ISO 22858:2020
(Main)Corrosion of metals and alloys — Electrochemical measurements — Test method for monitoring atmospheric corrosion
Corrosion of metals and alloys — Electrochemical measurements — Test method for monitoring atmospheric corrosion
This document specifies a test method for atmospheric corrosion measurements, using two-electrode electrochemical sensors. It is applicable to measurements of the corrosion rate of uncoupled metal surfaces (i.e. "free" corrosion rate), galvanic corrosion rate, conductance of thin film solutions and barrier properties of organic coatings. It specifies electrochemical sensors that are used with or without organic coatings. The sensors are applicable to corrosion measurements made in laboratory test chambers, outdoor exposure sites and service environments.
Corrosion des métaux et alliages — Mesures électrochimiques — Méthode d'essai pour la surveillance de la corrosion atmosphérique
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 22858
First edition
2020-08
Corrosion of metals and alloys —
Electrochemical measurements
— Test method for monitoring
atmospheric corrosion
Corrosion des métaux et alliages — Mesures électrochimiques —
Méthode d'essai pour la surveillance de la corrosion atmosphérique
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Summary of sensors . 2
5 Free corrosion current sensor . 3
5.1 Free corrosion current sensor description . 3
5.2 Sensor geometry . 3
5.3 Uniform corrosion current measurement . 3
5.3.1 Use and conditions for uniform corrosion measurements. 3
5.3.2 Method 1 — Sine wave excitation . 3
5.3.3 Method 2 — Triangle wave excitation . 3
5.3.4 Method 3 — Potential step excitation . 3
5.4 Localized corrosion current measurement . 4
5.5 Free corrosion rate and total free corrosion for sensors without coatings . 4
5.5.1 Free corrosion current and current density. 4
5.5.2 Free corrosion penetration rate . 4
5.5.3 Free corrosion mass loss rate . 5
5.5.4 Total free corrosion mass loss and corrosion penetration . 5
5.6 Free corrosion current and total charge for sensors with coatings . 5
5.6.1 Use and conditions for free corrosion measurements with coatings . 5
5.6.2 Free corrosion current for a coated sensor . 5
5.6.3 Free corrosion total charge for a coated sensor . 5
5.7 Free corrosion sensor preparation . 5
5.7.1 Considerations for free corrosion sensor surface preparation . 5
5.7.2 Free corrosion sensors without coatings . 6
5.7.3 Free corrosion sensors with coatings and surface treatments . 6
5.8 Specification and inspection — Free corrosion sensors . 6
5.8.1 Visual inspection . 6
5.8.2 Sensor range and span . 7
5.8.3 Electrical verification tests . 7
5.8.4 Corrosion verification tests . 7
6 Galvanic corrosion current sensor . 7
6.1 Galvanic corrosion current sensor description. 7
6.2 Sensor geometry . 7
6.3 Galvanic corrosion current measurements . 8
6.3.1 Methods for galvanic corrosion current measurement . 8
6.3.2 Method 1 — Zero-resistance ammeter . 8
6.3.3 Method 2 — Precision resistor . 8
6.4 Galvanic corrosion rate and total galvanic corrosion without coatings . 8
6.4.1 Galvanic corrosion current . 8
6.4.2 Galvanic corrosion rate for mass loss and corrosion penetration . 8
6.4.3 Total galvanic corrosion mass loss and corrosion penetration . 8
6.5 Galvanic corrosion rate and total galvanic corrosion with coatings . 9
6.5.1 Use and conditions for galvanic corrosion measurements with coatings . 9
6.5.2 Galvanic mass loss corrosion rate for a coated sensor . 9
6.5.3 Total galvanic mass loss for a coated sensor. 9
6.6 Galvanic corrosion sensor preparation . 9
6.6.1 Considerations for galvanic corrosion sensor preparation . . 9
6.6.2 Galvanic corrosion sensors without coatings . 9
6.6.3 Galvanic corrosion sensors with coatings and surface treatments . 9
6.7 Specification and inspection — Galvanic corrosion sensors . 9
6.7.1 Visual, span and range inspection . 9
6.7.2 Electrical verification tests . 9
6.7.3 Corrosion verification tests .10
7 Thin film conductance sensors .10
7.1 Conductance sensor description .10
7.2 Sensor geometry .10
7.3 Surface conductance measurement method .10
7.4 Surface conductance sensor preparation .10
7.5 Specification and inspection — Conductance sensor .10
7.5.1 Visual, span and range inspection .10
7.5.2 Electrical verification tests .10
7.5.3 Conductive solution verification tests .11
8 Coating barrier property sensors .11
8.1 Coating barrier property sensor description .11
8.2 Coating barrier property measurements .11
8.3 Coating barrier property sensor preparation .11
8.3.1 Sensor preparation for coating .11
8.3.2 Coating test condition.11
8.4 Specification and inspection — Coating barrier property sensor .12
8.4.1 Visual, span and range inspection .12
8.4.2 Electrical measurements .12
8.4.3 Sensing system impedance verification tests .12
9 Atmospheric testing with electrochemical sensors .12
9.1 Types of atmospheric tests .12
9.2 Test arrangement .12
9.3 Test duration .12
9.4 Sensor selection .12
9.5 Sampling time interval .12
9.6 Date and time information .13
10 Test report .13
10.1 Test report guidance .13
10.2 Sensor information .13
10.3 Surface preparation .13
10.4 Test description .13
10.5 Sensor inspection .13
10.6 Data storage .13
Annex A (informative) Example images of electrochemical sensors .14
Annex B (informative) Equivalent circuit analysis for two-electrode measurements .16
Annex C (informative) Example of reporting information .18
Bibliography .20
iv © ISO 2020 – All rights reserved
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
Introduction
The purpose of this document is to provide instructions on the use of electrochemical sensors
for monitoring atmospheric corrosion. These sensors are used to measure thin film electrolyte
conductance, corrosion current or coating condition over long periods. This method permits the
instantaneous evaluation of corrosion current that can be related to specific environmental conditions
in real time. The instantaneous corrosion current measurements are not accessible using electrical
resistance sensors or mass loss techniques. The technology described in this document complements
other standard techniques for assessing atmospheric corrosion such as mass loss coupons, electrical
resistance sensors or coated test panels (see ISO 8407 and ISO 4628-8). These continuous records of
material condition can be useful for studying atmospheric corrosion, evaluating materials or managing
[21][22][23][24][25][26][27][28][29]
assets .
This document was developed based on ANSI/NACE TM0416-2016.
This document is relevant to alloy and coating manufacturers and users in transportation, chemical
process, energy and infrastructure applications.
vi © ISO 2020 – All rights reserved
INTERNATIONAL STANDARD ISO 22858:2020(E)
Corrosion of metals and alloys — Electrochemical
measurements — Test method for monitoring atmospheric
corrosion
1 Scope
This document specifies a test method for atmospheric corrosion measurements, using two-electrode
electrochemical sensors.
It is applicable to measurements of the corrosion rate of uncoupled metal surfaces (i.e. “free” corrosion
rate), galvanic corrosion rate, conductance of thin film solutions and barrier properties of organic
coatings. It specifies electrochemical sensors that are used with or without organic coatings. The
sensors are applicable to corrosion measurements made in laboratory test chambers, outdoor exposure
sites and service environments.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 4618, Paints and varnishes — Terms and definitions
ISO 4628 (all parts), Paints and varnishes — Evaluation of degradation of coatings — Designation of
quantity and size of defects, and of intensity of uniform changes in appearance
ISO 8044, Corrosion of metals and alloys — Vocabulary
ISO 9223, Corrosion of metals and alloys — Corrosivity of atmospheres — Classification, determination
and estimation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4618, ISO 4628 (all parts),
ISO 8044, ISO 9223 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
electrical resistance sensor
device for measuring corrosion involving measurement of the ratio of the potential difference along a
conductor and the current through the conductor
Note 1 to entry: ISO 15091:2019, 3.1, defines “electrical resistance” as the “ratio of the potential difference along
a conductor and the current through the conductor”.
3.2
electrochemical sensor
device for measuring corrosion involving anodic and cathodic reactions
Note 1 to entry: ISO 8044:2020, 4.1, defines “electrochemical corrosion” as “corrosion involving at least one
anodic reaction and one cathodic reaction”.
3.3
electrode digit
single finger of an interdigitated electrode (3.5)
3.4
corrosion penetration
distance between the corroded surface of a metal and the original surface of the metal
Note 1 to entry: ISO 8044:2020, 3.11, defines “corrosion depth” as the “distance between a point on the surface of
a metal affected by corrosion and the original surface of the metal”.
3.5
interdigitated electrode
electronic conductors interlocked like fingers
3.6
sensor range
upper and lower measurement values
3.7
sensor span
difference between maximum and minimum measurement values
3.8
solution resistance
ratio of electrode potential increment to the corresponding current increment dependent on solution
3.9
thin film conductance
solution layer current transport capacity
Note 1 to entry: ISO 15091:2019, 3.3, defines “conductance” as the “reciprocal of the resistance”.
3.10
zero-resistance ammeter
instrument used for current measurement between two electrodes with no potential drop between them
4 Summary of sensors
The atmospheric corrosion measurements are made using three types of sensors to measure: a) free
corrosion current, b) galvanic corrosion current and c) surface conductance.
Electrochemical sensors for atmospheric corrosion shall have a planar gage area. They are composed of
two metallic electrodes separated by a dielectric material that electrically isolates the electrodes (see
Figures A.1 and A.2). The electrochemical sensors have interdigitated electrode geometries and may
be produced using composite laminate or thin film processes (see Annex A). The sensor gage area is
defined as the area of the electrodes exposed to the environment.
2 © ISO 2020 – All rights reserved
5 Free corrosion current sensor
5.1 Free corrosion current sensor description
Free corrosion rate measurements are obtained using two-electrode sensors that may have a variety
[22][23][24][25][27]
of alloys, geometries and excitation techniques . Two-electrode sensors for the
measurement of free corrosion rate may be made from any alloy of interest. Both electrodes of the free
corrosion current sensor shall be constructed of the same alloy. The electrical excitation of sensors shall
be done by applying a voltage between the two electrodes. Voltage may be applied using a potentiostat
or other electronic device designed to apply a controlled potential. During measurements, the voltage
between the two electrodes is controlled and the current response recorded. Between measurements,
the two electrodes of the sensor may be electrically shorted. The corrosion current measurement range
and span will be dependent on the expected corrosion rates of the given sensor alloy in the environment.
5.2 Sensor geometry
The separation distance between the electrodes should be no more than 300 μm (see Figures A.1 and A.2).
A high electrode digit length to width ratio minimizes the contribution of edge effects that distort current
and potential distributions, and small width electrodes support a more uniform active measurement
area under varying environmental conditions. An example length to width ratio is 10 and an example
electrode digit width is 2 mm. Geometry and sensing areas for each electrode should be the same.
5.3 Uniform corrosion current measurement
5.3.1 Use and conditions for uniform corrosion measurements
For alloys that undergo uniform corrosion, such as a low alloy steel, polarization resistance may be
obtained and free corrosion rate estimated by means of the Stern-Geary equation (see ISO 17475,
ISO/TR 16208 and ASTM G59-97). For a simple equivalent circuit model of a two-electrode sensor,
the polarization resistance may be approximated as half of the real impedance at a low frequency (see
Annex B). This assumes that the solution resistance is small relative to the polarization resistance. This
assumption may not be valid at low levels of corrosive contaminants or for very thin or discontinuous
solution layers. Uniform corrosion measurements should be validated with mass loss or other coupon
tests for environments classified as low corrosivity outdoor (C1) or medium corrosivity indoor (IC 3)
or less (see ISO 9223, ISO 9224, ISO 9226 and ISO 11844-1). High solution resistance could result in an
underestimation of corrosion rate. Polarization resistance shall be determined using the methods given
in 5.3.2 to 5.3.4.
5.3.2 Method 1 — Sine wave excitation
The current response may be measured using a voltage sine wave excitation. The amplitude should
be less than 30 mV. The excitation frequency shall be low enough, typically from 0,01 Hz to 10 Hz, to
obtain a reasonable estimate of the polarization resistance. This method may require verification
that the selected frequency yields, or correlates to, polarization resistances obtained using a full
electrochemical impedance scan (see ISO/TR 16208 and ASTM G102-89).
5.3.3 Method 2 — Triangle wave excitation
The current response may be measured using a triangle wave voltage excitation with an amplitude not
greater than 30 mV. The excitation signal shall have a ramp rate from 0,05 mV/sec to 10 mV/sec. This
method may require verification that the selected waveform produces polarization resistances that
correlate to those obtained using potentiodynamic scan methods (see ISO 17475 and ASTM G59-97).
5.3.4 Method 3 — Potential step excitation
The current response may be measured using potential steps and holds. A sufficient number of steps
shall be used to obtain a linear fit to the voltage versus current response data over a potential range
[22]
no greater than ±30 mV . For each step, the hold time should be sufficient to obtain a steady-state
current measurement. For each step, the current shall be measured after the hold time and be an
average of multiple readings. This method may require verification that the selected excitation produces
polarization resistances that correlate to polarization resistances obtained using potentiodynamic
scan methods (see ASTM G59-97).
5.4 Localized corrosion current measurement
For alloys that corrode by localized mechanisms, such as aluminium alloy pitting, the impedance should
be measured for a given sine wave voltage excitation, and the amplitude should be less than 30 mV. The
impedance may be either the real component or modulus. The excitation frequency shall be within the
range of 0,01 Hz to 10 Hz. In the case of localized corrosion processes, a constant of proportionality
that empirically relates the measured impedance to the corrosion current is needed to make absolute
estimates of corrosion rate.
5.5 Free corrosion rate and total free corrosion for sensors without coatings
5.5.1 Free corrosion current and current density
Free corrosion current density shall be reported as microampere per square centimetre (µA/cm ).
Current density shall be calculated using the free corrosion current and the area of one electrode. If one
electrode is smaller, then the smallest electrode area shall be used. See Formula (1).
I
corr
i = (1)
corr
A
where
i is the free corrosion current density, expressed as microampere per square centimetre (µA/cm );
corr
I is the free corrosion current, expressed as microampere (µA);
corr
A is the electrode area, expressed as square centimetre (cm ).
5.5.2 Free corrosion penetration rate
The free corrosion rate may also be converted to free corrosion penetration rate, but this shall only be
done for alloys with uniform corrosion and shall not be used for alloys with localized corrosion such as
pitting or intergranular corrosion. See Formula (2):
i
corr
rK=⋅⋅ W (2)
t 1 e
ρ
where
r is the corrosion penetration rate of a metal, expressed as micrometre per year (µm/a);
t
K is a constant of proportionality equal to 3,27, expressed as micrometre gram per microampere
centimetre year ((µm ⋅ g)/(µA ⋅ cm ⋅ a)) (see ASTM G102-89);
ρ is the density of the metal, expressed as kilogram per cubic metre (g/cm );
W is the atomic weight of the metal divided by the valence of the oxidized metal atom, this is used
e
as a dimensionless quantity (see ASTM G102-89).
Methods are available for obtaining corrosion mass loss and penetration rate for alloys that use
alloy equivalent weight and densities, but these methods are outside the scope of this document (see
ASTM G102-89).
4 © ISO 2020 – All rights reserved
5.5.3 Free corrosion mass loss rate
Free corrosion current density measurement may be converted to mass loss corrosion rate. See
Formula (3):
rK=⋅iW⋅ (3)
corr 2 corr e
where
r is the corrosion mass loss rate of metal, expressed in grams per square metre year
corr
(g/(m ⋅ a));
K is a constant of proportionality equal to 3,268, expressed as gram square centimetre
2 2
per microampere square metre year ((g ⋅ cm )/(µA ⋅ m ⋅ a)).
5.5.4 Total free corrosion mass loss and corrosion penetration
Time-based measurements of free corrosion current or corrosion rate may be integrated to obtain
estimates of total charge passed or total mass loss, respectively. Total charge or mass loss shall be
2 2
expressed in coulombs per square metre (C/m ) or grams per square metre (g/m ), respectively.
Measures of thickness loss may also be reported as micrometres (μm), but shall only be reported for
alloys with uniform corrosion. Total mass loss obtained from the sensor should be compared to the
mass loss of specimens produced from the same alloy as the sensor (see ISO 8407).
5.6 Free corrosion current and total charge for sensors with coatings
5.6.1 Use and conditions for free corrosion measurements with coatings
For use with coatings, the sensor should be mounted to form a planar surface that is greater than the
sensor gage area along each edge (see Figures A.1 and A.2). This may be achieved by casting, potting or
mounting the sensors.
The sensor responses are dependent on coating properties and coating defect area. The sensor response
may change during the test as the coating degrades. Therefore, the sensor response is not a measure of
uniform conditions over the complete gage area of the sensor.
5.6.2 Free corrosion current for a coated sensor
For a coated free corrosion sensor, the current should be measured for a given sine wave voltage
excitation and the amplitude should be less than 30 mV. The excitation frequency shall be within the
range of 0,01 Hz to 10 Hz.
Current shall be expressed as microamps (µA).
5.6.3 Free corrosion total charge for a coated sensor
Time-based measurements of current may be integrated to obtain estimates of total charge passed.
Total charge shall be expressed as coulombs (C).
5.7 Free corrosion sensor preparation
5.7.1 Considerations for free corrosion sensor surface preparation
The free corrosion sensors shall be prepared and cleaned as specified by the sensor supplier. Mechanical
surface preparation should be avoided for thin film sensing elements. Composite laminate sensors with
sufficient electrode thickness, approximately greater than 1 mm, may be mechanically finished using
abrasives such as 600-grit sandpaper.
5.7.2 Free corrosion sensors without coatings
For sensors used without coatings, the sensors shall be cleaned to remove soluble organic and inorganic
contaminants. Cleaning chemicals and processes shall be compatible with the sensor electrode and
dielectric materials.
5.7.3 Free corrosion sensors with coatings and surface treatments
5.7.3.1 Coatings and surface treatments for use with free corrosion sensors
A broad range of protective coatings and surface treatments can be used with the electrochemical
sensors. A general description of the use of electrochemical sensors with an organic coating system is
given in 5.7.3.2 to 5.7.3.4. The system can consist of a chemical pretreatment, primer and topcoat. Besides
paints and coatings, other materials that can be applied to the sensors are chemical pretreatments,
volatile corrosion inhibitors and protective oils or compounds.
5.7.3.2 Organic coating
Sensors to be used with organic coatings can be prepared using typical etching, cleaners and
pre-treatment processes. Coating performance is strongly dependent on surface cleanliness and
preparation. Some high temperature and cleaning process steps may damage the sensor electrodes.
It is recommended that sensor resistance to cleaning and pretreatment processes be verified prior to
coating tests. The sensors should be visually inspected after each step of the coating process.
5.7.3.3 Coating application and processing
Sensors used to test coatings should be processed, as much as possible, like actual parts. Coatings can
be applied by spray, brush or dipping processes. Masking shall be used to protect the electrical contacts
and connectors from the coating processes. Heat-cured coatings can be applied to the sensors, but
resistance of the sensor to cure temperatures shall be verified. Coatings should be applied and cured
according to the manufacturer’s specifications. Specific coating properties such as dry film thickness,
adhesion and curing should be tested on witness panels processed at the same time as the sensor
electrodes.
5.7.3.4 Coating scribe defect
Coating defects may be produced on free corrosion sensors using a mask, scribe tool or rotary
cutting tool (see ISO 4628-8 and ASTM D1654-08). The defects should be oriented transverse to the
interdigitated electrodes, and can be oblique or normal to the sensor length (see Figure A.1). One or
more defects may be applied to each sensor.
Coating defects can be formed by masking the sensor using thin strips of tape that extend across the
full width of the sensor.
Mechanical scribing of thick composite laminate electrodes can be done using standard manual or
automated methods for coated panels (see ISO 4628-8 and ASTM D1654-08). Mechanical scribing shall
not be performed on deposited or thin film electrodes. Scribes shall fully penetrate the coating to the
substrate and have a uniform width of exposed metal that should be 0,5 mm to 1,5 mm. After scribing,
the impedance shall be assessed to demonstrate that the electrodes are not electrically shorted by
metal smearing or debris in the scribe (see 5.8.3.2). If an electrical short is detected, the scribe should
be cleaned with a fine grit silicon carbide sandpaper (800 to 1 000 grit) or with an abrasive file.
5.8 Specification and inspection — Free corrosion sensors
5.8.1 Visual inspection
Sensors should be inspected prior to sensor processing and before use in atmospheric corrosion tests.
Sensors shall be visually inspected for any irregularities such as defects or pores in the dielectric
6 © ISO 2020 – All rights reserved
materials, damage to the electrodes, irregular geometry, electrode defects, surface contaminants or
other sensor-to-sensor anomalies that could affect performance and consistency of results.
5.8.2 Sensor range and span
The nominal expected range and span for the sensors should be established for the expected test
conditions either theoretically or experimentally before conducting exposure tests.
5.8.3 Electrical verification tests
5.8.3.1 Continuity test
Continuity between the gage area of the electrodes and sensor connector shall be checked. Care should
be taken not to scratch or otherwise damage the test surface during this measurement.
5.8.3.2 Electrical resistance test
Resistance measurements between the two electrodes of the interdigitated electrode sensor shall be
made on the as-produced or as-received sensors and shall be repeated again immediately prior to testing
after preparation is complete. The resistance should be measured using a multimeter. This test is done
to ensure that the two electrodes are electrically isolated from each other prior to use. The resistance
shall be greater than 100 MΩ. Resistance measurements shall be performed in environmentally
controlled conditions with a RH less than 50 % and a temperature between 20 °C and 27 °C.
5.8.4 Corrosion verification tests
Sensor operation may be verified by immersing in different corrosivity salt solutions (see ISO 11845).
Solution chemistry and concentration should be selected based on expected sensor use. Free corrosion
sensors shall be immersed in salt solution at a known temperature and excited using the excitation
methods of 5.3 or 5.4. Sensors used to verify performance should not be reused for atmospheric testing.
The time between exposure and testing should be noted. The time should be sufficient for the corrosion
sensor to reach a steady-state condition. Test conditions may be application specific. The solution
chemistry, concentration and temperature shall be recorded. Three conditions should be tested that
produce sensor response in each third (low, medium and high) of the desired corrosion rate range.
6 Galvanic corrosion current sensor
6.1 Galvanic corrosion current sensor description
Galvanic corrosion current measurements are obtained using two-electrode sensors with the
electrodes connected together using either: a) a zero-resistance ammeter, or b) a precision resistor (see
[21][23][24][26][29]
Figures A.1 and A.2) . The preferred technique for measuring galvanic corrosion is the
zero-resistance ammeter method. Two-electrode sensors for the measurement of galvanic corrosion
may be fabricated from any alloys of interest. The electrodes of the two-electrode sensor shall be
constructed of dissimilar materials to form a galvanic couple. Galvanic current range will depend on
the alloys, cathode and anode areas and environment. Spans for current measurements should be
selected based on these factors. The galvanic corrosion measurement range and span will depend on
the expected galvanic corrosion rates for the given sensor alloys and environment.
6.2 Sensor geometry
The separation distance between the electrodes and the electrode digit length to width ratio should be
as described by 5.2. The geometry of the electrodes and cathode to anode area ratio will influence the
spatial distribution and magnitude of the galvanic current. The area ratio may be selected based on the
known cathode and anode areas for a particular application.
6.3 Galvanic corrosion current measurements
6.3.1 Methods for galvanic corrosion current measurement
Electrical measurement of the galvanic current may be done using either of two methods:
a) zero-resistance ammeter (see 6.3.2);
b) precision resistor measurements (see 6.3.3).
6.3.2 Method 1 — Zero-resistance ammeter
Galvanic sensor excitation shall be done using a zero-resistance ammeter that applies a current to
control the potential difference between the electrodes of the galvanic couple to zero. The current
required to achieve this potential control is the galvanic current.
During the time interval between active zero-resistance ammeter measurements, the two electrodes of
the sensor shall be controlled to the same potential or electrically shorted.
6.3.3 Method 2 — Precision resistor
Galvanic current shall be determined by measuring the voltage drop across a precision resistor that
connects the two alloys of the galvanic couple. Knowing the voltage drop (V) and the value of the
precision resistor (R), the galvanic current (I ) may be calculated using Ohm’s Law: (I = V/R). The
g g
resistor produces a voltage difference between the anode and cathode. The resistance should be as
low as possible, while still allowing the current measurement to meet the galvanic current range and
precision requirements. The resistance should be between 10 ohms to 100 000 ohms with a tolerance
[31][32]
of 0,01 % .
Between measurements, the two electrodes of the sensor shall be electrically connected through the
precision resistors.
6.4 Galvanic corrosion rate and total galvanic corrosion without coatings
6.4.1 Galvanic corrosion current
Galvanic corrosion current density (i ) shall be reported as microampere per square centimetre (µA/
g
cm ). Current density will be measured using the area of the anode.
6.4.2 Galvanic corrosion rate for mass loss and corrosion penetration
Galvanic corrosion current density may be converted to mass loss rate using Formula (3) as described
in 5.5.3. Galvanic mass loss rate shall be expressed as grams per square metre year (g/(m ∙ a)). Galvanic
corrosion penetration rate may be expressed in micrometre per year (μm/a) using Formula (2) (see
5.5.2), but shall only be reported for alloys that uniformly corrode and shall not be reported for alloys
with localized corrosion such as pitting or intergranular corrosion.
6.4.3 Total galvanic corrosion mass loss and corrosion penetration
Time-based measurements of galvanic corrosion rate may be integrated to obtain estimates of total
mass loss. Total mass loss shall be expressed as grams per square metre (g/m ). Measures of thickness
loss may also be reported as micrometre (μm), but shall only be reported for alloys with uniform
corrosion.
8 © ISO 2020 – All rights reserved
6.5 Galvanic corrosion rate and total galvanic corrosion with coatings
6.5.1 Use and conditions for galvanic corrosion measurements with coatings
For use with coatings, the galvanic corrosion sensors should be mounted to form a planar surface that
is greater than the sensor gage area along each edge (see Figures A.1 and A.2). This may be achieved by
casting, potting or mounting the sensors.
The galvanic corrosion sensor responses are dependen
...
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