1、Designation: G96 90 (Reapproved 2018)Standard Guide forOnline Monitoring of Corrosion in Plant Equipment(Electrical and Electrochemical Methods)1This standard is issued under the fixed designation G96; the number immediately following the designation indicates the year of originaladoption or, in the
2、 case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers the procedure for conducting onlinecorrosion monitoring of metals in plan
3、t equipment underoperating conditions by the use of electrical or electrochemicalmethods. Within the limitations described, these test methodscan be used to determine cumulative metal loss or instanta-neous corrosion rate, intermittently or on a continuous basis,without removal of the monitoring pro
4、bes from the plant.1.2 The following test methods are included: Test MethodAfor electrical resistance, and Test Method B for polarizationresistance.1.2.1 Test Method A provides information on cumulativemetal loss, and corrosion rate is inferred. This test methodresponds to the remaining metal thickn
5、ess except as describedin Section 5.1.2.2 Test Method B is based on electrochemical measure-ments for determination of instantaneous corrosion rate butmay require calibration with other techniques to obtain truecorrosion rates. Its primary value is the rapid detection ofchanges in the corrosion rate
6、 that may be indicative ofundesirable changes in the process environment.1.3 The values stated in SI units are to be consideredstandard. The values in parentheses are for information only.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is
7、theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.Specific precautionary statements are given in 5.6.1.5 This international standard was developed in accor-dance
8、 with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Stand
9、ards:2D1125 Test Methods for Electrical Conductivity and Resis-tivity of WaterG1 Practice for Preparing, Cleaning, and Evaluating Corro-sion Test SpecimensG3 Practice for Conventions Applicable to ElectrochemicalMeasurements in Corrosion TestingG4 Guide for Conducting Corrosion Tests in Field Applic
10、a-tionsG15 Terminology Relating to Corrosion and Corrosion Test-ing (Withdrawn 2010)3G59 Test Method for Conducting Potentiodynamic Polariza-tion Resistance MeasurementsG102 Practice for Calculation of Corrosion Rates and Re-lated Information from Electrochemical Measurements3. Terminology3.1 Defini
11、tionsSee Terminology G15 for definitions ofterms used in this guide.4. Summary of Guide4.1 Test Method AElectrical ResistanceThe electricalresistance test method operates on the principle that theelectrical resistance of a measuring element (wire, strip, or tubeof metal) increases as its cross-secti
12、onal area decreases:R 5 lA(1)where:R = resistance, = resistivity of metal (temperature dependent),l = length, andA = cross-section area.In practice, the resistance ratio between the measuringelement exposed to corrosion and the resistance of a similar1This guide is under the jurisdiction of ASTM Com
13、mittee G01 on Corrosion ofMetals and is the direct responsibility of ASTM Subcommittee G01.11 onElectrochemical Measurements in Corrosion Testing.Current edition approved May 1, 2018. Published June 2018. Originallyapproved in 1990. Last previous edition approved in 2013 as G96 90 (2013). DOI:10.152
14、0/G0096-90R18.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The last approved version of this historical s
15、tandard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on P
16、rinciples for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1reference element protected from corrosion is measured, tocompensate for resistivity changes due to temperature. Basedon the initial
17、 cross-sectional area of the measurement element,the cumulative metal loss at the time of reading is determined.Metal loss measurements are taken periodically and manuallyor automatically recorded against a time base. The slope of thecurve of metal loss against time at any point is the correctionrat
18、e at that point. The more frequently measurements are taken,the better is the resolution of the curve from which thecorrosion rate is derived.4.1.1 The electrical resistance of the metal elements beingmeasured is very low (typically 2 to 10 m). Consequently,special measurement techniques and cables
19、are required tominimize the effect of cable resistance and electrical noise.4.1.2 Various probe element cross-sectional areas are nec-essary so that a wide range of corrosion rates can be monitoredwith acceptable resolution.4.2 Test Method BPolarization Resistance:4.2.1 The polarization resistance t
20、est method involves inter-action with the electrochemical corrosion mechanism of metalsin electrolytes in order to measure the instantaneous corrosionrate. Its particular advantage is its speed of response tocorrosion rate upsets. On a corroding electrode subject tocertain qualifications (see 12.1),
21、 it has been shown that thecurrent density associated with a small polarization of theelectrode is directly proportional to the corrosion rate of theelectrode.4.2.2 The polarization resistance equation is derived in TestMethod G59. See Practice G3 for applicable conventions. Forsmall polarization of
22、 the electrode (typically E up to 20 mV),the corrosion current density is defined as:icorr5BRp(2)where:B = a combination of the anodic and cathodic Tafel slopes(ba,bc), andRp= the polarization resistance with dimensions ohmcm2.B 5babc2.303 ba1bc!(3)4.2.3 The corrosion current density, icorr, can be
23、convertedto corrosion rate of the electrode by Faradays law if theequivalent weight (EW) and density, , of the corroding metalare known (see Practice G102):corrosion rate 5 K1icorrEW (4)where:K1= a constant.4.2.4 Equivalent weight of an element is the molecularweight divided by the valency of the re
24、action (that is, thenumber of electrons involved in the electrochemical reaction).4.2.5 In order to obtain an alloy equivalent weight that is inproportion with the mass fraction of the elements present andtheir valence, it must be assumed that the oxidation process isuniform and does not occur selec
25、tively; that is, each element ofthe alloy corrodes as it would if it were the only elementpresent. In some situations these assumptions are not valid.4.2.6 Effective equivalent weight of an alloy is as follows:1(lmnifiWi(5)where:fi= mass fraction of ithelement in the alloy,Wi= atomic weight of the i
26、thelement in the alloy,ni= exhibited valence of the ithelement under the condi-tions of the corrosion process, andm = number of component elements in the alloy (normallyonly elements above 1 mass % in the alloy areconsidered).Alloy equivalent weights have been calculated for manyengineering metals a
27、nd alloys and are tabulated in PracticeG102.4.2.7 Fig. 1 represents an equivalent circuit of polarizationresistance probe electrodes in a corroding environment. Thevalue of the double layer capacitance, Cdl, determines thecharging time before the current density reaches a constantvalue, i, when a sm
28、all potential is applied between the test andauxiliary electrode. In practice, this can vary from a fewNOTE 1Rs= Solution Resistance (ohmcm2) between test and auxiliary electrodes (increases with electrode spacing and solution resistivity).Ru= Uncompensated component of solution resistance (between
29、test and reference electrodes) (ohmcm2).Rp= Polarization Resistance Rp(ohmcm2).Cdl= Double layer capacitance of liquid/metal interface.i = Corrosion current density.FIG. 1 Equivalent Circuit of Polarization Resistance ProbeG96 90 (2018)2seconds up to hours. When determining the polarizationresistanc
30、e, Rp, correction or compensation for solutionresistance, Rs, is important when Rsbecomes significantcompared to Rp. Test Methods D1125 describes test methodsfor electrical conductivity and resistivity of water.4.2.8 Two-electrode probes, and three-electrode probes withthe reference electrode equidi
31、stant from the test and auxiliaryelectrode, do not correct for effects of solution resistance,without special electronic solution resistance compensation.With high to moderate conductivity environments, this effect ofsolution resistance is not normally significant (see Fig. 2).4.2.9 Three-electrode
32、probes compensate for the solutionresistance, Rs, by varying degrees depending on the positionand proximity of the reference electrode to the test electrode.With a close-spaced reference electrode, the effects of Rscan bereduced up to approximately ten fold. This extends the oper-ating range over wh
33、ich adequate determination of the polar-ization resistance can be made (see Fig. 2).4.2.10 A two-electrode probe with electrochemical imped-ance measurement technique at high frequency short circuitsthe double layer capacitance, Cdl, so that a measurement ofsolution resistance, Rs, can be made for a
34、pplication as acorrection. This also extends the operating range over whichadequate determination of polarization resistance can be made(see Fig. 2).4.2.11 Even with solution resistance compensation, there isa practical limit to the correction (see Fig. 2). At highersolution resistivities the polari
35、zation resistance technique can-not be used, but the electrical resistance technique may beused.NOTE 1See Appendix X1 for derivation of curves and Table X1.1 for description of points A, B, C and D.NOTE 2Operating limits are based on 20 % error in measurement of polarization resistance equivalent ci
36、rcuit (see Fig. 1).NOTE 3In the Stern-Geary equations, an empirical value of B = 27.5 mV has been used on the ordinate axis of the graph for “typical corrosion rateof carbon steel”.NOTE 4Conductivitymhos!cm51 000000Resistivity ohmcm!NOTE 5Effects of solution resistance are based on a probe geometry
37、with cylindrical test and auxiliary electrodes of 4.75 mm (0.187 in.) diameter,31.7 mm (1.25 ft) long with their axes spaced 9.53 mm (0.375 in.) apart. Empirical data shows that solution resistance (ohmscm2) for thisgeometry = 0.55 resistivity (ohmscm2).NOTE 6A two-electrode probe, or three-electrod
38、e probe with the reference electrode equidistant from the test and auxiliary electrode, includes % ofsolution resistance between working and auxiliary electrodes in its measurement of Rp.NOTE 7A close-space reference electrode on a three electrode probe is assumed to be one that measures 5 % of solu
39、tion resistance.NOTE 8In the method for Curve 1, basic polarization resistance measurement determines 2Rp+ Rs(see Fig. 1). High frequency measurement shortcircuits Cdlto measure Rs. By subtraction polarization resistance, Rpis determined. The curve is based on high frequency measurement at 834 Hz wi
40、thCdlof 40 F/cm2on above electrodes and 6 1.5 % accuracy of each of the two measurements.NOTE 9Curve 1 is limited at high conductivity to approximately 700 mpy by error due to impedance of Cdlat frequency 834 Hz.At low conductivityit is limited by the error in subtraction of two measurements where d
41、ifference is small and the measurements large.NOTE 10Errors increase rapidly beyond the 20 % error line (see Appendix X1, Table X1.1).FIG. 2 Guidelines on Operating Range for Polarization ResistanceG96 90 (2018)34.2.12 Other methods of compensating for the effects ofsolution resistance, such as curr
42、ent interruption, electrochemi-cal impedance and positive feedback have so far generally beenconfined to controlled laboratory tests.5. Significance and Use5.1 General corrosion is characterized by areas of greater orlesser attack, throughout the plant, at a particular location, oreven on a particul
43、ar probe. Therefore, the estimation ofcorrosion rate as with mass loss coupons involves an averagingacross the surface of the probe. Allowance must be made forthe fact that areas of greater or lesser penetration usually existon the surface. Visual inspection of the probe element, coupon,or electrode
44、 is required to determine the degree of interferencein the measurement caused by such variability. This variabilityis less critical where relative changes in corrosion rate are to bedetected.5.2 Both electrical test methods described in this guideprovide a technique for determining corrosion rates w
45、ithout theneed to physically enter the system to withdraw coupons asrequired by the methods described in Guide G4.5.3 Test Method B has the additional advantage of provid-ing corrosion rate measurement within minutes.5.4 These techniques are useful in systems where processupsets or other problems ca
46、n create corrosive conditions. Anearly warning of corrosive attack can permit remedial actionbefore significant damage occurs to process equipment.5.5 These techniques are also useful where inhibitor addi-tions are used to control the corrosion of equipment. Theindication of an increasing corrosion
47、rate can be used to signalthe need for additional inhibitor.5.6 Control of corrosion in process equipment requires aknowledge of the rate of attack on an ongoing basis. These testmethods can be used to provide such information in digitalformat easily transferred to computers for analysis.TEST METHOD
48、 AELECTRICAL RESISTANCE(1-6)46. Limitations and Interferences6.1 Results are representative for average metal loss on theprobe element. On wire-form measuring elements, pitting maybe indicated by rapid increases in metal loss reading after 50 %of probe life is passed. The larger cylindrical measurin
49、gelements are much less sensitive to the effect of pitting attack.Where pitting is the only form of attack, probes may yieldunreliable results.6.2 It should be recognized that the thermal noise andstress-induced noise on probe elements, and electrical noise onthese systems, occur in varying degrees due to the process andlocal environment. Care should be exercised in the choice ofthe system to minimize these effects. Electrical noise can beminimized by use of correct cabling, and careful location ofequipment and cable runs (where applicable)
