1、 Standard Practice Galvanic Anode Cathodic Protection of Internal Submerged Surfaces of 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 pre
2、clude 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 implicatio
3、n 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
4、 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 interpre
5、tation 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 o
6、f 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 healt
7、h 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 pra
8、ctices, 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 CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be revised or withdrawn
9、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. The user is cautioned to obtain the latest edition. Purchasers of NACE In
10、ternational standards may receive current information on all standards and other NACE International publications by contacting the NACE International FirstService Department, 15835 Park Ten Place, Houston, Texas 77084-5145 (telephone +1 281-228-6200). Revised 2015-11-28 NACE International 15835 Park
11、 Ten Place Houston, TX 77084-5145 +1 281-228-6200 ISBN 1-5790-189-7 2015 NACE International NACE SP0196-2015 Item No. 21077 SP0196-2015 i NACE International _ Foreword The purpose of this standard is to present the standard practices used in providing galvanic anode cathodic protection (CP) to the n
12、ormally submerged steel surfaces inside steel water storage tanks. This standard provides owners, engineers, and contractors with guidelines for the application of CP to the submerged surfaces of steel water storage tanks; for determining the effectiveness of these CP systems; and for the operation
13、and maintenance of these CP systems. This standard is applicable to steel water storage tanks of various sizes used in municipal water supply and fire protection, including elevated tanks and flat-bottom tanks at ground level. Although the practices presented in this standard generally are applicabl
14、e to all such tanks, the galvanic anode CP system described in this standard may not be practical for relatively large tanks. This standard was originally prepared in 1996 by NACE Task Group (TG) T-7L-1, a component of Unit Committee T-7L, “Cathodic Protection.” It was revised in 2004, 2011, and 201
15、5 by TG 284, “Cathodic Protection, Galvanic Anode for Internal Submerged Surfaces of Steel Water Storage TanksReview of NACE SP0196.” TG 284 is administered by Specific Technology Group (STG) 05, “Cathodic/Anodic Protection.” It is sponsored by STG 11, “Water Treatment,” and STG 35, “Pipelines, Tank
16、s, and Well Casings.” This standard is issued by NACE 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. The terms shall and must are used to state a requirement, an
17、d are considered mandatory. The term should is used to state something good and is recommended, but is not considered mandatory. The term may is used to state something considered optional. SP0196-2015 NACE International ii _ NACE International Standard Practice Galvanic Anode Cathodic Protection of
18、 Internal Submerged Surfaces of Steel Water Storage Tanks Contents 1. General 1 2. Definitions 1 3. Determination of Need for Cathodic Protection . 2 4. Design of Galvanic Anode Cathodic Protection Systems . 3 5. Installation of Galvanic Anode Cathodic Protection Systems . 7 6. Criteria for Cathodic
19、 Protection . 7 7. Operation and Maintenance 9 References 11 Appendix A: Tanks and Vessels Containing Other Waters (Nonmandatory) 12 TABLES Table 1: Typical Galvanic Anode Materials and Characteristics. 5 _ NACE International 1 _ Section 1: General 1.1 This standard presents standard practices for u
20、sing galvanic anodes to apply CP to the internal submerged surfaces of steel tanks used for the storage of potable and reclaimed water, drinking water, irrigation water, and fire protection water. Appendix A (nonmandatory) provides guidance for the use of CP for the internal surfaces of tanks and ve
21、ssels containing other waters. 1.2 Impressed current CP systems are used extensively for the internal surfaces of water storage tanks; however, this standard addresses only galvanic anode CP systems. For a description of impressed current CP systems, refer to NACE SP0388.1 1.3 The ground level and e
22、levated storage tanks considered in this standard are of welded, bolted, or riveted-steel construction, and include many shapes and sizes. 1.4 CP as described in this standard may be used alone to control corrosion of submerged steel surfaces or may be used as a complement to the protection provided
23、 by protective coatings or other procedures.2 CP cannot protect surfaces that are not submerged; these surfaces are typically protected by coatings alone. 1.5 CP may be installed to control corrosion in both newly constructed and existing tanks. When CP is used on existing tanks, it may be necessary
24、 to drain the tank during installation. 1.6 Tanks under consideration for application of CP are often associated with potable water and fire protection systems that may be subject to public health and safety regulations.3 This standard shall not infringe on those regulations. Proper disinfection of
25、the tanks may be required after installation. 1.7 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 principl
26、es of engineering and mathematics as acquired by professional education and related practical experience, is qualified to practice corrosion control, including CP, for water storage tanks. Such persons may be Registered Professional Engineers or persons recognized as being qualified or certified as
27、Corrosion Specialists or CP Specialists by NACE, if their professional activities include suitable experience in corrosion control and CP. 1.8 This standard may not be applicable in all situations. The corrosion engineer may consider alternative corrosion control methods. _ Section 2: Definitions An
28、ode: The electrode of an electrochemical cell at which oxidation occurs. (Electrons flow away from the anode in the external circuit. It is usually the electrode where corrosion occurs and metal ions enter solution.) Anode Circuit: The path from a single anode or multiple anodes connected through a
29、shunt, a resistor, and the connection to the tank. Cathode: The electrode of an electrochemical cell at which reduction is the principal reaction. (Electrons flow toward the cathode in the external circuit.) Cathodic Protection: A technique to reduce the corrosion rate of a metal surface by making t
30、hat surface the cathode of an electrochemical cell. Coating: (1) a liquid, liquefiable, or mastic composition that, after application to a surface, is converted into a solid protective, decorative, or functional adherent film; (2) (in a more general sense) a thin layer of solid material on a surface
31、 that provides improved protective, decorative, or functional properties. Conductivity: (1) a measure of the ability of a material to conduct an electric charge; (2) the current transferred across a material (e.g., coating) per unit area per unit potential gradient. (Conductivity is the reciprocal o
32、f resistivity). Corrosion: The deterioration of a material, usually a metal, that results from a chemical or electrochemical reaction with its environment. SP0196-2015 2 NACE International Corrosion Potential: The potential of a corroding surface in an electrolyte measured under open-circuit conditi
33、ons relative to a reference electrode. also known as electrochemical corrosion potential, free corrosion potential, open-circuit potential CP Coupon: A metal specimen made of similar material as the tank under investigation, which is connected to the external surface of, and immersed in, the electro
34、lyte adjacent to the tank being protected by cathodic protection. Current Density: The electric current flowing to or from a unit area of an electrode surface. Driving Potential: Difference in potential between the anode and the steel structure. For the purposes of this standard, driving potential i
35、s understood to be the potential difference between the galvanic anodes and the tank wall when the cathodic protection system is in operation. Electrical Isolation: The condition of being electrically separated from other metallic structures or the environment. Electrode: A material that conducts el
36、ectrons, is 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. Electrolyte: A chemical substance containing ions that migrate
37、 in an electric field. For the purpose 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 i
38、s 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 i
39、s direct current for cathodic protection.) Open-Circuit Potential: See corrosion potential. Polarization: The change from the corrosion potential as a result of current flow across the electrode/electrolyte interface. Polarized Potential: (1) (general use) The potential across the electrode/electrol
40、yte interface that is the sum of the corrosion potential and the applied polarization; (2) (cathodic protection use) the potential across the structure/electrolyte interface that is the sum of the corrosion potential and the cathodic polarization. Reference Electrode: An electrode having a stable an
41、d reproducible potential, which is used in the measurement of other electrode potentials. Shielding: (1) Protecting; protective cover against mechanical damage; (2) preventing or diverting cathodic protection current from its natural path. Tank-to-Water Potential: The potential difference between a
42、submerged metallic portion of the tank and the water (electrolyte), which is measured with a reference electrode in contact with the water. Voltage Drop: The voltage across a resistance when current is applied in accordance with Ohms Law. _ Section 3: Determination of Need for Cathodic Protection 3.
43、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 water quality and planned useful life of the tank. 3.1.2
44、 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. NACE International 3 3.1.3 CP is effective in controlling corrosion only on submerged metal surfaces during su
45、bmersion. It does not reverse structural damage already caused by corrosion. CP may not be effective under disbonded coating. 3.2 Coated Steel Tanks 3.2.1 CP should be used to reduce corrosion on submerged, coated steel tank surfaces where holidays existor are anticipatedand where corrosion would oc
46、cur as a result of those holidays. 3.2.2 The use of dissimilar metals inside the tank may contribute to the creation of a localized galvanic corrosion cell. Where the use of dissimilar metals cannot be avoided, the dissimilar metals should be electrically isolated from the steel tank shell where pos
47、sible. The application of a protective coating on the more noble metal components may be used in lieu of electrical isolation, provided that the coating can be properly applied and maintained. 3.2.3 If the water is sufficiently corrosive to justify the use of a coating, then CP is also justified and
48、 the combination provides a greater degree of protection than when either method is used alone. 3.2.4 Properly designed and maintained CP systems extend the time between recoating the submerged interior surfaces. 3.2.5 The current required for CP is much lower for coated steel tanks compared with bare or poorly coated steel tanks. 3.3 Uncoated Steel Tanks 3.3.1 It is feasible to design a CP system to provide protection for uncoated submerged surfaces of steel tanks (see Section 4). 3.3.2 It may be more practica
