Cathodic Protection for Steel Water Storage Tanks.pdf

David H. Kroon

Cathodic Protection for Steel Water Storage Tanks

POCKET FIELD GUIDE

Cathodic Protection for

Steel Water Storage Tanks

Pocket Field Guide

Cathodic Protection for

Steel Water Storage Tanks

Pocket Field Guide

David H. 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 brief excerpts or quotations for review purposes, without the written permission of the publisher.

Disclaimer

Tis 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. Te authors, editors, and publisher make no claim as to the accuracy of the book’s contents, or their applicability to any particular circumstance. Te 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 incurred by any person as a result of reliance on its contents. Te 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 Manager: Tony Petrites Technical Editor: Jenifer Walker Cover Art: Melanie Yamamoto Production Editor: Megan McCarthy

ISBN: 978-1-625-76224-5

v

Contents

Acknowledgments vii

Introduction ix

Chapter 1—Water Tank Corrosion and Control 1

Corrosion of Internal Wetted Surfaces 3

Chapter 2—Principles of Cathodic

Protection 11

Application of Cathodic Protection 13

Chapter 3—Cathodic Protection

Technology in Recent Years 19

Automatically Controlled Power Supplies 19

Sensor Technology 20

Mixed-Metal Oxide-Coated Titanium Anode Wires 21

Specialized Anode Design 23

NSF-Certifed Materials/Systems 23

Monitoring and Maintenance 24

New Challenges 24 vi

Chapter 4—Tank Rehabilitation 27

Chapter 5—Industry Standards 29

Glossary 31

References 35 vii

Acknowledgments

Reviewed by the AWWA Steel Tank Committee whose membership include the following:

Joe W. Davis, Tomas M. Dawson Jr. , Leslie D. Scott, and Gregory R. Stein.

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 corrosion of submerged surfaces inside water tanks. At that time, cathodic protection was identifed as an efec-tive way to prevent leaks. It wasn’t until the 1970s, however, when practical designs for water storage tanks were widely introduced, that cathodic pro-tection began to gain widespread use to prevent corrosion.

Even the best protective coating systems can-not prevent corrosion indefnitely. When cathodic protection is added to a coated tank, however, the advantage achieved by combining the benefts of a protective coating with cathodic protection is sig-nifcant for owners, doubling 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 steel water storage tank’s coating system for immersion service by as much as 20 years. With-out cathodic protection, system failure is likely to occur within 10 years.

Chapter 1

Water Tank Corrosion and Control

Steel water storage tanks are subject to corrosion on all of their external and internal surfaces. Te pri-mary focus of this pocket feld 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 constructed 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 sufcient to protect the steel reinforcements by providing a high pH environment over the life of the tank.

For stainless steel tanks, a diferent set of corrosive conditions needs to be evaluated. Tese include sufcient oxygen in the water to form a protective oxide flm and the presence of certain active ions, such as chlorides, that could lead to corrosion pit-ting of the stainless steel.

Tere 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. Te connec-tion of the carbon steel tank shell to diferent met-als should be avoided. Tis includes copper and stainless steel tubing, ladders, safety rails nozzles, noncompatible weld materials, and the like.

Te 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 used 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 fat bottom tank, consideration should be given to the appli-cation of 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 efec-tive alternative because the coating will be dam-aged during placement and welding. Information on tank bottom cathodic protection can be found in NACE International’s RP 0193-2001, “Exter-nal Cathodic protection of On-Grade Carbon Steel Storage Tank Bottoms.”

Inside the tank, corrosion is even more challeng-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 preparation is essential. Humidity and temperature 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 dif-cult because of impediments to inspection, includ-ing access, crevices, and sharp edges. Section 4.4 of AWWA D102-14 “Coating Steel Water-Storage Tanks” describes fve inside coating systems con-sisting of epoxies, polyurethanes, or polyurea. Te discussion that follows addresses corrosion and cathodic protection of the internal wetted steel 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 distributed on the metal surface. Tis is particularly true on tank interiors that are coated, where the corrosion attack is accelerated at holi-days or voids in the coating. Te attack is initiated by the development of anodic and cathodic areas on the submerged metal surfaces. Te anodic areas (e.g., location of coating holiday) will suf-fer accelerated corrosion (metal loss), whereas the cathodic areas will not corrode (see Figure 1 -1).

Te corrosion is often made even worse by the small anode-large cathode area efect (see Coating Pinhole Corrosion).

Tere are a number of mechanisms that can initiate 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 diferent from each other. Tus, some will be anodes, whereas others will be cathodes.

FIGURE 1-1 Anodic and Cathodic Areas on Tank Wall

Te 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 such 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

Te use of diferent metals in direct contact with each 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 metallurgy of the welding rod difers from the base plate metal (see Figure 1-2).

Crevice Corrosion

Tis 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 area’s cathodic. In water tanks, these develop most commonly between 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 water 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. Tis phenom-enon is often observed in poorly coated tanks where the lower submerged surfaces are heavily corroded (lower oxygen levels with 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 Ladder corrosion where deep vertical gouges (sometimes several inches or feet long) are observed on the submerged surfaces of the tanks (see Figure 1-3).

Tis common corrosion phenomenon is caused by the development of an initial corrosion pit gen-erating soft, fowing corrosion products. Gravity causes these products to migrate down the side of the tank wall and shield the lower surface from oxygen, rendering it anodic. Te 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 Wall

Coating Pinhole Corrosion

When the internal tank surfaces are coated with a dielectric material (e.g., epoxy), the corro-sion activity will be concentrated at the holidays (holes) in the coating. Te breaks in the coating may result from mechanical damage, improper surface preparation, or merely microscopic voids in the coating surface. Te corrosion currents will concentrate at 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 infuence the rate at which corrosion will proceed in water tanks. Te most signifcant factors are:

FIGURE 1-4 Corrosion at Coating Holidays water fow rates, relative surface area of anodes to cathodes, active ion concentrations, temperature, turbulence, and water level fuctuation.

Virtually all potable waters are corrosive with regard to steel. Tus, the question is not one of whether water tanks are subject to corrosion, but rather, what is the most efective and economic means of corrosion protection?

Chapter 2

Principles of Cathodic Protection

Te principles of cathodic protection are best under-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 fows from the anode through the water and to the cathode (see 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. Tis cell is the cathodic protection system (see Figure 2-1).

Cathodic protection requires an outside source of direct current (DC) that fows 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 fow from the anode through the water and to the tank surface. Tis 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. Te more active (electronegative) metal becomes the anode in an intentionally designed corrosion cell and sacrifces itself to protect the 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 exposed sub-merged metallic components.

Because cathodic protection current must fow 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 Protection

APPLICATION OF CATHODIC PROTECTION

Corrosion protection of water storage tanks through the application of cathodic protection has become an accepted practice for both existing and newly constructed tanks. Te increased use of cathodic protection for these structures is credited to the growing awareness that it is an economical means of controlling corrosion on the submerged surfaces of these structures and is the only method that pro-vides complete corrosion control.

Tere is no doubt that cathodic protection is a reliable, well-established, and cost-efective method to prevent corrosion inside water tanks. Cathodic protection should be included in all new tank and tank rehabilitation designs. Tis conclusion is con-sistent with the 1999 AwwaRF Report, “Main-taining Water Quality in Finished Water Storage Facilities,” in which it is recommended that cathodic protection be provided in the design of steel tanks.

Tere are several variables associated with each water tank that must be considered when designing cathodic protection. Among these are water chemis-try, coating, tank design, and tank function.

Variations in water chemistry that afect cathodic protection include source of water (e.g., lakes, riv-ers, wells), total dissolved solids, total hardness, dis-solved oxygen, total alkalinity, pH, temperature, and bacteria.

Variations in tank coating include type and con-dition. Tere are also a number of diferent tank designs and shapes, including ground storage reser-voirs, standpipes, and elevated water storage tanks.

In addition, a tank may function as a water-storing facility, pressure regulator, wash reservoir, or fre protection reservoir or any combination of these.

Even though there are diferent and varying conditions, standardization of cathodic protection designs is possible because there are several condi-tions in water tanks and treatment equipment that favor this approach.

First, the electrolyte (water) in which the metal is submerged is relatively uniform. Second, although there are diferent structure shapes, they are all basi-cally a combination of cylinders and curves. Both of these two conditions are diferent from those found on underground pipelines, where there is considerable irregularity in soil conditions and pip-ing confguration. Tird, the most responsible fact for permitting standardized design is that cathodic protection systems for water tanks are most often impressed current. Great fexibility can be built into an impressed current system to meet a variety of current requirements without adding signifcantly to the cost. Magnesium and other types of sacrif-cial anode material are often used in smaller tanks or those located remotely from AC power.

When preparing to design a cathodic protection system for a water storage tank, the designer must frst determine the following: total current required for protection; water resistivity; anode material;

anode geometry; and service life.

Te total current required for cathodic protec-tion is determined simply by multiplying the total submerged surface area by a preselected current den-sity. Although current density requirements can vary in fresh waters from as little as 0.1 milliamperes to

5.0 milliamperes per square foot of bare submerged surface area, a maximum design current density of

2.5 milliamperes per square foot will sufce for the majority of all steel tanks. If less current is actually required, the rectifer output can easily be reduced in the feld. Some of the factors that increase the actual operating current are temperature, turbulence, and chlorides. Factors such as alkalinity will reduce the current requirement. Te most important factor is the type and condition of the coating. Although there are a number of coatings for use in the interi-ors of water storage tanks, epoxy coating systems are the most common.

Te proper design of a cathodic protection system makes it necessary to measure the water conductiv-ity (or resistivity, which is the inverse of conductiv-ity). Conductivity is a major factor governing the electrical circuit resistance, which governs the volt-age at which the system will operate to deliver the required current. Te higher the voltage and current, the greater the power consumption; the greater the power consumption, the higher the operating costs.

A large anode results in lower circuit resistance, which then results in lower voltage required to deliver the same current at lower operating costs.

But a larger anode typically results in a higher initial cost; therefore, the designer must strike the optimum economic balance between capital cost and operating cost. Because most treated waters fall in the resistivity range of 2,000 to 10,000 ohm-centimeters, it is usually possible to adjust the num-ber and size of the anodes to reduce the circuit resistance such that the maximum system voltage requirements are between 10 to 80 volts.

Most components in cathodic protection systems for water storage tanks and water treatment equip-ment have a design life of 20 years. Te electrical components and the rectifer units in cathodic pro-tection systems are capable of continuous operation over this time with little attention. Te major excep-tion is the cathodic protection anodes that are con-sumed by the system operation. Anodes in impressed current systems can be easily designed for 20 years or more, whereas sacrifcial anodes usually have a maxi-mum life of 10 years.

When protective coatings are used with cathodic protection and in accordance with AWWA Stan-dards and manufacturer specifcations, the coating system can be expected to have a useful life of 15 to 20 years (when used with cathodic protection), and repainting of the submerged areas of the tank can be done much less frequently.

A coating serves an entirely diferent function inside a cathodically protected tank. When a coating is used as the exclusive means for corrosion control, voids, peeling, and/or cracking of only one percent to three percent of the coated surface would indi-cate failure of the paint system as a means for corro-sion control. In fact, in most cases, accelerated attack will occur.

In a cathodically protected tank, similar coat-ing deterioration will be easily compensated for by a small increase in current output of the cathodic pro-tection system. Te coating’s major function with cathodic protection is to reduce the power cost and rate of anode consumption.

Chapter 3

Cathodic Protection

Technology in Recent Years

In recent years, the greatest advancements in cathodic protection technology have been in six key areas.

AUTOMATICALLY CONTROLLED

POWER SUPPLIES

When frst applied decades ago, cathodic protection had a reputation for being high maintenance. Sys-tems had to be hand-calibrated regularly to adjust for water level and temperature changes in a water storage tank. In addition, in cold weather climates, cathodic protection was considered to be a seasonal, not permanent, solution.

Now monitoring sensors can be located inside water tanks to detect environmental changes, including the amount of water turbulence and aer-ation generated during tank flling and potential water chemistry changes. Similar to a thermostat, automatic controllers (see Figure 3-1) connected to these sensors use the information to calibrate the cathodic protection system automatically, without human intervention. Systems adjust continuously to maintain optimum protection levels, requiring only annual maintenance checkups.

SENSOR TECHNOLOGY

Sensor technology is used for more than simply defning the cathodic protection level needed at a given moment and signaling the power supply to adjust for it. Sensors, which usually are reference electrodes installed in a tank, can also provide data about the internal coating condition to help owners determine the percent of exposed steel without the expense of internal inspection.

Owners can also chart current output to measure the rate of deterioration and more accurately predict when rehabilitation will be necessary. Without such data, owners must rely mostly on visual inspection.

In addition, older technology sensors usually provide reliable results for no more than 3 to 5 years. Today, FIGURE 3 -1 Automatically Controlled Rectifer for Water

Tanks standard sensors have a minimum 10-year life span but often produce reliable, reproducible results for much longer periods.

MIXED-METAL OXIDE-COATED TITANIUM

ANODE WIRES

Te small amount of DC current needed to polarize submerged internal tank surfaces to prevent corro-sion is delivered by anode wires that must be sub-merged in the water. Older cathodic protection systems relied primarily on electrodes made of high silicon iron. When low temperatures caused icy con-ditions, aluminum electrodes were chosen, but they had to be replaced each spring. Both types of anodes have been replaced by mixed-metal oxide-coated

FIGURE 3 -2 Mixed-Metal -Oxide-Coated Titanium Anode

Wire

FIGURE 3 -3 Horizontal Hoop Design for Ground Storage

Reservoir

FIGURE 3 -4 Horizontal Hoop Design for Elevated Tank titanium anode wires, which are easier to handle and install, have a life of 20+ years, and include NSF 61 certifed systems (see Figure 3-2).

SPECIALIZED ANODE DESIGN

Cathodic protection system installation is driven primarily by climate. In cold climates, water stor-age tanks are subject to ice buildup that can damage system wiring, especially if the wiring is not prop-erly suspended. Modern cathodic protection systems with a horizontal hoop anode design prevent dam-age caused by ice formation and provide more even distribution of protective current; therefore, the sys-tems require less current to achieve the same result.

Because such systems are suspended from the sides of the tank by eye rings, no hand-holes need to be cut in the top of a tank to access roof suspension sys-tems (see Figures 3-3 and 3-4).

NSF-CERTIFIED MATERIALS/SYSTEMS

Many of today’s cathodic protection materials and systems are certifed by NSF under ANSI/NSF 61, Drinking Water System Components—Heath Efects, which provides owners extra assurance that their systems aren’t endangering the drinking water supply. NSF certifcation is an ongoing audit and supply-chain management system that requires quarterly independent third-party audits of materi-als to assure ongoing compliance. Only systems that are part of a supply-chain management system and participate in ongoing testing can be NSF certifed.

MONITORING AND MAINTENANCE

Regardless of the type of cathodic protection system installed, it should be checked every 2 months by local personnel to ensure that it is operating prop-erly. Te DC current and voltage outputs of the sys-tem and potential set point should be recorded each month and placed on record for future reference.

If any of the measurements are out of range, steps should be taken to restore proper operation.

Annually, a complete system inspection should be performed to include a potential profle of the sub-merged tank surface. Advancements in cathodic protection coupon technology allow for recording polarized potentials (IR drop-free instant of) even when the cathodic protection current cannot be interrupted, as may be the case for direct connected sacrifcial anode systems.

NEW CHALLENGES

Cathodic protection technology will continue to address changing methods of steel water storage tank design and construction. Use of complex, dis-similar metals on internal tank surfaces is placing new demands on protective coating systems. Tis has increased the need for cathodic protection.

Likewise, new piping and mixing systems can pose additional corrosion risks. For example, inside some carbon steel tanks are nozzles, safety rails, lad-ders, and other components made of uncoated stain-less steel. Interconnection of these diferent alloys can create dissimilar metal corrosion cells, or spots, where the two dissimilar metals are submerged and in contact with each other. Te cell can result in cor-rosion to the coated carbon steel tank foor and shell.

In these environments, cathodic protection is even more critical to prevent metal loss from corrosion.

Tat is why cathodic protection is so important for preventing corrosion on interior submerged surfaces of steel water storage tanks.

Chapter 4

Tank Rehabilitation

Cathodic protection is a reliable, cost-efective way to prevent corrosion inside water tanks, but it accounts for only about one percent of tank reha-bilitation costs. Te remaining costs cover protec-tive coating and application (see Figure 4-1).

Capital

Costs

Protective Coatings and Application

Cathodic Protection

*Estimated average costs. Actual costs and resu l ts may vary.

Construction/Fabrication

Other

1%

24%

75%

20%

80%

Maintenance and

Life-Cycle Costs

P e rc e n t o f U ti li ty B u d g e t

Cost of Fighting Water Tank Corrosion Cathodic protection accounts for only about 1 percent of the capital costs of bui ld ing a tank.

FIGURE 4-1 Relative Cost of Corrosion Protection

Coating costs have risen in recent years, result-ing mostly from design enhancements that improved performance, lowered operational costs, and adhered to more stringent volatile organic car-bon (VOC) limits. For example, newer coatings are designed to reduce the current demand (and elec-tricity costs) on a cathodic protection system. In turn, cathodic protection increases coating life by preventing corrosion in voids or damaged areas.

Chapter 5

INDUSTRY STANDARDS

Cathodic protection, used in conjunction with pro-tective coatings, is considered standard industry prac-tice for controlling internal tank corrosion, according to AWWA and NACE International, two organiza-tions that have adopted corrosion control standards.

NACE International developed the frst cathodic protection standard in 1988, which it updated with NACE S0388-2014, the current version. In 1996, NACE completed the Galvanic Anode Cathodic Protection of Internal Submerged Surfaces of Steel Water Storage Tanks standard for sacrifcial anode use for cathodic protection. Te latest edition of this standard was updated in 2011.

AWWA published its frst impressed current cathodic protection standard in 1996. Te current version was released in 2011 as ANSI/AWWA D104-

11. Te AWWA series was completed in 2010 with ANSI/AWWA D106-10, Sacrifcial Anode Cathodic Protection Systems for the Interior Submerged Sur-faces of Steel Water Storage Tanks.

As these standards were evolving, technical work groups representing the two organizations sought to ensure that the AWWA and NACE standards, although diferent, did not contradict each other. In subsequent revisions, European and Australian stan-dards will be reviewed for consistency.

Glossary

Alkalinity—Alkalinity measures the ability of a sub-stance to resist a change in pH. In other words, alka-linity measures water’s ability to neutralize strong acids or bases; thus, alkalinity is also referred to as the “bufering capacity” of the water. Alkalinity is normally measured by the equivalent of milligrams of calcium carbonate (CaCO3) per liter of water.

Anode—(1) Corrosion: Te electrode of a corro-sion cell that has a greater tendency to corrode or oxidize.

(2) Cathodic Protection: Te expendable materials that are buried and through which direct current fows into the soil. Common materials used for this purpose are graphite, high silicon iron, magnesium, zinc, and scrap iron.

Cathode—Te electrode of a corrosion cell where a net reduction reaction occurs. In corrosion pro-cesses, the cathode is usually that area which does not corrode.

Cathodic Protection—Reduction or prevention of corrosion of a metal surface by making it cathodic by the use of sacrifcial anodes or impressed current cathodic protection systems. When cathodic pro-tection is applied, the structure is part of an elec-trical circuit in which direct current fows from an external anode into the surrounding electrolyte and onto the structure to be protected. Tis current opposes the corrosion cell currents discharged at the anodic (-) areas. Te entire surface of the struc-ture is changed to a cathodic (+) or protected state.

Hence the name “cathodic protection.”

Coating—A liquid, liquefable, or mastic composi-tion that, after application to a surface, is converted into a solid protective, decorative, or functional adherent flm.

Conductivity—A measure of the ability of a mate-rial to carry an electric current. In water, this depends on the total concentration of the ion-ized substances dissolved and the temperature at which the measurement is made. It is the recipro-cal of resistivity and is usually expressed in µS/cm (µmhos/cm).

Corrosion—Te deterioration of a material, usu-ally a metal, that results from a reaction with its environment.

Current Density—Te 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 or from an electrolyte.

Electrolyte—A chemical substance containing ions that migrate in an electric feld. For the purposes of this standard, electrolyte refers to the water, includ-ing the dissolved chemicals, in the tank.

Epoxy—Type of resin formed by the reaction of ali-phatic or aromatic polyols (such as bisphenol) with epichlorohydrin and characterized by the presence of reactive oxirane end groups.

Holiday—A discontinuity in a protective coat-ing that exposes unprotected surface to the environment.

Impressed Current—An electric current supplied by a device using a power source that is external to the electrode system. (An example is direct current for cathodic protection.)

Impressed Current Anode—An anode, usually composed of substantially inert material, that is supplied with impressed current systems.

Polarization—Te change from the open-circuit potential as a result of current across the electrode/ electrolyte interface.

Reference Electrode—An electrode whose open-circuit potential is constant under similar condi-tions of measurement, which is used for measuring the relative potentials of other electrodes.

Resistivity—A measure of the specifc resistance of a material to the passage of electric current. It is usually expressed in ohm-centimeters (ohm-cm) and is the reciprocal of conductivity.

Sacrifcial Anode—A metal that provides sacrif-cial protection to another metal that is more noble when electrically coupled in an electrolyte. Tis type of anode is the electron source in a sacrifcial anode cathodic protection system.

Tank-to-Water Potential—Te voltage diference between a submerged metallic portion of the tank and the electrolyte (water), which is measured with a reference electrode in contact with the electrolyte.

Voltage Drop—Te voltage across a resistance when current is applied in accordance with Ohm’s law.

References

ANSI/NSF 61 (latest revision), “Drinking Water System Components—Health Efects” (New York, NY: ANSI and Ann Arbor, MI: NSF International).

AWWA Standard D104-11, Automatically Con-trolled, Impressed Current Cathodic Protec-tion for the Interior Submerged Surfaces of Steel Water Storage Tanks (catalog no. 441040).

AWWA Standard D106-10, Sacrifcial Anode Cathodic Protection Systems for the Interior Submerged Surfaces of Steel Water Tanks (catalog no. 44106).

AWWA Manual of Water Supply Practices M58:

Internal Corrosion in Water Distribution Systems, 2011 (catalog no. 30058).

D.H. Kroon. Corrosion in Water Systems an Overview, Water and Wastewater International, October/November 1986.

J.B. Bushman, D.H. Kroon. Cathodic Protection of Water Storage Tanks, Journal - AWWA, January 1984.

NACE International SP0196 (latest revision), “Galvanic Anode Cathodic Protection of

Internal Submerged Surfaces of Steel Water Storage Tanks” (Houston, TX: NACE).

NACE International SP0388 (latest revision), “Impressed Current Cathodic Protection of Internal Submerged Surfaces of Carbon Steel Water Storage Tanks” (Houston, TX: NACE).