ASTM G96-90(2013)
(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
5.1 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.
5.2 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 G4.
5.3 Test Method B has the additional advantage of providing corrosion rate measurement within minutes.
5.4 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.
5.5 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.
5.6 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
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 5.
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.
1.4 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 2013)
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. 1, 2013. Published August 2013. Originally
the cumulative metal loss at the time of reading is determined.
approved in 1990. Last previous edition approved in 2008 as G96–90 (2008). DOI:
Metal loss measurements are taken periodically and manually
10.1520/G0096-90R13.
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 (2013)
or automatically recorded against a time base.The slope of the i
corr
corrosionrate 5 K EW (4)
curve of metal loss against time at any point is the correction ρ
rateatthatpoint.Themorefrequentlymeasurementsaretaken,
where:
the better is the resolution of the curve from which the
K = a constant.
corrosion rate is derived.
4.1.1 The electrical resistance of the metal elements being
4.2.4 Equivalent weight of an element is the molecular
measured is very low (typically 2 to 10 mΩ). Consequently,
weight divided by the valency of the reaction (that is, the
special measurement techniques and cables are required to
number of electrons involved in the electrochemical reaction).
minimize the effect of cable resistance and electrical noise.
4.2.5 In order to obtain an alloy equivalent weight that is in
4.1.2 Various probe element cross-sectional areas are nec-
proportion with the mass fraction of the elements present and
essarysothatawiderangeofcorrosionratescanbemonitored
their valence, it must be assumed that the oxidation process is
with acceptable resolution.
uniformanddoesnotoccurselectively;thatis,eachelementof
4.2 Test Method B–Polarization Resistance:
the alloy corrodes as it would if it were the only element
4.2.1 Thepolarizationresistancetestmethodinvolvesinter- present. In some situations these assumptions are not valid.
actionwiththeelectrochemicalcorrosionmechanismofmetals
4.2.6 Effective equivalent weight of an alloy is as follows:
in electrolytes in order to measure the instantaneous corrosion
rate. Its particular advantage is its speed of response to
(5)
m
n f
i i
corrosion rate upsets. On a corroding electrode subject to
(
W
l
i
certain qualifications (see 12.1), it has been shown that the
current density associated with a small polarization of the
where:
electrode is directly proportional to the corrosion rate of the
f = mass fraction of i element in the alloy,
i th
electrode.
W = atomic weight of the i element in the alloy,
i th
4.2.2 The polarization resistance equation is derived inTest
n = exhibited valence of the i element under the condi-
i th
Method G59. See Practice G3 for applicable conventions. For
tions of the corrosion process, and
smallpolarizationoftheelectrode(typically ∆Eupto20mV),
m = number of component elements in the alloy (normally
the corrosion current density is defined as:
only elements above 1 mass% in the alloy are
considered).
B
i 5 (2)
corr
R
p
Alloy equivalent weights have been calculated for many
engineering metals and alloys and are tabulated in Practice
where:
G102.
B = a combination of the anodic and cathodic Tafel slopes
4.2.7 Fig. 1 represents an equivalent circuit of polarization
(b,b ), and
a c
resistance probe electrodes in a corroding environment. The
R = the polarization resistance with dimensions ohm·cm .
p
value of the double layer capacitance, C , determines the
dl
b b
a c
charging time before the current density reaches a constant
B 5 (3)
2.303 ~b 1b !
a c
value, i, when a small potential is applied between the test and
4.2.3 The corrosion current density, i , can be converted auxiliary electrode. In practice, this can vary from a few
corr
to corrosion rate of the electrode by Faraday’s law if the seconds up to hours. When determining the polarization
equivalent weight (EW) and density, ρ, of the corroding metal resistance, R , correction or compensation for solution
p
are known (see Practice G102): resistance, R , is important when R becomes significant
s s
−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 (2013)
compared to R . Test Methods D1125 describes test methods 4.2.10 A two-electrode probe with electrochemical imped-
p
for electrical conductivity and resistivity of water. ance measurement technique at high frequency short circuits
4.2.8 Two-electrodeprobes,andthree-electrodeprobeswith
the double layer capacitance, C , so that a measurement of
dl
the reference electrode equidistant from the test and auxiliary
solution resistance, R , can be made for application as a
s
electrode, do not correct for effects of solution resistance,
correction. This also extends the operating range over which
without special electronic solution resistance compensation.
adequate determination of polarization resistance can be made
Withhightomoderateconductivityenvironments,thiseffectof
(see Fig. 2).
solution resistance is not normally significant (see Fig. 2).
4.2.11 Even with solution resistance compensation, there is
4.2.9 Three-electrode probes compensate for the solution
a practical limit to the correction (see Fig. 2). At higher
resistance, R , by varying degrees depending on the position
s
solution resistivities the polarization resistance technique can-
and proximity of the reference electrode to the test electrode.
not be used, but the electrical resistance technique may be
Withaclose-spacedreferenceelectrode,theeffectsofR canbe
s
used.
reduced 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).
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 (2013)
4.2.12 Other methods of compensating for the effects of equipment and cable runs (where applicable) to avoid electri-
solution resistance, such as current interruption, electrochemi- cally noisy sources such as power cables, heavy duty motors,
calimpedanceandpositivefeedbackhavesofargenerallybeen switchgear, and radio transmitters.
confined to controlled laboratory tests. 6.2.1 The electrical resistivity of metals increases with
increased temperature.Although basic temperature compensa-
5. Significance and Use
tionisobtainedbymeasuringtheresistanceratioofanexposed
test element and protected reference element, the exposed
5.1 Generalcorrosionischaracterizedbyareasofgreateror
element will respond more rapidly to a change in temperature
lesser attack, throughout the plant, at a particular location, or
than does the protected reference element. This is a form of
even on a particular probe. Therefore, the estimation of
thermal noise. Various probes have different sensitivities to
corrosionrateaswithmasslosscouponsinvolvesanaveraging
such thermal noise. Where temperature fluctuations may be
across the surface of the probe. Allowance must be made for
significant, preference should be given to probes with the
the fact that areas of greater or lesser penetration usually exist
lowest thermal noise sensitivity.
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
5.3 Test Method B has the additional advantage of provid-
element, reducing the measured metal loss, or showing some
ing corrosion rate measurement within minutes.
apparent metal gain. Certain probe configurations are less
5.4 These techniques are useful in systems where process
sensitive to this than others, depending on the path length
upsets or other problems can create corrosive conditions. An
between one end of the exposed probe element and the other.
early warning of corrosive attack can permit remedial action
6.4 When first introduced into a system, initial transient
before significant damage
...
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