1、 Item No. 24235 NACE International Publication 35108 This Technical Committee Report has been prepared by NACE International Task Group 211* on Cathodic Protection: Report on the Application of the 100 mV Polarization Criterion. One Hundred Millivolt (mV) Cathodic Polarization Criterion May 2008, NA
2、CE 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, marketing, purchasing, or using product
3、s, 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 Patent, or as indemnifying or pro
4、tecting 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 subject. Unpredictable circumstan
5、ces 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 regulatory documents and for determ
6、ining 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 referred to within this report. Users o
7、f 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 requirements prior to the use of this repor
8、t. 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 NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE reports may r
9、eceive 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 NACE International technical committee report is intended for use by engineers during evaluat
10、ion of criteria for levels of cathodic protection (CP). Throughout this report, reference is made to pertinent available standards. Of particular relevance are NACE SP01691and TM04972for pipelines, as well as numerous other standards for other types of structures, some of which are listed in the Sum
11、mary section of this report. The report discusses the theoretical basis for the 100 millivolt (mV) cathodic polarization criterion, the effects of other factors such as temperature, mill scale, moisture, and anaerobic bacteria, measurement of the polarization, and the applicability of the criterion
12、in situations such as areas susceptible to stress corrosion cracking, mixed-metal systems, and areas susceptible to stray currents. It also includes the results of an industry questionnaire on the use of the 100 mV polarization criterion and opinions on its effectiveness. This technical committee re
13、port was prepared by Task Group (TG) 211Cathodic Protection: Report on the Application of the 100 mV Polarization Criterion. This TG is composed of corrosion researchers, corrosion engineers, corrosion consultants, facility owners, and representatives of both industry and government. TG 211 is admin
14、istered by Specific Technology Group (STG) 35Pipelines, Tanks, and Well Casings. It is also sponsored by STG 05Cathodic/Anodic Protection. This technical committee report is issued by NACE International under the auspices of STG 35. _ * Chair Drew Hevle, El Paso Corporation, Houston, TX. NACE Intern
15、ational 2 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 mitigation technology, whether considered successful or not. Statements used to convey this inform
16、ation 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 general application of this technology, and must not be construed as such. Introduction In Februa
17、ry 2000, NACE Work Group T-10A-3b sent questionnaires to 49 CP practitioners and users regarding the use of the 100 mV cathodic polarization criterion.3In this report, a summary of the 14 responses to this questionnaire is included as Appendix A. The number of companies employing the 100 mV cathodic
18、 polarization criterion increased 300% in the 10 years prior to the questionnaire. While most report using this criterion for only a small percentage of their systems (11% on average), the trend toward more widespread use is clearly established when it offers advantages to users. As usage of this cr
19、iterion has increased, however, so have questions pertaining to its validity and applicability. This report is intended to address these concerns on both a theoretical and empirical basis. Definitions Alkalization: The process of becoming more alkaline. Amphoteric Metal: A metal that is susceptible
20、to corrosion in both acid and alkaline environments. Blister: An undesirable, dome-shaped projection on the surface of a coating resulting from the local loss of adhesion and lifting of the film from an underlying coat or from the base substrate. Cathodic Blistering: Formation of coating blisters as
21、 the result of an alkaline environment under the coating caused by the cathodic reaction. Cathodic Disbondment: The destruction of adhesion between a coating and the coated surface caused by products of a cathodic reaction. Close-Interval Potential Survey (CIPS) (also Close-Interval Survey CIS): A p
22、otential survey performed on a buried or submerged metallic pipeline in order to obtain valid DC structure-to-electrolyte potential measurements at a regular interval sufficiently small to permit a detailed assessment. Corrosion Potential (Ecorr): The potential of a corroding surface in an electroly
23、te relative to a reference electrode under open-circuit conditions (also known as rest potential, open-circuit potential, or freely corroding potential). Depolarization: The removal of factors resisting the current in an electrochemical cell. For the purposes of this report, depolarization refers to
24、 a reduction in the level of protection as a result of a reduction or elimination of CP current. Depolarized Potential: The potential of a cathodically protected surface in an electrolyte relative to a reference electrode after influencing CP current sources have been turned off for a sufficient dur
25、ation of time for depolarization to have occurred. Depolarized Close-Interval Potential Survey: A CIS performed after influencing CP current sources have been turned off for a sufficient duration of time for depolarization to have occurred. This is often called a native-state CIS if it is performed
26、prior to the initial application of CP. Holiday: A discontinuity in a protective coating that exposes unprotected surface to the environment. Native Corrosion Potential: The Corrosion Potential prior to application of CP. Stray Current: Current through paths other than the intended circuit. Telluric
27、 Current: Current in the earth as a result of geomagnetic fluctuations. Theoretical Considerations The validity of a 100 mV cathodic polarization criterion in reducing corrosion on ferrous structures in soils, immersion in natural waters, and in reinforced concrete is well supported by both thermody
28、namic and kinetic corrosion fundamentals, although the degree of effectiveness can vary. Thermodynamic Considerations The Pourbaix4equilibrium diagram shown in Figure 1 depicts the thermodynamic equilibrium for iron and its various oxides in a deaerated aqueous solution on a potential/pH format. In
29、plotting a test electrode potential versus an applied current, the potential reading normally includes an error component referred to as the IR voltage NACE International 3 drop. If that error is measured and subtracted from the electrode potential measurement, the potential is normally referred to
30、as the electrode potential. In the case of the data in Figure 1, there is no external current affecting the electrode and no IR voltage drop; therefore, it is an electrode potential. Figure 1 IMMUNITYPASSIVITYFe O34Fe O23FeO H4FeO4-O2OH-Fe+H+CORROSION10-6100H0pH2-1 0 1 3 54768910112131415-1.116-1.31
31、6-0.916-0.716-0.516+0.084-0.116-0.316+0.284+0.484+0.684+0.884+1.084-1.0-0.8-0.62-0.561-0.502-0.443-0.4-0.200.20.40.60.81.01.21.4Fe+Fe0Potential-pH equilibrium diagram of the system iron/water at 25 C (77 F). The diagram shows potential with respect to (wrt) a standard hydrogen electrode (SHE) and Cu
32、/CuSO4reference electrodes. The lines between the corrosion and immunity zones represent the Feo/Fe+equilibrium for various equilibrium concentrations of ferrous ions (Fe+). These lines can be described by the Nernst Equation, in which Equation (1) relates the iron potential to the ferrous ion activ
33、ity (approximated here by concentration). ESHE= 0.440 V + 0.0295 log Fe+ (1) where: SHE is standard hydrogen electrode F+is the concentration of ferrous ions in mol/L At a potential of 0.617 VSHE, the concentration of Fe+ is 106mol/L of solution. Accordingly, an order of magnitude change in the equi
34、librium concentration of Fe+ would result in a 29.5 mV change in solution potential. Conversely, if the solution potential shifted electronegatively by 29.5 mV, the equilibrium concentration of Fe+ would decrease by a factor of 10. By inference, it follows that 100 mV of cathodic polarization would
35、result in a decrease in Fe+ equilibrium concentration of more than three orders of magnitude. However, kinetic rate information cannot be derived from thermodynamic equilibrium information, and this does not imply that the corrosion rate would likewise decrease by three orders of magnitude. In fact,
36、 as Figure 25indicates, the reduction in corrosion rate in some experiments conducted on iron in aqueous solutions showed only a 60% reduction in corrosion rate with 100 mV of cathodic polarization. NACE International 4 Figure 2 -2 0 2 4 6 8 10 12 1614-1.6-1.2-0.8-0.400.40.81.21.623passivation10.310
37、00mm/an*ba300100301010.31 mm/an*-2 0 2 4 6 8 10 12 1614-1.6-1.2-0.8-0.400.40.81.21.62immunitycorrosionpH* mm/an = mm/y1 1Pourbaix diagram for iron in water at 25 C (77 F) showing corrosion rates determined experimentally in which ais the hydrogen line, and bis the oxygen line. Kinetic Considerations
38、 Mears and Brown,6in defining their theory of CP, concluded that “for CP to be entirely effective, the local cathodes on the corroding specimen must be polarized to the potential of the unpolarized local anodes.” Typical anodic and cathodic polarization characteristics for a corrosion cell in soil a
39、nd fresh water can be represented by the polarization diagram depicted in Figure 3. This diagram, sometimes called an Evans diagram, which illustrates a corrosion cell under cathodic control, is typical for steel in soils or water when the rate of corrosion is controlled by the rate of diffusion of
40、dissolved oxygen to the cathode surface. When a CP current density (icp) is applied to this corrosion cell, the corrosion potential shifts in the electronegative direction because of polarization, and the corrosion current density (icorr) retreats to i corras the anode polarized potential shifts ele
41、ctronegatively toward its open-circuit potential (Ea,oc). It follows that the corrosion rate reduction for a given potential shift directly depends on the slope of the anodic polarization curve. The slope of the anodic curve, expressed in volts per decade of current, is also called its Tafel slope a
42、nd represents the amount of cathodic polarization required to reduce the corrosion rate by a factor of 10. 1,000 mm/an* NACE International 5 Figure 3 ioEcorr0.001 0.01 0.1 1.0 10-0.8-0.7-0.6-0.5-0.4log Current Density ( A/cm )2icp100mVa =50mV/decadeicorricorrEa,oc-0.3-0.2Polarization schematic showi
43、ng effect of 100 mV cathodic polarization on the corrosion rate of steel in a neutral pH aerated water. VCSEis potential in volts with reference to a saturated copper-copper sulfate reference electrode. The anodic Tafel slope (a) for steel in soil, as determined experimentally by Jones,7is 57 mV per
44、 decade of current. Dexter, et al.8also determined experimentally an anodic Tafel slope for steel in seawater of 30 mV per decade. Other tests on steel electrodes performed by Kuznetsova, et al.9in sand having variable moisture content indicated an anodic Tafel slope of approximately 60 mV, which wa
45、s “independent of moisture content.” Freiman, et al.10found the anodic Tafel slope in soil with 10% moisture content and for tap water to range between 30 and 50 mV per decade. Based on a Tafel slope range between 30 and 60 mV per decade, 100 mV of cathodic polarization would result in corrosion rat
46、e reduction by a factor of 2,150 and 46, respectively. It follows that a 100 mV cathodic polarization criterion is likely to reduce the corrosion rate (icorr icorr) significantly on ferrous structures in earth environments in which the anode polarization slope is relatively small. It also generally
47、obeys the Mears and Brown6definition of effective CP because for corrosion cells under cathodic control, the corrosion potential (Ecorr) is close in value to the open-circuit anode potential (Ea,oc). (a) Laboratory Testing In the mid 1950s, Schaschl and Marsh11investigated the effect of dissolved ox
48、ygen in aqueous solutions on the corrosion of steel and on the current required for CP. For the specific conditions of pH 8 and an oxygen concentration of 2.2 ppm, Figure 4 shows that a 100 mV shift in potential from Ecorr(0.665 VSCEto 0.765 VSCE), where SCErefers to a saturated calomel electrode, r
49、esults in a near-zero corrosion rate, and 0.765 VSCE corresponds to 0.840 VCSE, where CSErefers to a saturated copper-copper sulfate reference electrode. 50 m /decade 100 mV NACE International 6 Figure 4 Applied Current Density, mA/sq.ft.2105 20 301-0.6-0.5-0.7-0.8-0.9-1.0-1.100.10.20.30.40.5pH 8; Dissolved O , 2.2ppmFreely Corroding Rate, 0.42 in/hr.Minimum C.D., 11 mA/sq.ft.2Ecorr100mVPotential CurvePoint ofProtection(1mA/ft 1 A/cm )22 Relationship between Britton curve and the corrosion rate-applied current curve. Vo
