ASTM G217-16(2022)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: G217 − 16 (Reapproved 2022)
Standard Guide for
Corrosion Monitoring in Laboratories and Plants with
Coupled Multielectrode Array Sensor Method
This standard is issued under the fixed designation G217; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope G4 Guide for Conducting Corrosion Tests in Field Applica-
tions
1.1 This guide outlines the procedure for conducting corro-
G16 Guide for Applying Statistics to Analysis of Corrosion
sionmonitoringinlaboratoriesandplantsbyuseofthecoupled
Data
multielectrode array sensor (CMAS) technique.
G46 Guide for Examination and Evaluation of Pitting Cor-
1.2 For plant applications, this technique can be used to
rosion
assess the instantaneous non-uniform corrosion rate, including
G48 Test Methods for Pitting and Crevice Corrosion Resis-
localized corrosion rate, on a continuous basis, without re-
tance of Stainless Steels and Related Alloys by Use of
moval of the monitoring probes, from the plant.
Ferric Chloride Solution
G96 Guide for Online Monitoring of Corrosion in Plant
1.3 For laboratory applications, this technique can be used
to study the effects of various testing conditions and inhibitors Equipment (Electrical and Electrochemical Methods)
G102 Practice for Calculation of Corrosion Rates and Re-
on non-uniform corrosion, including pitting corrosion and
crevice corrosion. lated Information from Electrochemical Measurements
G193 Terminology and Acronyms Relating to Corrosion
1.4 Units—The values stated in SI units are to be regarded
G199 Guide for Electrochemical Noise Measurement
as the standard. No other units of measurement are included in
this standard.
3. Terminology
1.5 This standard does not purport to address all of the
3.1 Definitions—The terminology used herein, if not spe-
safety concerns, if any, associated with its use. It is the
cifically defined otherwise, shall be in accordance with Termi-
responsibility of the user of this standard to establish appro-
nology G193. Definitions provided herein and not given in
priate safety, health, and environmental practices and deter-
Terminology G193 are limited only to this guide.
mine the applicability of regulatory limitations prior to use.
3.2 Definitions of Terms Specific to This Standard:
1.6 This international standard was developed in accor-
3.2.1 coupled multielectrode array sensor, CMAS,
dance with internationally recognized principles on standard-
n—device with multiple working electrodes that are coupled
ization established in the Decision on Principles for the
through an external circuit such that all the electrodes operate
Development of International Standards, Guides and Recom-
at the same electrode potential to simulate the electrochemical
mendations issued by the World Trade Organization Technical
behavior of a single-piece metal.
Barriers to Trade (TBT) Committee.
3.2.2 non-uniform corrosion, n—corrosion that occurs at
2. Referenced Documents various rates across the metal surface, with some locations
exhibiting higher anodic rates while others have higher ca-
2.1 ASTM Standards:
thodic rates, thereby requiring that the electron transfer occurs
G1 Practice for Preparing, Cleaning, and Evaluating Corro-
between these sites within the metal.
sion Test Specimens
3.2.2.1 Discussion—Non-uniform corrosion includes both
localized corrosion and uneven general corrosion (1). Non-
uniform corrosion also includes the type of general corrosion
This guide is under the jurisdiction ofASTM Committee G01 on Corrosion of
Metals and is the direct responsibility of Subcommittee G01.11 on Electrochemical that produces even surfaces at the end of a large time interval,
Measurements in Corrosion Testing.
but uneven surfaces during small time intervals.
Current edition approved May 1, 2022. Published May 2022. Originally
3.2.3 uneven general corrosion, n—corrosion that occurs
approved in 2016. Last previous edition approved in 2016 as G217–16. DOI:
10.1520/G0217-16R22.
overthewholeexposedsurfaceoralargeareaatdifferentrates.
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 boldface numbers in parentheses refer to a list of references at the end of
the ASTM website. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G217 − 16 (2022)
3.2.3.1 Discussion—In this guide, general corrosion is fur- warning of corrosive attack can permit remedial action before
ther divided into even general corrosion, or uniform corrosion, significant damage occurs to process equipment. The two
which is defined as the corrosion that proceeds at exactly the methods described in Guide G96 are suitable for uniform
same rate over the surface of a material (see Terminology corrosion, but may not be sensitive enough for non-uniform
G193) and uneven general corrosion. Uneven general corro- corrosion, especially localized corrosion. This guide describes
sion is defined as the general corrosion that produces uneven a new method for monitoring non-uniform corrosion, espe-
surfaceorwave-likesurfaceonametalthathasanevensurface cially localized corrosion.
before the corrosion (2, 3).
4.2 The CMAS technique measures the net anodic current
3.2.4 zero-voltage ammeter, ZVA, n—device that imposes a
or net cathodic current from each of the individual electrodes
a c
negligibly low voltage drop when inserted into a circuit for
(I or I in Fig. 1), which is the characteristic of non-
ex ex
measurement of current.
uniform corrosion such as localized corrosion and uneven
3.2.4.1 Discussion—The ZVA defined in this guide also
general corrosion.Therefore, the CMAS technique can be used
meets the definition of the zero-resistance ammeter (ZRA) in
to estimate the rate of uneven general corrosion and localized
Guide G199.Atypical ZRAis built with inverting operational
corrosion (see Section 5).
amplifiers to limit the voltage drop in the current-measuring
4.3 Unlike uniform corrosion, the rate of non-uniform
circuit to a low value. Both ZRA and a simple device formed
corrosion,especiallylocalizedcorrosion,canvarysignificantly
with a shunt resistor and a voltmeter can be used as a ZVA as
from one area to another area of the same metal exposed to the
long as they do not impose a significant voltage drop (<1 mV)
same environment. Allowance shall be made for such varia-
in the current-measuring circuit (see Annex A2 for more
tions when the measured non-uniform corrosion rate is used to
information).
estimate the penetration of the actual metal structure or the
actual wall of process equipment. This variability is less
4. Significance and Use
critical when relative changes in corrosion rate are to be
4.1 Guide G96 describes a linear-polarization method and
detected, for example, to track the effectiveness of corrosion
an electrical resistance method for online monitoring of corro-
inhibitors in an inhibited system.
sion in plant equipment without the need to enter the system
physically to withdraw coupons. These two online monitoring 4.4 The same as the method described in Guide G96, the
techniques are useful in systems in which process upsets or CMAS technique described in this guide provides a technique
other problems can create corrosive conditions. An early for determining corrosion rates without the need to enter the
NOTE 1—The upper section shows the electron flows from the corroding area to the less corroding areas inside a metal when localized corrosion takes
place; the lower section shows the electron flows after the anodic and cathodic areas are separated into individual small electrodes and coupled through
a c
an external circuit that measures the anodic current (I ) and cathodic current (I ) through each of the individual electrodes (4).
ex ex
FIG. 1 Principle of CMAS Probe
G217 − 16 (2022)
system physically to withdraw coupons as required by the trodes. The quantitative localized or non-uniform corrosion
methods described in Guide G4. rates from the individual electrodes may be determined from
the anodic currents (4, 5, 8). The reason to use a ZVA to
4.5 The same as the methods described in Guide G96, the
measure the current for each electrode is that the ZVAdoes not
CMAS technique is useful in systems in which process upsets
impose a potential drop between the electrode under measure-
or other problems can create corrosive conditions. An early
ment and the coupling joint, which ensures that all the
warning of corrosive attack can permit remedial action before
electrodes are at the same electrode potential so that the
significant damage occurs to process equipment.
multiple electrodes simulate the behavior of a one-piece metal.
4.6 The CMAS technique provides the instantaneous corro-
A zero-resistance ammeter (ZRA) is one type of ZVA and can
sion rate within 10 s to 40 s making it suitable for automatic
be used for the current measurements in a CMAS probe. A
corrosion inhibitor dosing control.
resistor inserted in the circuit and a voltmeter can also be used
as the ZVA for the measurements of the current in a CMAS
4.7 TheCMAStechniqueisanonlinetechniqueandmaybe
probebecausethecurrentfromaCMASelectrodeisextremely
used to provide real-time measurements for internal corrosion
small (typically <1 µA) and produces negligibly low-voltage
of pipelines and process vessels, external corrosion of buried
drop across the resistor (<0.1 mV if the resistor is 100 Ω).
pipes and structures, and atmospheric corrosion of metal
5.1.3 On an anodic electrode, the corrosion current (total
structures.
dissolution current), I , is equal to the sum of the externally
corr
a
flowing anodic current, I (see Fig. 1) and the internally
ex
5. Description of Guide
a
flowing anodic currents, I (see AnnexA1 for more informa-
in
5.1 Coupled Multielectrode Array Sensor (CMAS) Prin-
tion). Therefore,
ciple:
a a
I 5 I 1I (1)
corr ex in
5.1.1 Coupled multielectrode array is a system with mul-
a
tiple working electrodes that are electrically coupled through 5.1.4 Because the I for the anodic electrode, especially
in
when the anodic electrode is the most anodic electrode among
an external circuit so that all of the electrodes operate at the
same potential to simulate the electrochemical behavior of a all the anodic electrodes of the CMAS probe, is often much
a
smaller than its I at the coupling potential in a non-uniform
single-piece metal. The coupled multielectrode arrays have
ex
been used for studying the spatial and temporal electrochemi- corrosion or localized corrosion environment, the externally
flowing current from such anodic electrode of the probe is
cal behaviors of metals during corrosion processes (5-7). The
CMAS is a coupled multielectrode array used as a sensor for often used to estimate the non-uniform or localized corrosion
current:
monitoring corrosion. The outputs from a coupled multielec-
trode array are the addressable individual currents from all a
I 'I (2)
corr ex
electrodes.TheoutputsfromatypicalCMASprobeareusually
5.1.5 In the case of uniform corrosion, however, there
the maximum corrosion rate and maximum penetration depth
would be no physical separation between the anodic electrodes
derived from the individual currents from the multiple elec-
and the cathodic electrodes. The behavior of the most anodic
trodes without the need to know the spatial location of the
electrodewouldbesimilartotheotherelectrodesintheCMAS
particular electrodes (4, 8).
a
probe. In this case, the I on the anodic electrode would be
in
5.1.2 When a metal undergoes non-uniform corrosion, par-
a
large and I would be zero, and Eq 2 may not be used to
ex
ticularly localized corrosion such as pitting corrosion or
calculate the corrosion rate.Therefore, CMAS technique is not
crevice corrosion in a corrosive environment, electrons are
suitable for monitoring the rate of corrosion where the corro-
released from the anodic sites where the metal corrodes and
sion is uniformly progressing at all times. The CMAS probe is
travel within the metal to the cathodic sites where the metal
suitable for monitoring non-uniform corrosion, including un-
corrodes less or does not corrode (see upper section of Fig. 1)
even general corrosion such as the case for carbon steel in
(4).Suchphenomenonoccursbecauseoflocalvariationsinthe
seawater and localized corrosion (see AnnexA1 for theoretical
microstructure of the metal surface and in the environment or
basis). In cases of general corrosion in which the corrosion is
the development of scale layers on the metal surface. If the
characterized as both uniform corrosion and uneven general
metal is separated into multiple small pieces (or mini-
corrosion,theCMASprobemeasurestheunevenportionofthe
electrodes), some of the mini-electrodes have properties that
corrosion. For example, the CMAS probe measures more than
are close to the anodic sites and others have properties that are
56 % of the corrosion rate for carbon steel in a 0.2 %
close to the cathodic sites of the corroding metal. When these
hydrochloric acid (HCl) solution and more than 22 % of the
mini-electrodes are coupled by connecting each of them to a
corrosion rate for carbon steel in a 2 % HCl solution (see
common joint through a multichannel zero-voltage ammeter
Annex A1 for more information).
(ZVA), the electrodes that exhibit anodic properties simulate
the anodic areas, and the electrodes that exhibit the cathodic 5.2 Determination of Corrosion Rate (5, 8):
properties simulate the cathodic areas of the corroding metal 5.2.1 In a corrosion management program for engineering
(see lower section of Fig. 1). The electrons released from the structures, field facilities, or plant equipment, the most impor-
anodic electrodes are forced to flow through the coupling joint tant parameter is the remaining life (often the remaining wall
to the cathodic electrodes. Thus, the ZVAmeasures the anodic thickness) of the systems. If localized corrosion is of concern,
a
currents (I ) to the more corroding electrodes and cathodic the remaining wall thickness in the most corroded area is often
ex
c
currents (I ) from the less corroding or noncorroding elec- used to evaluate the remaining life. Therefore, the maximum
ex
G217 − 16 (2022)
corrosion depth (the corrosion-induced wall thinning at the
Q 5 *I ~t!dt (6)
i i
most corroded area) for non-uniform corrosion (localized
where:
corrosion and uneven general corrosion) is often the most
th
important parameter in an operator’s mind. Because the corro-
Q = cumulative anodic charge of the i electrode.
i
sion depth is a parameter that takes a long time (often many
5.3.2 Similar to the maximum localized corrosion rate, the
years) to accumulate, the corresponding parameter that is
following equation may be used to calculate the maximum
important to the day-to-day operation would be the maximum
cumulative localized corrosion depth or penetration (cm):
non-uniform corrosion rate.
CD 5 Q EW⁄~F ρ A! (7)
max max
5.2.2 The maximum non-uniform corrosion rate may be
derived from the maximum anodic current (assuming no
where:
internal current effect) (5):
Q = highest cumulative anodic charge (coulombs) of all
max
CR 5 I EW⁄ F ρ A (3) the electrodes.
~ !
max max
5.3.3 The cumulative anodic charge of each electrode is
where:
calculated individually using Eq 6.
CR = calculated maximum penetration rate (cm/s),
max
5.3.4 Similar to the average corrosion rate, the average
I = maximum anodic current or the most anodic
max
anodic charge, Q , may be calculated by:
avg
current,
F = Faraday constant (96485 C/mol),
Q 5 Σ Q ⁄n, i from 1 to n (8)
~ !
avg i
A = surface area of the electrode (cm ),
where:
ρ = density of the alloy or electrode (g/cm ), and
Q = anodic charge from the i electrode, and
EW = equivalent weight (g/mol) (see Practice G102).
i th
n = number of electrodes in the CMAS probe.
5.2.3 Eq 3 assumes that the corrosion on the most corroded
5.3.5 In Eq 8, only anodic charge is included. If the charge
electrode is uniform over the entire surface. Because the
from an electrode is cathodic, its anodic charge is set to zero in
surface area of one electrode is usually between 1 mm and
the calculation. Thus, the average non-uniform corrosion
0.03 mm in a typical CMAS probe, the prediction of the
penetration depth, CD , may be calculated by:
penetration rate or localized corrosion rate by assuming uni- avg
form corrosion on the small electrode is realistic in most
CD 5 Q EW⁄ F ρ A (9)
~ ! ~ !
avg avg
applications.
5.3.6 The localized penetration depth ratio, f , may be
depth
5.2.4 The average non-uniform corrosion rate, CR , may
avg
defined as the ratio of the maximum non-uniform penetration
be derived from the average anodic current, I , which is the
avg
depth to the average non-uniform corrosion penetration depth:
sum of anodic currents divided by the number of electrodes on
f 5 CD ⁄CD (10)
the CMAS probe: depth max avg
5.3.7 Similar to the pitting factor described in Guide G46,
CR 5 I EW~F ρ A! (4)
avg avg
Eq 4 is valid only if the areas of all the electrodes are equal. the localized corrosion penetration depth ratio indicates the
Otherwise, a weighted average according to the areas of the
severity of localized corrosion. The f represents how much
depth
electrodes should be used.
deeperthedeepestpitisthantheaveragedepthsofthepits.The
average non-uniform corrosion depth (CD )in Eq 10 is not
avg
5.2.5 The localized rate ratio, f , is defined as the ratio of
rate
the average depth of metal loss as defined in Guide G46.Ina
the maximum localized corrosion rate to the average non-
uniform corrosion case, the uniform corrosion depth may be
uniform corrosion rate. It can be expressed by:
high, but the average non-uniform corrosion depth may be
f 5 CR ⁄CR (5)
rate max avg
zero.
The average non-uniform corrosion rate is not the average
metal corrosion rate because it does not include the portion
6. Limitations and Interferences
of uniform corrosion rate. In a uniform corrosion case, the
uniform corrosion rate may be high, but the average non- 6.1 The technique is not applicable if the corrosion is purely
uniform corrosion rate may be zero.
uniform. In this case, all electrodes are corroding at the same
a
pace and the I (see Fig. 1) from any of the electrodes will be
ex
5.2.6 The localized rate ratio indicates how much higher the
zero and the corrosion current, I , is equal to the internal
corr
non-uniform corrosion rate on the most corroding electrode
currents(seeEq1).Apurelyuniformcorrosioncaseisrareand
(which simulate the penetration rate of the fastest growing pit
most of the general corrosion cases observed in laboratories
onthesurfaceofametalinthecaseofpittingcorrosion)isthan
and industrial fields exhibit a certain degree of uneven general
the average corrosion rate.
corrosion. Examples of such cases are carbon steel or alumi-
5.3 Determination of Corrosion Penetration Depth (5, 8):
num in a dilute hydrochloric acid solution where uniform
5.3.1 The corrosion depth or penetration is related to the corrosion is dominant but uneven general corrosion is still
total damage accumulated in a given time period. The corro- present (see Annex A1). In cases in which uniform corrosion
th
sion depth of the i electrode may be derived from the and uneven general corrosion are both present, the CMAS
cumulative anodic charge that can be obtained by integrating technique underestimates the corrosion rate because this tech-
the corrosion current through the electrode from time zero to nique only measures the uneven portion of the general corro-
time, t: sion rate.
G217 − 16 (2022)
6.2 CMAS probes with flush electrodes embedded in insu- process temperature to which the probes are exposed. In a heat
latorshavehighsusceptibilitytotheeffectofcrevicecorrosion. transfer environment, actual plant metal temperatures may be
This may be alleviated by applying a coating on the electrodes significantly different from that of the test probe.
before the electrodes are embedded in the insulator.
6.9 Corrosion rates may be affected by flow velocity.
Consequently, probe electrodes should be installed in a veloc-
6.3 Electron-conductive deposits (such as iron sulphides)
can cause a short-circuiting (bridging) effect for measurement ity typical of the plant conditions. Caution should be exercised
in any laboratory tests to reproduce typical velocities and keep
methods based on electrical resistance or electrochemical
principles.Forexample,theelectron-conductivedepositscause the test fluid representative of plant conditions by preventing
an unrepresentative buildup of corrosion products in solution
the electrical resistance method (see Guide G96) to show a
lower metal loss rate or even a negative metal loss rate. For the or depletion of dissolved oxygen. Localized corrosion usually
occurs at locations with stagnant flow or deposits. The sensing
CMAS technique, closely packed small electrodes are also
susceptible to the bridging of the electrodes by the electron- surface should be exposed to the stagnant flow if it is used to
monitor the worst-case localized corrosion in a plant system.
conducting deposits. This may be alleviated by using individu-
ally insulated electrodes that project above the surface of the
6.10 Where flow dynamics or process fluid separation at a
probe (also called fingered electrodes) so that the interfacial
pipe or vessel wall are particularly critical to the corrosion
path between each electrode becomes significantly larger (9).
process, a flush-mounted probe may be more desirable than a
probe with electrodes positioned near the center of the pipe or
6.4 The CMAS technique may be more affected by electri-
vessel.
cal or magnetic noises from the process and local environment
because the current signals from a CMAS probe are typically
6.11 The CMAS measurement only determines metal lost
low (1 pAto 1 µA) for each electrode. The noise effect may be
caused by electrochemical corrosion and not metal lost by
minimized by the use of shielded cables and a noise-rejection
mechanical removal (such as erosion) or local chemical reac-
CMAS instrument and proper selection of the installation
tion (such as dry-air oxidation). In the case of erosion
location.TheCMASprobe,CMASinstrument,andthecabling
corrosion, only the electrochemical-induced corrosion compo-
should be such that they are away from the electrically noisy
nent will be measured by the CMAS technique.This technique
sources such as power cables, heavy duty motors, and heavy
is also not capable of detecting the pitting that results from
duty pumps.
non-electrochemically active inclusions in the metal in which
case the formation of the pit does not involve the release of
6.5 The CMAS technique calculates the maximum non-
electrons.
uniform corrosion or localized corrosion rate by assuming that
the corrosion on the most anodic electrode is uniform. In
6.12 When first introduced into a system, the initial corro-
reality, the electrode size may not be as small as a single pit
sionratesonanewlypolishedprobeareusuallyhigherthanthe
during the initiation stage and the corrosion on the most anodic
longer-term corrosion. Establishment of the probe-sensing
electrodes may not be uniform. The CMAS probes with
electrode surface typical of the plant system by passivation,
smaller electrode surface areas (<1 mm ) should be used to
oxidation, deposits, or inhibitor film buildup may vary from
minimize the size effect on the measurement of the non-
hours to several days. Therefore, the corrosion rates occurring
uniform corrosion rate. When the size is small (<1 mm ), the
on the probe electrodes during the first few hours or days of
corroded area is more likely to occupy the whole surface
exposure may not be typical of corrosion occurring in the
during the propagation stage of localized corrosion, especially
system. Preconditioning of electrodes for the surface to corre-
after the corroded area has grown to a certain depth that may
spond to the metal surface after the chemical treatment of the
compromise the integrity of the plant equipment.
plant may reduce this transient effect.
6.6 As an electrochemical method, the presence of other
7. Apparatus
secondary reactions that are not directly related to corrosion
7.1 CMAS Probes:
but involve charge transfer may affect the measurements. In
7.1.1 The CMAS is composed of multiple electrodes with
cases in which the other secondary reactions may be present,
chemical and metallurgical properties closely matching those
the potential of the coupling joint of the CMAS probe (the
of the plant systems or metals under study. For corrosion
potential of the CMAS electrodes) should be measured and
monitoring or corrosion studies in a laboratory, the number of
compared with the electrode potentials of the secondary
electrode can be as many as 55 or even 100 (6, 7, 10, 11). For
reactions that are published or measured in the laboratories
corrosion monitoring in a plant, however, the number of
with electrodes similar to those of the CMAS probe. If the
electrodes in a CMAS probe is usually 8 to 25 because the
coupling potential of the CMAS probe is close to the potentials
costs of the CMAS probe and CMAS instruments usually
of the secondary reactions, the corrosion rate data from the
increase with the increase in the number of electrodes (12, 13,
CMAS probe should be carefully evaluated before use.
14).
6.7 Repolishing of the CMAS probe surfaces should be
7.1.2 The electrodes in a CMAS probe are usually flush
avoided if the system being monitored is experiencing corro-
mounted in an epoxy or a high-temperature insulator and the
sion under deposits. This will improve the representativeness
length of the electrodes is usually more than 2.5 cm. The
of the probe surfaces to the plant surfaces of interest.
probe’s sensing surface can be polished and reused until a
6.8 Since the corrosion rate is usually temperature depth that corresponds to the length of the electrodes embed-
dependent, results will
...