ASTM G96-90(1996)E1

Standard Guide for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)

SCOPE
1.1 This guide outlines the procedure for conducting on-line 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 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 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.

General Information

• Status: Historical
• Publication Date: 29-Mar-1990
• Technical Committee: G01 - Corrosion of Metals

Relations

• Replaced By: ASTM G96-90(2001)e1 - Standard Guide for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)
• Effective Date: 30-Mar-1990
• Referred By: ASTM G217-16(2022) - Standard Guide for Corrosion Monitoring in Laboratories and Plants with Coupled Multielectrode Array Sensor Method
• Effective Date: 30-Mar-1990
• Referred By: ASTM G215-17 - Standard Guide for Electrode Potential Measurement
• Effective Date: 30-Mar-1990
• Referred By: ASTM B826-09(2020) - Standard Test Method for Monitoring Atmospheric Corrosion Tests by Electrical Resistance Probes
• Effective Date: 30-Mar-1990
• Referred By: ASTM G170-06(2020)e1 - Standard Guide for Evaluating and Qualifying Oilfield and Refinery Corrosion Inhibitors in the Laboratory
• Effective Date: 30-Mar-1990
• Referred By: ASTM G148-97(2018) - Standard Practice for Evaluation of Hydrogen Uptake, Permeation, and Transport in Metals by an Electrochemical Technique
• Effective Date: 30-Mar-1990
• Referred By: ASTM G185-06(2020)e1 - Standard Practice for Evaluating and Qualifying Oil Field and Refinery Corrosion Inhibitors Using the Rotating Cylinder Electrode
• Effective Date: 30-Mar-1990
• Referred By: ASTM G208-12(2020) - Standard Practice for Evaluating and Qualifying Oilfield and Refinery Corrosion Inhibitors Using Jet Impingement Apparatus
• Effective Date: 30-Mar-1990
• Referred By: ASTM G199-09(2020)e1 - Standard Guide for Electrochemical Noise Measurement
• Effective Date: 30-Mar-1990

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
e1
Designation: G 96 – 90 (Reapproved 1996)
Standard Guide for
On-Line Monitoring of Corrosion in Plant Equipment
(Electrical and Electrochemical Methods)
This standard is issued under the fixed designation G 96; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Editorial corrections were made throughout in October 1996.
1. Scope G 4 Method for Conducting Corrosion Coupon Tests in
Plant Equipment
1.1 This guide outlines the procedure for conducting on-line
G 15 Terminology Relating to Corrosion and Corrosion
corrosion monitoring of metals in plant equipment under
Testing
operating conditions by the use of electrical or electrochemical
G 59 Practice for Conducting Potentiodynamic Polarization
methods. Within the limitations described, these test methods
Resistance Measurements
can be used to determine cumulative metal loss or instanta-
G 102 Practice for Calculation of Corrosion Rates and
neous corrosion rate, intermittently or on a continuous basis,
Related Information from Electrochemical Measurements
without removal of the monitoring probes from the plant.
1.2 The following test methods are included: Test Method A
3. Terminology
for electrical resistance, and Test Method B for polarization
3.1 Definitions—See Terminology G 15 for definitions of
resistance.
terms used in this guide.
1.2.1 Test Method A provides information on cumulative
metal loss, and corrosion rate is inferred. This test method
4. Summary of Guide
responds to the remaining metal thickness except as described
4.1 Test Method A–Electrical Resistance—The electrical
in Section 5.
resistance test method operates on the principle that the
1.2.2 Method B is based on electrochemical measurements
electrical resistance of a measuring element (wire, strip, or tube
for determination of instantaneous corrosion rate but may
of metal) increases as its cross-sectional area decreases:
require calibration with other techniques to obtain true corro-
l
sion rates. Its primary value is the rapid detection of changes in
R5s (1)
A
the corrosion rate that may be indicative of undesirable
changes in the process environment.
where:
1.3 This standard does not purport to address all of the
R 5 resistance,
safety concerns, if any, associated with its use. It is the
s5 resistivity of metal (temperature dependent),
responsibility of the user of this standard to establish appro-
l 5 length, and
priate safety and health practices and determine the applica-
A 5 cross-section area.
bility of regulatory limitations prior to use. Specific precau-
In practice, the resistance ratio between the measuring
tionary statements are given in 5.6.
element exposed to corrosion and the resistance of a similar
reference element protected from corrosion is measured, to
2. Referenced Documents
compensate for resistivity changes due to temperature. Based
2.1 ASTM Standards:
on the initial cross-sectional area of the measurement element,
D 1125 Test Methods for Electrical Conductivity and Re-
the cumulative metal loss at the time of reading is determined.
sistivity of Water
Metal loss measurements are taken periodically and manually
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
or automatically recorded against a time base. The slope of the
rosion Test Specimens
curve of metal loss against time at any point is the correction
G 3 Practice for Conventions Applicable to Electrochemical
rate at that point. The more frequently measurements are taken,
Measurements in Corrosion Testing
the better is the resolution of the curve from which the
corrosion rate is derived.
This guide is under the jurisdiction of ASTM Committee G-1 on Corrosion of
4.1.1 The electrical resistance of the metal elements being
Metals and is the direct responsibility of ASTM Subcommittee G01.12 on In-Plant
measured is very low (typically 2 to 10 mV). Consequently,
Corrosion Tests.
special measurement techniques and cables are required to
Current edition approved March 30, 1990. Published May 1990.
Annual Book of ASTM Standards, Vol 11.01.
minimize the effect of cable resistance and electrical noise.
Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
G96
i
4.1.2 Various probe element cross-sectional areas are nec-
corr
corrosion rate 5 K EW (4)
r
essary so that a wide range of corrosion rates can be monitored
with acceptable resolution.
where:
4.2 Test Method B–Polarization Resistance:
K 5 a constant.
4.2.1 The polarization resistance test method involves inter-
4.2.4 Equivalent weight of an element is the molecular
action with the electrochemical corrosion mechanism of metals
weight divided by the valency of the reaction (that is, the
in electrolytes in order to measure the instantaneous corrosion
number of electrons involved in the electrochemical reaction).
rate. Its particular advantage is its speed of response to
4.2.5 In order to obtain an alloy equivalent weight that is in
corrosion rate upsets. On a corroding electrode subject to
proportion with the mass fraction of the elements present and
certain qualifications (see 12.1), it has been shown that the
their valence, it must be assumed that the oxidation process is
current density associated with a small polarization of the
uniform and does not occur selectively; that is, each element of
electrode is directly proportional to the corrosion rate of the
the alloy corrodes as it would if it were the only element
electrode.
present. In some situations these assumptions are not valid.
4.2.2 The polarization resistance equation is derived in
4.2.6 Effective equivalent weight of an alloy is as follows:
Practice G 59. See Practice G 3 for applicable conventions. For
small polarization of the electrode (typically DE up to 20 mV),
(5)
m
n f
i i
the corrosion current density is defined as:
(
W
l
i
B
i 5 (2)
corr
R
p where:
f 5 mass fraction of i element in the alloy,
i th
where:
W 5 atomic weight of the i element in the alloy,
i th
B 5 a combination of the anodic and cathodic Tafel slopes
n 5 exhibited valence of the i element under the condi-
i th
( b ,b ), and
a c
tions of the corrosion process, and
R 5 the polarization resistance with dimensions ohm·cm .
p
m 5 number of component elements in the alloy (normally
only elements above 1 mass % in the alloy are
b b
a c considered).
B 5 (3)
2.303 ~b 1 b !
a c
Alloy equivalent weights have been calculated for many
engineering metals and alloys and are tabulated in Practice
4.2.3 The corrosion current density, i , can be converted
corr
G 102.
to corrosion rate of the electrode by Faraday’s law if the
equivalent weight (EW) and density, r, of the corroding metal 4.2.7 Fig. 1 represents an equivalent circuit of polarization
are known (see Practice G 102): resistance probe electrodes in a corroding environment. The
−2
NOTE 1—R 5 Solution Resistance (ohm·cm ) between test and auxiliary electrodes (increases with electrode spacing and solution resistivity).
s
−2
R 5 Uncompensated component of solution resistance (between test and reference electrodes) (ohm·cm ).
u
R 5 Polarization Resistance R (ohm·cm ).
p p
Cdl 5 Double layer capacitance of liquid/metal interface.
i 5 Corrosion current density.
FIG. 1 Equivalent Circuit of Polarization Resistance Probe
G96
value of the double layer capacitance, C , determines the and proximity of the reference electrode to the test electrode.
dl
charging time before the current density reaches a constant With a close-spaced reference electrode, the effects of R can be
s
value, i, when a small potential is applied between the test and reduced up to approximately ten fold. This extends the oper-
auxiliary electrode. In practice, this can vary from a few ating range over which adequate determination of the polar-
seconds up to hours. When determining the polarization ization resistance can be made (see Fig. 2).
resistance, R , correction or compensation for solution resis- 4.2.10 A two-electrode probe with electrochemical imped-
p
tance, R , is important when R becomes significant compared ance measurement technique at high frequency short circuits
s s
to R . Test Methods D 1125 describes test methods for electri- the double layer capacitance, C , so that a measurement of
p dl
cal conductivity and resistivity of water. solution resistance, R , can be made for application as a
s
4.2.8 Two-electrode probes, and three-electrode probes with correction. This also extends the operating range over which
the reference electrode equidistant from the test and auxiliary adequate determination of polarization resistance can be made
electrode, do not correct for effects of solution resistance, (see Fig. 2).
without special electronic solution resistance compensation. 4.2.11 Even with solution resistance compensation, there is
With high to moderate conductivity environments, this effect of 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
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 5 27.5 mV has been used on the ordinate axis of the graph for “typical corrosion rate
of carbon steel”.
~μmhos! 1 000 000
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 5 0.55 3 resistivity (ohms·cm ).
NOTE 6—A two-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
NOTE 9—Curve 1 is limited at high conductivity to approximately 700 mpy by error due to impedance of C at frequency 834 Hz. At low conductivity
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
4.2.12 Other methods of compensating for the effects of 6.2.1 The electrical resistivity of metals increases with
solution resistance, such as current interruption, electrochemi- increased temperature. Although basic temperature compensa-
cal impedance and positive feedback have so far generally been tion is obtained by measuring the resistance ratio of an exposed
confined to controlled laboratory tests. test element and protected reference element, the exposed
element will respond more rapidly to a change in temperature
5. Significance and Use
than does the protected reference element. This is a form of
5.1 General corrosion is characterized by areas of greater or
thermal noise. Various probes have different sensitivities to
lesser attack, throughout the plant, at a particular location, or
such thermal noise. Where temperature fluctuations may be
even on a particular probe. Therefore, the estimation of
significant, preference should be given to probes with the
corrosion rate as with mass loss coupons involves an averaging
lowest thermal noise sensitivity.
across the surface of the probe. Allowance must be made for
6.2.2 If probe elements are flexed due to excessive flow
the fact that areas of greater or lesser penetration usually exist
conditions, a strain gage effect can be produced introducing
on the surface. Visual inspection of the probe element, coupon,
stress noise onto the probe measurement. Suitable probe
or electrode is required to determine the degree of interference
element shielding can remove such effects.
in the measurement caused by such variability. This variability
6.3 Process fluids, except liquid metals and certain molten
is less critical where relative changes in corrosion rate are to be
salts, do not normally have sufficient electrical conductivity to
detected.
produce a significant shorting effect on the electrical resistance
5.2 Both electrical test methods described in this guide
of the exposed probe element. Conductive deposits (such as
provide a technique for determining corrosion rates without the
iron sulphide) can cause some short circuiting effect on the
need to physically enter the system to withdraw coupons as
element, reducing the measured metal loss, or showing some
required by the methods described in Guide G 4.
apparent metal gain. Certain probe configurations are less
5.3 Test Method B has the additional advantage of provid-
sensitive to this than others, depending on the path length
ing corrosion rate measurement within minutes.
between one end of the exposed probe element and the other.
5.4 These techniques are useful in systems where process
6.4 When first introduced into a system, initial transient
upsets or other problems can create corrosive conditions. An
corrosion rates on a probe element may be different from the
early warning of corrosive attack can permit remedial action
longer term corrosion rates.
before significant damage occurs to process equipment.
6.4.1 Establishment of a probe element surface typical of
5.5 These techniques are also useful where inhibitor addi-
the plant by passivation, oxidation, deposits, or inhibitor film
tions are used to control the corrosion of equipment. The
bui
...