1、Designation: G96 90 (Reapproved 2013)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 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:2D1125 Test Methods for Electric
8、al 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 Applica-tionsG15 Terminology Relating to Co
9、rrosion and Corrosion Test-ing (Withdrawn 2010)3G59 Test Method for Conducting Potentiodynamic Polariza-tion Resistance MeasurementsG102 Practice for Calculation of Corrosion Rates and Re-lated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 DefinitionsSee Terminology G15 for definiti
10、ons 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-sectional area decreases:R 5 lA(1)where:R
11、= 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 similarreference element protected from corrosion is measured, tocompensate for resistivity c
12、hanges due to temperature. 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 manually1This guide is under the jurisdiction of ASTM Committee G01 on Corrosion ofMetals a
13、nd is the direct responsibility of ASTM Subcommittee G01.11 onElectrochemical Measurements in Corrosion Testing.Current edition approved Aug. 1, 2013. Published August 2013. Originallyapproved in 1990. Last previous edition approved in 2008 as G9690 (2008). DOI:10.1520/G0096-90R13.2For referenced AS
14、TM 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 standard is referenced onwww.astm
15、.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1or automatically recorded against a time base. The slope of thecurve of metal loss against time at any point is the correctionrate at that point. The more frequently measurements ar
16、e 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 are required tominimize the effect of cable resistan
17、ce 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 test method involves inter-action with the electroche
18、mical 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), it has been shown that thecurrent density associate
19、d 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 the electrode (typically E up to 20 mV),the corrosi
20、on 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 convertedto corrosion rate of the electrode by Farad
21、ays 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 reaction (that is, thenumber of electrons involved in
22、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 selectively; that is, each element ofthe alloy corrodes a
23、s 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 ithelement in the alloy,ni= exhibited valence of the
24、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 and alloys and are tabulated in PracticeG102.4.2.7 Fi
25、g. 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 small potential is applied between the test andauxilia
26、ry electrode. In practice, this can vary from a fewseconds up to hours. When determining the polarizationresistance, Rp, correction or compensation for solutionresistance, Rs, is important when Rsbecomes significantcompared to Rp. Test Methods D1125 describes test methodsfor electrical conductivity
27、and resistivity of water.4.2.8 Two-electrode probes, and three-electrode probes withthe reference electrode equidistant from the test and auxiliaryelectrode, do not correct for effects of solution resistance,NOTE 1Rs= Solution Resistance (ohmcm2) between test and auxiliary electrodes (increases with
28、 electrode spacing and solution resistivity).Ru= 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 P
29、olarization Resistance ProbeG96 90 (2013)2without 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 probes compensate for the solutionresistance, Rs, by
30、 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 which adequate determination of the polar-ization resi
31、stance 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 application as acorrection. This also extends the ope
32、rating 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 polarization resistance technique can-not be used, but the
33、 electrical resistance technique may beused.4.2.12 Other methods of compensating for the effects ofsolution 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
34、corrosion is characterized by areas of greater orlesser attack, throughout the plant, at a particular location, 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 forthe fact
35、that areas of greater or lesser penetration usually existNOTE 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 circuit (see Fig. 1).NOTE 3In the Stern-G
36、eary 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 with cylindrical test and auxiliary ele
37、ctrodes 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-electrode probe with the reference electrode eq
38、uidistant 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 solution resistance.NOTE 8In the method for
39、 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 withCdlof 40 F/cm2on above electrodes and
40、 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 difference is small and the measurements
41、 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 (2013)3on the surface. Visual inspection of the probe element, coupon,or electrode is required to determine the degree of interference
42、in 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 without theneed to physically enter the system to wit
43、hdraw 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 can create corrosive conditions. Anearly warning of co
44、rrosive 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 rate can be used to signalthe need for additional in
45、hibitor.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 AELECTRICAL RESISTANCE(1-6)46. Limitations and Inte
46、rferences6.1 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 measuringelements are much less sensitive to the effect of p
47、itting 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
48、 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) to avoid electri-cally noisy sources such as power cables, heavy duty motors,switchgear, and radio transm
49、itters.6.2.1 The electrical resistivity of metals increases withincreased temperature. Although basic temperature 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 t
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