Cathodic Protection of Cathodic Protection of Underground Petroleum Underground Petroleum Storage Tanks and Piping Storage Tanks and Piping SystemsSystemsAMERICAN PETR!EUM AMERICAN PETR!EUM INSTITUTEINSTITUTERECMMEN"E" PRACTICE RECMMEN"E" PRACTICE #$%$%&Cathodic Protection of Underground Cathodic Protection of Underground Petroleum Storage Tanks and Piping Petroleum Storage Tanks and Piping SystemsSystemsComponents of UST systems that are in contact Components of UST systems that are in contact with the ground, are constructed of steel, and with the ground, are constructed of steel, and routinely carry product, must be cathodically routinely carry product, must be cathodically protected unless a corrosion expert determines protected unless a corrosion expert determines that the site is not corrosive enough to cause a that the site is not corrosive enough to cause a release during the system's operating life. release during the system's operating life. Otherwise, new systems must have cathodic Otherwise, new systems must have cathodic protection when installed and existing systems protection when installed and existing systems must be upgraded with cathodic protection within must be upgraded with cathodic protection within ! years, except that upgrading by interior lining ! years, except that upgrading by interior lining combined with inspection may be used in lieu of combined with inspection may be used in lieu of adding cathodic protection.adding cathodic protection.Corrosion of 'uried Steel Corrosion of 'uried Steel StructuresStructuresCorrosion may be defined as the deterioration of Corrosion may be defined as the deterioration of metal due to a reaction with its environment. metal due to a reaction with its environment. "xternal corrosion of buried steel structures is an "xternal corrosion of buried steel structures is an electrochemical process. electrochemical process. #or the process to occur, areas with different #or the process to occur, areas with different electrical potentials must exist on the metal electrical potentials must exist on the metal surface. These areas must be electrically surface. These areas must be electrically connected and in contact with an electrolyte.connected and in contact with an electrolyte. There are, therefore, There are, therefore, four components in each four components in each electrochemical corrosion cell$ an anode, a electrochemical corrosion cell$ an anode, a cathode, a metallic path connecting the anode cathode, a metallic path connecting the anode and cathode, and an electrolyte. and cathode, and an electrolyte. "lectrochemical Corrosion Cell"lectrochemical Corrosion CellThe role of each component in the The role of each component in the corrosion process is as follows$corrosion process is as follows$%t the anode the base metal goes into solution%t the anode the base metal goes into solution &corrodes' by releasing electrons and forming positive&corrodes' by releasing electrons and forming positive metal ions. metal ions.(o metal loss occurs at the cathode. )owever, other(o metal loss occurs at the cathode. )owever, other chemical reactions occur that consume the electronschemical reactions occur that consume the electrons released at the anode. released at the anode.*ositive current flows through the metal path connecting*ositive current flows through the metal path connecting the cathode and anode.the cathode and anode. "lectrons generated by the"lectrons generated by the chemical corrosion reactions at the anode are conductedchemical corrosion reactions at the anode are conducted through the metal to the cathode through the metal to the cathode where they arewhere they are consumed. consumed.*ositive current flows through the electrolyte from the*ositive current flows through the electrolyte from the anode to the cathode to complete the electrical circuit. +nanode to the cathode to complete the electrical circuit. +n the case of buried structures the electrolyte is moist soil. the case of buried structures the electrolyte is moist soil.Corrosion ProcessesCorrosion Processes(al)anic Corrosion (al)anic CorrosionCorrosion is usually not limited to a single point as shown in #ig. .Corrosion is usually not limited to a single point as shown in #ig. . +n the case of general corrosion, thousands of microscopic corrosion+n the case of general corrosion, thousands of microscopic corrosion cells occur randomly over the metal surface resulting in relativelycells occur randomly over the metal surface resulting in relatively uniform metal loss.uniform metal loss. +n the case of pitting, the individual corrosion cells tend to be larger+n the case of pitting, the individual corrosion cells tend to be larger and distinct anode and cathode areas can often be identified. ,etaland distinct anode and cathode areas can often be identified. ,etal loss may be concentrated within relatively small areas andloss may be concentrated within relatively small areas and substantial areas of the surface may be unaffected by corrosion. substantial areas of the surface may be unaffected by corrosion.-oth metal composition and environmental factors may determine-oth metal composition and environmental factors may determine which areas on a metal surface become anodes or cathodes.which areas on a metal surface become anodes or cathodes. Steel is an inherently non.homogeneous material, and for aSteel is an inherently non.homogeneous material, and for a particular environment potential differences between ad/acent areasparticular environment potential differences between ad/acent areas can result from uneven distribution of alloying elements orcan result from uneven distribution of alloying elements or contaminants within the metal structure.contaminants within the metal structure. 0ifferences between the weld material and the steel plate can also0ifferences between the weld material and the steel plate can also cause corrosion cells in welded structures. cause corrosion cells in welded structures.(al)anic Corrosion(al)anic Corrosion*hysical and chemical properties of the soil *hysical and chemical properties of the soil &electrolyte' may also influence the location of &electrolyte' may also influence the location of anodic and cathodic areas on the metal surface. anodic and cathodic areas on the metal surface. #or example, differing oxygen concentrations at #or example, differing oxygen concentrations at different areas on a buried steel structure may different areas on a buried steel structure may generate potential differences. %reas with lower generate potential differences. %reas with lower oxygen concentrations become anodic areas oxygen concentrations become anodic areas and areas with higher oxygen concentrations and areas with higher oxygen concentrations become cathodic areas. become cathodic areas. This may result in more severe corrosion attac1 This may result in more severe corrosion attac1 at the bottom of a buried tan1 than at the top of at the bottom of a buried tan1 than at the top of the tan1 since oxygen concentration in soil is the tan1 since oxygen concentration in soil is primarily dependent on diffusion from the soil primarily dependent on diffusion from the soil surface ig. 2'. surface ig. 2'. #ig. 2. Corrosion caused by differences in Oxygen and moisture content of soils.(al)anic Corrosion(al)anic CorrosionThe same mechanism can also contribute to corrosion inThe same mechanism can also contribute to corrosion in areas where clay or debris contact a steel tan1 buried inareas where clay or debris contact a steel tan1 buried in a sand bac1fill, or where a tan1 is buried in two differenta sand bac1fill, or where a tan1 is buried in two different types of soil ig. 3'. types of soil ig. 3'.Soil characteristics substantially affect the type and rateSoil characteristics substantially affect the type and rate of corrosion occurring on buried structures. #or example,of corrosion occurring on buried structures. #or example, dissolved salts influence the current.carrying capacity ofdissolved salts influence the current.carrying capacity of the soil electrolyte and help determine reaction rates atthe soil electrolyte and help determine reaction rates at anode and cathode areas. Soil moisture content, p) &aanode and cathode areas. Soil moisture content, p) &a measure of acidity', and the presence of sulfides alsomeasure of acidity', and the presence of sulfides also influence corrosion. These factors, and perhaps others,influence corrosion. These factors, and perhaps others, interact in a complex fashion to influence corrosion atinteract in a complex fashion to influence corrosion at each location. each location.#ig. 3. Corrosion caused by dissimilar soils Stray Current And 'imetallic Stray Current And 'imetallic CorrosionCorrosion+n addition to galvanic corrosion, stray current corrosion and+n addition to galvanic corrosion, stray current corrosion and bimetallic corrosion may also be encountered on buried steelbimetallic corrosion may also be encountered on buried steel structures. 4i1e galvanic corrosion, these corrosion processes alsostructures. 4i1e galvanic corrosion, these corrosion processes also involve electrochemical reactions. involve electrochemical reactions.Stray currents are electric currents that travel through the soilStray currents are electric currents that travel through the soil electrolyte.electrolyte. The most common, and potentially the most damaging, strayThe most common, and potentially the most damaging, stray currents are direct currents. These currents are generated fromcurrents are direct currents. These currents are generated from grounded 0C electric power operations including electric railroads,grounded 0C electric power operations including electric railroads, subways, welding machines, and impressed current cathodicsubways, welding machines, and impressed current cathodic protection systems.protection systems. Stray currents may enter a buried metal structure and travel throughStray currents may enter a buried metal structure and travel through the low resistance path of the metal to an area on the structurethe low resistance path of the metal to an area on the structure closer to the current source. Current leaves the structure at thatcloser to the current source. Current leaves the structure at that point to return to the source through the soil electrolyte.point to return to the source through the soil electrolyte. Corrosion occurs at the area where current leaves the structureCorrosion occurs at the area where current leaves the structure ig. 5'. ig. 5'. Stray Current Corrosion-imetallic corrosion-imetallic corrosion-imetallic corrosion occurs when two metals -imetallic corrosion occurs when two metals with different compositions are connected in a with different compositions are connected in a soil electrolyte. soil electrolyte. #or example, bimetallic corrosion can occur #or example, bimetallic corrosion can occur where a bron6e chec1 valve is /oined to steel where a bron6e chec1 valve is /oined to steel piping or where galvani6ed pipe is connected to piping or where galvani6ed pipe is connected to a steel tan1. +n the bron6e chec1 valve7steel pipe a steel tan1. +n the bron6e chec1 valve7steel pipe example, the steel pipe becomes the anode and example, the steel pipe becomes the anode and the bron6e chec1 valve is the cathode. Since the bron6e chec1 valve is the cathode. Since current ta1es the path of least resistance, the current ta1es the path of least resistance, the most severe corrosion attac1 on the steel pipe most severe corrosion attac1 on the steel pipe often occurs in the area immediately ad/acent to often occurs in the area immediately ad/acent to the chec1 valve ig. 8'. the chec1 valve ig. 8'. -imetallic corrosion-imetallic corrosionCorrosion ControlCorrosion ControlCorrosion of buried steel structures may be Corrosion of buried steel structures may be eliminated by proper application of cathodic eliminated by proper application of cathodic protection. protection. Cathodic protection is a techni9ue for preventing Cathodic protection is a techni9ue for preventing corrosion by ma1ing the entire surface of the corrosion by ma1ing the entire surface of the metal to be protected act as the cathode of an metal to be protected act as the cathode of an electrochemical cell. electrochemical cell. Corrosion is not eliminated. +t is simply transferred Corrosion is not eliminated. +t is simply transferred from the metal surface to an external anode. from the metal surface to an external anode. There are two methods of applying cathodic There are two methods of applying cathodic protection to underground metal structures$protection to underground metal structures$:Sacrificial or galvanic anodes. Sacrificial or galvanic anodes.:+mpressed current. +mpressed current.Sacrificial or (al)anic AnodesSacrificial or (al)anic AnodesSacrificial or galvanic anode systems employ a Sacrificial or galvanic anode systems employ a metal anode more negative in the galvanic series metal anode more negative in the galvanic series than the metal to be protected &see Tablefor a than the metal to be protected &see Tablefor a partial galvanic series'. The anode is electrically partial galvanic series'. The anode is electrically connected to the structure to be protected and connected to the structure to be protected and buried in the soil. buried in the soil. % galvanic corrosion cell develops and the active % galvanic corrosion cell develops and the active metal anode corrodes &is sacrificed' while the metal anode corrodes &is sacrificed' while the metal structure cathode is protected. metal structure cathode is protected. %s the protective current enters the structure, it %s the protective current enters the structure, it opposes, overcomes, and prevents the flow of opposes, overcomes, and prevents the flow of any corrosion current from the metal structure. any corrosion current from the metal structure. The protective current then returns to the The protective current then returns to the sacrificial anode through a metallic conductor. sacrificial anode through a metallic conductor. *ig+ $, Sacrificial Anode Cathodic protectionSacrificial anode cathodic protectionSacrificial anode cathodic protection%dvantages of sacrificial anode cathodic protection systems%dvantages of sacrificial anode cathodic protection systems include$ include$. . (o external power supply is necessary. (o external power supply is necessary.2. 2. +nstallation is relatively easy. +nstallation is relatively easy.3. 3. Costs are low for low current re9uirement situations. Costs are low for low current re9uirement situations.5. 5. ,aintenance costs are minimal after installation. ,aintenance costs are minimal after installation.8. 8. +nterference problems &stray currents' on structures other than the one being+nterference problems &stray currents' on structures other than the one being protected are rare. protected are rare.;. ;. Sacrificial anodes may be attached directly to new coated tan1s by tan1Sacrificial anodes may be attached directly to new coated tan1s by tan1 manufacturers. manufacturers.inc anodes are best utili6ed in low resistivity soils where their lower current output will result in longer anode life.lower current output will result in longer anode life. ,agnesium anodes, because of their higher driving potential,agnesium anodes, because of their higher driving potential &..; volts compared to .. volts for 6inc', are fre9uently used&..; volts compared to .. volts for 6inc', are fre9uently used in higher resistivity soils or soils with high concentrations ofin higher resistivity soils or soils with high concentrations of carbonates, bicarbonates, or phosphates.carbonates, bicarbonates, or phosphates. 4ow driving voltages and low current outputs &usually less4ow driving voltages and low current outputs &usually less than !.! amperes per anode' generally limit sacrificialthan !.! amperes per anode' generally limit sacrificial anodes to cathodic protection of well.coated structures.anodes to cathodic protection of well.coated structures. Sacrificial anode corrosion protection systems areSacrificial anode corrosion protection systems are particularly suited to new installations involving coatedparticularly suited to new installations involving coated tanks and/or distribution piping. tanks and/or distribution piping.#actory.+nstalled %nodes#actory.+nstalled %nodesUnderground storage tan1s are available from tan1Underground storage tan1s are available from tan1 manufacturers with sacrificial anode cathodic protectionmanufacturers with sacrificial anode cathodic protection systems already attached.systems already attached. Current re9uirements are minimi6ed by factoryCurrent re9uirements are minimi6ed by factory application of long.lasting corrosion resistant coatings.application of long.lasting corrosion resistant coatings. Sacrificial anodes are attached at the plant with anodeSacrificial anodes are attached at the plant with anode weight and si6e determined by the tan1 surface area. weight and si6e determined by the tan1 surface area.(onconductive bushings and flanges are provided with(onconductive bushings and flanges are provided with each tan1 to isolate the tan1 from any extra currenteach tan1 to isolate the tan1 from any extra current demand resulting from electrical contact between thedemand resulting from electrical contact between the tan1 and associated metal piping. These bushings andtan1 and associated metal piping. These bushings and flanges must be used to protect the tan1 and maintainflanges must be used to protect the tan1 and maintain the supplier's warranty.the supplier's warranty. #actory.+nstalled %nodes#actory.+nstalled %nodesThese tan1s may also be obtained with factory.installed test leadsThese tan1s may also be obtained with factory.installed test leads which facilitate field monitoring of anode current output and tan1.to. which facilitate field monitoring of anode current output and tan1.to.soil potential. soil potential.(ote$ Steel product piping isolated from the tan1 by non.conductive(ote$ Steel product piping isolated from the tan1 by non.conductive bushings and flanges will re9uire its own cathodic protectionbushings and flanges will re9uire its own cathodic protection system. system.#actory.installed cathodic protection systems for new tan1s are#actory.installed cathodic protection systems for new tan1s are designed to satisfy re9uirements for most soil situations. Thedesigned to satisfy re9uirements for most soil situations. The purchaser should compare conditions present at the proposedpurchaser should compare conditions present at the proposed installation sites with the conditions under which long.terminstallation sites with the conditions under which long.term corrosion protection is provided by the factory.installed system. corrosion protection is provided by the factory.installed system.Tan1s with factory.installed cathodic protection systems must beTan1s with factory.installed cathodic protection systems must be handled carefully during transportation and installation to protecthandled carefully during transportation and installation to protect against coating damage or rupture of anode pac1ages.against coating damage or rupture of anode pac1ages. %node wires, test leads, tan1 coating, and tan1 isolation bushings%node wires, test leads, tan1 coating, and tan1 isolation bushings should be inspected for obvious damage prior to final installation.should be inspected for obvious damage prior to final installation. %s with any cathodic protection system, a regular post.installation%s with any cathodic protection system, a regular post.installation monitoring program is necessary to determine that corrosionmonitoring program is necessary to determine that corrosion protection is being maintained. protection is being maintained.#ield.+nstalled %nodes#ield.+nstalled %nodes#ield.installed sacrificial anode cathodic protection systems may#ield.installed sacrificial anode cathodic protection systems may also be used to protect new or existing well.coated tan1s and piping.also be used to protect new or existing well.coated tan1s and piping. % coal tar coating routinely applied to steel petroleum storage tan1s% coal tar coating routinely applied to steel petroleum storage tan1s should not be considered to be an ade9uate coating for cathodicshould not be considered to be an ade9uate coating for cathodic protection purposes. protection purposes. ,any of the corrosion resistant coatings used on cathodically,any of the corrosion resistant coatings used on cathodically protected petroleum pipelines may also be used as coatings forprotected petroleum pipelines may also be used as coatings for buried petroleum storage tan1s and dispensing piping. "ffectiveburied petroleum storage tan1s and dispensing piping. "ffective coatings reduce current re9uirements of these structures to levelscoatings reduce current re9uirements of these structures to levels that can be supplied over a long term by sacrificial anodes. that can be supplied over a long term by sacrificial anodes.Current re9uirements for new or existing buried structures may beCurrent re9uirements for new or existing buried structures may be calculated if the soil resistivity, total surface area to be protected,calculated if the soil resistivity, total surface area to be protected, and coating 9uality are 1nown.and coating 9uality are 1nown. #or existing structures it is also possible to 9uantitatively determine#or existing structures it is also possible to 9uantitatively determine current re9uirements by electrically connecting buried structures tocurrent re9uirements by electrically connecting buried structures to a variable 0C power source and measuring structure.to.soila variable 0C power source and measuring structure.to.soil potentials versus a copper.copper sulfate electrode. The observedpotentials versus a copper.copper sulfate electrode. The observed current output needed to achieve a desired cathodic protectioncurrent output needed to achieve a desired cathodic protection potential may then be used for designing a cathodic protectionpotential may then be used for designing a cathodic protection system for the structures. system for the structures.%node Types and *lacement%node Types and *lacementThree sacrificial anode materials are commonly used forThree sacrificial anode materials are commonly used for soil installations$ soil installations$:)igh potential magnesium alloy. )igh potential magnesium alloy.:Standard magnesium alloy. Standard magnesium alloy.:>inc. >inc.,agnesium and 6inc anodes prepac1aged in chemical,agnesium and 6inc anodes prepac1aged in chemical bac1fill are readily available in a number of si6e andbac1fill are readily available in a number of si6e and weight configurations to meet various current output andweight configurations to meet various current output and anode life design re9uirements.anode life design re9uirements. The use of a chemical bac1fill with anodes is necessaryThe use of a chemical bac1fill with anodes is necessary for reliable operation in soil environments.for reliable operation in soil environments. Chemical bac1fill promotes anode efficiency, lengthensChemical bac1fill promotes anode efficiency, lengthens anode life, and 1eeps the anode environment moist. anode life, and 1eeps the anode environment moist.The number of sacrificial anodes re9uired to provideThe number of sacrificial anodes re9uired to provide cathodic protection to buried structures dependscathodic protection to buried structures depends primarily upon total current re9uirements? however,primarily upon total current re9uirements? however, current distribution factors should also be considered. current distribution factors should also be considered.@here several anodes are needed for a structure, better@here several anodes are needed for a structure, better current distribution &more uniform polari6ation' may becurrent distribution &more uniform polari6ation' may be achieved by distributing the anodes uniformly around theachieved by distributing the anodes uniformly around the structure.structure. +n the case of new tan1 installations, sacrificial anodes+n the case of new tan1 installations, sacrificial anodes are often placed parallel to the tan1 walls, at or near theare often placed parallel to the tan1 walls, at or near the level of the tan1 bottom.level of the tan1 bottom. #or petroleum distribution systems, anodes are typically#or petroleum distribution systems, anodes are typically installed below the piping at a depth of about five feet.installed below the piping at a depth of about five feet. +nstallation of anodes at these locations for tan1s and+nstallation of anodes at these locations for tan1s and piping serves two purposes$ piping serves two purposes$:+t helps assure anodes remain in a moist environment. +t helps assure anodes remain in a moist environment.:+t provides better current distribution to the lower portion of the+t provides better current distribution to the lower portion of the buried structures where corrosion is typically most severe.buried structures where corrosion is typically most severe. "lectrical Connections and +solation"lectrical Connections and +solation% low resistance electrical connection must be made% low resistance electrical connection must be made between the structure to be protected and the wire lead forbetween the structure to be protected and the wire lead for the sacrificial anodes.the sacrificial anodes. #or new installations and where safety factors permit,#or new installations and where safety factors permit, powder weld connections, such as Cadweld or Thermitpowder weld connections, such as Cadweld or Thermit type, should be used.type, should be used. +n some situations safety factors may dictate the use of+n some situations safety factors may dictate the use of mechanical connections. ,echanical connections shouldmechanical connections. ,echanical connections should be made carefully since poor electrical contact willbe made carefully since poor electrical contact will increase the effective wire resistance in the circuit and limitincrease the effective wire resistance in the circuit and limit the amount of useful current available from the anode.the amount of useful current available from the anode. %ll exposed metal at structure.lead wire connections%ll exposed metal at structure.lead wire connections should be waterproofed and coated with electricalshould be waterproofed and coated with electrical insulating material. This will prevent bimetallic corrosioninsulating material. This will prevent bimetallic corrosion between the copper wire and the steel tan1 if the sacrificialbetween the copper wire and the steel tan1 if the sacrificial anode should fail at some time in the future. anode should fail at some time in the future.Current output of sacrificial anodes is limited andCurrent output of sacrificial anodes is limited and cathodic protection designs typically do not providecathodic protection designs typically do not provide sufficient current for protecting ad/acent buriedsufficient current for protecting ad/acent buried structures. Therefore, the structure to be protectedstructures. Therefore, the structure to be protected should be electrically isolated from other buried metalshould be electrically isolated from other buried metal structures, including piping and electrical conduit.structures, including piping and electrical conduit. *roperly installed insulating unions and bushings provide*roperly installed insulating unions and bushings provide effective electrical isolation. 4ocation of these insulatingeffective electrical isolation. 4ocation of these insulating fittings will depend upon the cathodic protection designfittings will depend upon the cathodic protection design and the extent of the structure to be cathodicallyand the extent of the structure to be cathodically protected. These insulating fittings should be inspectedprotected. These insulating fittings should be inspected after installation to insure no damage has occurred.after installation to insure no damage has occurred. *articular care should be ta1en to assure that the*articular care should be ta1en to assure that the structure to be protected is isolated from any buriedstructure to be protected is isolated from any buried metallic power conduit. #or example, petroleummetallic power conduit. #or example, petroleum distribution piping protected by sacrificial anodes shoulddistribution piping protected by sacrificial anodes should have an isolating union at each dispensing unit, andhave an isolating union at each dispensing unit, and underground storage tan1s with submersible pumpsunderground storage tan1s with submersible pumps should have isolating bushings between the pump risershould have isolating bushings between the pump riser pipe and the tan1. pipe and the tan1."valuating Corrosion *rotection"valuating Corrosion *rotectionThere are several criteria for determining if ade9uate cathodicThere are several criteria for determining if ade9uate cathodic protection has been achieved on buried structures.protection has been achieved on buried structures. The most common criterion for ensuring ade9uate sacrificial anodeThe most common criterion for ensuring ade9uate sacrificial anode protection for underground petroleum storage tan1s and piping is theprotection for underground petroleum storage tan1s and piping is the measurement of a negative potential of at least !.A8 volts betweenmeasurement of a negative potential of at least !.A8 volts between the tan1 &or piping' and the surrounding soil &the electrolyte'. the tan1 &or piping' and the surrounding soil &the electrolyte'. TheThe sacrificial anodes should remain attached during the measurement. sacrificial anodes should remain attached during the measurement.=outine measurements of structure.to.soil potential for evaluating the=outine measurements of structure.to.soil potential for evaluating the extent of corrosion protection are often difficult to obtain at petroleumextent of corrosion protection are often difficult to obtain at petroleum storage and distribution sites because the buried structures arestorage and distribution sites because the buried structures are covered by asphalt or concrete pavement. @hile valid potentialcovered by asphalt or concrete pavement. @hile valid potential measurements are possible through asphalt or concrete, specialmeasurements are possible through asphalt or concrete, special e9uipment and trained personnel may be re9uired to obtain ande9uipment and trained personnel may be re9uired to obtain and interpret these results. ,ore reliable results can be obtained byinterpret these results. ,ore reliable results can be obtained by installing permanent access manholes through the pavement to theinstalling permanent access manholes through the pavement to the soil at the time cathodic protection is installed. The soil accesssoil at the time cathodic protection is installed. The soil access manholes should be positioned to enable measurement of potentialsmanholes should be positioned to enable measurement of potentials over the structure at points farthest from the sacrificial anodes or inover the structure at points farthest from the sacrificial anodes or in shielded areas. shielded areas.%s an alternative to ma1ing structure.to.soil potential%s an alternative to ma1ing structure.to.soil potential measurements, long.life reference cells may bemeasurements, long.life reference cells may be permanently installed in the soil near protectedpermanently installed in the soil near protected structures. Specially designed copper.copper sulfate orstructures. Specially designed copper.copper sulfate or pac1aged 6inc reference cells are commercially availablepac1aged 6inc reference cells are commercially available for this purpose. These reference cells are typicallyfor this purpose. These reference cells are typically located within several inches of the structure in areaslocated within several inches of the structure in areas where current distribution may be poor, at points farthestwhere current distribution may be poor, at points farthest from attached anodes, or under the tan1. %cceptablefrom attached anodes, or under the tan1. %cceptable potential readings at such locations, are a goodpotential readings at such locations, are a good indication that ade9uate corrosion protection has beenindication that ade9uate corrosion protection has been achieved on the entire external structure surface. achieved on the entire external structure surface.4ead wires attached to the buried reference cells should4ead wires attached to the buried reference cells should be run to a permanent test station at the site to allowbe run to a permanent test station at the site to allow easy attachment to the instruments used for testing.easy attachment to the instruments used for testing. +t may also be convenient or necessary to attach a test+t may also be convenient or necessary to attach a test lead to the buried structure and run it to the test stationlead to the buried structure and run it to the test station to facilitate periodic potential monitoring. to facilitate periodic potential monitoring.Structure.to.soil potential measurements are made by electricallyStructure.to.soil potential measurements are made by electrically connecting the buried structure to the negative terminal of aconnecting the buried structure to the negative terminal of a suitable high resistance voltmeter &8!,!!! ohms internalsuitable high resistance voltmeter &8!,!!! ohms internal resistance or higher'. The positive terminal of the voltmeter isresistance or higher'. The positive terminal of the voltmeter is connected to a properly placed reference cell. @hen a buried 6incconnected to a properly placed reference cell. @hen a buried 6inc reference cell is used, .. volts must be added to the observedreference cell is used, .. volts must be added to the observed voltmeter reading to calculate the potential reading that wouldvoltmeter reading to calculate the potential reading that would result from using a copper.copper sulfate reference cell. result from using a copper.copper sulfate reference cell.@here sacrificial anodes have been installed, their proper@here sacrificial anodes have been installed, their proper operation should be confirmed by a cathodic protection testeroperation should be confirmed by a cathodic protection tester within ; months after installation andyear thereafter. +f these testwithin ; months after installation andyear thereafter. +f these test confirm proper operation, subse9uent inspection intervals can beconfirm proper operation, subse9uent inspection intervals can be extended to 3 years. )owever, if underground wor1 is performedextended to 3 years. )owever, if underground wor1 is performed at the protected site, or if the UST system is repaired, cathodicat the protected site, or if the UST system is repaired, cathodic protection should be remonitored within ; months after wor1protection should be remonitored within ; months after wor1 completion andyear thereafter before again extending thecompletion andyear thereafter before again extending the inspection interval.inspection interval. *rocedures used and data obtained should be clearly recorded*rocedures used and data obtained should be clearly recorded and included in permanent cathodic protection records for theand included in permanent cathodic protection records for the location, along with the name of the person ma1ing thelocation, along with the name of the person ma1ing the measurements and the date measurements were made. =ecordsmeasurements and the date measurements were made. =ecords of the last two inspections and of any post.repair testing must beof the last two inspections and of any post.repair testing must be maintained. maintained.SECTIN . , IMPRESSE" CURRENT SECTIN . , IMPRESSE" CURRENT PRTECTINPRTECTIN5. 0escription5. 0escription+mpressed current cathodic protection is often the most+mpressed current cathodic protection is often the most economical means of controlling corrosion in existingeconomical means of controlling corrosion in existing underground bare steel petroleum storage tan1s andunderground bare steel petroleum storage tan1s and distribution piping systems. +t may also be used to providedistribution piping systems. +t may also be used to provide corrosion protection for other buried structures at the samecorrosion protection for other buried structures at the same site such as used oil tan1s, heating oil tan1s, andsite such as used oil tan1s, heating oil tan1s, and automobile lifts. The additional cost for protecting otherautomobile lifts. The additional cost for protecting other structures is often minimal. structures is often minimal.Cathodic protection re9uires a source of direct current. #orCathodic protection re9uires a source of direct current. #or impressed current systems, direct current is typicallyimpressed current systems, direct current is typically provided by a rectifier attached to a conventional %C powerprovided by a rectifier attached to a conventional %C power source. The rectifier converts alternating current to directsource. The rectifier converts alternating current to direct current.current. 0C output from the rectifier flows to the buried impressed0C output from the rectifier flows to the buried impressed current anode, through the soil electrolyte and onto thecurrent anode, through the soil electrolyte and onto the surface of the structure as shown in #ig. ;. surface of the structure as shown in #ig. ;.=ectifier Selection and Operation=ectifier Selection and Operation% typical cathodic protection rectifier has two ma/or% typical cathodic protection rectifier has two ma/or components$components$ :a step.down transformer to reduce the %C supply voltage, anda step.down transformer to reduce the %C supply voltage, and :rectifying elements to provide 0C output.rectifying elements to provide 0C output. Units may be obtained with eitherUnits may be obtained with either selenium or siliconselenium or silicon rectifier elements rectifier elements. -oth types of elements are widely. -oth types of elements are widely used and provide dependable service.used and provide dependable service. "ither may show cost or performance advantages for a"ither may show cost or performance advantages for a particular application. Small selenium units are often lessparticular application. Small selenium units are often less expensive than similar silicon units but silicon rectifiersexpensive than similar silicon units but silicon rectifiers are generally more efficient. Silicon units are particularlyare generally more efficient. Silicon units are particularly susceptible to damage from power surges? therefore,susceptible to damage from power surges? therefore, protective devices should be included in these units toprotective devices should be included in these units to prevent lightning damage. Selenium rectifiers are notprevent lightning damage. Selenium rectifiers are not recommended if ambient temperatures are expected torecommended if ambient temperatures are expected to exceed 3!B# &88BC'. exceed 3!B# &88BC'.=ectifier Selection and Operation=ectifier Selection and OperationThe rectifier output capacity selected will depend upon$The rectifier output capacity selected will depend upon$ &a' estimated or measured current re9uirements of the structure to&a' estimated or measured current re9uirements of the structure to be protected, andbe protected, and &b' the voltage needed to cause current to flow from the anodes to&b' the voltage needed to cause current to flow from the anodes to the buried structure.the buried structure. =ectifiers with a moderate excess capacity should be=ectifiers with a moderate excess capacity should be selected to allow for ad/ustments during the life of theselected to allow for ad/ustments during the life of the cathodic protection system and to prevent damage duecathodic protection system and to prevent damage due to voltage overloads. to voltage overloads.=ectifiers may be damaged by voltage overloads.=ectifiers may be damaged by voltage overloads. "xcess rectifier voltage outputs of from 8 to 3! percent"xcess rectifier voltage outputs of from 8 to 3! percent above expected needs are suggested to allow forabove expected needs are suggested to allow for possible line voltage fluctuations. Some excess rectifierpossible line voltage fluctuations. Some excess rectifier current output capacity is suggested, particularly forcurrent output capacity is suggested, particularly for increase the current re9uirements of a structure. increase the current re9uirements of a structure.Careful consideration should be given to selectingCareful consideration should be given to selecting installation locations for rectifiers. "fforts should beinstallation locations for rectifiers. "fforts should be made to place the units in areas not susceptible tomade to place the units in areas not susceptible to accidental or malicious damage, since no corrosionaccidental or malicious damage, since no corrosion protection is provided by an impressed current system ifprotection is provided by an impressed current system if the rectifier is inoperative.the rectifier is inoperative. @henever possible, rectifiers should be mounted within@henever possible, rectifiers should be mounted within a secure building in a location that allows regular anda secure building in a location that allows regular and convenient monitoring. %ll wiring to rectifiers mustconvenient monitoring. %ll wiring to rectifiers must comply with local and national electrical codes. comply with local and national electrical codes.=ectifiers are available with optional e9uipment that=ectifiers are available with optional e9uipment that facilitates periodic monitoring and provides an indicationfacilitates periodic monitoring and provides an indication that power is available to impressed current anodes.that power is available to impressed current anodes. +t is suggested that the e9uipment listed below should be+t is suggested that the e9uipment listed below should be mounted on the front panel of each rectifier cabinet$ mounted on the front panel of each rectifier cabinet$:Creen and red signal lights to indicate proper7improper rectifierCreen and red signal lights to indicate proper7improper rectifier operation. operation.:% volt.ammeter for measuring rectifier voltage and current% volt.ammeter for measuring rectifier voltage and current output. output.=ectifier Selection and Operation=ectifier Selection and Operation:% non.resettable elapsed time meter with a minimum hour capacity% non.resettable elapsed time meter with a minimum hour capacity of DD,DDD hours. of DD,DDD hours.:% warning sign stating that the cabinet contains a cathodic% warning sign stating that the cabinet contains a cathodic protection rectifier and that corrosion of underground structuresprotection rectifier and that corrosion of underground structures can result if power to the unit is shut off. can result if power to the unit is shut off.:Optional audio alarm signals, which sound if a rectifier is notOptional audio alarm signals, which sound if a rectifier is not operating properly, may be useful in some situations. operating properly, may be useful in some situations.The %C power source for each rectifier should be anThe %C power source for each rectifier should be an isolated circuit with an individual circuit brea1er on theisolated circuit with an individual circuit brea1er on the main electrical panel. This brea1er switch should have amain electrical panel. This brea1er switch should have a tab.loc1out brac1et installed, if permitted by local code, totab.loc1out brac1et installed, if permitted by local code, to prevent inadvertent switching.off of power to the rectifier.prevent inadvertent switching.off of power to the rectifier. =ectifier cabinets should be sealed or padloc1ed to=ectifier cabinets should be sealed or padloc1ed to prevent unauthori6ed entry to the on.off %C circuit brea1erprevent unauthori6ed entry to the on.off %C circuit brea1er and exposed high voltage contacts within the cabinet. and exposed high voltage contacts within the cabinet.%node +nstallation and Connections%node +nstallation and ConnectionsSeveral materials are used as impressed current Several materials are used as impressed current anodes. The most common are$anodes. The most common are$a. Craphite. a. Craphite.b. )igh silicon cast iron. b. )igh silicon cast iron.c. *latini6ed niobium, tantalum, or titanium.c. *latini6ed niobium, tantalum, or titanium. Craphite or high silicon cast iron anodes are Craphite or high silicon cast iron anodes are generally preferred for soil installations. generally preferred for soil installations. *latini6ed niobium, tantalum, and titanium are *latini6ed niobium, tantalum, and titanium are best suited for water rather than soil best suited for water rather than soil installations.installations.%node +nstallation and Connections%node +nstallation and Connections"ach anode material has an optimum current density"ach anode material has an optimum current density that provides maximum anode service life.that provides maximum anode service life. %nodes may be located in remote ground beds, in deep%nodes may be located in remote ground beds, in deep wells, or distributed closely about the structure to bewells, or distributed closely about the structure to be protected. The latter two designs may be re9uired to$ protected. The latter two designs may be re9uired to$:%void physical interference with existing facilities. %void physical interference with existing facilities.:*rovide better current distribution. *rovide better current distribution.:%void stray current interference with off.site structures. %void stray current interference with off.site structures.The number of anodes used in a particular remoteThe number of anodes used in a particular remote ground bed or deep.well cathodic protection design willground bed or deep.well cathodic protection design will be determined primarily by total current re9uirements ofbe determined primarily by total current re9uirements of the structure &s' to be protected and the optimum currentthe structure &s' to be protected and the optimum current density of the anode material selected.density of the anode material selected. #or a distributed anode design, additional anodes may#or a distributed anode design, additional anodes may be installed to provide more uniform current distribution. be installed to provide more uniform current distribution.+mpressed current anodes are typically installed in+mpressed current anodes are typically installed in carbonaceous bac1fill. +f the bac1fill is installed properly,carbonaceous bac1fill. +f the bac1fill is installed properly, much of the current reaching the anode is conducted tomuch of the current reaching the anode is conducted to the bac1fill, by electrical contact. This promotesthe bac1fill, by electrical contact. This promotes consumption of the bac1fill instead of the anode andconsumption of the bac1fill instead of the anode and substantially lengthens the effective anode life.substantially lengthens the effective anode life. Carbonaceous bac1fill also tends to reduce total circuitCarbonaceous bac1fill also tends to reduce total circuit resistance by lowering anode.to.soil resistance. resistance by lowering anode.to.soil resistance.CAUTIO! T"e negative lead of t"e rectifier must beCAUTIO! T"e negative lead of t"e rectifier must be attac"ed to t"e structure to be protected. If t"eattac"ed to t"e structure to be protected. If t"e structure is mistakenly attac"ed to t"e positive lead# itstructure is mistakenly attac"ed to t"e positive lead# it will serve as an anode and rapid corrosion failure canwill serve as an anode and rapid corrosion failure can result. result.% weld connection powder &for example, Cadweld or% weld connection powder &for example, Cadweld or Thermit type' is the preferred means for connecting theThermit type' is the preferred means for connecting the negative rectifier lead wire to the underground structure?negative rectifier lead wire to the underground structure? however, good mechanical connections may behowever, good mechanical connections may be substituted if necessary. %ll connections and wire splicessubstituted if necessary. %ll connections and wire splices should be carefully waterproofed and covered withshould be carefully waterproofed and covered with electrical insulating material. electrical insulating material.%node +nstallation and Connections%node +nstallation and Connections%ll underground wire attached to the positive %ll underground wire attached to the positive rectifier terminal is at a positive potential with rectifier terminal is at a positive potential with respect to ground. +f not completely insulated, respect to ground. +f not completely insulated, the wire may discharge current &act as an the wire may discharge current &act as an anode', which will result in corrosion of the wire anode', which will result in corrosion of the wire and eventual failure of the cathodic protection and eventual failure of the cathodic protection installation. Therefore, all anode lead wires, installation. Therefore, all anode lead wires, header cables, and any wire splices should be header cables, and any wire splices should be carefully inspected prior to bac1filling. carefully inspected prior to bac1filling. -ac1fill should be free of sharp stones or other -ac1fill should be free of sharp stones or other material that could damage wire insulation.material that could damage wire insulation."valuating Corrosion *rotection"valuating Corrosion *rotection%t least two criteria are applicable for determining if impressed%t least two criteria are applicable for determining if impressed current cathodic protection has achieved corrosion protection ofcurrent cathodic protection has achieved corrosion protection of underground petroleum storage tan1s and distribution piping$ underground petroleum storage tan1s and distribution piping$% negative, potential of at least !.A8 volts measured between the% negative, potential of at least !.A8 volts measured between the structure and a saturated copper.copper sulfate reference electrodestructure and a saturated copper.copper sulfate reference electrode contacting the soil electrolyte. This reading should be made with thecontacting the soil electrolyte. This reading should be made with the rectifier turned on. rectifier turned on.% polari6ation decay voltage shift of at least !.! volts, after the% polari6ation decay voltage shift of at least !.! volts, after the rectifier is turned off, between the structure and a saturated copper. rectifier is turned off, between the structure and a saturated copper.copper sulfate reference electrode contacting the soil electrolyte. %ncopper sulfate reference electrode contacting the soil electrolyte. %n immediate voltage shift will occur when the current is turned off. Theimmediate voltage shift will occur when the current is turned off. The voltage reading noted after this immediate shift should be used asvoltage reading noted after this immediate shift should be used as the initial reading to begin measuring the !.! volt decay. the initial reading to begin measuring the !.! volt decay.&Optional' % negative voltage shift of at least !.3! volts between the&Optional' % negative voltage shift of at least !.3! volts between the structure and a saturated copper.copper sulfate reference electrodestructure and a saturated copper.copper sulfate reference electrode contacting the soil electrolyte when the protective current is firstcontacting the soil electrolyte when the protective current is first applied. #or well.coated structures, this shift will occur very rapidly.applied. #or well.coated structures, this shift will occur very rapidly. #or bare or poorly coated structures, the shift may continue over#or bare or poorly coated structures, the shift may continue over several hours. several hours.Considerations for measuring structure.to.soil electrolyteConsiderations for measuring structure.to.soil electrolyte potentials discussed in Section 3 are also applicable topotentials discussed in Section 3 are also applicable to structures protected by impressed current systems.structures protected by impressed current systems. *ermanent soil access manholes over underground structures*ermanent soil access manholes over underground structures or buried long.life reference electrodes should be included asor buried long.life reference electrodes should be included as part of each impressed current cathodic protection installation.part of each impressed current cathodic protection installation. Constant potential rectifiers are available that use buriedConstant potential rectifiers are available that use buried reference electrodes to automatically control rectifier currentreference electrodes to automatically control rectifier current output to the anodes, and thereby maintain a constant presetoutput to the anodes, and thereby maintain a constant preset potential on a structure. These rectifiers are particularly usefulpotential on a structure. These rectifiers are particularly useful for installations sub/ect to periodic stray currents, or in areasfor installations sub/ect to periodic stray currents, or in areas where there are wide variations in soil resistivity due to soilwhere there are wide variations in soil resistivity due to soil moisture changes. moisture changes.@hen an impressed current system is energi6ed, a 9ualified@hen an impressed current system is energi6ed, a 9ualified person familiar with cathodic protection e9uipment and theoryperson familiar with cathodic protection e9uipment and theory should measure structure.to.soil potentials.should measure structure.to.soil potentials. Structure.to.structure potentials between protected structuresStructure.to.structure potentials between protected structures and other buried structures at the site &including alland other buried structures at the site &including all underground utility systems' should be measured to verifyunderground utility systems' should be measured to verify that bonding or insulation provisions included in the cathodicthat bonding or insulation provisions included in the cathodic protection design have been met.protection design have been met. "valuating Corrosion *rotection"valuating Corrosion *rotection(ecessary ad/ustments of rectifier output should be(ecessary ad/ustments of rectifier output should be made based on these results and the structure.to.soilmade based on these results and the structure.to.soil potential readings. *rocedures used and data obtainedpotential readings. *rocedures used and data obtained during this initial survey should be recorded induring this initial survey should be recorded in permanent cathodic protection records for the location.permanent cathodic protection records for the location. The name of the person ma1ing the measurements andThe name of the person ma1ing the measurements and the date measurements were made should also be notedthe date measurements were made should also be noted on the record sheet. on the record sheet.,etallic bonds between cathodically protected structures,etallic bonds between cathodically protected structures and ad/acent buried structures belonging to a differentand ad/acent buried structures belonging to a different owner may occasionally be necessary to satisfyowner may occasionally be necessary to satisfy interference problems. Such bonds should only be madeinterference problems. Such bonds should only be made after obtaining permission from the owner of the foreignafter obtaining permission from the owner of the foreign structure, and usually re9uires a mutual testing programstructure, and usually re9uires a mutual testing program witnessed by representatives of both structure owners. witnessed by representatives of both structure owners.%n impressed current system$%n impressed current system$a. ,ust be tested within ; months of installation and at leasta. ,ust be tested within ; months of installation and at least every 3 years thereafter? every 3 years thereafter?b. ,ust be tested within ; months of any repair to the USTb. ,ust be tested within ; months of any repair to the UST system? and system? andc. ,ust be inspected every ;! days to be sure the system isc. ,ust be inspected every ;! days to be sure the system is operating properly.operating properly. =ecords of the last two triennial inspections, of the last three=ecords of the last two triennial inspections, of the last three ;!.day inspections, and of any post.repair testing must be;!.day inspections, and of any post.repair testing must be maintained. +n addition, maintained. +n addition,%nnual surveys of structure.to.soil potentials and structure.to. %nnual surveys of structure.to.soil potentials and structure.to.structure potentials of an impressed current system are alsostructure potentials of an impressed current system are also necessary to ensure continued satisfactory operations.necessary to ensure continued satisfactory operations. Surveys should be conducted using procedures outlined inSurveys should be conducted using procedures outlined in this section for the initial startup survey. =esults obtainedthis section for the initial startup survey. =esults obtained should be added to the permanent cathodic protection recordsshould be added to the permanent cathodic protection records for the location. %ny ad/ustments or other maintenance shouldfor the location. %ny ad/ustments or other maintenance should be performed promptly. be performed promptly.