1、1995NACE InternationalRP0388Impressed Current Cathodic Protection of Internal SubmergedSurfaces of Steel Water Storage TanksNACE Standard RP0388-95Item No.21040StandardRecommended PracticeImpressed Current Cathodic Protectionof Internal Submerged Surfacesof Steel Water Storage TanksThis NACE Interna
2、tional standard represents a consensus of those individual members who have reviewed thisdocument,its scope,and provisions.Its acceptance does not in any respect preclude anyone,whether he hasadopted the standard or not,from manufacturing,marketing,purchasing,or using products,processes,orprocedures
3、 not in conformance with this standard.Nothing contained in this NACE International standard is to beconstrued as granting any right,by implication or otherwise,to manufacture,sell,or use in connection with anymethod,apparatus,or product covered by Letters Patent,or as indemnifying or protecting any
4、one against liabilityfor infringement of Letters Patent.This standard represents minimum requirements and should in no way beinterpreted as a restriction on the use of better procedures or materials.Neither is this standard intended to applyin all cases relating to the subject.Unpredictable circumst
5、ances may negate the usefulness of this standard inspecific instances.NACE International assumes no responsibility for the interpretation or use of this standard byother parties and accepts responsibility for only those official NACE International interpretations issued by NACEInternational in accor
6、dance with its governing procedures and policies which preclude the issuance ofinterpretations by individual volunteers.Users of this NACE International standard are responsible for reviewing appropriate health,safety,environmental,and regulatory documents and for determining their applicability in
7、relation to this standard prior to its use.ThisNACE International standard may not necessarily address all potential health and safety problems or environmentalhazards associated with the use of materials,equipment,and/or operations detailed or referred to within thisstandard.Users of this NACE Inte
8、rnational standard are also responsible for establishing appropriate health,safety,and environmental protection practices,in consultation with appropriate regulatory authorities if necessary,toachieve compliance with any existing applicable regulatory requirements prior to the use of this standard.R
9、P0388 Copyright by NACE International(Thu Aug 24 09:22:51 2000)CAUTIONARY NOTICE:NACE International standards are subject to periodic review,and may be revised orwithdrawn at any time without prior notice.NACE International requires that action be taken to reaffirm,revise,orwithdraw this standard no
10、 later than five years from the date of initial publication.The user is cautioned to obtainthe latest edition.Purchasers of NACE International standards may receive current information on all standards andother NACE International publications by contacting the NACE International Membership Services
11、Department,P.O.Box 218340,Houston,Texas 77218-8340(telephone+1 281/228-6200).Approved November 1988Reaffirmed April 1990Reaffirmed March 1995NACE InternationalP.O.Box 218340Houston,Texas 77218-8340+1 713/492-0535 1995,NACE InternationalForewordThe purpose of this NACE standard is to present the reco
12、mmended practices for providing impressed currentcathodic protection to the normally submerged steel surfaces inside water storage tanks.It containsrecommendations for the design and installation of these cathodic protection systems and methods for determiningthe effectiveness of these systems.Recom
13、mendations for the operation and maintenance of both automatic andmanual systems are also provided.This standard is applicable to relatively large water storage tanks used inmunicipal water supply and fire protection,including elevated as well as flat-bottom tanks at ground level.Althoughthe general
14、 principles outlined in this standard are applicable to all such tanks,the impressed current cathodicprotection system described in this standard may not be practical for smaller tanks.This standard was originally prepared in 1988 by Task Group T-7L-1,a component of Unit Committee T-7L onCathodic Pr
15、otection.The task group was composed of corrosion engineers and others experienced in the design,installation,and maintenance of impressed current cathodic protection systems for water storage tanks.It wasreaffirmed by T-7L in 1990 and again in 1995,and is issued by NACE International under the ausp
16、ices of GroupCommittee T-7 on Corrosion by Waters.This standard represents a consensus of those individual members who have reviewed this document,its scope,andprovisions.Its acceptance does not in any respect preclude anyone,whether he has adopted the standard or not,from manufacturing,marketing,pu
17、rchasing,or using products,processes,or procedures not in conformance withthis standard.Nothing contained in this NACE International standard is to be construed as granting any right,byRP0388 Copyright by NACE International(Thu Aug 24 09:22:51 2000)implication or otherwise,to manufacture,sell,or use
18、 in connection with any method,apparatus,or product coveredby Letters Patent,or as indemnifying or protecting anyone against liability for infringement of Letters Patent.Thisstandard represents minimum requirements and should in no way be interpreted as a restriction on the use ofbetter procedures o
19、r materials.Section 1:General1.1This standard presents recommended practices for applying impressed current cathodic protection to theinternal submerged surfaces of steel tanks used for the storage of natural waters.1.2It is recognized that galvanic anode systems are used successfully for cathodic p
20、rotection of the internal surfacesof water storage tanks;however,this standard addresses only impressed current systems.1.3Natural waters,as used in this standard,include both potable and nonpotable fresh waters associated with watersupply systems and fire protection.1.4The ground-level and elevated
21、 storage tanks considered in this standard are of welded,bolted,or riveted steelconstruction and include many shapes and sizes.1.5Cathodic protection as described in this standard may be used alone to control corrosion of submerged steelsurfaces or used as a complement to the protection afforded by
22、protective coatings or other procedures.Cathodicprotection cannot protect surfaces that are not submerged.These surfaces must be protected by coatings alone.1.6Cathodic protection may be installed to control corrosion in both newly constructed and existing tanks.Whencathodic protection is used on ex
23、isting tanks,it is not necessary to prepare the surfaces to be protected;however,it may be necessary to drain the tank during installation.1.7It is recognized that the tanks under consideration are often associated with potable water and fire protectionsystems,which may be subject to public health a
24、nd safety regulations.This standard should not infringe uponthose regulations.Proper disinfection of the tanks may be required after installation.Any applicable regulationsRP0388 Copyright by NACE International(Thu Aug 24 09:22:51 2000)such as those from the U.S.Environmental Protection Agency(EPA)(
25、1)and NSF-61(2,3)should be checked.1.8The provisions of this standard should be applied under the direction of a competent corrosion engineer.Theterm corrosion engineer,as used in this standard,refers to a person who by reason of his knowledge of thephysical sciences and the principles of engineerin
26、g and mathematics,as acquired by professional education andrelated practical experience,is qualified to practice corrosion control and cathodic protection for water storagetanks.Such persons may be registered professional engineers or persons recognized as being qualified oraccredited as Corrosion S
27、pecialists by NACE International if their professional activities include suitableexperience in corrosion control and cathodic protection.1.9This standard may not be applicable in all situations.The responsible corrosion engineer may consider alternatecorrosion control methods.Section 2:DefinitionsA
28、node:The electrode of an electrochemical cell at which oxidation occurs.Electrons flow away from the anode inthe external circuit.Corrosion usually occurs and metal ions enter the solution at the anode.Calcareous Coating:A layer consisting of calcium carbonate and other salts deposited on the surfac
29、e.When thesurface is cathodically polarized as in cathodic protection,this layer is the result of the increased pH adjacent tothe protected surface.Cathode:The electrode of an electrochemical cell at which reduction is the principal reaction.Electrons flowtoward the cathode in the external circuit.C
30、athodic Protection:A technique to reduce the corrosion of a metal surface by making that surface the cathode ofan electrochemical cell.Coating:All components comprising the protective coating system,the sum of which provides effective electricalinsulation of the coated structure from the electro-lyt
31、e.Conductivity:A measure of the ability of a material to carry an electric current.In water,this depends on the totalconcentration of the ionized substances dissolved and the temperature at which the measurement is made.It isthe reciprocal of resistivity and is usually expressed in mmhos/cm.Corrosio
32、n:The deterioration of a material,usually a metal,that results from a reaction with its environment.Current Density:The current to or from a unit area of an electrode surface.Electrode:A conductor used to establish contact with an electrolyte and through which current is transferred to orfrom an ele
33、ctrolyte.RP0388 Copyright by NACE International(Thu Aug 24 09:22:52 2000)Electrode Potential:The potential of an electrode in an electrolyte as measured against a reference electrode.(Theelectrode potential does not include any resistance losses in potential in either the electrolyte or the external
34、 circuit.It represents the reversible work to move a unit of charge from the electrode surface through the electrolyte to thereference electrode.)Electrolyte:A chemical substance containing ions that migrate in an electric field.For the purposes of thisstandard,electrolyte refers to the water,includ
35、ing the dissolved chemicals,in the tank.Galvanic Anode:A metal that provides sacrificial protection to another metal that is more noble when electricallycoupled in an electrolyte.This type of anode is the current source in one type of cathodic protection.Holiday:A discontinuity in a protective coati
36、ng that exposes unprotected surface to the environment.Impressed Current:An electric current supplied by a device employing a power source that is external to theelectrode system.(An example is direct current for cathodic protection.)Impressed Current Anode:An anode,usually composed of substantially
37、 inert materials,that is supplied withimpressed current.IR Drop:The voltage across a resistance in accordance with Ohms Law.Polarization:The change from the open-circuit potential as a result of current across the electrode/electrolyteinterface.Reference Electrode:An electrode whose open-circuit pot
38、ential is constant under similar conditions ofmeasurement and that is used for measuring the relative potentials of other electrodes.Resistivity:A measure of the specific resistance of a material to the passage of electric current.It is usuallyexpressed in ohm-centimeters(ohm-cm)and is the reciproca
39、l of conductivity.Tank-to-Water Potential:The voltage difference between a submerged metallic portion of the tank and theelectrolyte(water),which is measured with a reference electrode in contact with the electrolyte.Section 3:Determination of the Need for Cathodic Protection3.1 Introduction3.1.1Ste
40、el tank surfaces submerged in natural waters are subject to corrosion.The methods and procedures used tocontrol corrosion should be governed by the rate of corrosion and the cost of maintaining the tank,including thehydraulic and aesthetic effects of corrosion debris in the tank and piping.3.1.2All
41、coatings are subject to damage and deterioration.Therefore,corrosion control by use of coatings alone isRP0388 Copyright by NACE International(Thu Aug 24 09:22:52 2000)possible only if the coating is maintained to provide complete coverage of the surface to be protected.3.1.3Cathodic protection is e
42、ffective in controlling corrosion only on metal surfaces during submersion.However,cathodic protection does not reverse structural damage already caused by corrosion.3.2 Cathodic Protection of Coated Tanks3.2.1Control of corrosion on submerged,coated steel tank surfaces requires that cathodic protec
43、tion be used toprevent corrosion where holidays exist or are likely to occur in the coating and where corrosion is likely to occuras a result of coating holidays.3.2.2If the water is sufficiently corrosive to justify the use of a coating,then cathodic protection is also justified andprovides a great
44、er degree of protection than when either method is used alone.3.2.3Properly designed and maintained cathodic protection systems can extend the useful life of the coating.3.2.4The current required for cathodic protection is lowered significantly when coatings are also used.3.3 Cathodic Protection of
45、Uncoated Steel Tanks3.3.1It is feasible to design a cathodic protection system to provide complete protection for uncoated submergedsurfaces of steel tanks(see Section 4).3.3.2For existing steel tanks with submerged surfaces that are not coated or that have coatings in poor condition,itmay be econom
46、ical to rely entirely on cathodic protection.3.4 Economic Considerations3.4.1 Coated Tanks3.4.1.1RP0388 Copyright by NACE International(Thu Aug 24 09:22:52 2000)The cost of cathodic protection should be compared with the cost of recoating and metal loss.In addition toextending the life of existing c
47、oatings,cathodic protection prevents the need for recoating,repairs,leakage,and replacement caused by metal loss where coating damage has occurred.Both coating repair andrecoating can be deferred for a considerable time.3.4.2 Uncoated Tanks3.4.2.1It is possible to prevent metal loss in steel tanks w
48、here the surfaces are not coated or where the coating hasdeteriorated to the point that the surfaces can be considered uncoated.The cost of installing cathodicprotection,projected energy,anode replacement,and maintenance costs,and surveillance costs should becompared with the cost of coating mainten
49、ance,metal loss,service disruption,reduction in design life,andwater quality degradation.3.5 Operational Considerations3.5.1Taking the tank out of service for coating maintenance or corrosion repair can be eliminated or postponed if thecathodic protection system is designed and operated properly.3.5
50、.2Benefits in both system capacity and availability for fire protection result when the tank is kept in continuousservice.Section 4:Design of Impressed Current Cathodic Protection Systems4.1 Introduction4.1.1This section outlines the elements that should be considered when designing impressed curren
