NACE_SP_0388

Standard Practice Impressed Current Cathodic Protection of Internal Submerged Surfaces of Carbon Steel Water Storage Tanks This NACE International standard represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone, whether he or she has adopted the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with this standard. Nothing contained in this NACE International standard is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This standard represents minimum requirements and should in no way be interpreted as a restriction on the use of better procedures or materials. Neither is this standard intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this standard in specific instances. NACE International assumes no responsibility for the interpretation or use of this standard by other parties and accepts responsibility for only those official NACE International interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of interpretations 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 relation to this standard prior to its use. This NACE International standard may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this standard. Users of this NACE International standard are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this standard. CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be revised or withdrawn at any time in accordance with NACE technical committee procedures. NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial publication and subsequently from the date of each reaffirmation or revision. The user is cautioned to obtain the latest edition. Purchasers of NACE International standards may receive current information on all standards and other NACE International publications by contacting the NACE International First Service Department, 1440 South Creek Dr., Houston, Texas 77084-4906 (telephone +1 281/228-6200). Reaffirmed 2007-09-20 Revised 2001-11-07 Approved November 1988 Reaffirmed April 1990 Reaffirmed March 1995 Approved November 1988 NACE International 1440 South Creek Drive Houston, Texas 77084-4906 +1 281/228-6200 ISBN 1-57590-138-2 © 2007, NACE International NACE SP0388-2007 (formerly RP0388-2001) Item No. 21040 Copyright NACE International Provided by IHS under license with NACE Not for ResaleNo reproduction or networking permitted without license from IHS - - ` , , ` ` ` , , , , ` ` ` ` - ` - ` , , ` , , ` , ` , , ` - - - SP0388-2007 NACE International i _________________________________________________________________________ Foreword The purpose of this NACE standard is to present the recommended practices for providing impressed current cathodic protection (CP) to the normally submerged steel surfaces inside water storage tanks. It contains recommendations for the design and installation of these cathodic protection systems and methods for determining the effectiveness of these systems. Recommendations for the operation and maintenance of both automatic and manual systems are provided. This standard is applicable to relatively large water storage tanks used in municipal water supply and fire protection, including elevated and on-grade tanks. Although the general principles outlined in this standard are applicable to all such tanks, the impressed current cathodic protection system described in this standard may not be practical for smaller tanks. This standard is intended for use by engineers, water utilities, tank erectors and other contractors, and owner operators of steel water storage tanks. This standard was originally prepared in 1988 by Task Group T-7L-1, a component of Unit Committee T-7L on Cathodic Protection. 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 was reaffirmed by T-7L in 1990 and 1995, revised in 2001 by Task Group 167 (formerly T-7L-14), and reaffirmed by Specific Technology Group (STG) 05 in 2007. Task Group 167 is administered by STG 05 on Cathodic/Anodic Protection. This standard is issued by NACE International under the auspices of STG 05. In NACE standards, the terms shall, must, should, and may are used in accordance with the definitions of these terms in the NACE Publications Style Manual, 4th ed., Paragraph 7.4.1.9. Shall and must are used to state mandatory requirements. The term should is used to state something good and is recommended but is not mandatory. The term may is used to state something considered optional. _________________________________________________________________________ Copyright NACE International Provided by IHS under license with NACE Not for ResaleNo reproduction or networking permitted without license from IHS --`,,```,,,,````-`-`,,`,,`,`,,`--- SP0388-2007 ii NACE International _________________________________________________________________________ NACE International Standard Practice Impressed Current Cathodic Protection of Internal Submerged Surfaces of Steel Water Storage Tanks Contents 1. General .......................................................................................................................... 1 2. Definitions ...................................................................................................................... 1 3. Determination of the Need for Cathodic Protection ....................................................... 2 4. Design of Impressed Current Cathodic Protection Systems ......................................... 3 5. Installation of Impressed Current Cathodic Protection Systems ................................... 5 6. Criteria for Cathodic Protection and Measurement Procedures .................................... 6 7. Automatic Impressed Current Systems ......................................................................... 7 8. Operation and Maintenance .......................................................................................... 8 References .......................................................................................................................... 9 _________________________________________________________________________ Copyright NACE International Provided by IHS under license with NACE Not for ResaleNo reproduction or networking permitted without license from IHS - - ` , , ` ` ` , , , , ` ` ` ` - ` - ` , , ` , , ` , ` , , ` - - - SP0388-2007 NACE International 1 _________________________________________________________________________ Section 1: General 1.1 This standard presents recommended practices for applying impressed current cathodic protection to the internal submerged surfaces of steel tanks used for the storage of natural waters. These tanks may be provided with an interior barrier coating. It is not intended for use with metallic coatings or linings, such as zinc or aluminum. 1.2 It is recognized that galvanic anode systems can, at times, be used for cathodic protection of the internal surfaces of water storage tanks; however, this standard addresses only impressed current systems. NOTE: Information regarding galvanic cathodic protection of water storage tanks may be found in NACE Standard RP0196. 1 1.3 Natural waters, as used in this standard, include both potable and nonpotable fresh waters associated with water supply systems and fire protection. 1.4 The ground level and elevated storage tanks considered in this standard are of welded, bolted, or riveted steel construction and include many shapes and sizes. 1.5 Cathodic protection, as described in this standard, may be used alone to control corrosion of submerged steel surfaces or used as a complement to the protection afforded by protective coatings. Cathodic protection cannot protect surfaces that are not submerged. These surfaces must be protected by coatings alone. Cathodic protection does not reverse structural damage already caused by corrosion. 1.6 Cathodic protection may be installed to control corrosion in both newly constructed and existing tanks. When cathodic protection is used on existing tanks, it is not necessary to prepare the surfaces to be protected; however, it may be necessary to drain the tank during installation. 1.7 It is recognized that the tanks under consideration are often associated with potable water and fire protection systems, which may be subject to public health and safety regulations. This standard should not infringe upon those regulations. Proper disinfection of the tanks may be required after installation. Any applicable regulations such as those from the U.S. Environmental Protection Agency (EPA) (1) and ANSI (2) /NSF (3) -61 2 should be checked. In the United States, all materials in contact with potable water or exposed to the interior of potable water tanks must be classified in accordance with ANSI/NSF-61. 1.8 The provisions of this standard should be applied under the direction of a competent corrosion engineer. The term “corrosion engineer,” as used in this standard, refers to a person who by reason of knowledge of the physical sciences and the principles of engineering and mathematics, as acquired by professional education and related practical experience, is qualified to practice corrosion control and cathodic protection for water storage tanks. Such persons may be registered professional engineers or persons certified by NACE International as Cathodic Protection or Corrosion Specialists, if their professional activities include suitable experience in corrosion control and cathodic protection. 1.9 This standard may not be applicable in all situations. The responsible corrosion engineer may consider alternate corrosion control methods. _________________________________________________________________________ Section 2: Definitions Anode: The electrode of an electrochemical cell at which oxidation occurs. Electrons flow away from the anode in the 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 surface. When the surface is cathodically polarized as in cathodic protection, this layer is the result of the increased pH adjacent to the protected surface. Cathode: The electrode of an electrochemical cell at which reduction is the principal reaction. Electrons flow toward the cathode in the external circuit. Cathodic Disbondment: The destruction of adhesion between a coating and the coated surface caused by products of a cathodic reaction. Cathodic Protection: A technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. _________________________________________ (1) U.S. Environmental Protection Agency (EPA), 401 M Street SW, Washington, DC 20460. (2) American National Standards Institute (ANSI), 25 W 43rd St., Fourth Floor, New York, NY 10036. (3) NSF International, P.O. Box 130140, Ann Arbor, MI 48113-0140. Copyright NACE International Provided by IHS under license with NACE Not for ResaleNo reproduction or networking permitted without license from IHS - - ` , , ` ` ` , , , , ` ` ` ` - ` - ` , , ` , , ` , ` , , ` - - - SP0388-2007 2 NACE International Coating: A liquid, liquefiable, or mastic composition that, after application to a surface, is converted into a solid protective, decorative, or functional adherent film. Conductivity: A measure of the ability of a material to carry an electric current. In water, this depends on the total concentration of the ionized substances dissolved and the temperature at which the measurement is made. It is the reciprocal of resistivity and is usually expressed in µS/cm (µmhos/cm). Corrosion: 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 or from an electrolyte. Electrode Potential: The potential of an electrode in an electrolyte as measured against a reference electrode. (The electrode potential does not include any resistance losses in potential in either the electrolyte or the external circuit. It represents the reversible work to move a unit of charge from the electrode surface through the electrolyte to the reference electrode.) Electrolyte: A chemical substance containing ions that migrate in an electric field. For the purposes of this standard, electrolyte refers to the water, including the dissolved chemicals, in the tank. Galvanic Anode: A metal that provides sacrificial protection to another metal that is more noble when electrically coupled in an electrolyte. This type of anode is the electron source in one type of cathodic protection. Holiday: A discontinuity in a protective coating that exposes unprotected surface to the environment. Impressed Current: An electric current supplied by a device employing 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 materials, that is supplied with impressed current. IR Drop: The voltage across a resistance in accordance with Ohm’s Law. Polarization: The 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 conditions of measurement, which 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 usually expressed in ohm-centimeters (ohm-cm) and is the reciprocal of conductivity. Tank-to-Water Potential: The voltage difference between a submerged metallic portion of the tank and the electrolyte (water), which is measured with a reference electrode in contact with the electrolyte. _________________________________________________________________________ Section 3: Determination of the Need for Cathodic Protection 3.1 Introduction 3.1.1 Steel tank surfaces submerged in natural waters are subject to corrosion. The methods and procedures used to control corrosion should be governed by the rate of corrosion and the cost of maintaining the tank, including the hydraulic and aesthetic effects of corrosion debris in the tank and piping. 3.1.2 All coatings are subject to damage and deterioration. Therefore, corrosion control by use of coatings alone on the submerged surfaces of a steel water storage tank is usually not possible. 3.1.3 Cathodic protection is effective in controlling corrosion only on the submerged metal surfaces. 3.2 Cathodic Protection of Coated Tanks 3.2.1 In almost all cases, natural waters are sufficiently corrosive to require the use of protective coatings and cathodic protection for corrosion control. Protective coatings and cathodic protection are synergistic, with the combination of the two methods providing a greater degree of corrosion protection than either method used alone. 3.2.2 Properly designed and maintained, cathodic protection systems can extend the useful life of the water tank and its coating system. 3.2.3 The current required for cathodic protection is lowered significantly when coatings are used. 3.2.4 The coating system should be compatible with cathodic protection. Copyright NACE International Provided by IHS under license with NACE Not for ResaleNo reproduction or networking permitted without license from IHS --`,,```,,,,````-`-`,,`,,`,`,,`--- SP0388-2007 NACE International 3 3.3 Cathodic Protection of Uncoated Steel Tanks 3.3.1 It is feasible to design a cathodic protection system to provide complete protection for uncoated submerged surfaces of steel tanks (see Section 4). 3.3.2 For existing steel tanks with submerged surfaces that are not coated or that have coatings in poor condition, it may be economical to rely entirely on cathodic protection. 3.4 Economic Considerations 3.4.1 Coated Tanks The cost of cathodic protection should be compared with the cost of recoating and metal loss. Cathodic protection defers the need for coating repair and recoating for considerable time. This effectively extends the life of the coating system. In addition, cathodic protection prevents repair costs from metal loss. 3.4.2 Uncoated Tanks It is possible to prevent metal loss in steel tanks where the surfaces are not coated or where the coating has deteriorated to the point that the surfaces can be considered uncoated. The cost of cathodic protection should be compared with the cost of metal loss, service disruption, reduction in design life, and water quality degradation. 3.5 Operational Considerations 3.5.1 Taking the tank out of service for coating maintenance or corrosion repair can be eliminated or postponed if the cathodic protection system is designed and operated properly. 3.5.2 Two benefits of keeping the tank in continuous service are system capacity and water availability for fire protection. _________________________________________________________________________ Section 4: Design of Impressed Current Cathodic Protection Systems 4.1 Introduction 4.1.1 This section outlines the elements that should be considered when designing impressed current cathodic protection systems for steel water storage tanks. 4.1.2 In the design of an impressed current system, the following items must be considered: (a) Design of the tank (accessibility, crevices, shielded areas), (b) Present and future condition of the coating and the generic type of coating employed, (c) Voltage and current capacity of the power source, (d) Impressed current anode material and configuration, (e) Impressed current anode life and ease of replacement, (f) Type of power source control, (g) Hardware, (h) Reference cell location(s), (i) Power costs, (j) Possible evolution of hydrogen and ventilation req