1、Ideal crop marks David H. Kroon Cathodic Protection for Steel Water Storage Tanks POCKET FIELD GUIDE Ideal crop marks 1P-300-20831-5/17 IW Cathodic protection is a low-cost way to safeguard investments in storage tanks and protective coating systems. Designed to be carried in a shirt pocket, this gu
2、ide is a ready reference for water operators, providing infor- mation on tank corrosion control, principles of cathodic protection, and tank rehabilitation.Cathodic Protection for Steel Water Storage Tanks Pocket Field GuideCathodic Protection for Steel Water Storage Tanks Pocket Field Guide David H
3、. Kroon, P.E.Copyright 2017 American Water Works Association All rights reserved. No part of this publication may be repro- duced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any informa- tion or retrieval system, except in the form of brie
4、f excerpts or quotations for review purposes, without the written permission of the publisher. Disclaimer This 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 serv
5、ices. The authors, editors, and publisher make no claim as to the accuracy of the books contents, or their applicability to any particular circumstance. The editors, authors, and pub- lisher accept no liability to any person for the information or advice provided in this book or for loss or damages
6、incurred by 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. Managing Editor: Melissa Valentine Product Mana
7、ger: Tony Petrites Technical Editor: Jenifer Walker Cover Art: Melanie Yamamoto Production Editor: Megan McCarthy ISBN: 978-1-625-76224-5v Contents Acknowledgments . . . . . . . . . . . . . . . . . vii Introduction . . . . . . . . . . . . . . . . . . . . . . ix Chapter 1Water Tank Corrosion and Cont
8、rol . . . . . . . . . . . . . . . . . . . . . . 1 Corrosion of Internal Wetted Surfaces 3 Chapter 2Principles of Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . 11 Application of Cathodic Protection 13 Chapter 3Cathodic Protection Technology in Recent Years . . . . . . . . . 19 Autom
9、atically Controlled Power Supplies 19 Sensor Technology 20 Mixed-Metal Oxide-Coated Titanium Anode Wires 21 Specialized Anode Design 23 NSF-Certified Materials/Systems 23 Monitoring and Maintenance 24 New Challenges 24vi Chapter 4Tank Rehabilitation . . . . . . 27 Chapter 5Industry Standards . . . .
10、 . . . 29 Glossary . . . . . . . . . . . . . . . . . . . . . . . 31 References . . . . . . . . . . . . . . . . . . . . . . 35vii Acknowledgments Reviewed by the AWWA Steel Tank Committee whose membership include the following: Joe W. Davis, Thomas M. Dawson Jr., Leslie D. Scott, and Gregory R. Stein
11、. And by the AWWA Corrosion Control Committee whose membership include the following: Graham Bell, Sylvia Hall, Mike Horton, and Andy Romer. With comments from: Rajendra D. Vaidya and Al Fancher.ix Introduction As early as 1943, an AWWA technical commit- tee concluded it was necessary to control cor
12、rosion of submerged surfaces inside water tanks. At that time, cathodic protection was identified as an effec- tive way to prevent leaks. It wasnt until the 1970s, however, when practical designs for water storage tanks were widely introduced, that cathodic pro- tection began to gain widespread use
13、to prevent corrosion. Even the best protective coating systems can- not prevent corrosion indefinitely. When cathodic protection is added to a coated tank, however, the advantage achieved by combining the benefits of a protective coating with cathodic protection is sig- nificant for owners, doubling
14、 or even tripling the life of the coating. Consequently, tank owners view cathodic protection as a low-cost way to safeguard investments in their storage tanks and protective coating systems. By pairing protective coatings and cathodic pro- tection, it is possible to extend the life of a typi- cal s
15、teel water storage tanks coating system for immersion service by as much as 20 years. With- out cathodic protection, system failure is likely to occur within 10 years.1 Chapter 1 Water Tank Corrosion and Control Steel water storage tanks are subject to corrosion on all of their external and internal
16、 surfaces. The pri- mary focus of this pocket field guide is to provide guidance for cathodic protection of the internal wetted surfaces of steel tanks, but, for comprehen- sive asset protection, corrosion and corrosion con- trol of all surfaces should be considered. Most storage tanks are construct
17、ed of either steel or steel and reinforced concrete (compos- ite tanks). For composite tanks, it is important that corrosive admixtures be avoided and that the depth of concrete cover over the rebar be sufficient to protect the steel reinforcements by providing a high pH environment over the life of
18、 the tank. For stainless steel tanks, a different set of corrosive conditions needs to be evaluated. These include sufficient oxygen in the water to form a protective oxide film and the presence of certain active ions, such as chlorides, that could lead to corrosion pit- ting of the stainless steel.
19、 There are distinct zones associated with a water tank, each of which requires corrosion protection by a combination of material selection, protective coatings, and/or cathodic protection. The connec- tion of the carbon steel tank shell to different met- als should be avoided. This includes copper a
20、nd 2 stainless steel tubing, ladders, safety rails nozzles, noncompatible weld materials, and the like. The exterior surfaces of the tank shell and roof exposed to the atmosphere are best protected from corrosion by the use of protective coatings. Many coating systems that have been successfully use
21、d include epoxies, polyurethanes, and alkyds, some of which incorporate zinc-rich primers. Section 4.3 of AWWA D102-14 “Coating Steel Water- Storage Tanks” describes seven outside coating systems. For the external surfaces of a flat bottom tank, consideration should be given to the appli- cation of
22、cathodic protection to prevent corrosion of the soil side. Accelerated corrosion can occur because of corrosive soils or tank pad material, high moisture content, or connection to copper grounding. Applying a protective coating alone to the soil side of the bottom plates is not an effec- tive altern
23、ative because the coating will be dam- aged during placement and welding. Information on tank bottom cathodic protection can be found in NACE Internationals RP 0193-2001, “Exter- nal Cathodic protection of On-Grade Carbon Steel Storage Tank Bottoms.” Inside the tank, corrosion is even more challeng-
24、 ing. Connection (metal-to-metal contact) to cop- per, brass, and stainless steel appurtenances must be avoided. Protective coatings and cathodic pro- tection should both be used. Coating the inside of a tank is not an easy process. Proper surface 3 preparation is essential. Humidity and temperature
25、 for proper cure must be closely monitored, recog- nizing that the side of the tank exposed to the sun can be quite a bit hotter from radiant heating. Sur- face preparation and quality application of coating the roof, support columns, and purlins is diffi- cult because of impediments to inspection,
26、includ- ing access, crevices, and sharp edges. Section 4.4 of AWWA D102-14 “Coating Steel Water-Storage Tanks” describes five inside coating systems con- sisting of epoxies, polyurethanes, or polyurea. The discussion that follows addresses corrosion and cathodic protection of the internal wetted ste
27、el sur- faces of carbon steel water storage tanks. CORROSION OF INTERNAL WETTED SURFACES In fresh water tanks, corrosion activity on internal wetted surfaces usually results in concen- trated pitting attack, which leads to quicker wall penetration than if the corrosion was more uni- formly distribut
28、ed on the metal surface. This is particularly true on tank interiors that are coated, where the corrosion attack is accelerated at holi- days or voids in the coating. The attack is initiated by the development of anodic and cathodic areas on the submerged metal surfaces. The anodic areas (e.g., loca
29、tion of coating holiday) will suf- fer accelerated corrosion (metal loss), whereas the cathodic areas will not corrode (see Figure 1-1). 4 The corrosion is often made even worse by the small anode-large cathode area effect (see Coating Pinhole Corrosion). There are a number of mechanisms that can in
30、itiate and sustain corrosion of the submerged steel in water tanks. Uniform Corrosion Although steel visually appears to be homoge- nous, close inspection reveals that it is quite irregu- lar, consisting of numerous grains of metal that are electrically different from each other. Thus, some will be
31、anodes, whereas others will be cathodes. FIGURE 1-1 Anodic and Cathodic Areas on Tank Wall5 The corrosion attack will usually appear as ran- domly, closely spaced pits. Stressed Metal Usually steel that is under stress will be anodic to unstressed steel. In tanks, these stresses can be caused by suc
32、h things as welding (where the area immediately adjacent to the weld becomes stressed), bending or forming without stress reliev- ing, and bolting and riveting (usually the fastener will be anodic to the adjacent plate). Dissimilar Metal Corrosion The use of different metals in direct contact with e
33、ach other will establish a corrosion cell where the more noble metal will be cathodic and the more active metal will be anodic. Examples of such cells in tanks include the use of copper or stainless steel heater coils, stainless steel ladders, stainless steel nozzles, and weld seams where the metall
34、urgy of the welding rod differs from the base plate metal (see Figure 1-2). Crevice Corrosion This corrosion cell develops at crevices that create oxygen concentration or ion entrapment cells. Gen- erally, the corroding (anodic) area will be in the crevice with the nearby surface areas cathodic. In
35、water tanks, these develop most commonly between 6 the head and plate of bolted or riveted plates and between the overlapping areas of unsealed plates. Differential Oxygen Concentration When steel is immersed in water and where some of the steel surface is exposed to a relatively oxygen-enriched wat
36、er as compared with other steel surfaces, the area deprived of oxygen will be anodic with respect to those surfaces exposed to the abundantly oxygenated water. This phenom- enon is often observed in poorly coated tanks where the lower submerged surfaces are heavily corroded (lower oxygen levels with
37、 depth), whereas the upper areas show little corrosion. Even more common is the appearance of vertical striation FIGURE 1-2 Galvanic Corrosion between Coated Carbon Steel and Stainless Steel Ladder7 corrosion where deep vertical gouges (sometimes several inches or feet long) are observed on the subm
38、erged surfaces of the tanks (see Figure 1-3). This common corrosion phenomenon is caused by the development of an initial corrosion pit gen- erating soft, flowing corrosion products. Gravity causes these products to migrate down the side of the tank wall and shield the lower surface from oxygen, ren
39、dering it anodic. The shielded sur - face begins to corrode, generating more corrosion products with the process continuing over and over down the wall of the tank. FIGURE 1-3 Vertical Corrosion of Interior Tank Wall8 Coating Pinhole Corrosion When the internal tank surfaces are coated with a dielec
40、tric material (e.g., epoxy), the corro- sion activity will be concentrated at the holidays (holes) in the coating. The breaks in the coating may result from mechanical damage, improper surface preparation, or merely microscopic voids in the coating surface. The corrosion currents will concentrate at
41、 the holidays and result in higher corrosion current densities at these locations. Even though a good coating will reduce the total metal loss, a complete penetration of the metal surface will occur more quickly than if the tank were not coated (see Figure 1-4). Many other factors can influence the
42、rate at which corrosion will proceed in water tanks. The most significant factors are: FIGURE 1-4 Corrosion at Coating Holidays9 water flow rates, relative surface area of anodes to cathodes, active ion concentrations, temperature, turbulence, and water level fluctuation. Virtually all potable water
43、s are corrosive with regard to steel. Thus, the question is not one of whether water tanks are subject to corrosion, but rather, what is the most effective and economic means of corrosion protection?11 Chapter 2 Principles of Cathodic Protection The principles of cathodic protection are best under -
44、 stood by thinking about the corrosion cell. At the anode, metal ions go into solution as the result of an oxidation reaction; at the cathode, a reduction reaction occurs, protecting the metal and preventing corrosion. Corrosion current flows from the anode through the water and to the cathode (see
45、Figure 1-1). With cathodic protection, all submerged steel surfaces are a cathode in a macro electrochemi- cal cell that is built, where an anode for consump- tion is intentionally provided, with the submerged tank interior acting as the cathode. This cell is the cathodic protection system (see Figu
46、re 2-1). Cathodic protection requires an outside source of direct current (DC) that flows through the water onto the surface of the tank. One source of this cur- rent may be a DC power supply connected to a rel- atively inert anode that is suspended in the water inside the tank. DC is forced to flow
47、 from the anode through the water and to the tank surface. This is impressed current cathodic protection. Another source of DC may be obtained by con- necting a more active metal (e.g., magnesium) to the steel shell. The more active (electronegative) metal becomes the anode in an intentionally desig
48、ned corrosion cell and sacrifices itself to protect the 12 submerged surfaces of the tank. A familiar applica- tion of cathodic protection is household water heat- ers. A small magnesium, zinc, or aluminum rod is installed inside the tank to provide cathodic pro- tection and prevent corrosion of the
49、 exposed sub- merged metallic components. Because cathodic protection current must flow from the anode through the electrolyte and onto the surface to be protected, it can only be used to pre- vent corrosion when the structure is submerged, bur- ied, or embedded in concrete. Cathodic protection is not applicable to the control of atmospheric corro- sion. It will not protect areas above the water line on the tank shell, the head space, or any surfaces that are not in contact with the stored water. FIGURE 2-1 Water Tank Cathodic Protection13 APPLICATION OF CATHODIC PROTECTION Corros
