ASTM G96-90(2008)
(Guide)Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)
Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)
SIGNIFICANCE AND USE
General corrosion is characterized by areas of greater or lesser attack, throughout the plant, at a particular location, or even on a particular probe. Therefore, the estimation of corrosion rate as with mass loss coupons involves an averaging across the surface of the probe. Allowance must be made for the fact that areas of greater or lesser penetration usually exist on the surface. Visual inspection of the probe element, coupon, or electrode is required to determine the degree of interference in the measurement caused by such variability. This variability is less critical where relative changes in corrosion rate are to be detected.
Both electrical test methods described in this guide provide a technique for determining corrosion rates without the need to physically enter the system to withdraw coupons as required by the methods described in Guide G 4.
Test Method B has the additional advantage of providing corrosion rate measurement within minutes.
These techniques are useful in systems where process upsets or other problems can create corrosive conditions. An early warning of corrosive attack can permit remedial action before significant damage occurs to process equipment.
These techniques are also useful where inhibitor additions are used to control the corrosion of equipment. The indication of an increasing corrosion rate can be used to signal the need for additional inhibitor.
Control of corrosion in process equipment requires a knowledge of the rate of attack on an ongoing basis. These test methods can be used to provide such information in digital format easily transferred to computers for analysis. TEST METHOD A—ELECTRICAL RESISTANCE
(1-6)4 Top
SCOPE
1.1 This guide covers the procedure for conducting online corrosion monitoring of metals in plant equipment under operating conditions by the use of electrical or electrochemical methods. Within the limitations described, these test methods can be used to determine cumulative metal loss or instantaneous corrosion rate, intermittently or on a continuous basis, without removal of the monitoring probes from the plant.
1.2 The following test methods are included: Test Method A for electrical resistance, and Test Method B for polarization resistance.
1.2.1 Test Method A provides information on cumulative metal loss, and corrosion rate is inferred. This test method responds to the remaining metal thickness except as described in Section .
1.2.2 Test Method B is based on electrochemical measurements for determination of instantaneous corrosion rate but may require calibration with other techniques to obtain true corrosion rates. Its primary value is the rapid detection of changes in the corrosion rate that may be indicative of undesirable changes in the process environment.
1.3 The values stated in SI units are to be considered standard. The values in parentheses are for information only.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in 5.6.
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Designation: G96 − 90(Reapproved 2008)
Standard Guide for
Online Monitoring of Corrosion in Plant Equipment
(Electrical and Electrochemical Methods)
ThisstandardisissuedunderthefixeddesignationG96;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope D1125Test Methods for Electrical Conductivity and Resis-
tivity of Water
1.1 This guide covers the procedure for conducting online
G1Practice for Preparing, Cleaning, and Evaluating Corro-
corrosion monitoring of metals in plant equipment under
sion Test Specimens
operatingconditionsbytheuseofelectricalorelectrochemical
G3Practice for Conventions Applicable to Electrochemical
methods. Within the limitations described, these test methods
Measurements in Corrosion Testing
can be used to determine cumulative metal loss or instanta-
G4Guide for Conducting Corrosion Tests in FieldApplica-
neous corrosion rate, intermittently or on a continuous basis,
tions
without removal of the monitoring probes from the plant.
G15TerminologyRelatingtoCorrosionandCorrosionTest-
1.2 Thefollowingtestmethodsareincluded:TestMethodA
ing (Withdrawn 2010)
for electrical resistance, and Test Method B for polarization
G59TestMethodforConductingPotentiodynamicPolariza-
resistance.
tion Resistance Measurements
1.2.1 Test Method A provides information on cumulative
G102Practice for Calculation of Corrosion Rates and Re-
metal loss, and corrosion rate is inferred. This test method
lated Information from Electrochemical Measurements
responds to the remaining metal thickness except as described
in Section 5.
3. Terminology
1.2.2 Test Method B is based on electrochemical measure-
3.1 Definitions—See Terminology G15 for definitions of
ments for determination of instantaneous corrosion rate but
terms used in this guide.
may require calibration with other techniques to obtain true
corrosion rates. Its primary value is the rapid detection of
4. Summary of Guide
changes in the corrosion rate that may be indicative of
4.1 Test Method A–Electrical Resistance—The electrical
undesirable changes in the process environment.
resistance test method operates on the principle that the
1.3 The values stated in SI units are to be considered
electricalresistanceofameasuringelement(wire,strip,ortube
standard. The values in parentheses are for information only.
of metal) increases as its cross-sectional area decreases:
1.4 This standard does not purport to address all of the
l
safety concerns, if any, associated with its use. It is the R 5 σ (1)
A
responsibility of the user of this standard to establish appro-
where:
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. Specific precau- R = resistance,
tionary statements are given in 5.6. σ = resistivity of metal (temperature dependent),
l = length, and
2. Referenced Documents
A = cross-section area.
2.1 ASTM Standards: In practice, the resistance ratio between the measuring
element exposed to corrosion and the resistance of a similar
reference element protected from corrosion is measured, to
This guide is under the jurisdiction ofASTM Committee G01 on Corrosion of
compensate for resistivity changes due to temperature. Based
Metals and is the direct responsibility of ASTM Subcommittee G01.11 on
Electrochemical Measurements in Corrosion Testing. on the initial cross-sectional area of the measurement element,
Current edition approved Aug. 15, 2008. Published September 2008. Originally
the cumulative metal loss at the time of reading is determined.
´1
approvedin1990.Lastpreviouseditionapprovedin2001asG96–90(2001) .DOI:
Metal loss measurements are taken periodically and manually
10.1520/G0096-90R08.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G96 − 90 (2008)
or automatically recorded against a time base.The slope of the where:
curve of metal loss against time at any point is the correction
K = a constant.
rateatthatpoint.Themorefrequentlymeasurementsaretaken,
4.2.4 Equivalent weight of an element is the molecular
the better is the resolution of the curve from which the
weight divided by the valency of the reaction (that is, the
corrosion rate is derived.
number of electrons involved in the electrochemical reaction).
4.1.1 The electrical resistance of the metal elements being
4.2.5 In order to obtain an alloy equivalent weight that is in
measured is very low (typically 2 to 10 mΩ). Consequently,
proportion with the mass fraction of the elements present and
special measurement techniques and cables are required to
their valence, it must be assumed that the oxidation process is
minimize the effect of cable resistance and electrical noise.
uniformanddoesnotoccurselectively;thatis,eachelementof
4.1.2 Various probe element cross-sectional areas are nec-
the alloy corrodes as it would if it were the only element
essarysothatawiderangeofcorrosionratescanbemonitored
present. In some situations these assumptions are not valid.
with acceptable resolution.
4.2.6 Effective equivalent weight of an alloy is as follows:
4.2 Test Method B–Polarization Resistance:
4.2.1 Thepolarizationresistancetestmethodinvolvesinter-
(5)
m
n f
actionwiththeelectrochemicalcorrosionmechanismofmetals
i i
(
in electrolytes in order to measure the instantaneous corrosion W
l
i
rate. Its particular advantage is its speed of response to
where:
corrosion rate upsets. On a corroding electrode subject to
f = mass fraction of i element in the alloy,
i th
certain qualifications (see 12.1), it has been shown that the
W = atomic weight of the i element in the alloy,
i th
current density associated with a small polarization of the
n = exhibited valence of the i element under the condi-
i th
electrode is directly proportional to the corrosion rate of the
tions of the corrosion process, and
electrode.
m = number of component elements in the alloy (normally
4.2.2 The polarization resistance equation is derived inTest
only elements above 1 mass% in the alloy are
Method G59. See Practice G3 for applicable conventions. For
considered).
smallpolarizationoftheelectrode(typically ∆Eupto20mV),
Alloy equivalent weights have been calculated for many
the corrosion current density is defined as:
engineering metals and alloys and are tabulated in Practice
B
G102.
i 5 (2)
corr
R
p
4.2.7 Fig. 1 represents an equivalent circuit of polarization
where:
resistance probe electrodes in a corroding environment. The
B = a combination of the anodic and cathodic Tafel slopes value of the double layer capacitance, C , determines the
dl
(b,b ), and charging time before the current density reaches a constant
a c
R = the polarization resistance with dimensions ohm·cm . value, i, when a small potential is applied between the test and
p
auxiliary electrode. In practice, this can vary from a few
b b
a c
B 5 (3)
seconds up to hours. When determining the polarization
2.303 ~b 1b !
a c
resistance, R , correction or compensation for solution resis-
p
4.2.3 The corrosion current density, i , can be converted
corr tance, R , is important when R becomes significant compared
s s
to corrosion rate of the electrode by Faraday’s law if the
to R .TestMethodsD1125describestestmethodsforelectrical
p
equivalent weight (EW) and density, ρ, of the corroding metal
conductivity and resistivity of water.
are known (see Practice G102):
4.2.8 Two-electrodeprobes,andthree-electrodeprobeswith
i the reference electrode equidistant from the test and auxiliary
corr
corrosionrate 5 K EW (4)
ρ electrode, do not correct for effects of solution resistance,
−2
NOTE 1—R =Solution Resistance (ohm·cm ) between test and auxiliary electrodes (increases with electrode spacing and solution resistivity).
s
−2
R =Uncompensated component of solution resistance (between test and reference electrodes) (ohm·cm ).
u
R =Polarization Resistance R (ohm·cm ).
p p
C =Double layer capacitance of liquid/metal interface.
dl
i=Corrosion current density.
FIG. 1 Equivalent Circuit of Polarization Resistance Probe
G96 − 90 (2008)
without special electronic solution resistance compensation. 4.2.11 Even with solution resistance compensation, there is
Withhightomoderateconductivityenvironments,thiseffectof a practical limit to the correction (see Fig. 2). At higher
solution resistance is not normally significant (see Fig. 2). solution resistivities the polarization resistance technique can-
4.2.9 Three-electrode probes compensate for the solution not be used, but the electrical resistance technique may be
resistance, R , by varying degrees depending on the position used.
s
and proximity of the reference electrode to the test electrode. 4.2.12 Other methods of compensating for the effects of
Withaclose-spacedreferenceelectrode,theeffectsofR canbe solution resistance, such as current interruption, electrochemi-
s
reduced up to approximately ten fold. This extends the oper- calimpedanceandpositivefeedbackhavesofargenerallybeen
ating range over which adequate determination of the polar- confined to controlled laboratory tests.
ization resistance can be made (see Fig. 2).
5. Significance and Use
4.2.10 A two-electrode probe with electrochemical imped-
ance measurement technique at high frequency short circuits 5.1 Generalcorrosionischaracterizedbyareasofgreateror
the double layer capacitance, C , so that a measurement of lesser attack, throughout the plant, at a particular location, or
dl
solution resistance, R , can be made for application as a even on a particular probe. Therefore, the estimation of
s
correction. This also extends the operating range over which corrosionrateaswithmasslosscouponsinvolvesanaveraging
adequate determination of polarization resistance can be made
across the surface of the probe. Allowance must be made for
(see Fig. 2). the fact that areas of greater or lesser penetration usually exist
NOTE 1—See Appendix X1 for derivation of curves and Table X1.1 for description of points A, B, C and D.
NOTE 2—Operating limits are based on 20% error in measurement of polarization resistance equivalent circuit (see Fig. 1).
NOTE 3—In 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 rate
of carbon steel”.
µmhos 1000000
~ !
NOTE 4—Conductivity 5
cm Resistivity ~ohm·cm!
NOTE 5—Effects 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 (ohms·cm ) for this
geometry=0.55×resistivity (ohms·cm ).
NOTE 6—Atwo-electrode probe, or three-electrode probe with the reference electrode equidistant from the test and auxiliary electrode, includes% of
solution resistance between working and auxiliary electrodes in its measurement of R .
p
NOTE 7—A close-space reference electrode on a three electrode probe is assumed to be one that measures 5% of solution resistance.
NOTE 8—In the method for Curve 1, basic polarization resistance measurement determines 2R + R (see Fig. 1). High frequency measurement short
p s
circuits C to measure R . By subtraction polarization resistance, R is determined. The curve is based on high frequency measurement at 834 Hz with
dl s p
C of 40 µ F/cm on above electrodes and 6 1.5% accuracy of each of the two measurements.
dl
NOTE9—Curve1islimitedathighconductivitytoapproximately700mpybyerrorduetoimpedanceof C atfrequency834Hz.Atlowconductivity
dl
it is limited by the error in subtraction of two measurements where difference is small and the measurements large.
NOTE 10—Errors increase rapidly beyond the 20% error line (see Appendix X1, Table X1.1).
FIG. 2 Guidelines on Operating Range for Polarization Resistance
G96 − 90 (2008)
onthesurface.Visualinspectionoftheprobeelement,coupon, 6.2.2 If probe elements are flexed due to excessive flow
or electrode is required to determine the degree of interference conditions, a strain gage effect can be produced introducing
in the measurement caused by such variability.This variability stress noise onto the probe measurement. Suitable probe
islesscriticalwhererelativechangesincorrosionratearetobe element shielding can remove such effects.
detected.
6.3 Process fluids, except liquid metals and certain molten
5.2 Both electrical test methods described in this guide
salts, do not normally have sufficient electrical conductivity to
provideatechniquefordeterminingcorrosionrateswithoutthe
produceasignificantshortingeffectontheelectricalresistance
need to physically enter the system to withdraw coupons as
of the exposed probe element. Conductive deposits (such as
required by the methods described in Guide G4.
iron sulfide) can cause some short–circuiting effect on the
element, reducing the measured metal loss, or showing some
5.3 Test Method B has the additional advantage of provid-
apparent metal gain. Certain probe configurations are less
ing corrosion rate measurement within minutes.
sensitive to this than others, depending on the path length
5.4 These techniques are useful in systems where process
between one end of the exposed probe element and the other.
upsets or other problems can create corrosive conditions. An
6.4 When first introduced into a system, initial transient
early warning of corrosive attack can permit remedial action
corrosion rates on a probe element may be different from the
before significant damage occurs to process equipment.
longer term corrosion rates.
5.5 These techniques are also useful where inhibitor addi-
6.4.1 Establishment of a probe element surface typical of
tions are used to control the corrosion of equipment. The
the plant by passivation, oxidation, deposits, or inhibitor film
indication of an increasing corrosion rate can be used to signal
build up may vary from hours to several days.
the need for additional inhibitor.
6.5 Since the corrosion rate is usually temperature depen-
5.6 Control of corrosion in process equipment requires a
dent, results will be comparable only for the alloy at the
knowledgeoftherateofattackonanongoingbasis.Thesetest
p
...
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