ISO 22858:2020

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

• Status: Published
• Publication Date: 16-Aug-2020
• ICS: 77.060 - Corrosion of metals
• Technical Committee: ISO/TC 156 - Corrosion of metals and alloys
• Drafting Committee: ISO/TC 156/WG 11 - Electrochemical test methods
• Current Stage: 6060 - International Standard published
• Start Date: 17-Aug-2020
• Due Date: 11-Sep-2020
• Completion Date: 17-Aug-2020

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 22858:2020(E)
©
ISO 2020

---------------------- Page: 1 ----------------------
ISO 22858:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 22858:2020(E)

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
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ISO 22858:2020(E)

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

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ISO 22858:2020(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
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
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
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.
© ISO 2020 – All rights reserved v

---------------------- Page: 5 ----------------------
ISO 22858:2020(E)

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

---------------------- Page: 6 ----------------------
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”.
© ISO 2020 – All rights reserved 1

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ISO 22858:2020(E)

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

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ISO 22858:2020(E)

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
© ISO 2020 – All rights reserved 3

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ISO 22858:2020(E)

[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
2
Free corrosion current density shall be reported as microampere per square centimetre (µA/cm ).
Current density shall be calculated using the fre
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 22858
ISO/TC 156
Corrosion of metals and alloys —
Secretariat: SAC
Electrochemical measurements
Voting begins on:
2020­05­25 — Test method for monitoring
atmospheric corrosion
Voting terminates on:
2020­07­20
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 22858:2020(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
©
NATIONAL REGULATIONS. ISO 2020

---------------------- Page: 1 ----------------------
ISO/FDIS 22858:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH­1214 Vernier, Geneva
Phone: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/FDIS 22858:2020(E)

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 . 6
5.7.1 Considerations for free corrosion sensor surface preparation . 6
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 . 7
5.8.1 Visual inspection . 7
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 . 8
6.3 Galvanic corrosion current measurements . 8
6.3.1 Methods for galvanic corrosion current measurement . 8
6.3.2 Method 1 — Zero­resistance 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 . 9
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
© ISO 2020 – All rights reserved iii

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ISO/FDIS 22858:2020(E)

6.7 Specification and inspection — Galvanic corrosion sensors . 9
6.7.1 Visual, span and range inspection . 9
6.7.2 Electrical verification tests .10
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 .11
7.5.1 Visual, span and range inspection .11
7.5.2 Electrical verification tests .11
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 .12
8.3.1 Sensor preparation for coating .12
8.3.2 Coating test condition.12
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 .13
9.5 Sampling time interval .13
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 .14
Annex A (informative) Example images of electrochemical sensors .15
Annex B (informative) Equivalent circuit analysis for two-electrode measurements .17
Annex C (informative) Example of reporting information .19
Bibliography .21
iv © ISO 2020 – All rights reserved

---------------------- Page: 4 ----------------------
ISO/FDIS 22858:2020(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non­governmental, in liaison with ISO, also take part in the work.
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
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
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.
© ISO 2020 – All rights reserved v

---------------------- Page: 5 ----------------------
ISO/FDIS 22858:2020(E)

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 4628­8). 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 TM0416­2016.
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

---------------------- Page: 6 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 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”.
© ISO 2020 – All rights reserved 1

---------------------- Page: 7 ----------------------
ISO/FDIS 22858:2020(E)

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.
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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 . Two­electrode 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 11844­1). 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 G102­89).
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
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[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 G59­97).
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 with
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