1、Item No. 24218NACE International Publication 42102This Technical Committee Report has been preparedby NACE International Task Group 029*onCorrosion in Manholes.Corrosion in Power and Communication Manholes June 2002, NACE InternationalThis NACE International technical committee report represents a c
2、onsensus of those individual memberswho have reviewed this document, its scope, and provisions. Its acceptance does not in any respect precludeanyone from manufacturing, marketing, purchasing, or using products, processes, or procedures not includedin this report. Nothing contained in this NACE Inte
3、rnational report is to be construed as granting any right, byimplication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or productcovered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of LettersPatent. This report
4、should in no way be interpreted as a restriction on the use of better procedures or materialsnot discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictablecircumstances may negate the usefulness of this report in specific instances. NACE Internatio
5、nal assumes noresponsibility for the interpretation or use of this report by other parties.Users of this NACE International report are responsible for reviewing appropriate health, safety,environmental, and regulatory documents and for determining their applicability in relation to this report prior
6、 toits use. This NACE International report may not necessarily address all potential health and safety problems orenvironmental hazards associated with the use of materials, equipment, and/or operations detailed or referredto within this report. Users of this NACE International report are also respo
7、nsible for establishing appropriatehealth, safety, and environmental protection practices, in consultation with appropriate regulatory authorities ifnecessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of thisreport.CAUTIONARY NOTICE: The user is cau
8、tioned to obtain the latest edition of this report. NACEInternational reports are subject to periodic review, and may be revised or withdrawn at any time without priornotice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACEInternational reports may receive curre
9、nt information on all NACE International publications by contacting theNACE International Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906(telephone +1281228-6200).ForewordThis state-of-the-art report presents methods of identifyingcorrosion in power and communicatio
10、ns manholes andidentifies methods used to mitigate the effects of corrosionwithin the confines of the manhole. People involved withthe responsibility of providing corrosion control in power andcommunications manholes are typically aware of bondingand grounding systems and conduct their work incompli
11、ance with the specifications set forth by the NationalElectrical Safety Code (NESC)1and state and local electricsafety codes.This state-of-the-art report was prepared by NACE TaskGroup (TG) 029 on Corrosion in Manholes. This TG isadministered by Specific Technology Group (STG) 42 onEnergyElectric Po
12、wer and Communication. The TG isalso sponsored by STG 03 on Protective Coatings andLiningsImmersion/Buried. This report is published byNACE under the auspices of STG 42._*Chairman George Schick, Westwood, NJ.NACE International2GeneralThe types of corrosion encountered in manholes include butare not
13、limited to:(a) Galvanic corrosion,(b) Stray current corrosion,(c) Chemical corrosion,(d) Bacterial corrosion,(e) Cathodic corrosion,(f) Stress corrosion cracking, and(g) Hydrogen stress cracking.Many types of metals are used in the construction of themanhole and the manhole equipment. Coupling of th
14、esematerials may create galvanic cells in the common electro-lyte and lead to early failure of one or more of the compon-ents that are electrically joined.Metals joined together in a common electrolyte assume apolarized potential between the open-cell potential of thesemetals. In the application of
15、cathodic protection the pot-ential of the most active metal is considered.The pH of the electrolyte found in manholes can range fromacid to alkaline, and is a factor that is typically consideredwhen designing mitigation systems. The resistivity of theelectrolyte, measured in ohm-centimeters, is also
16、 consid-ered in the design of mitigation systems.Stray currents can have a severe effect on manhole struc-ture and equipment. The presence of stray currents hasbeen observed during testing and considered in the designof any mitigation system.Chemical corrosion is always a possibility in manholecondu
17、it systems that are subject to flooding or ground waterrun-off.Corrosion caused by bacteria may affect many of themetals found in the underground environment. If tests con-firm the presence of sulfate-reducing bacteria, they areeliminated before additional corrective actions are consid-ered.Galvanic
18、 CorrosionGalvanic corrosion occurs when dissimilar metals are in acommon electrolyte and connected with a metallic path.Lead sheath cable in contact with galvanized steel supporthardware is a common galvanic corrosion cell found inpower and communications manholes. In flooded man-holes, all metal s
19、tructures that are electrically continuous inthe common electrolyte are included in the corrosion cell.Galvanic or concentration cell corrosion may take place in ametallic structure in nonflooded manholes that are subjectto high humidity and/or the local accumulation of debris.The effects of galvani
20、c corrosion can be mitigated usingvarious methods. A corrosion cell consists of an anode,cathode, electrolyte, and a metallic path connecting theanode and cathode. The removal of any one of these ele-ments eliminates the corrosion cell. The use of compatiblematerials adds to the service life of the
21、manhole and theequipment installed in the manhole.Coating manhole equipment, support hardware, and thestructure can mitigate the effects of corrosion, especially ifthe cathode of the corrosion cell is coated. However,coating defects on the anode of the corrosion cell causedby improper coating applic
22、ation, mechanical damage, oraging may allow localized failures. When the anode area(at coating defect sites) is much smaller than the cathodearea, the corrosion rate is high and may cause rapid pene-tration.Stray Current CorrosionStray currents have been observed during testing. Onesource of stray c
23、urrents is rectifiers providing cathodic pro-tection to other underground structures. Stray current mayalso be produced by direct current (DC)-powered tractionsystems or high-voltage direct current (HVDC) powersystems.Stray current may be picked up by underground structuresand conducted back to the
24、vicinity of its source. Corrosionoccurs where the stray current leaves the structure. Currentleaving the structure at the manhole can lead to the corro-sion of manhole equipment, support hardware, and, insome cases, of rebar or other metallic materials used in theconstruction of the manhole.Chemical
25、 CorrosionChemicals that enter manholes either through the manholeopening or by migrating through the conduit system mayattack the structures within the manhole. A common chem-ical corrosive agent is the fertilizer used on lawns. Re-moving the chemically contaminated electrolyte, cleaningthe manhole
26、, and then flushing the flooded duct(s) canreduce the effects of chemical corrosion. In some casesthe conduits entering the manholes have been sealed.Flushing of occupied ducts may have a significant effect inreducing cable failures.High chloride content in concrete, a result of additives in theconc
27、rete to promote faster curing, can break down the pas-sive film on the rebar and cause advanced corrosion. Thechloride can also increase galvanic corrosion if dissimilarmetals are connected for grounding purposes. The loss ofconnectivity of any part of the electrical grounding circuitrepresents a si
28、gnificant safety hazard.Degradation of copper and copper-based alloys occurswhen free chlorine gas exists. The operation of impressed-current cathodic protection systems can generate freechlorine that attacks exposed copper. This condition isespecially prevalent when impressed-current anodes areused
29、 in ducts containing chloride-bearing water.NACE International3Bacterial CorrosionCorrosion induced by bacteria is a widely recognized phen-omenon and can lead to the rapid failure of manhole equip-ment and hardware. Anaerobic bacteria (e.g., sulfate-reducing bacteria) are the most common type of ba
30、cteriafound in manholes. Anaerobic bacteria derive their nutri-ents from organic material in the surrounding environmentand accelerate corrosion during the process of their lifecycle. These bacteria may cause severe corrosion leadingto the early failure of galvanized steel support hardware andequipm
31、ent, including the more noble metals such as copperand tin-coated copper bonding ribbon. In sulfate-reducingbacteria-infested manholes, the potential of copper shifts inthe negative direction and may become anodic to iron andsteel. This can lead to galvanic corrosion failures of copperand tin-coated
32、 copper bonding ribbons.Cathodic CorrosionCathodic corrosion may occur in areas where the structure-to-electrolyte potential is highly negative and the environ-ment contains alkaline metal salts. Under the influence ofcathodic current, the alkaline metal ions migrate to the cath-ode surface, creatin
33、g an alkaline environment. Amphotericmetals (e.g., lead and aluminum) corrode at these locations.Such areas are generally stray current pick-up points orcathodically overprotected structures.Stress Corrosion Cracking (SCC)Stress corrosion cracking (SCC) can occur when highlystressed SCC-susceptible
34、alloys are exposed to specificchemicals that can act as SCC agents. The metallic comp-onents in manholes most likely to fail by SCC are copper-based alloys, primarily brasses. Coaxial terminals, pressurevalves, and some air pipe connecting hardware are usuallymade of brass. A thin layer (approximate
35、ly 5 m 0.2 mils)of tin coating on the brass has not offered reliable protectionagainst SCC. The higher the zinc content of the brass, themore susceptible it is to SCC. In general, a zinc contentgreater than 15% can lead to SCC if the component isstressed or contains residual manufacturing stresses.
36、Themost aggressive SCC agent for copper-based alloys isammonia or amines. Fertilizers and household cleaningsolutions are the most likely sources of these chemicals.Some stressed stainless steel components can also fail bySCC. In these cases, the chemical agent is typicallychloride.Hydrogen Stress C
37、rackingStressed high-strength steels and stainless steels (espec-ially the 400 series) are subject to failure by hydrogen stresscracking. Stainless steel hardware in a manhole (e.g.,splice case bolts and v-band clamps on apparatus cases) isoften in contact with steel, galvanized steel, or galvanized
38、cast iron. In the resulting galvanic couple, the stainlesssteel is the cathode, and the hydrogen forms on its surface.This hydrogen can enter the stressed metal structure, caus-ing hydrogen stress cracking. Sulfides on the metal surfacekeep the hydrogen in its atomic form for a longer period oftime,
39、 and thus facilitate its entry into the metal structure.Molybdenum disulfide, which is often used as an antiseizecompound on splice case bolts and v-band clamps, accel-erates the hydrogen stress cracking of these components.Methods of MitigationOne or more types of corrosion can be found in any onem
40、anhole. The methods used to mitigate the effects of cor-rosion are determined by the type of corrosion encountered.Generally, the severity of corrosion of rebar and other man-hole hardware decreases as the amount of chloride in theconcrete mix decreases. A lower chloride content has typic-ally been
41、specified for locations with a history of corrosionproblems.To reduce concrete damage caused by drilling and cutting,manholes have sometimes been manufactured with rack-mounting inserts and grounding inserts.Galvanic corrosion has been mitigated by the selection ofmaterials that are close in the gal
42、vanic series, the use ofcoatings, removal of any one component of the corrosioncell, or the application of either a sacrificial or impressedcurrent cathodic protection system. The use of pumps ordrains to remove the electrolyte from the manhole has beensuccessful. To be effective, these devices have
43、 been keptin sound operating condition.Stray current corrosion has been controlled with the use ofdrainage bonds or cathodic protection. Effects on otherunderground structures have been considered whenimpressed current cathodic protection has been selected.Insulating joints have been used in communi
44、cation cables.To prevent chlorine gas degradation of copper and copper-based alloys, the operation of impressed current cathodicprotection systems has been closely regulated and moni-tored in order to prevent overdriving the anodes, which hasgenerated free chlorine gas (see Chemical Corrosion).SCC o
45、f any metal has been controlled by removing theapplied stresses or eliminating the manufacturing stressesby heat treatment. Another method of control is selectingalloys that are not susceptible to SCC in the manholeenvironment.Hydrogen stress cracking has also been controlled by theremoval of applie
46、d and manufacturing stresses. In mostcases the problem has been resolved by discontinuing theuse of sulfide-containing antiseize compounds. The selec-tion of materials that are not susceptible to hydrogen stressNACE International4cracking has also been a viable alternative to prevent thistype of cor
47、rosion failure.Corrosion influenced by sulfate-reducing bacteria has beenmitigated by spraying the infected areas (manhole walls,cables, support hardware, bonding ribbon, closures, etc.)with an oxidizing agent (e.g., sodium hypochlorite house-hold bleach). The disappearance of the black color indi-cates the effectiveness of the mitigation process.References1. IEEE C2 (latest revision), “National Electrical SafetyCode” (New York, NY: Institute of Electrical and ElectronicsEngineers(1)._(1)Institute of Electrical and Electronics Engineers (IEEE), Three Park Avenue, New York, NY 10016-5997.
