1、 Corrosion Control for Buried Water MainsPocket Field GuideReviewed by the AWWA Corrosion Committee whose membership included the following:Graham Bell, Richard Bonds, Steve Cooper, Larry Dunn, Matthew Dykema, Andrew Ferrigno, John Grocki, John Higdon, Mike Horton, Bryan Hughes, Rodney Jackson, Jame
2、s Keith, Kevin Kelly, Gregory Kirmeyer, David Kroon, Stephen Lamb, Gene Oliver (chairman), Steven Piper, George Richards, Jeff Rog, Andrew Romer, Lois Sherry, Allen Skaja, and Greg SmithAndrew E. Romer and Bayard Bosserman, IICorrosion Control for Buried Water MainsPocket Field GuidePrinted on recyc
3、led paperCopyright 2009 American Water Works Association.All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechani-cal, including photocopy, recording, or any information or retrieval system, except in the form of brief excer
4、pts or quotations for review purposes, without the written permission of the publisher.DisclaimerThis book is provided for informational purposes only, with the understanding that the publisher, editors, and authors are not thereby engaged in rendering engineering or other professional services. The
5、 authors, editors, and publisher make no claim as to the accuracy of the books contents, or their applicability to any particular circum-stance. The editors, author, and publisher accept no liability to any person for the information or advice provided in this book, or for loss or damages incurred b
6、y any person as a result of reliance on its contents. The reader is urged to consult with an appropriate licensed professional before taking any action or making any interpretation that is within the realm of a licensed professional practice.Project Manager/Editor: Melissa ValentineProduction Editor
7、: Cheryl ArmstrongLibrary of Congress Cataloging-in-Publication DataRomer, Andrew E.AWWA back to basics guide to corrosion control for buried water mains / prepared by Andrew E. Romer and Bayard Bosserman, II.p. cm.Reviewed by the AWWA Corrosion Committee, including Graham Bell and others.Includes b
8、ibliographical references.ISBN 978-1-58321-725-21. Water-pipes-Corrosion-Prevention-Handbooks, manuals, etc. 2. Corrosion and anti-corrosives-Handbooks, manuals, etc. 3. Underground pipelines-Protection-Handbooks, manuals, etc. I. Bosserman, Bayard E. II. Bell, Graham E. C. III. AWWA Corrosion Commi
9、ttee. IV. Title. V. Title: Guide to corrosion control for buiried water mains.TD491.R66 2009628.15-dc222009000411Printed in the United States of America.American Water Works Association6666 West Quincy AvenueDenver, CO 80235-3098ISBN 1-58321-725-81WHAT IS CORROSION?Corrosion occurs when a substance
10、is deterio-rated by its environment. A common example is when iron rusts. There are many types of corro-sion. This guide focuses on the process of corro-sion of buried water mains.Mechanism of CorrosionCorrosion occurs by an electrochemical pro-cess. An anode (negative electrode), a cathode (positiv
11、e electrode), electrolyte (corrosive envi-ronment, such as certain soils and waters), and a metallic circuit connecting the anode and the cathode are required for corrosion to occur. Dissolution of metal occurs at the anode where the current enters the electrolyte and flows to the cathode (Figure 1)
12、.The three basic forms of corrosion that com-monly occur on underground ferrous metal pipe-lines are (1) general corrosion (uniform attack); (2) galvanic corrosion (dissimilar-metal corro-sion); and (3) concentration cell corrosion. In addition to these “normal” forms of deteriora-tion, stray curren
13、t corrosion can also occur on an underground ferrous metal structure. This form of corrosion is related to uncontrolled direct2Figure 1 Galvanic corrosion cell (adapted from Figure 2-2 AWWA M27)currents flowing in the earth. The currents flow onto the underground ferrous metal structures at certain
14、locations, causing no detrimental effect. Similarly, the currents cause no damage while they are flowing along the structure. Eventually, however, the currents must leave the structure, return to the earth, and flow to their source of generation. It is where the currents leave the pipe-line that ser
15、ious corrosion damage occurs. Stray current corrosion and normal corrosion activity are similar in that corrosion always occurs at the anodic areas. The basic difference between the two is that an external current causes stray cur-rent corrosion; the current is generated by the 3corrosion cell when
16、normal corrosion activity takes place.General uniform corrosion is the uni-form anodic dissolution of metal over the entire exposed surface area. The corrosion rate is nearly constant at all locations. Underground uncoated ferrous metal pipelines can be expected to dete-riorate by general corrosion
17、at reasonably rapid rates when they are exposed to low resistivity, aggressive soils. For example, uncoated ferrous metal pipelines exposed to soils having resistiv-ities less than 1,000 ohm-cm can be expected to develop corrosion leaks in as short as five years.Galvanic corrosion will generally occ
18、ur if two electrochemically dissimilar metals or alloys are metallically connected and exposed to a com-mon electrolyte. Oxidation occurs at the anode. Reduction occurs at the cathode. There is no net electrical discharge. (See Table 1.) Anodic mate-rials are at the top of the list, progressing down
19、 to cathodic materials. The less noble material (anode) suffers accelerated attack, and the more noble metal/alloy is cathodically protected by the galvanic current. Any material in Table 1 will act as an anode to materials listed below it. For example, accelerated corrosion of the steel would 4be e
20、xpected to occur if brass and steel are metal-lically connected and exposed to an aggressive soil. Similarly, underground steel pipelines that are connected to large copper grounds would be expected to deteriorate by galvanic action if the soil has a relatively low resistivity.Electrochemical attack
21、 of a metal or alloy because of differences in the environment is called concentration cell corrosion. At least five types of concentration cells exist. Of these, the differential aeration or oxygen concentration cell is the one generally responsible for corrosion of underground steel structures. Ar
22、eas on a pipeline surface in contact with electrolyte having a high oxygen content are generally cathodic to those areas in contact with electrolyte having a lower oxygen content.On cross-country electrically continuous underground pipelines, concentration cell cor-rosion can occur over relatively l
23、ong distances. This is caused by what are often referred to as long-line corrosion currents. For example, ferrous metal pipe exposed to a loam generally will be cathodic to areas where the pipeline is in con-tact with clay. Pipe buried under a river will be anodic to aerated soil adjacent to the str
24、eam.5TABLE 1 Practical galvanic series of commonly produced metalsAnodic Metal Volts*Commercially pure magnesium 1.75Magnesium alloy (6% Al, 3% Zn, 0.15% Mn)1.55Zinc 1.15Aluminum alloy (5% zinc) 1.05Mild steel (clean and shiny) 0.5 to 0.7Mild steel (rusted) 0.2 to 0.5Cast and ductile iron 0.5Tape-co
25、ated steel 0.5Lead 0.5Stainless steel, AISI 316 0.25Mild steel in concrete 0.2Copper, brass, bronze 0.2Cast ironhigh silicon 0.2Mill scale on steel 0.2Stainless steel, AISI 304 0.15Cathodic Graphite +0.3*Reference to a coppercopper sulfate reference cell.Localized soil differences can also be involv
26、ed in the concentration cell corrosion of under-ground pipelines. For example, ferrous metal in contact with undisturbed, low oxygen con-tent soil will generally be anodic to ferrous metal in contact with the aerated backfill. Corrosion damage to the underside of the pipeline is accel-erated by the
27、large cathode-to-anode area ratio that exists. Similarly, ferrous metal in contact with lumps of clay will be anodic to nearby fer-rous metal if the major backfill material is a 6sandy loam. Localized concentration cell corro-sion is believed to be a major cause of corrosion leaks in underground pip
28、elines. New pipelines act as anodes and protect old pipelines; of course, this increases the corrosion on the new pipeline. Concentration cells also develop between highly stressed surfaces such as under bolt heads.Materials Susceptible to CorrosionDo not bury copper lines next to uncoated ferrous m
29、etal lines in low resistivity soils. The ferrous metal pipe will act as an anode and will rapidly corrode. To protect the ferrous metal line, the following items should be done in decreasing order of preference: (1) do not use copper piping, use PVC or ferrous metal; (2) coat the copper line with ho
30、t- or cold-applied coal-tar tape or polyethylene wrap and install in an insulating coupling between the copper and ferrous metal line at the point of connection; and (3) coat the ferrous metal line for a distance of at least 50 pipe diameters on both sides of the connection to the copper line with c
31、ement mortar, coal-tar enamel, or hot- or cold-applied coal-tar tape. It is best to perform both (2) and (3).7CORROSION OF THE EXTERIOR OF BURIED WATER MAINSWhat is a Corrosive Soil?One of the important factors affecting corrosion activity along an underground pipeline is the resistivity of the soil
32、. Corrosiveness of the envi-ronment is generally an inverse function of resis-tivity. Low resistivity favors the flow of current and increases the probability of anodic dissolu-tion; corrosion may not be a problem in very high resistivity electrolytes. The effect of soil resistivity on the anticipat
33、ed corrosion activity for steel can be predicted using the information given in the Table 2. These data, however, should not be used as an absolute criterion for corrosivity. Often, severe corrosion damage occurs in soils having relatively high resistivities. This is especially true in heterogeneous
34、 soils (e.g., an environment con-sisting of lumps of clay mixed with sand). TABLE 2 Soil resistivity v. corrosion activityResistivity range (ohm-cm) Corrosion activity0 to 2,000 Severe2,000 to 10,000 Moderate10,000 to 30,000 MildGreater than 30,000 Limited8The latter should be considered when widely
35、 varying, high resistivities are measured along a pipeline.In addition to the mineral content, mois-ture greatly affects a soils resistivity. Resistivity decreases with an increase in moisture content up to a point near saturation. For example, a typ-ical clay containing about 5 percent moisture can
36、 have a resistivity approaching 1,000,000 ohm-cm; the soil can have a resistivity as low as 7,000 ohm-cm if the moisture content is increased to about 20 percent. This suggests that corrosion activity should be most severe during the rainy season. For this reason, resistivities should not be measure
37、d when the soil is abnormally dry.The pH of nearly all soils and groundwaters vary within the range of 3.5 to 10. The major-ity of these environments, however, have a pH in the range of 6.5 to 7.5; that is, most are essen-tially neutral. Alkaline soils having a pH in the range of 7.5 to 10 (e.g., al
38、kaline loams and alka-line salt marshes), and acidic soils having a pH in the range of 3.5 to 6 (e.g., cinder fills and muck) also exist.Although the exact influence of pH and the other interrelated factors that affect the corrosion 9of underground steel pipelines are not com-pletely understood, it
39、is reasonable to believe that decreasing pH increases the corrosion activity. This general statement is supported by the obser-vation that the corrosion rate for steel increases as the pH decreases when the soil resistivity is con-stant. This effect is much more pronounced in acidic than alkaline so
40、ils. For example, the cor-rosion rate for steel in alkaline soils (i.e., pH 7) is affected much more by soil resistivity than pH; in acidic soils (pH 1,8002,1002,1002,5002,5003,0003,00085210pH 022446.56.57.57.5 8.58.55300*03Redox +100 mv+50 to +100 mv0 to +50 mvNegative ()03.545Sulfides PositiveTrac
41、eNegative3.520Moisture Poor drainage, continuously wetFair drainage, generally moistGood drainage, generally dry210*If sulfides are present and low or negative redox results are obtained, three points shall be given for this range.13Protecting the Exterior of Buried Water MainsCoatings. Several coat
42、ings are available for use with buried ferrous metal pipelines. The follow-ing are AWWA standards for coating or wrap-ping various types of pipe:C116/A21.16: Protective Fusion-Bonded sEpoxy Coatings Interior and Exterior Surfaces Ductile-Iron and Gray Iron Fittings for Water Supply ServiceC203: Coal
43、-Tar Protective Coatings and sLinings for Steel Water Pipelines, Enamel and Tape, Hot-AppliedC205: Cement-Mortar Protective Lining sand Coating for Steel Water Pipe, 4 In. (100 mm) or Larger, Shop AppliedC209: Cold-Applied Tape Coatings for the sExterior of Special Sections, Connections, and Fitting
44、s for Steel Water PipeC210: Liquid-Epoxy Coating Systems for sthe Interior and Exterior of Steel Water Pipelines14C213: Fusion-Bonded Epoxy Coating for sthe Interior and Exterior of Steel Water PipelinesC214: Tape Coating Systems for the Exterior sof Steel Water PipelinesC215: Extruded Polyolefin Co
45、atings for the sExterior of Steel Water PipelinesC216: Heat-Shrinkable Cross-Linked sPolyolefin Coatings for the Exterior of Special Sections, Connections, and FittingC217: Petrolatum and Petroleum Wax Tape sCoatings for the Exterior of Connections and Fittings for Steel Water PipelinesC218: Coating
46、 the Exterior of Aboveground sSteel Water Pipelines and FittingsC222: Polyurethane Coatings for the sInterior and Exterior of Steel Water Pipe and FittingsC224: Nylon-11 Based Polyamide Coating sSystem for Interior and Exterior of Steel Water Pipe, Connections, Fittings, and Special SectionsC225: Fu
47、sed Polyolefin Coating Systems for sthe Exterior of Steel Water Pipelines15C602: Cement-Mortar Lining of Water sPipelines in Place4 In. (100 mm) and LargerBarriers. Effective protection of ductile-iron pipe in corrosive soil environments can be achieved economically by encasing the pipe with either
48、8-mil (0.008-in.) low-density poly-ethylene or 4-mil (0.004-in.) high-density cross-laminated polyethylene (as described in ANSI/AWWA C105/A21.5). This system of protec-tion was first used experimentally in the United States in 1951 for protection of gray-iron pipe in corrosive environments.Polyethy
49、lene encasement is not a coating, although it offers some of the qualities of a coat-ing, such as dielectric strength. It is mainly an environmental improvement for the pipe. At the trench, ductile-iron pipe is snugly wrapped with a tube or sheet of polyethylene, which acts as an unbonded film, preventing direct contact of the pipe with the corrosive soil. The polyethylene film also effectively reduces the environment to a very th
