1、Item No. 24207 NACE International Publication 6A100 This Technical Committee Report has been prepared by NACE International Task Group T-6A-39* on Coatings in Conjunction with Cathodic Protection. Coatings Used in Conjunction with Cathodic Protection July 2000, NACE International This NACE Internati
2、onal technical committee report 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 from manufacturing, marketing, purchasing, or using products, processes, or procedures not included in
3、 this report. Nothing contained in this NACE International report 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 liabil
4、ity for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness
5、 of this report in specific instances. NACE International assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE International report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determi
6、ning their applicability in relation to this report prior to its use. This NACE International report 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 re
7、port. Users of this NACE International report 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 pri
8、or to the use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE International reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 ye
9、ars old. Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International Membership Services Department, P.O. Box 218340, Houston, Texas 77218-8340 (telephone +1281228-6200). Foreword The use of protective coatings
10、in conjunction with cathodic protection has an extensive record of proven performance in the protection of structures including underground pipelines, ships, underground storage tanks, pilings, interiors of aboveground storage tanks, and tank bottoms. The use of both a protective coating and a catho
11、dic protection system presents unique design, installation, and maintenance considerations. This technical committee report provides users of pro-tective coatings and cathodic protection with information on the strengths and weaknesses of each protective system and explains how a greater level of co
12、rrosion protection can be achieved on buried or immersed sub-strates if both systems are used together. This report does not claim that one protective system is superior, but instead focuses on the interdependent relationship between coatings and cathodic protection. The inform-ation presented in th
13、is report is relevant to both interior and exterior coatings used in all industries, but specific applications are also noted. This technical committee report was prepared by NACE Task Group T-6A-39, which is under the guidance of Unit Committee T-6A on Coating and Lining Materials for Immersion Ser
14、vice. It is issued under the auspices of Group Committee T-6 on Protective Coatings and Linings. Members of NACE Unit Committee T-10A on Cathodic Protection also assisted in reviewing this report. _ *Chairman, Kat Coronado, Ameron, Houston, TX. NACE International 2 Introduction The decision to use p
15、rotective coatings, cathodic protection, or both is often based on economic, safety, and environmental issues as well as the particular conditions of the structure under consideration. A well-trained and competent corrosion-control specialist weighs all factors to determine the methods of protection
16、 to be used for a particular structure and environment. If a structure is coated but not cathodically protected, holidays or small defects can develop, resulting in pitting and possibly corrosion leaks. If the same structure is cathodically protected without coating, the operational costs would like
17、ly be prohibitive. The use of coatings in conjunction with cathodic protection is often considered the most effective and economically viable alternative. The parameters for selection of a coating system and the design of a cathodic protection system can be determined after the mechanisms of perform
18、ance and protection for each system are studied independently. The desired corrosion-control objectives of a system can be realized only if the cathodic protection system is compatible with the coating selected. Most protective coatings are compatible with cathodic protection systems. However, catho
19、dic protection can adversely affect a coating if both are used together in an improperly designed system. The determination of whether the cathodic protection system or the coating system is the primary means of protection occurs after determining whether one is compatible with the other. Protective
20、 Coatings Coatings have successfully protected aboveground, underground, and immersed steel structures in contact with various electrolytes. Because coating maintenance for buried or submerged structures is expensive, cathodic protection is often used to complement a coating system to maximize maint
21、enance intervals. For the purposes of this report, a protective coating is defined as any material or system that electrically and chemically isolates the conductive substrate to which it is applied from the intended service environment. Coatings used in conjunction with cathodic protection are ofte
22、n referred to as barrier coats or electrical isolators. Some common characteristics of all protective coatings typically used with cathodic protection are as follows: 1. All coatings have varying dielectric properties that reduce the tendency of the electrolyte to complete the electrical circuit bet
23、ween adjacent anodic and cathodic sites on a substrate, thereby mitigating corrosion. 2. All coatings are permeable, and have micro-pores and fissures that eventually permit water vapor to reach the substrate. 3. Every coating system has a finite life and event-ually degrades, allowing oxygen, water
24、, and chemicals to reach the substrate. Some protective coatings become brittle and crack, exposing the substrate to its environment. Mechanical and operational damage can occur to both submerged and buried coated structures. These holidays or defective areas can result in the initiation of corrosio
25、n. Surface Preparation Even though a commercial blast-cleaned surface1is accepted for some coating systems, it is not typically considered to be adequate surface preparation for coatings in immersion service. Surface preparation for coatings used in submerged or buried service is provided in accorda
26、nce with the specification or the manufacturers recommendations.(1)Adhesion is typically considered to be particularly impor-tant for proper performance of a coating in immersion service. Although a particular type of blast cleaning is usually specified, a surface profile is often not specified. How
27、ever, provision of an appropriate profile height after blasting is normally considered essential for good adhesion and adequate performance, especially in severe service. Coating specialists believe that disbondment or blistering of a coating, in test or in service, is a result of many factor(s) inc
28、luding insufficient surface profile and/or cleanliness. Cathodic Protection Cathodic protection is a method of corrosion control that can protect an exposed substrate for an extended period of time by making that substrate the cathode of an electrochemical cell. It is a common method of con-trolling
29、 the effects of corrosion on steel structures that are underground, underwater, totally immersed in an electro-lyte, or within concrete structures. Cathodic protection can be achieved with a properly designed sacrificial anode system, or with an impressed current system consisting of a direct curren
30、t (DC) power source and impressed current anodes. DC is applied to a structure to prevent the flow of ions from the substrate to the electrolyte, rendering the substrate cathodic. _ (1)NACE Standard RP01782provides surface preparation requirements for tanks and vessels to be coated for immersion ser
31、vice. NACE International 3 Corrosion control can be achieved at various levels of polarization, depending on environmental conditions. However, in the absence of specific data that demon-strate adequate achievement of cathodic protection, the criteria presented in NACE Standard RP01693are used to ev
32、aluate cathodic protection systems that protect pipe-lines. Further information on cathodic protection criteria for underground and ongrade storage tanks is found in NACE Standards RP02854and RP0193,5respectively, as well as in ISO(2)and U.S. Navy(3)standards. The location of the structure to be pro
33、tected is an important consideration in the installation of a cathodic protection system. A cathodic protection system can impact the effectiveness of corrosion protection on nearby structures, and, conversely, be affected by outside electrical sources (e.g., other cathodic protection systems, stray
34、 electrical currents, and contact with foreign structures). Using Coatings and Cathodic Protection Together The dynamic forces acting on protective coatings in atmospheric, splash-and-spill, subsoil, marine, and immersion services are similar to the dynamic forces acting on coatings applied to under
35、ground coated structures under cathodic protection. For example, the environment of an underground coated structure can be similar to that of a structure in immersion conditions if the soil is saturated with water. The environment of an underground coated structure can also be similar to that of a s
36、tructure in atmospheric conditions. Underground coated structures under cathodic protection are also affected by a possibly potent electrical driving force that alters the chemical and dynamic forces normally oper-ating on a protective coating, which can greatly affect the performance of that coatin
37、g. The effects of electrical stress can be extremely adverse, especially when exposed to excessive cathodic potentials. However, these conditions are often present in right-of-ways in which there are several pipelines with a variety of coating types, ages, and conditions. One or more of these pipes
38、might be uncoated. For these reasons, the cathodic protection potentials might be excessive on some of the pipes in order to provide proper protection to the others. Even in a single pipeline right-of-way, the location of a cathodic protection system sometimes affects the amount of protection provid
39、ed. A cathodic protection system cannot always be placed so that adequately low protection potentials are achieved. Most coating manufacturers accept cathodic disbondment tests by outside sources, if the testers adhere to the parameters cited in ASTM(4)G 8.6Many companies vary the test criteria in a
40、n effort to “accelerate” the test or observe a different set of results than can be observed by the standard tests. If a coating can survive at least -3.0 V DC for 30, 60, or 90 days (or even six months) in a test situation, then that coating is often considered able to withstand any electrical stre
41、ss that could occur in the field. However, there has been very little work done to determine the ability of a coating to resist electrical stress, particularly overvoltage potentials, over a given period of time. Simpson and Robinson state that “exper-ience with fast/high voltage tests (up to 3 mont
42、hs at -5.0 volts) led to certain erroneous results in earlier exper-iments.” This method of testing was discarded as being unrepresentative because it was extreme and did not allow the system to reach equilibrium.7 Also, lab meas-urements of the dielectric strength of a coating do not simulate the p
43、lacement of that coating on a nonuniform substrate in a soil environment of a varying resistivity and changing physical conditions. Dr. Harold F. Hasse conducted an 11-year cathodic disbondment test program on a variety of thin-film (150- to 200-m 6- to 8-mil) coatings.8Of the 11 coatings tested at
44、an applied cell potential of -2.3 V DC, four of the coating systems were rated “failed” and two were rated “poor.” Some “failed” and “poor” systems were still noted as being very effective barriers in reducing cathodic current requirements, but all except one eventually failed from blistering or coa
45、ting degradation. The results of another laboratory test of typical pipeline tape coatings subjected to -1.1 V DC and -2.0 V DC showed that at -2.0 V DC, coatings break down at between 3 weeks and 4.5 years.9While some coatings survive overvoltage and destructive tests, most coatings are eventually
46、unable to survive this type of electrical stress. Although some coat-ings survive destructive tests, it is not certain whether they are truly corrosion-resistant coatings or merely good electrical cable sheaths. Coatings intended for service in an urban environment, where large amounts of stray curr
47、ent are likely, are very often tested using higher test voltages because of the unusually high electrical stress that these coatings encounter in service. These tests can identify one or several coatings that pass such an accelerated test in the short run. However, the objective of testing is to eva
48、luate coatings for their performance in regard to a multitude of tests and/or field criteria, not just electrical resistance. The severity of testing can also be greatly increased by affecting variables other than current, including temper- ature, composition of the electrolyte, and duration of the_
49、 (2)International Organization for Standardization (ISO), 1 rue de Varmbe, Case Postal 56, CH-1121 Geneve 20, Switzerland. (3)Office of Naval Research, 800 N Quincy St., Arlington VA 22217-5660. (4)ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959. NACE International 4 test, or by agitation of the electrolyte. Coatings are also tested under conditions that test water absorption/ permeation. Coatings for use on structures that operate at higher temperatures are typically tested at or abov
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