of vessel batteries. This corrosion damage led to the vessel flooding. As discussed in Chapter 2, corrosion of metals which are in neutral media in a passive state, such as stainless steel and aluminum, almost always has a form of pitting which occurs as a result of different kinds of corrosion factors, including stray current attack. However, when elevated rate of pitting penetration into the metal occurs, this can point to the influence of stray current. Abnormal consumption of aluminum sacrificial anodes for cathodic protection of offshore platforms may indicate stray current attack. Although this indication may be to some extent needless: the presence of the welding generator out of the platform unambiguously pinpoints the presence of stray currents [8], as it was noted in the previous chapter. Abnormal corrosion of zinc sacrificial anodes for protection of boats and brightness of the anode surface is also an indication of stray current corrosion attack [9]. The corroded surface of bronze propellers and other metallic parts of the boats also becomes sometimes shining. The brightness of the anodes and of metallic parts is explained by a high intensiveness of the metal dissolution (electrolysis) under the influence of external anodic current. It is probable that the anodic process of metal dissolution is followed by acidification of the water layer adjacent to metal surface which prevents corrosion products deposition on this surface. The absence or low quantities of corrosion products at the damaged areas of a metallic structure is considered in Ref. [10] as an indication of stray current attack. Anodic current dissolves the grain boundaries of lead. This specific influence makes the anodic current responsible for the intercrystalline corrosion of lead sheaths of
Chapter 4 Detection, Identification and Estimation of Stray Currents
89
telephone cable in soil [11]. Moreover, ions of chlorine tend to move toward the anodic areas, and their concentration at these areas is higher than the average chloride concentration in the soil. So, intercrystalline corrosion of lead and more than 5% concentration of lead chloride near the corroded areas (when general salinity of the soil is low) may furnish a proof of anodic stray current attack of lead sheaths [11]. Thus, stray current attack can be often revealed at the initial inspection stage of the damaged underground or underwater metallic structures. The results of such inspection are most trustworthy when they are supported by the data on the presence of some potential stray current sources disposed nearby the damaged structure. This common information may significantly facilitate and simplify the choice of ways of stray current mitigation and elaboration methods for protection from electrocorrosion. However, in most cases special methods and appropriate devices are necessary for reliable detection and identification of stray currents and their location, direction and distribution.
4.2 Detection and Estimation of Stray Currents by Measurements of Metal - Soil Potential and of Voltage Drop on Underground Structures 4.2.1
Instruments and Techniques of Potential Measurements
Copperecopper sulfate reference electrode (CSE) is commonly accepted for metal potential measurements at underground pipelines, cables and other metallic structures. The body of the electrode (Figure 4-4) is made of transparent polymer material or of glass. A plug of porous material serves as a body bottom. The electrode is filled with saturated solution of copper sulfate. Surplus crystals of CuSO4 are added to the solution To voltmeter
Copper rod Insulating plug Transparent body
Crystals of copper sulfate
Copper sulfate saturated solution Porous plug
FIGURE 4-4 Copperecopper sulfate reference electrode (CSE).
90 ELECTROCORROSION AND PROTECTION OF METALS
Pipe line
+
V
-
Voltmeter
Reference electrode
FIGURE 4-5 Scheme of potential measurement on underground pipeline.
for keeping saturation concentration of copper sulfate. A rod made of high purity electrolytic copper that passes through the insulating plug is dipped into the copper sulfate solution. The pores of the bottom plug filled with the solution play the part of an electrolytic bridge when the bottom is pressed against the ground in the process of measurements. The potential of CSE at temperature of 25 C is equal to 320 mV with respect to standard hydrogen electrode; the deviations of measurements do not surpass 5 mV. A scheme of potential measurement on underground pipeline is shown in Figure 4-5. The reference electrode is installed directly above the pipeline and recessed into the earth by several centimeters. Voltmeter that is used for the measurements must have a high input resistance, surpassing the total resistance of the reference electrode and other components of the measurement circuit by several orders of magnitude. Input resistance of modern digital voltmeters for such measurements is higher than 20 MU that makes possible not to consider the circuit resistance in the process of measurements. Most of the modern pH-meters are provided with an option of voltage measurements, so these devices can also be used as voltmeters with high input resistance. Negative terminal of voltmeter is usually connected to the reference electrode, and positive terminal is connected to pipeline offset or to some other accessible part of the underground pipeline.
4.2.2
Detection of Stray Currents Presence on Unprotected Underground Structures
The potential value of carbon steel in such electrolytes as soil and water depends on many factors, including the composition and the state of the metal surface, electrolyte composition, air access etc. This is the cause of carbon steel potential variations in these media over a wide range, from 0.2 to 0.8 V (CSE), as it is shown in Table 1-7. Corrosion factors accelerating the corrosion process of carbon steel (differential aeration, crevices, bacteria etc.) may even more enlarge the range of potential variations. So, individual values of the measured potentials do not allow judging the presence of stray currents. It was noted in Chapter 3 that stray currents coming to underground pipeline from such sources as DC railway systems and mining operations have a dynamic character that is manifested in their fluctuations. Fluctuations of stray currents lead to corresponding fluctuations of potentials. Maximal amplitudes values of the potential in both, negative and positive sides can by an order of magnitude surpass the potential measured
Chapter 4 Detection, Identification and Estimation of Stray Currents
91
at the absence of stray current attack [12,13]. Measured potential amplitudes in the earth depend on the distance of the reference electrode from the stray current source [12]. The more remote is the reference electrode from the current source, the lower are the measured potential amplitudes. Results of simultaneous measurements of the potential fluctuations when copper/ copper sulfate reference electrodes were located at a short and at a remote distance from the stray current source are shown in Figure 4-6 [12]. It is seen that the fluctuation amplitudes at a remote distance are by an order of magnitude lower than at a short distance, but the profiles of the fluctuations are similar. These potential fluctuations do not provide quantitative information on the stray current values, but they are evidence of the dynamic stray current presence in the soil regardless of the reference electrode distance from the current source. The areas where the amplitudes are the highest in the negative side may indicate that these are the areas of stray current pick up. The areas of maximal fluctuation amplitudes to the positive side may be suspected as most hazardous and require special inspection. However, it must be taken into account that these changes in the fluctuations may be related with other factors, such as soil resistivity etc.
(A) 10
(B)
8 6 4 2 0 –2 –4 –6 –8 –10 13:00
13:03
1 0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1 13:00
13:03
13:06
13:06
13:09
13:09
13:12
13:12
13:14
13:14
Time, h:min FIGURE 4-6 Earth gradients on an ungrounded embedded track system. The CSE reference electrodes were located at distances: (A) short (7.6 m) and (B) remote (53.3 m) from the stray current source [12].
92 ELECTROCORROSION AND PROTECTION OF METALS
The fluctuation may also indicate the sources of stray current. For example, attenuation of the fluctuation in the noon hours or in the night can indicate that the industrial machinery is the stray current origin [14]. It is possible often to reveal correlation between the electric cars schedule and the voltage fluctuations by regular registration of fluctuations 24 h a day. At the side where the current is picked up by the pipeline, it has predominantly cathodic direction and potential values attain maximal negative values. These values are diminishing along the underground structure and attain minimal negative or, in the considering range of the measured potentials, maximal positive values at the area of current flow from the metallic structure toward the substation. At this, anodic side the metallic structure undergoes the most severe corrosion attack. It was already noted in Chapter 3 that an alternative zone between the cathodic and anodic zones, where the fluctuating currents periodically change their direction, is also hazardous from the point of view of stray current corrosion attack. Detection of static stray current presence in the soil requires a great number of systematic potential measurements along the pipeline. The results of potential distribution may indicate the dangerous areas of the pipeline. Those are the areas of the most positive potential values. Presence in the soil of stray current and its direction can be detected by using a circuit that includes a voltmeter with high input resistance and two reference electrodes [13]. The scheme of the measurements is shown in Figure 4-7. Two similar reference electrodes are placed approximately 8 m apart, along a line which is perpendicular to the pipeline. If the potential of right electrode is positive with respect to the left one, the stray current in the soil is directed to the left, toward the pipeline. Data on stray current direction make possible the detection of cathodic and anodic areas on the pipeline. Different methods of potential measurements with two reference electrodes along underground pipelines are considered in [15]. Among them a method called side-drain measurements is described. In this method first electrode is placed directly above the pipeline and the second electrode is disposed at a right angle to the pipeline, at a distance surpassing the pipeline depth in about 2.5 times. The measurements are made on both sides of the pipeline. The results of these measurements provide information on the stray current direction and on the most dangerous areas on the pipeline. Voltmeter
-
V
+
Reference electrodes
Pipeline
Stray current direction
FIGURE 4-7 Detection of stray current presence and direction.
Chapter 4 Detection, Identification and Estimation of Stray Currents
93
However, these data still do not allow assessing the danger of the revealed stray currents which is determined by the stray current value. Data on soil resistivity at the area of stray current attack are necessary for assessing the values of stray currents.
4.2.3
Measurements of Soil Resistivity
Soil resistivity is one of the most important parameters determining the corrosion hazard to the underground metallic structures. Magnitudes and distribution character of stray currents in the soil highly depend on the soil resistivity value that may vary from tens to thousands Um. The most accepted field measurements of the soil resistivity are carried out by Wenner four electrode method in accordance with Standard ASTM G57e06 [16]. Schemes of soil resistivity measurements based on this method are given in many literature sources [17,18]. According to these schemes (Figure 4-8) two reference electrodes and two metal electrodes are dipped into the soil in a straight line above the underground pipeline at equal distances a. The recommended length of distance a can be chosen from inequality: 2h 3a 4h;
where h is the pipeline depth at the measured area. Direct current battery of voltage about 80 V provides current I in the circuit of the external metal electrodes. Two internal reference electrodes are measuring in this field the potential difference DV at a distance a in the electric field produced by the metal electrodes. Soil resistivity ⍴ can be found from the measurement results by following formula given in [17]: r¼
2paDV I
To prevent the interference of stray current with the results of the measurements it is recommended to carry out a second similar test with a reversed current direction.
4.2.4
Estimation of Static Stray Current Values on Unprotected Underground Pipelines by Potential Difference Measurements
The assessment of static stray current can be carried out by measurement of potential difference between two reference electrodes. Like in the side-drain method, one electrode is placed directly above the pipeline and the second electrode is placed at some A V Metal electrode
a
Reference electrodes
a
Metal electrode
Soil surface
a
FIGURE 4-8 Scheme of four electrode method for soil resistivity measurement.
94 ELECTROCORROSION AND PROTECTION OF METALS
distance y apart the pipeline, at a right angle to it. According to formula proposed in [17] the measured difference of potentials DV between the electrodes: rj y 2 þ h2 ln 2p h2
DV ¼
where ⍴ ¼ soil resistivity, h ¼ depth of buried pipeline at the measured area, y ¼ distance between the points of the potential measurement, and j ¼ stray current entering or leaving the pipe per its length unit. If to accept that y ¼ 10 h, the formula for estimation of the stray current value is simplified: DV ¼ 0:734rj
4.2.5
Direct Measurements of Static Stray Current Along Unprotected Pipelines
Magnitude and direction of static stray current flowing along a pipeline can be measured with the help of a very sensitive voltmeter that has a full-scale deflection 1e10 mV and accurately calibrated constant lead wires. The span L chosen for the voltage drop measurement between the contact points A and B (left part of Figure 4-9) depends on the pipeline diameter and its wall thickness. The greater is the diameter and the thicker is the wall of the pipe, the longer must be the span L, which may attain 100e200 m. In all cases electric resistance RAB of section AB must be no less than 0.001 U and the magnitude of this resistance must be accurately measured or calculated. The section of the pipe must be uniform, without any flange joints and fittings. Contact resistance between the pipe and the lead wires in points A and B must be excluded. If voltage difference DV ¼ VB VA ¼ 0, this indicates that stray current attack of the pipeline is absent. In the case of stray current I flow along the pipeline it is equal to ratio of voltage drop DV ¼ VB VA to section resistance RAB: I ¼ DV=RAB
If additional wires are necessary for the measurements, their resistance must be measured, and a consequent correction must be introduced into the ratio. Pipeline +
V
A
−
+ V − B
C
L
D
L
Direction of stray current flow FIGURE 4-9 Stray current measurement along underground pipeline.
Chapter 4 Detection, Identification and Estimation of Stray Currents
95
The considered method can be also used for detection areas of stray current discharge or picking up by the pipeline. For this purpose, two circuits that are identical to the above considered circuit are created at the ends of the inspecting area, as it is shown in Figure 4-9. Voltage drop measurements are carried out simultaneously at both circuits. If DVAB ¼ DVCD, neither discharge nor picking up of stray current occurs at the pipe section AD. If DVAB > DVCD, or DVAB < DVCD, this indicates, respectively, that current discharge or current pick up occurred at this pipeline section. The difference between ratios DVAB/RAB and DVCD/RCD provides information on the magnitude of discharged or picked up current.
4.2.6
Finding Dynamic Stray Current Direction on Unprotected Pipelines by Repeated Measurements
Current measured at CD
Dynamic stray currents are easier to identify than static currents owing to their fluctuations. However, the same fluctuations make their measurement and value estimation much more problematic, as compared with the static stray currents. Consequently, it is more difficult to detect on the pipeline areas of dynamic stray current discharge and pick up. Considered above method of voltage drop measurement at two calibrated pipeline spans can be used for this purpose, however, series of at least 20e40 simultaneous measurements at sections AB and CD are necessary [14]. Instant current values obtained from simultaneous voltage measurements at sections AB and CD are plotted on the graph (Figure 4-10). If the currents are statistically equal at sections AB and CD, the straight line drawn across the obtained values will pass at an angle of 45 . If the angle is greater than 45 degrees, as it is shown in Figure 4-10, the current value at section CD is greater than at section AB. This indicates that stray current pick up occurred at some areas at section BD. If the angle is less than 45 degrees, this means that current discharge took place at this section.
M Current measured at AB FIGURE 4-10 Location of areas of dynamic stray currents discharging or picking up [14].
96 ELECTROCORROSION AND PROTECTION OF METALS
Stray current source + V-2 -
+
V-1
-
Reference electrode Underground pipeline FIGURE 4-11 Setup for determination of stray current direction.
Intersection point M of the straight line with the ordinate, above axis AB, serves as an indication that some steady component of current is present at section CD that is not influenced by the current source. If a straight line cannot be drawn across the plotted points, this is an indication that two or more sources of stray currents are present around. When the chosen resistance magnitudes of sections AB and CD are equal each other, instant values of voltage drops can be plotted on the graph instead of calculated current values. As was noted above, owing to potential fluctuations, dynamic stray current on a pipeline can be easily detected by potential to soil measurements with a single reference electrode and a high resistance input voltmeter. However, these results do not provide any information on the direction of the detected currents. When the source of stray current is known, its influence on the pipeline can be analyzed with the help of graphs called beta curves. The procedure of beta curves plotting is the following. Along with installation of a reference electrode and voltmeter V-1 of high input resistance (Figure 4-11), the pipeline is connected through voltmeter V-2 with the current source. In the moments of connection instant values of voltage difference between the stray current source and the pipeline (DV2) and of pipeline potential to soil (DV1) are simultaneously measured. If in the moment of measurement, the current flows down from the current source to the pipeline, its direction will be fixed on voltmeter V-2 and potential swing to the negative side will occur on the scale of voltmeter V-1. As the current will have an opposite direction, the potential on the V-1 will swing to the positive side. Because of several tens of measurements linear dependences are obtained of pipeline potential shift DV1 on voltage change DV2. Such dependences are called beta curve [13,15,19]. When stray current is directed from the current source to the pipeline and picked up by the pipeline, the straight line has a form shown in Figure 4-12a [19]: voltage shift DV2 to the positive side leads to shift of the pipe-to-soil potential DV1 to the negative side. When the stray current has an opposite direction and flows down from the pipeline, the straight line has a form shown in Figure 4-12b. When voltage DV2 increases, the pipeline potential DV1 shifts to the positive side. In other words, in this case stray current discharges from the pipeline at the considered area and the pipeline undergoes corrosion attack by anodic stray currents.
Chapter 4 Detection, Identification and Estimation of Stray Currents
(A)
97
(B)
+4.0
V2, volts
+3.0 +2.0 +1.0
0 -1.4
-1.2
-1.0
-0.8
-0.6
-0.4 -0.4
-0.2 V1, volts
0
FIGURE 4-12 a, b Forms of beta curves: a e pick up area; b e discharge area [19].
The obtained beta curves indicate not only the direction of the stray current on the pipeline, but also the influence grade of the current source on the pipeline at different areas. The smaller is the straight-line angle to the horizontal axis, the stronger is the influence, because the higher is the potential shift DV1 on the pipeline. This angle b is defined by ratio: b ¼ DV 1 =DV 2
At the area where this slope of the straight line is the smallest the stray current is maximal, and this area is the most disposed to stray current corrosion. When the straight line is close to vertical, any influence of the stray current source on the pipeline does not take place. When a straight line cannot be drawn through the plotted points this means that there are some additional sources of stray current, which must be detected.
4.2.7
Detection of Stray Currents and Their Sources on Protected Pipelines
The problem of stray current detection on cathodically protected pipelines located close to each other is more complicated than the above considered problem of current detection on unprotected pipelines in consequence of stray current interference with the protection currents. The above considered methods of stray current detection by voltage drop measurements can be used for protected pipelines on condition that the cathodic protection current of all closely spaced pipelines will be interrupted. An interrupter must be installed for this purpose in the cathodic protection circuits of the considered pipeline. Such requirement is feasible, since these devices are commonly used in modern cathodic protection systems for performing protection potential control and monitoring along the pipelines. One of the most used systems of cathodic protection control called close-interval potential survey (CIPS or CIS) consists of numerous measurements of potentials
98 ELECTROCORROSION AND PROTECTION OF METALS
Pipeline-to -soil potential, V
metal-to-soil along the underground metallic structures. These measurements are carried out at regular intervals and assess the protection potential level on the structure, which normally must be no less negative than 0.85 V. The protection potential value may fail in some areas of the structure due to influence of different factors, referred to relative positions of pipelines and ground beds, to characteristics and operational conditions of cathodic protection systems, to soil features etc. One of the main causes of such a failure is the attack by stray currents. Any source of stray current may upset normal operation of cathodic protection. Cathodically protected foreign pipelines disposed close to the considered protected structure are one of common current sources, especially at crossing areas of these objects. An example of such influence is given in Figure 4-13. At the absence of stray currents the potential metal-to-soil along a cathodically protected pipeline 1 is equal to or more negative than 0.85 V. However, in the area of crossing with a foreign cathodically protected pipeline the potential drastically shifts to the positive side, to the values which are much more positive than 0.85 V. Localization of this hazardous area on the base of data shown in Figure 4-13 enables revealing the stray current source, if other suspected sources of stray currents do not exist in this region. When too negative potential value of the protected pipeline is revealed, this may point to interference of stray current pick up from a ground bed of a foreign cathodic protection system. In these cases, measures must be undertaken for search hazardous areas along the pipeline where the current leaves the pipeline and, consequently, where the potential values are more positive than 0.85 V. Reliable detection of stray current source can be executed by application of an interrupter introduced into the cathodic protection circuit of the pipeline which is suspected as a source of stray current. The interrupter is set to operate by unequal periods of switching “off” and “on,” for example, respectively, 40 and 20 s or 20 and 10 s. CSE reference electrode is installed above the crossing area for potential measurement at interrupter positions “on” and “off.” Potential reading at the position “on” is carried out
-1.2 -1.0 -0.8 -0.6 -0.4
-0.85 V Hazardous area of the interfered pipeline Foreign cathodically protected pipeline
1
2
Interfered cathodically protected pipeline
FIGURE 4-13 Metal-to-soil distribution of potential along a cathodically protected pipeline crossing with a foreign cathodically protected pipeline.
Chapter 4 Detection, Identification and Estimation of Stray Currents
99
just before switching off the circuit, and the second reading must be made immediately after the interruption. If the potential metal-to-soil along influenced pipeline returns to the necessary range, this indicates that pipeline (2) is a single source of stray current. It was noted in the previous chapter that the influence of protected pipelines on other underground structures depends to a large extent on the coating quality of interfering pipelines. The most dangerous current source is a protected uncoated underground structure. If pipeline (1) in Figure 4-13 is coated and pipeline (2) is uncoated, it is most probable that the last one will be a strong current source with respect to pipeline (1). The lower is the coating quality of a protected underground structure, the more this structure is dangerous as a stray current source. Vice versa, a cathodically protected pipeline with a high-quality coating will have a minimal effect on metallic structures located in close proximity. A detailed analysis of mutual influence of several crossing pipelines of different kinds is given in [20]. Presence of dynamic stray currents hinders the detection of static stray currents. As was mentioned above, the source of the dynamic current can be revealed by registration of the potential fluctuations during 24 h. Results of such registration are given as an example in article [20] by M.J. Szeliga. The measurements of potentials (Figure 4-14) were carried out on a cathodically protected pipeline located close to electric transit system. At night hours the potential of the pipeline was stable and corresponded to the required values of the protection potential (0.85 V to CSE). The most intensive potential fluctuations occurred in the morning and afternoon rush hours. In these periods of time the potential shifted to the positive side, to values which are more positive than 0.85 V with respect to CSE reference electrode. Static stray currents sources can be detected in this area in the night hours with the aid of interrupter, as it was described above.
PIPE TO COPPER SULPHATE POTENTIAL-VOLTS
–2.0
–1.0
0
+1.0
MORNING RUSH HOUR
AFTERNOON RUSH HOUR
+2.0 MIDNIGHT
6 AM
NOON
6 PM
MIDNIGHT
FIGURE 4-14 Influence of dynamic stray current on pipe-to-soil potential of a pipeline in different periods of time during 24 h [20].
100
ELECTROCORROSION AND PROTECTION OF METALS
Coordination of all operations, including temporary disconnection of closely spaced cathodic protection circuits, is necessary in the process of detection of stray current sources. Therefore, cooperation of all owners of cathodically protected underground structures disposed in the examined area must be available.
4.2.8
Detection of Well Casing Stray Currents
The annular gaps between the casing strings and the well are filled by cement for preventing corrosion damage of well casing. However, cement not always can be introduced over the whole casing height and some dip parts of the strings often remain in contact with a mixture of underground water and mud liquid used in drilling operations. This mixture may possess high aggressiveness. Macrocells are often formed between the areas filled with cement and areas contacting directly with the aggressive liquid. Therefore, cathodic protection and constant control of corrosion state of the external surface of casing strings is necessary. There are two features that differentiate the methods of corrosion control and monitoring of well casing from the above considered corrosion control methods of horizontal underground metallic structures: practically impossible access to the external surface of strain casing and relatively easy access to its internal surface. So, direct potential measurements of the external surface cannot be carried out, but a probe with contacting elements can be introduced inside the internal casing string, and it can attain any area along the string height. In this regard a method of the external surface control with a special measuring probe was developed. Descriptions of the measurements which are carried out with this probe are given in Ref. [21]. A schematic of such a probe is shown in Figure 4-15. It includes two groups of contacting elements, A and B. The elements are electrically shorted inside the groups; the groups are insulated from one another and connected to a high-sensitive voltmeter with a scale of mV. In the measurement operations the probe is introduced inside the casing string and all contacting elements are pressed against the internal string surface by springs providing a reliable electrical contact with the metal of the internal string surface. Voltage drops between the macrocells M and N, which are spaced on the external string surface by distance L, are measured. The recommended distance L is in the range of 8 m. The measurements can be carried out along the whole vertical casing string. In each of the measurements the voltmeter determines not only the voltage drop value, which is usually of the order of several mV, but also cathodic and anodic areas of the macrocells. These measurements allow mapping the distribution of cathodic and anodic areas along the string axis. Example of potential distribution, called voltage profile along an unprotected casing string [21] obtained at the absence (curve 1) and at the influence of stray current attack (curve 2) is shown in Figure 4-16. The curves start from the depth of about 190 m; up to this depth the string has a double well filled with a dense layer of cement and any
Voltmeter + Well casing M A
L
Contacts
B N Pipe of plastic Springs FIGURE 4-15 Measuring probe for corrosion state assessment of casing string external surface.
-600
Δ V, μ volts 1000
1800
Depth of double wall casing
0
100
Depth, m
n 200
300
m d c
k g
400
500
2
600
f
1
700
800
e
FIGURE 4-16 Profiles of voltage drop on unprotected casing string: (1) absence of stray currents; (2) attack by stray currents [21].
102
ELECTROCORROSION AND PROTECTION OF METALS
potential drop does not exists at this section. A slope cd obtained at the absence of stray current indicates that anodic area of the casing string is located at a depth of about 300e370 m: at this string section the voltage drop has a negative value. The results of these measurements are not accurate enough, since they do not consider the influence of microcells and assume that the measured values do not change during the measuring operations and for some time after the measurements. However, these results provide information on the most dangerous steady-state macrocells. Well casing and connected to it facilities may be attacked by direct stray currents of the same origin as other, above considered underground metallic structures: high voltage transmission lines, welding devices, electric traction systems, grounding of electrical equipment, telluric currents. Areas of stray current pick up or discharge disposed close to the ground surface can be detected by the previously considered methods in which CSE reference electrodes and high input resistance voltmeters are used [22]. However, the major source of stray currents on the well casing are other cathodically protected casing strings, especially ground beds of their cathodic protection systems. The depth of oil and gas well casing attains thousands of meters that requires using deep ground beds disposed at a significant (by the order of hundreds meters) distance from the protected well casing. This increases the probability of stray current interference to the adjacent well casing which is not connected with the mentioned ground bed. Sometimes preliminary visual inspection of local corrosion damages may indicate that they originate from stray current attack. In a special Report from Alberta Oil Tool [23] data are conveyed on a specific view of pits resulting from arcs induced by stray currents: “Arcs originating from sucker rods leave a deep, irregular shaped pit with smooth sides, sharp edges and a small cone in the base of the pit. Arcs originating from the tubing leave deep pits with smooth sides and sharp edges that are random in dimension and irregular in shape. Stray current corrosion pits are usually singular and isolated in a row down one side of the sucker rod near the upsets.” The results of visual inspection are not reliable enough and may be used only for corrosion control of accessible equipment. The considered method with a special measuring probe is best suited for stray current detection on the well casing. This is seen in Figure 4-16 from the profile of curve 2, obtained at the same casing string attacked by stray current. Voltage drop in this case is four to six times greater than at the stray current absence. Cathodic area (efg) of current pick up and hazardous anodic area of current discharge (kmn) are clearly defined at curve 2. The method makes possible detection of the areas of stray current discharge and pick up at any depth of the well casing. As electrical resistance value of the L long tube is known, the axial current values can be calculated at every section of the string, where the voltage drop is measured. Since the directions of currents are also known, the values of currents along the string as well as the values of currents pick up and discharge at every section can be determined.
Chapter 4 Detection, Identification and Estimation of Stray Currents 103
For increasing the accuracy during measuring operation, the internal volume of the string must be filled with a liquid of high resistivity, such as demineralized water or some nonconductive oil. The considered method can be also used for stray current detection on a cathodically protected well casing.
4.2.9
Detection of Telluric Currents
Telluric currents are also considered as a kind of stray currents [3]. As distinct from the stray currents that come from electric traction systems, telluric currents can penetrate to the underground structures not only through the coating defects, but also owing to the induction effect. In the last case, the metal-to-soil potential fluctuations are not determined by current pick up or discharge from the metallic structure. Moreover, coating does not protect from the penetration of telluric currents. Potential fluctuations produced by telluric currents may be like the fluctuations of dynamic stray currents. In the periods of solar activity, the action of telluric current can be detected by finding the correlation between the magnetic field strength and the fluctuations of the metal-to-soil potential in the inspected metallic structure [24], provided that these two parameters are measured simultaneously. On the other hand, when inspection of the underground structure is carried out in an area known as a region of telluric current activity, it is preferable to carry out the measurements in the periods of a minimal solar activity. This makes possible avoiding the interference of telluric currents since these currents deteriorate results of the measurements. The absence of correlation in fluctuations of the metal-to-soil potential with the traffic schedule of electrified traction systems may indicate the presence of telluric currents. There are data [25] that in the presence of telluric currents the profile of potentialtime fluctuations on the coated underground pipelines remains similar at a length of tens and even of hundreds of kilometers. So, the information on the character of telluric currents on the coated pipeline can be obtained by measurement in a limited number of locations. Detection of telluric stray currents can be carried out by measuring near ground and far ground potentials [26]. Sometimes telluric current can be detected by the method of the voltage drop measurements along the pipeline. The most reliable information on the attack of cathodically protected pipeline by telluric currents can be obtained in close-interval potential survey [26,27]. Modern techniques make it possible to measure pipe-to-soil potential considering IR drop at the measured potential points [28]. When using these techniques is related with some difficulties, the information can be obtained by installation of measuring coupons in the most hazardous areas of the pipeline, as it is described in [29,30]. The coupons which reproduce defects in the pipeline coating are made of the same steel as the pipeline.
104
ELECTROCORROSION AND PROTECTION OF METALS
They are located close to the pipeline and electrically connected with it by cables. So, the coupons are cathodically protected, like the pipeline, but the cables can be switched off in the moments of current measurement. CSE reference electrodes are located close to the surface of each of the coupons that makes possible measuring IR free potential values. Coupons that are not connected with the pipeline and provided with CSE electrodes are also installed close to the pipeline for reference. This method can be successfully used for potential control along buried pipelines under the conditions of attack by manmade stray currents as well as by telluric currents [30].
References [1] S.Y. Li, S. Jung, K.W. Park, S.M. Lee, Y.G. Kim, Mater. Chem. Phys. 103 (1) (2007) 9e13. [2] V.S. Sastri, E. Ghall, M. Elboujdanini, Corrosion Prevention and Protection, John Willey & Sons, Ltd, N.Y, 2007, 562 p. [3] P.R. Roberge, Handbook of Corrosion Engineering, Mc Crow-Hill, N.Y, 2000, 1128 p. [4] M. Zee, Catastrophic Failure of Aging Underground Pipelines Is Inevitable under Certain Corrosion Conditions, Exova, Pittsburgh, 2016. www.puc.state.pa.us/.../gassafe/.../Gas_Safety_Seminar_2016. [5] J.M. Malo, V. Salinas, J. Uruchurtu, Mater. Perform. 33 (8) (1994) 63. [6] In: J.R. Davis (Ed.), Corrosion: Understanding the Basics, ASM, Materials Park, Ohio, 2000, 563 p. [7] Report on the Investigation of the Flooding of the UK Charter Fishing Vessel Random Harvest, 1999. https://assets.publishing.service.gov.uk/.../random_harvest. [8] J. Britton, Mater. Perform. 30 (2) (1991) 30e33. [9] D. Pascoe, Corrosion, A Boatman’s Primer on the Essential, 2015, www.yachtsurvey.com/corrosion. htm. [10] W.B. Holtsbaum, Stray currents in underground corrosion, in: Corrosion, Environments and Industry, ASM Handbook, vol. 13C, Materials Park, Ohio, 2006, pp. 107e114. [11] U.R. Evans, The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications, Edward Arnold Ltd, London, 1960. [12] S. Greenberger, T. Elliott, Electric rail corrosion and control, in: corrosion, environments and industries, ASM Handbook 13C (2006) 548e558. [13] J.R. Walters, Stray-current corrosion, in: L.L. Schreir, R.A. Jarman, G.T. Burstein (Eds.), Corrosion, third ed. Corrosion Control, vol. 2, 1994, pp. 10:122e10:129. [14] M.E. Parker, E.G. Peattie, in: Pipeline Corrosion and Cathodic Protection, Gulf Publishing Company, Houston, TX, 1999, 166 p. [15] AUCSC, Education and Training for Corrosion Control, Appalachian Underground Corrosion Short Course, West Virginia University, Morgantown, 2014. [16] Standard ASTM G57 e 06, Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-electrode Method, 2012. [17] H.H. Uhlig, R.W. Revie, Corrosion and Corrosion Control, John Willey & Sons, N.Y, 1985, 464 p. [18] E.I. Dizenko, V.F. Novosiolov, P.I. Tugunov, V.A. Yufin, Anticorrosion Protection of Piping and Vessels (In Russian), Nedra, Moscow, 1978, 199 p. [19] J.H. Fitzgerald III, Stray-current analysis, in: R.W. Revie (Ed.), Uhlig’s Corrosion Handbook, third ed., John Willey & Sons, Inc. ECS, Hoboken, New Jersey, 2011, pp. 1013e1020.
Chapter 4 Detection, Identification and Estimation of Stray Currents 105
[20] M.J. Szeliga, Stray current corrosion, in: R.L. Blanchetti (Ed.), Peabody’s “Control of Pipeline Corrosion”, second ed., NACE International, Houston, 2001, pp. 211e236. [21] W. Prinz, B. Leutner, Cathodic protection of well casing, in: W. v. Baeckmann, W. Schwenk, W. Prinz (Eds.), Handbook of Cathodic Protection: Theory and Practice of Electrochemical Protection Processes, third ed., Gulf Publishing Co, Houston, 1997, pp. 415e426. [22] W.B. Holtsbaum, Well casing external corrosion and cathodic protection, in: Corrosion, Environments and Industry, ASM Handbook, vol. 13C, Materials Park, Ohio, 2006, pp. 97e106. [23] A Special Report from Alberta Oil Tool, October 2001, 14 p. www.albertaoiltool.com/pdf/Rod_ Failure.pdf. [24] D.H. Boteler, L. Trichtchenko, Telluric influence, in: R.W. Revie (Ed.), Oil an Gas Pipelines, Integrity and Safety Handbook, J. Villey & Sons, Inc., 2015, pp. 275e288. [25] Stray currents in underground corrosion, in: W.B. Holtsbaum (Ed.), ASM Handbook, in: S.D. Cramer, B.S. Covino (Eds.), Corrosion: Environments and Industries, vol. 13C, 2006, pp. 107e114. [26] A. Kowalski, The close-interval potential survey (CIS/CIPS) method for detecting corrosion in underground pipelines, in: M.E. Orazem (Ed.), Underground Pipeline Corrosion, Elsevier, Amsterdam, N.Y, 2014, pp. 227e246. [27] D. Hevle, A. Kowalski, Close-interval survey techniques, in: S.D. Cramer, B.S. Covino (Eds.), ASM Handbook, Corrosion: Environments and Industries, vol. 13C, 2006, pp. 84e88. [28] W. v Baeckmann, W. Schwenk, Fundamentals and practice of electrical measurements, in: W. v. Baeckmann, W. Schwenk, W. Prinz (Eds.), Handbook of Cathodic Protection: Theory and Practice of Electrochemical Protection Processes, third ed., Gulf Publishing Co, Houston, 1997, pp. 79e138. [29] P.J. Nicholson, Mater. Perform. 46 (10) (2007) 24e28. [30] C.D. Stears, O.C. Moghissi, L. Bone III, Mater. Perform. 37 (2) (1998) 23e31.