1、Designation: G 96 90 (Reapproved 2008)Standard Guide forOnline Monitoring of Corrosion in Plant Equipment(Electrical and Electrochemical Methods)1This standard is issued under the fixed designation G 96; the number immediately following the designation indicates the year of originaladoption or, in t
2、he 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 pl
3、ant 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 p
4、robes 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 thic
5、kness 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 ra
6、te 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 i
7、s theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. Specific precau-tionary statements are given in 5.6.2. Referenced Documents2.1 ASTM Standards:2D 1125 Test Methods for Elect
8、rical Conductivity and Re-sistivity of WaterG1 Practice for Preparing, Cleaning, and Evaluating Cor-rosion Test SpecimensG3 Practice for ConventionsApplicable to ElectrochemicalMeasurements in Corrosion TestingG4 Guide for Conducting Corrosion Tests in Field Appli-cationsG15 Terminology Relating to
9、Corrosion and CorrosionTestingG59 Test Method for Conducting Potentiodynamic Polar-ization Resistance MeasurementsG 102 Practice for Calculation of Corrosion Rates andRelated Information from Electrochemical Measurements3. Terminology3.1 DefinitionsSee Terminology G15 for definitions ofterms used in
10、 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-sectional area decreases:R 5slA(1)where:R = resistance,s = re
11、sistivity 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 similarreference element protected from corrosion is measured, tocompensate for resistivity changes due to temp
12、erature. Basedon the initial 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 an
13、y point is the correctionrate at that point. The more frequently measurements are taken,1This guide is under the jurisdiction of ASTM Committee G01 on Corrosion ofMetals and is the direct responsibility of ASTM Subcommittee G01.11 onElectrochemical Measurements in Corrosion Testing.Current edition a
14、pproved Aug. 15, 2008. Published September 2008. Originallyapproved in 1990. Last previous edition approved in 2001 as G 9690(2001)1.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume infor
15、mation, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.the better is the resolution of the curve from which thecorrosion rate is derived.4.1.1 The electrical resistance
16、 of the metal elements beingmeasured is very low (typically 2 to 10 mV). Consequently,special measurement techniques and cables 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 corrosi
17、on rates can be monitoredwith acceptable resolution.4.2 Test Method BPolarization Resistance:4.2.1 The polarization resistance test method involves inter-action with the electrochemical corrosion mechanism of metalsin electrolytes in order to measure the instantaneous corrosionrate. Its particular a
18、dvantage is its speed of response tocorrosion rate upsets. On a corroding electrode subject tocertain qualifications (see 12.1), 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 pol
19、arization resistance equation is derived in TestMethod G59. See Practice G3for applicable conventions. Forsmall polarization of the electrode (typically DE 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), an
20、dRp= the polarization resistance with dimensions ohmcm2.B 5babc2.303 ba1 bc!(3)4.2.3 The corrosion current density, icorr, can be convertedto corrosion rate of the electrode by Faradays law if theequivalent weight (EW) and density, r, of the corroding metalare known (see Practice G 102):corrosion ra
21、te 5 K1icorrrEW (4)where:K1= a constant.4.2.4 Equivalent weight of an element is the molecularweight divided by the valency of the reaction (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 mas
22、s fraction of the elements present andtheir valence, it must be assumed that the oxidation process isuniform and does not occur selectively; 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 e
23、quivalent weight of an alloy is as follows:1(lmnifiWi(5)where:fi= mass fraction of ithelement in the alloy,Wi= atomic weight of the ithelement 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 (normal
24、lyonly elements above 1 mass % in the alloy areconsidered).Alloy equivalent weights have been calculated for manyengineering metals and alloys and are tabulated in PracticeG 102.4.2.7 Fig. 1 represents an equivalent circuit of polarizationresistance probe electrodes in a corroding environment. Theva
25、lue of the double layer capacitance, Cdl, determines thecharging time before the current density reaches a constantvalue, i, when a small potential is applied between the test andauxiliary electrode. In practice, this can vary from a fewseconds up to hours. When determining the polarizationresistanc
26、e, Rp, correction or compensation for solution resis-tance, Rs, is important when Rsbecomes significant comparedto Rp. Test Methods D 1125 describes test methods for electri-cal conductivity and resistivity of water.4.2.8 Two-electrode probes, and three-electrode probes withthe reference electrode e
27、quidistant from the test and auxiliaryelectrode, do not correct for effects of solution resistance,without special electronic solution resistance compensation.NOTE 1Rs= Solution Resistance (ohmcm2) between test and auxiliary electrodes (increases with electrode spacing and solution resistivity).Ru=
28、Uncompensated component of solution resistance (between 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 ProbeG 96 90 (2008)2With
29、high to moderate conductivity environments, this effect ofsolution resistance is not normally significant (see Fig. 2).4.2.9 Three-electrode probes compensate for the solutionresistance, Rs, by varying degrees depending on the positionand proximity of the reference electrode to the test electrode.Wi
30、th a close-spaced reference electrode, the effects of Rscan bereduced up to approximately ten fold. This extends the oper-ating range over which 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 te
31、chnique at high frequency short circuitsthe double layer capacitance, Cdl, so that a measurement ofsolution resistance, Rs, can be made for application as acorrection. This also extends the operating range over whichadequate determination of polarization resistance can be made(see Fig. 2).4.2.11 Eve
32、n with solution resistance compensation, there isa practical limit to the correction (see Fig. 2). At highersolution resistivities the polarization resistance technique can-not be used, but the electrical resistance technique may beused.4.2.12 Other methods of compensating for the effects ofsolution
33、 resistance, such as current 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 locat
34、ion, oreven on a particular probe. Therefore, the estimation ofcorrosion rate as with mass loss coupons involves an averagingacross the surface of the probe. Allowance must be made forNOTE 1See Appendix X1 for derivation of curves and Table X1.1 for description of points A, B, C and D.NOTE 2Operatin
35、g limits are based on 20 % error in measurement of polarization resistance equivalent circuit (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 000
36、 000Resistivity ohmcm!NOTE 5Effects of solution resistance are based on a probe geometry 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
37、 thisgeometry = 0.55 3 resistivity (ohmscm2).NOTE 6A two-electrode probe, or three-electrode 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 re
38、ference electrode on a three electrode probe is assumed to be one that measures 5 % of solution 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
39、 resistance, Rpis determined. The curve is based on high frequency measurement at 834 Hz withCdlof 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
40、Hz.At low conductivityit is limited by the error in subtraction of two measurements where difference 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 ResistanceG 96 90 (20
41、08)3the fact that areas of greater or lesser penetration usually existon the surface. Visual inspection of the probe element, coupon,or electrode is required to determine the degree of interferencein the measurement caused by such variability. This variabilityis less critical where relative changes
42、in corrosion rate are to bedetected.5.2 Both electrical test methods described in this guideprovide a technique for determining corrosion rates without 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 adva
43、ntage of provid-ing corrosion rate measurement within minutes.5.4 These techniques are useful in systems where processupsets or other problems can create corrosive conditions. Anearly warning of corrosive attack can permit remedial actionbefore significant damage occurs to process equipment.5.5 Thes
44、e techniques are also useful where inhibitor addi-tions are used to control the corrosion of equipment. Theindication of an increasing corrosion 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 o
45、ngoing basis. These testmethods can be used to provide such information in digitalformat easily transferred to computers for analysis.TEST METHOD AELECTRICAL RESISTANCE(1-6)36. Limitations and Interferences6.1 Results are representative for average metal loss on theprobe element. On wire-form measur
46、ing elements, pitting maybe indicated by rapid increases in metal loss reading after 50 %of probe life is passed. The larger cylindrical measuringelements 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 shoul
47、d 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
48、 use of correct cabling, and careful location ofequipment and cable runs (where applicable) to avoid electri-cally noisy sources such as power cables, heavy duty motors,switchgear, and radio transmitters.6.2.1 The electrical resistivity of metals increases withincreased temperature. Although basic t
49、emperature compensa-tion is obtained by measuring the resistance ratio of an exposedtest element and protected reference element, the exposedelement will respond more rapidly to a change in temperaturethan does the protected reference element. This is a form ofthermal noise. Various probes have different sensitivities tosuch thermal noise. Where temperature fluctuations may besignificant, preference should be given to probes with thelowest thermal noise sensitivity.6.2.2 If probe elements are flexed due to excessive flowconditions, a strain gage eff
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