1、 Item No. 24242 NACE International Publication 35110 This Technical Committee Report has been prepared by NACE International Task Group 327,*“AC Corrosion State-of-the-Art: Corrosion Rate, Mechanism, and Mitigation Requirements.” AC Corrosion State-of-the-Art: Corrosion Rate, Mechanism, and Mitigati
2、on Requirements January 2010, NACE International This NACE International (NACE) 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, marketi
3、ng, purchasing, or using products, processes, or procedures not included in this report. Nothing contained in this NACE 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 P
4、atent, or as indemnifying or protecting anyone against liability 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 s
5、ubject. Unpredictable circumstances may negate the usefulness of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and reg
6、ulatory documents and for determining their applicability in relation to this report prior to its use. This NACE 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 referre
7、d to within this report. Users of this NACE 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 requirement
8、s prior to the use of this report. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old.
9、Purchasers of NACE reports may receive current information on all NACE International publications by contacting the NACE FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281-228-6200). Foreword This technical committee report represents the current understandi
10、ng of the corrosion phenomenon associated with alternating current (AC) interference on buried steel pipelines. The purpose of this state-of-the-art report is to begin the development of corrosion protection criteria with regard to AC corrosion. In the past 20 years, AC corrosion has become recogniz
11、ed as a threat to the integrity of underground structures, especially to buried pipelines sharing the right-of-way with high-tension electrical lines. Every attempt was made to incorporate as many published accounts into this report as possible, including multiple international sources. However, giv
12、en the increased awareness of AC corrosion by pipeline operators and the corrosion community at large, a considerable amount of literature on the subject exists, and some most recent publications might have been left out of the report. The report addresses AC corrosion characteristics and proposed m
13、echanisms and describes the currently used approaches to protection and monitoring. It is intended for use by pipeline operators and others concerned with control of AC corrosion. _ * Chair Mark Yunovich, Honeywell Process Solutions, Houston, TX. NACE International 2 The report also identifies exist
14、ing knowledge gaps and briefly outlines the path forward. Four case studies are presented in Appendix A. The issue of AC interference (and AC corrosion) mitigation is deliberately presented in brief; AC mitigation is the primary focus of Task Group (TG) 025, “Alternating Current (AC) Power Systems,
15、Adjacent: Corrosion Control and Related Safety Procedures to Mitigate the Effects.” This technical committee report has been prepared by TG 327, “AC Corrosion State-of-the-Art: Corrosion Rate, Mechanism, and Mitigation Requirements.” TG 327 is administered by Specific Technology Group (STG) 35, “Pip
16、elines, Tanks, and Well Casings.” This report is issued by NACE International under the auspices of STG 35. NACE technical committee reports are intended to convey technical information or state-of-the-art knowledge regarding corrosion. In many cases, they discuss specific applications of corrosion
17、mitigation technology, whether considered successful or not. Statements used to convey this information are factual and are provided to the reader as input and guidance for consideration when applying this technology in the future. However, these statements are not intended to be recommendations for
18、 general application of this technology, and must not be construed as such. Introduction The phenomenon of AC corrosion has been considered by many authors since the early 1900s. However, the mechanisms of AC corrosion are still not completely understood. The body of recent (post-1980) literature in
19、dicates that AC corrosion or AC-enhanced corrosion (ACEC) is a bona fide effect (reported corrosion rates up to 20 mpy 0.5 mm/y, with pitting rate considerably higher); there appears to be a tacit agreement that at prevailing commercial current frequencies (such as 50 or 60 Hz) corrosion is possible
20、, even on cathodically protected pipelines. AC corrosion on cathodically protected pipelines is not well understood, despite discussion about it dating back to the late 19thcentury. For many years, corrosion experts did not consider corrosion attributed to alternating currents on metallic structures
21、 very important. In 1891, Mengarini1concluded that corrosion (“chemical decomposition”) by AC (1) is less than that caused by the equivalent direct current (DC), (2) is proportional to the AC, (3) there exists a threshold AC density below which no “decomposition of electrolyte” occurs, and (4) the e
22、xtent of corrosion decreases with increased AC frequency. In 1916, McCollum, et al.2published a research paper that concluded iron does not suffer from attack when a limiting frequency of the current (somewhere between 15 and 60 Hz) is reached. AC corrosion was not well understood for two reasons: (
23、1) the electrochemical phenomenon of corrosion is normally attributed to DC, and (2) the instruments normally used to measure the electric parameters in direct currents cannot correctly detect the presence of AC with frequencies between 50 and 100 Hz.3Recently, concern for AC corrosion mitigation ha
24、s been increasing because AC interference has been shown to affect cathodically protected underground structures and increase safety concerns (i.e., high AC step-and-touch potentials). Factors that contribute to AC interference on pipelines include (1) the growing number of high-voltage power lines,
25、 (2) AC operated high-speed traction systems, (3) high isolation resistance of modern pipeline coatings, and (4) coating integrity.3 Characteristics of AC Corrosion Corrosion Rate There is a scarcity of data on the magnitude of the corrosion rate of steel in soils influenced by AC. The general under
26、standing is that higher alternating currents lead to higher risk of AC corrosion. Ragault4reiterates this notion and states that field investigations of conditions on a coated, cathodically protected pipeline with AC density ranging between 84 and 1,100 A/m2(7.8 and 102 A/ft2) (with on-potentials be
27、tween 2.0 and 2.5 V) did not show any clear relationship between AC density and corrosion rate (found to be between 12 and 54 mpy 0.3 and 1.4 mm/y). Wakelin, et al.5reports that three field studies and inspections found rates ranging from 17 to 54 mpy (0.4 to 1.4 mm/y) for AC densities between 75 an
28、d 200 A/m2 (7 and 19 A/ft2). A German field-based coupon study6revealed rates scattered between 2 and 4 mpy (0.05 and 0.1 mm/y) at 100 A/m2(9.3 A/ft2) and 12 mpy (0.3 mm/y) at 400 A/m2 (37 A/ft2); the rate of pitting was more NACE International 3 scatteredbetween 8 and 56 mpy (0.2 and 1.4 mm/y), and
29、 it showed much less pronounced dependence on the AC density. A 1964 work by Bruckner7(sponsored by the American Gas Association AGA)(1)showed that in soils with pH between 6 and 7, the corrosion rate of steel is below 3 mpy (0.08 mm/y) at 155 A/m2(14.4 A/ft2) and between 10 and 20 mpy (0.25 and 0.5
30、 mm/y) at 775 A/m2(72 A/ft2). A paper by Song, et al.8reports corrosion rates as measured on coupons installed for 6 and 12 months next to a buried cathodically protected pipeline, which were exposed to AC with a carrier frequency of 60 Hz, but with an appreciable contribution of a 180 Hz harmonic.
31、The rates were found to be linearly increasing with AC density (less than 10 mpy 0.25 mm/y for densities below 100 A/m29.3 A/ft2 and between 5 and 25 mpy (0.13 and 0.64 mm/y) for densities between 100 and 500 A/m2 9.3 and 46 A/ft2); on-potentials were generally over 0.9 V.The German field study6obse
32、rved pitting corrosion rates of 210 mpy (5.3 mm/y) associated with AC densities between 20 and 200 A/m2(1.9 and 19 A/ft2). A number of recent publications presented the results of laboratory and field evaluation using coupons and probes. Short-term field testing by Nielsen and Galsgaard9recorded pea
33、k AC corrosion rates as high as 10 mm/y (400 mpy); Gregoor and Pourbaix10reported short-term laboratory-based rates between 0.01 and 0.25 mm/y per each A/m2(4.2 and 106 mpy/A/ft2) of AC density, with actual observed rates falling between 0.65 and 3.4 mm/y (26 and 130 mpy). Shoeneich11reported corros
34、ion rates from buried coupons exposed to 1 to 91 A/m2(0.09 to 8.5 A/ft2) of AC density under cathodic protection (CP) potentials more negative than 0.95 V (vs. copper/copper sulfate reference electrode CSE); the observed rate did not exceed 0.02 mm/y (0.8 mpy). A laboratory study by Yunovich and Tho
35、mpson12revealed that in the absence of CP, corrosion rates ranged from 3.5 to 8.2 mpy (0.09 to 0.21 mm/y). Song, et al.8suggest that for a given AC density, the corrosion rate tends to decrease with time and that there may even be an “incubation” period of one or more months, depending on the curren
36、t density. Morphological Characteristics Goran13studied AC in the field on steel test coupons. The test coupons were cathodically protected and exposed to different AC densities. The series of tests consisted of one with 10 V AC applied to the test coupons for approximately two years, and another on
37、e with 30 V AC during CP for 1.5 years. From the results and observations, he concluded the appearance of corrosion could be divided into three groups: Small point-shaped attacks evenly distributed across the surface (uneven surface); Large point-shaped attacks evenly distributed across the surface
38、(rough surface); and A few large, deep local attacks on an otherwise uncorroded surface (“pocked” surface). Nielsen and Cohn14 describe a distinct tubercle of “stone-hard soil,” comprising a mixture of corrosion products and soil that is often observed to grow from the coating defect in connection w
39、ith AC corrosion incidents. The specific resistivity of such a tubercle can be expected to be lower than the specific resistivity of the surrounding soil. In addition, the effective area of the tubercle is considerably greater than the original coating defect. Both processes tend to decrease the spr
40、eading resistance of the associated coating defect during the corrosion process, making the corrosion process autocatalytic in nature. Ragault4describes 31 AC corrosion cases on a polyethylene-coated gas transmission line and notes that the corrosion product consisted mostly of magnetite mixed with
41、soil. Williams15also indicates that the corrosion product on steel under AC influence was mainly magnetite. Some examples of the photographic evidence culled from the investigations of underground pipeline failures attributed to AC corrosion are shown in Figures 1 and 2. (1)American Gas Association
42、(AGA), 400 N. Capitol St. NW, Washington, DC 20001. NACE International 4 Figure 1: Leak site on underground natural gas transmission pipeline (attributed to AC corrosion), before and after cleaning. The arrow indicates the leak.16 Figure 2: External corrosion site on a natural gas transmission pipel
43、ine (attributed to AC corrosion), before and after cleaning.17Bolzoni, et al.,18who studied AC influence in solutions, report that the AC led to growth of thick but nonadhering corrosion products; the research results suggested that corrosion caused by AC was likely to be localized. If one follows t
44、he checklist presented in the 2000 CEOCOR(2)proceedings3to determine whether the corrosion attack is caused by AC based on the morphology of the damaged site, the answer is not clear. The questionnaire posts such questions as (1) is there a coating defect, (2) is the shape of corrosion damage a roun
45、ded pit, (3) is the size of the pit much larger than the size of the associated coating defect, (4) is the soil resistivity low/very low, and several others. After one answers these and several other questions, the authors conclude that if “many” of the answers are “yes,” then it “probably” is an AC
46、 corrosion case. Wakelin, et al.5describes Canadian AC case histories and offers another similar checklist to determine whether the cause of corrosion could be attributed to AC. The approach is to eliminate all other culprits (e.g., microbiologically influenced corrosion MIC) and evaluate the charac
47、teristics of the damaged region, paying particular attention to whether the pit has a rounded bottom and whether soil and corrosion products had formed a hard dome over the pit. AC Density Studies performed in the 1950s and 1960s indicated that the AC-enhanced corrosion of steel is low, being in the
48、 range of 0.1 to 1% of a similar amount of DC-enhanced corrosion. Within this range (below 1%), Pookote (2)CEOCOR, c/o CIBE, rue aux laines, 70, B-1000 Brussels, Belgium. NACE International 5 and Chin19observed an increase in corrosion rate upon increasing AC density. Funk and Schoeneich6reported th
49、e results of a two-year field study, which showed similar trends for both general corrosion rate and pitting. Gummow, et al.20compiled an extensive literature survey on the subject of AC corrosion in 1998, when similar findings by many other researchers were presented. A 2005 study by Goidanich, et al.21reached a similar conclusion that DC equivalent percentage (defined as the ratio between observed corrosion rate to that expected for the DC of the same magnitude) for carbon steel in simulated soil solution is lower than 4%. The researcher
