Patent ID: 12235205

DETAILED DESCRIPTION

Cathodic protection measurements on underground or submerged pipelines for crude oil, natural gas, refined products, petrochemical, water pipelines, bulk storage tanks, and the like, often include corrosion influences from dissimilar-metal contacts, e.g., contacts between different, or dissimilar, metals, contacts with electrical grounding systems, and even contacts with the regional AC power grid copper neutral system. These mixed-metal potential differences or voltages may cause substantial challenges in determining whether adequate cathodic protection is being applied to control external corrosion rates on exposed metal surfaces in electrolyte contact. The metal structures could be buried in soils on land or operate in submerged conditions, related to river crossings, lake crossings and even open ocean environments. In some cases, the reactions may take place on internal tank or vessel metal surfaces, and measurement devices are needed on the insides of such vessels.

Stationary reference cell technology has been used for the past 15 years or more, and often includes a set of metal coupons installed on the reference cell body (see U.S. Pat. No. 9,804,078). Such devices are typically called stationary coupon reference cells, or permanent coupon reference cells. The metal coupons in most cases are made of carbon steel, similar in metallurgy to welded steel pipeline metal, and to bulk above-ground storage tank (AST) bottom plate steel, but may also be made of ductile iron, when the metal structures being studied for corrosion processes and risk are also made of ductile iron. Some coupon materials may include 316 stainless steel. In examples, all existing coupon reference cells described herein are made using a single type of metal coupon.

Examples of this disclosure thus provide not just one type of metal coupons on the stationary reference cell body, but two or more different metal coupon types. As an example, in the on-land corrosion control industry, there are many situations in which the copper neutral system of the AC power grid becomes electrically joined to buried pipeline metal, bulk AST external bottom metal, and other buried metal structures such as, e.g., reinforcing steel in concrete footings and foundations, structural steel piling, steel equipment skids set on soils, pipe rack metal supports drilled or otherwise placed into soils, and the like, in facilities such as e.g., refineries, natural gas compression stations, petrochemical plants, water treatment plants, and the like. Because copper is a better electrical conductor compared to carbon steel, and to most other industrial metals from which pipelines and other subsurface structures may be built, copper becomes a desired metal to which cathodic protection current is applied, with the electrically connected carbon steel pipelines and storage tank bottoms suffering a reduced application of cathodic protection current. Copper is more electrochemically noble or passive than carbon steel, and as a result does not usually require cathodic protection to resist in-soil external corrosion. However, copper receives the cathodic protection in preference to carbon steel surfaces based on copper being far more electrically conductive than the carbon steel. These factors commonly lead to under-protection of the underground carbon steel structures for which the cathodic protection current is actually intended. Copper also typically does not electrochemically polarize very strongly over time, whereas carbon steel achieves good electrochemical polarization when enough protective current has been, or is being, delivered. For carbon steel that is not joined to copper or another more noble metal, this polarization effect reduces the amount of protective cathodic protection current needed once polarization has occurred. As a result, bare copper surfaces continuously receive large quantities of cathodic protection current, reducing the amount of protective current that electrically joined carbon steel metal can ever receive. That current is then transmitted through the AC neutral wire grid along a current return flow path to the particular cathodic protection system involved. None of this current flow provides protection to the intended metal structures, most commonly formed of carbon steel.

In various examples, using a stationary reference cell, as further discussed below, may substantially improve the ability of the corrosion control industry personnel to recognize dissimilar-metal interactions, to gauge the prevalence and severity of the interactions including overprotection conditions, and to properly mitigate the interactions when possible. This may significantly reduce the cost of the sizing, installation and operation of cathodic protection systems, and may more clearly identify the need for installation of electrical decoupling devices sometimes used to break electrical connections between copper and steel structures for DC current flows. Examples of this disclosure may also provide more targeted, better quality cathodic protection applications to many buried or submerged metallic structures.

In various examples of the current disclosure, the coupons built into stationary reference cells may be put together in pairs, so that a first coupon can receive cathodic protection current. The second metal coupon, made of an identical metal as the first coupon, is not provided cathodic protection, and the second coupon may remain in an unprotected, or native, electrochemical condition. Cathodic protection effectiveness may be measured as a direct-current (DC) voltage magnitude and polarity, or as the potential difference between the metal structure surface in soil contact and the actual reference cell placed in soil or other electrolyte (fresh water, brackish water or salt water, for instance) contact. For most applications in soils and fresh water, the applicable reference cell may include a copper rod immersed in copper sulfate ion solution and deionized water, all of which being surrounded by a semi-permeable membrane. For brackish and saltwater exposures, the applicable reference cell may include a silver rod immersed in a silver chloride ion solution and deionized water mixture, all surrounded by a semi-permeable membrane.

When the copper-copper sulfate reference cell is used, three measurement criteria may be relied on to estimate whether a buried or submerged carbon steel structure is receiving adequate cathodic protection. The first criterion of the three measurement criteria, referred to herein as Criterion I, includes using a cathodic protection current continuously applied to the metal surfaces in electrolyte contact, where the potential difference, measured as a DC voltage, between the metal surface of the submerged carbon steel structure and a reference cell is −0.850 V or more negative, e.g., −0.850V, −0.900 V and the like. This Criterion I measurement may be performed after any “IR drop” voltage contributions have been removed from the original measurement. IR drops are the artificial voltages created by the distance that measurement circuit current travels through soils or other electrolyte path, by that same current traveling across contact resistances between the steel surface and soil/other electrolyte interface, the touch point of meter wiring to the reference cell lead wire, and the measurement current traveling through wire paths which complete the measurement circuit. One benefit of a stationary reference cell is that the stationary reference cell is buried substantially close to the structure being tested, thus reducing the IR drop contribution from a soil/electrolyte path length in comparison to a portable reference cell being placed very close to ground or other electrolyte surface, and farther from the structure.

The second criterion of the three measurement criteria, referred to herein as Criterion II, is measured within about one-quarter of a second (0.25 s) or less after the cathodic protection current flow has been temporarily interrupted to the buried or submerged structure being protected. In other words, the cathodic protection current is interrupted, and the Criterion II measurement is performed substantially immediately thereafter. When the cathodic protection of the buried/submerged structure is sufficiently protected, the measured “instant-off” or current-interrupted voltage may be −0.850 V DC or less (more negative), e.g., −0.850V, −0.900 V and the like. A benefit of current interruption is that substantially all the IR drop voltage contributions disappear from the measurement, since cathodic protection current is no longer flowing.

The third criterion of the three measurement criteria, referred to herein as Criterion III, is referred to as the polarization shift comparison. Criterion III relies on two different cathodic protection voltage measurements, wherein the current-interrupted voltage that is obtained during Criterion II as described above is compared to a native or depolarized structure-to-electrolyte voltage. The native voltage may be obtained either from the same structure after cathodic protection was turned off for a period of days to weeks, or by measuring the voltage on a piece of native coupon metal that is similar or identical to the material of the buried or submerged structure to be protected in the same type of electrolyte or soil and substantially close to the submerged structure. However, the native coupon is not electrically connected to buried/submerged structure or to the source of cathodic protection. The goal of Criterion III is to establish whether the difference between the current-interrupted voltage and the native or unprotected voltage is 100 millivolts (0.1 V) DC or greater. Criterion III is particularly valuable for old, poorly coated steel pipelines, or for bare carbon steel tank bottom external metal surfaces. Bare steel structures in soils or water typically require far greater amounts of cathodic protection current to be applied continuously, and do not obtain the same degree of electrochemical protection, expressed by more negative structure-to-electrolyte voltages, compared to well-coated steel structures. Criterion III is not typically considered to be practical for application when dissimilar-metal contacts, which include carbon steel, are present in a structure or set of structures. It is, however, often used as an acceptable criterion for the cathodic protection of copper, or of stainless steel, when they are not connected with dissimilar metals. The device and methods described herein offer new ways to do such measurement and data interpretation and will allow much better consideration of this particular criterion with respect to dissimilar-metal interactions and especially for carbon steel structures joined with other metals.

Coupons made of other metals that are dissimilar compared to the underground structure to be cathodically protected may also be used. For example, buried stainless steel pipelines, commonly of 304 or 316 stainless steel, are used in many petrochemical and refinery plant settings, and in other industrial settings as well. These stainless steel pipelines, although typically coated with high-dielectric-strength materials, typically cause interactions with carbon steel and even copper underground structures, because the stainless steel metal, when exposed to electrolyte contact, is more cathodic than either copper or carbon steel. Coupon reference cells according to the current disclosure may help confirm the presence of the dissimilar-metal interactions, measure the degrees of interaction based on coupon voltages found, and compare the DC currents flowing to and from each particular coupon. Another substantial risk to stainless steel metal pipelines is that of cathodic over-protection. Stainless steel structures placed in underground or submerged service are susceptible to cathodic protection (CP) over-protection, which may cause hydrogen-assisted cracking in the metal. This is especially concerning when impressed-current cathodic protection is applied to mixed-metal systems in complex facilities, which often include carbon steel, copper and stainless steel structures electrically joined. When a cathodic protection system is used to protect large carbon steel structures in buried or submerged service, the protection delivered to the carbon steel surfaces may be so strong that nearby stainless steel may suffer hydrogen embrittlement damage. Measurements of native stainless steel voltages can be used in comparison to the CP-applied and current-interrupted voltages, and then be compared to the carbon steel coupons, the copper coupons and the other metals in use. A CP practitioner may more quickly and effectively recognize the over-protection risk imposed on the stainless steel structure, and mitigation steps can be implemented. Similarly, other structures susceptible to cathodic over-protection such as American Water Works Association C301 pre-stress-wire-wrapped, reinforced concrete steel cylinder pressure pipe may be evaluated using the same approach, whether associated with dissimilar metals or not. The present invention may also allow accurate measurement and improved evaluation of such over-protection risks for stainless steel and other at-risk metallic structures in underground or submerged services.

A coupon set to be used for vessels and tanks in water treatment plants may also include carbon steel and aluminum coupon pairs and may be combined with a copper coupon or other metal pair. Many vessels and tanks for water supply are built using aluminum, and the dissimilar-metal contacts of aluminum with steel, with copper, and with ductile iron (commonly used for buried water pipeline work) may cause the more electrochemically active aluminum to corrode as the anode in each of these dissimilar-metal pairs. Aluminum also exhibits significantly increased corrosion rates at water pH levels outside a range of about 4 to 8.5. The aluminum coupons may be monitored in comparison to the other metal coupons, for current flows and voltages, to show when the aluminum is electrochemically more anodic and under increased corrosion attack.

In each coupon reference cell setup described in examples of this disclosure, switching may be provided to allow the joining of two or more current-interrupted coupons, and/or of the native coupons, so that the mixed-metal behaviors of each dissimilar-metal group may be temporarily monitored. Another example includes permanently joining dissimilar metal coupons to one another, so that the mixed-metal voltages may be measured continuously. This may show the slow voltage changes that arise from changing soil moisture conditions, soil temperature conditions, spring and summer growing-season soil conditions versus fall and winter no-growth or less-growth soil conditions, and the like. It may also show the voltages to be very similar to the nearby metal structure complex, which also includes joined carbon steel and copper metals in the electrolyte, for example. Examples of this disclosure include an improved stationary coupon reference cell device, and the example methods disclosed herein allow for improving the interpretation and understanding of dissimilar-metal interactions, and thereby improving cathodic protection of below-grade metal assets.

FIG.1is a schematic diagram of a test head terminal block, in accordance with various examples of the disclosure. InFIG.1, the test head terminal block000is a system that encompasses various terminals that may be used to test and effectively monitor the adequacy of cathodic protection being applied to a metallic structure, e.g., cathodically protect an underground or submerged metallic structure. The test head terminal block000may include a plurality of terminals such as, e.g., structure terminals201and202, which are coupled to the underground or submerged structure. For example, the underground or submerged structure may be a pipeline or other underground or submerged metallic structure. Terminals203-210are test terminals that include substantially a single metal. The metals in each of terminals203-210may be the same or different from each other, but the metal in each terminal is a single metal such as, e.g., copper, or stainless steel. Terminals211and212are mixed-metal terminals, e.g., terminals that include more than one metal. Terminals213and214a metal native terminals, e.g., terminals that are unconnected to the underground or submerged structure to be protected. Terminal215is a reference cell terminal, e.g., a terminal connected to a reference cell. The reference cell may be used to, e.g., measure voltages of the underground or submerged structure.

FIG.2is a schematic diagram of a cathodic protection measurement circuit, in accordance with various examples of the disclosure.FIG.2illustrates a circuitry connecting various elements from the test head terminal block000illustrated inFIG.1, the test head terminal block000being coupled to the structure101that is, e.g., the underground or submerged structure to be cathodically protected such as, e.g., an underground or submerged metallic pipeline or other structure.FIG.2also illustrates a plurality of metal test coupons001-007housed on a reference cell301. As such, voltages at each of the metal test coupons001-007may be measured with respect to the reference cell terminal215that is connected to the reference cell301. In the example configuration illustrated inFIG.2, the metal test coupon001is electrically coupled to metal test terminals206and203in series, and then to the structure terminal201via the metal test terminals206and203. The metal test terminals206and203are electrically coupled to one another via inline resistor501A, at which current flow and polarity (direction of DC current travel) can be measured, and the metal test terminal203and the structure terminal201are coupled together via switch401A which may turned on and off. Accordingly, the connection between the metal test terminal203and the structure terminal201can be controlled and turned on and off as desired.

Similarly, the metal test coupon002is electrically coupled to metal test terminals207and204in series, and then to the structure terminal201via the metal test terminals207and204. The metal test terminals207and204are electrically coupled to one another via inline resistor501B, at which current flow and polarity can be measured, and the metal test terminal204and the structure terminal201are coupled together via switch401B. Accordingly, the connection between the metal test terminal203and the structure terminal201can be controlled and turned on and off as desired. The metal test coupon003is electrically coupled to metal test terminals208and205in series, and then to the structure terminal201via the metal test terminals208and205. The metal test terminals208and205are electrically coupled via inline resistor501C, at which current flow and polarity can be measured, and the metal test terminal205and the structure terminal201are coupled together via switch401C. Accordingly, the structure terminal201may be coupled to any one or more of the metal test coupons001,002and003by closing or opening any one or more of the switches401A,401B and401C. As a result, the submerged or underground structure to be protected101may be electrically coupled to any one of the metal test coupons001,002and002by opening or closing any one or more of the switches401A,401B and401C. Based on the circuitry illustrated inFIG.2, various dynamic measurements may be performed between the metal test terminals203-208, the metal test coupons001-003, and the structure101.

FIG.2also illustrates that the metal test coupon004is electrically coupled to metal test terminals211and209in series and then to the structure terminal202via the metal test terminals211and209. The metal test terminals211and209are electrically coupled to one another via inline resistor501D, at which current flow and polarity can be measured, and the metal test terminal209and the structure terminal202are coupled together via switch401D. The metal test coupon005is electrically coupled to metal test terminals212and210in series and then to the structure terminal202via metal test terminals212and210. The metal test terminals212and210are electrically coupled to one another via inline resistor501E, at which current flow and polarity can be measured, and the metal test terminal210and the structure terminal202are electrically coupled together via switch401E. In this configuration, the metal test terminals212and211are electrically coupled together via inline resistor501F at which current flow and polarity can be measured. As a result of this configuration, any one of the metal test coupons004and005may be coupled to the structure terminal202, and ultimately to the underground structure101, via activation of switches401D and401E. In addition, due to the inline resistor501F connecting metal test terminals212and211, both of the metal test coupons004and005may be concurrently coupled to the structure terminal202and to the underground structure101.

FIG.2further illustrates that the metal test coupon006is electrically coupled to metal native terminal213which remains unconnected to the structure101, and the metal test coupon007is electrically coupled to metal native terminal214via switch401F. The native terminals213and214may not include the same metal, and are electrically coupled together via inline resistor501G, at which current flow and polarity can be measured. Neither of the metal test coupons006and007are connected to the underground structure101. Accordingly, any voltage measured at either one of the native terminals213and214is reflective of the interaction of each terminal with the ground or medium in which the structure101is buried or submerged.

FIG.3illustrates another example configuration of a cathodic protection measurement circuit, in accordance with various examples of the disclosure. InFIG.3, the underground structure101is electrically coupled to structure terminals201and202, and each structure terminal201and202is coupled to a given circuit. In an example, structure terminal201is electrically coupled to metal test coupons001,002and003via a plurality of inline resistors501A,501B and501C, at which current flow and polarity can be measured, respectively, and a plurality of switches401A,401B and401C, respectively. Specifically, structure terminal201is coupled to metal test coupon001via metal test terminals203and206, where metal test terminal203is coupled to the structure terminal201via switch401A and to metal test terminal206via inline resistor501A. Similarly, structure terminal201is coupled to metal test coupon002via metal test terminals204and207, where metal test terminal204is coupled to the structure terminal201via switch401B and to metal test terminal207via inline resistor501B. Structure terminal201is also coupled to metal test coupon003via metal test terminals205and208, where metal test terminal205is coupled to the structure terminal201via switch401C and to metal test terminal208via inline resistor501C.

In another example, structure terminal202is electrically coupled to metal test coupons004and005via a plurality of inline resistors501D and501E, respectively, and switches401D and401E, respectively. Specifically, structure terminal202is electrically coupled to metal test coupon004via metal test terminals209and211, where metal test terminal209is coupled to the structure terminal202via switch401D and to metal test terminal211via inline resistor501D. Structure terminal202is also electrically coupled to metal test coupon005via metal test terminals210and212, where metal test terminal210is coupled to the structure terminal202via switch401E and to metal test terminal212via inline resistor501E. InFIG.3, metal test terminals211and212may be electrically coupled to each other via an inline resistor501F. As a result of the above-described configuration, the underground structure101may be electrically coupled to the structure terminals201and202as well as to any one of the metal test coupons001-005, as desired. Accordingly, any voltages or currents flowing between the underground structure101and any one of the test coupons001-005may be tested and measured as desired.

FIG.3further illustrates the reference cell terminal215electrically coupled to a reference cell301. Accordingly, any voltages flowing through any of the above circuit configurations may be measured with respect to the reference cell301, so as to provide a consistent baseline of voltage measurements. AlthoughFIG.3does not show connections between the reference cell terminal215and any other circuit, it is understood that any voltages and currents measured for each of the circuits illustrated inFIG.3are measured with respect to the reference cell301and connected thereto via the reference cell terminal215.

Also inFIG.3, a native circuit, which is a circuit that is unconnected to the underground structure101, includes native metal terminals213and214, and native coupons006and007. Native coupons006and007may have, e.g., dissimilar metals. The metal native terminal213may be directly electrically coupled to the native coupon006, and may be coupled to the native metal terminal214via inline resistor501G. The native metal terminal214may be electrically coupled to the native coupon007via switch401F. Accordingly, any voltages and currents measured, with respect to the reference cell301, for the above native circuit, are indicative of the state of the unprotected mixed-metal structure101as buried or submerged in the electrolyte.

InFIGS.2and3, the test head terminal block000and the circuits that include the structure terminals201and202may be coupled to a processor150that includes, e.g., a memory130. Accordingly, operation of the circuits discussed may be controlled via operation of the processor150and the memory130.

FIG.4is a flow chart illustrating a method of measuring cathodic protection of a metallic structure using a plurality of coupons, in accordance with various examples of the disclosure. The metallic structure may be, e.g., an underground pipeline buried or submerged in a soil or electrolyte. The method400includes operation410, which includes electrically coupling a first coupon to the metallic structure. The first material may be or include a same material as the metallic structure. For example, the first material may be or include stainless steel such as, e.g.,316steel. Operation420includes electrically coupling a second coupon to the metallic structure, and may include a second material that is more or less noble than the first material. For example, the second material may be or include copper. With reference toFIG.2, operations410and420may be performed by coupling coupons004and005to the underground or submerged metallic structure101by closing switches401D and401E, respectively. Operation430includes electrically coupling the first coupon to the second coupon. With reference toFIG.2, operation430may be performed by coupling coupon004with coupon005via the resistor501F. Operation440includes applying a first cathodic protection current to the metallic structure. For example, applying the first cathodic protection current includes applying stepwise increases in the first cathodic protection current.

Operation450includes interrupting the application of the first cathodic protection current. Operation460includes, subsequent to interrupting the application of the first cathodic protection current, measuring the voltage between the electrically coupled first and second coupons, metallic structure and the reference cell, respectively. In various examples, prior to measuring the voltage between the electrically coupled first and second coupons, operation460includes measuring a first voltage of the first coupon with respect to the reference cell, and measuring a second voltage of the second coupon with respect to the reference cell, both measurements being performed while the application of the first cathodic protection current is interrupted. With reference toFIG.3, operation460may be performed via reference cell terminal215coupled to the reference cell301, so that the voltages are measured with respect to reference cell301. Measuring the voltage may be performed within a given period of time such as, e.g., a period of time in a range of 0.1 second to 0.5 second, after the interruption of the application of the first cathodic protection current. The measured voltage may be a direct current (DC) voltage.

Operation470includes determining whether a degree of cathodic protection applied to the metallic structure is sufficient based on the measured voltage, subsequent to the interruption of cathodic protection current. When the measured voltage is equal to or more negative than −0.850 V, then during operation470, the degree of cathodic protection is determined to be sufficient. When the measured voltage is more positive than −0.850 V, then the method400further includes applying a second cathodic protection current to the metallic structure, the second cathodic protection current being greater than the first cathodic protection current. Applying the second cathodic protection current may include applying a stepwise increase in the second cathodic protection current. When the second cathodic protection current is applied, the method400further includes interrupting the application of the second cathodic protection current, and may further include repeating operations460-470. Measuring the second voltage may be performed within a given period of time such as, e.g., a period of time in a range of 0.1 second to 0.5 second, after the interruption of the application of the second cathodic protection current.

In examples, the method400further includes measuring a structure-to-electrolyte voltage between the structure and another reference cell at a second location of the structure, calculating a difference between the structure-to-electrolyte voltage and the third voltage, and determining a level of cathodic protection of the structure at the second location based on the calculated difference as discussed above with respect to operations410-470.

FIG.5is a flow chart illustrating a method of measuring cathodic protection of a metallic structure using a plurality of coupons, in accordance with various examples of the disclosure. The method500includes operation510, which includes electrically coupling a first coupon and a second coupon to the metallic structure, the first coupon and the metallic structure including a same first material, and the second coupon including a second material that is more or less noble than the first material. With reference toFIG.2, operation510may be performed by coupling coupons001and002to the structure101. In various examples, before electrically coupling the first coupon and the second coupon to the metallic structure, operation510includes measuring a current flowing to the first coupon and determining a current density of the current flowing to the first coupon based on the measured current, and measuring a current flowing to the second coupon and determining a current density of the current flowing to the second coupon based on the measured current. Once the above discussed current densities are determined, a current flow may be increased and applied to the first and second coupons such that the current densities for both coupons are substantially equal to each other. In this case, substantially equal may refer to a difference that is equal to or less than 10%.

Operation520includes applying a first cathodic protection current to the metallic structure. In the case of multiple applications of cathodic current, applying the first cathodic protection current includes, or may be performed by, applying a plurality of stepwise increases to the first cathodic protection current. Operation530includes contemporaneously, concurrently or simultaneously, measuring a first current flowing between the coupled first coupon and second coupon and the metallic structure. With reference toFIG.3, operation530includes measuring the current via a calibrated measurement resistor. When the applied first current is an applied stepwise current, then operation530includes measuring the first current and calculating the first current density for each stepwise increase.

Operation540includes determining a first current density based on the measured first current. For example, determining the first current density may include calculating the current density by dividing the measured current with the surface area of the first coupon. Operation550includes contemporaneously, concurrently or simultaneously, measuring a second current flowing between a second coupon and or other electrolyte via the reference cell, and measuring what should be zero current flow to the third coupon, native in characteristic, being unconnected to the metallic structure and including the first material. During operation550, when a plurality of stepwise increases in the first cathodic protection current are applied, operation550includes measuring the second and subsequent current flow for each stepwise increase. Operation560includes determining a second and subsequent current density based on the measured second and subsequent current flows. For example, determining the second current density may include calculating the current density by dividing the measured current with the surface of the first coupon. The same set of steps may also be performed using the second coupon, of a different metal than the first coupon. Operation570includes determining whether a degree of cathodic protection of the metallic structure is sufficient based on the calculated first current density, and the calculated second current density, depending on the types of metal involved and their protection criteria to be applied. For example, operation570includes determining that the degree of cathodic protection of the metallic structure is sufficient when the first coupon current density is substantially equal to the second coupon current density, and the less noble metal shows adequate protection based on the measured voltages found at each particular rate of current flow.

Operation570may further include calculating a difference between the first coupon current density and the second coupon current density, and when the calculated difference between the first current density and the second current density is equal to or less than the absolute value of 10% of the first current density, the degree of cathodic protection of the metallic structure is determined to be sufficient. When the difference between the first current density and the second current density is greater than 10%, as the more noble coupon will have a greater current density applied than the less noble coupon, then operation570further includes applying a second cathodic protection current to the metallic structure, the second cathodic protection current being greater than the first cathodic protection current. Applying the second cathodic protection current may include applying a stepwise increase compared to the first cathodic protection current. As increased currents are applied, voltage measurements are also taken, to track the changes in effectiveness of cathodic protection at each coupon.

FIG.6is a flow chart illustrating a method of measuring a flow of AC current from a metallic structure to an electrical grounding structure using a plurality of coupons coupled to a stationary reference cell, in accordance with various examples of the disclosure. The method600includes operation610, which includes mounting a first metal coupon and a second metal coupon on a coupon reference cell. Operation610also includes placing the coupon reference cell adjacent to the metallic structure, which may be a specialty AC mitigation media often formed of either zinc ribbon or of bare copper cable, the first coupon being electrically connected to the specialty mitigation media and including a first material, the second metal coupon being electrically connected to the specialty mitigation media and including a second material. Operation620includes measuring an AC current flow of the first coupon and of the second coupon. Operation630includes determining a first current density and a second current density based on the measured first current flow and second current flow, respectively. Operation640includes comparing the measured AC current flow of the first coupon and the determined first current density to the measured AC current flow of the second coupon and the determined second current density, and continuing to perform these measurements and calculations over a period of time. Operation650includes determining which of the first coupon and the second coupon represents a lower-electrical-resistance grounding structure over time and mitigation service based on the comparisons performed during operation640.

This disclosure described some examples of the present technology with reference to the accompanying drawings, in which only some of the possible examples were shown. Other aspects can, however, be embodied in many different forms and should not be construed as limited to the examples set forth herein. Rather, these examples were provided so that this disclosure was thorough and complete and fully conveyed the scope of the possible examples to those skilled in the art.

Although specific examples were described herein, the scope of the technology is not limited to those specific examples. One skilled in the art may recognize other examples or improvements that are within the scope of the present technology. Therefore, the specific structure, acts, or media are disclosed only as illustrative examples. Examples according to the technology may also combine elements or components of those that are disclosed in general but not expressly exemplified in combination, unless otherwise stated herein. The scope of the technology is defined by the following claims and any equivalents therein.