Corrosion of metals and alloys -- Electrochemical measurement of ion transfer resistance to characterize the protective rust layer on weathering steel

This document specifies a method for the electrochemical measurement of ion transfer resistance of the rust layer formed on weathering steel alloys in order to assess their protective properties against corrosion thereafter[3]. This method uses an electrochemical AC impedance measurement[4][5][6][7][8], together with harmonic analysis, to identify the ion transfer resistance, and a rust thickness measurement to characterize the stability of the protective rust layer in terms of corrosion protection under used environments.

Corrosion des métaux et alliages -- Mesurage électrochimique de la résistance au transfert d'ions pour caractériser la couche de rouille protectrice sur l'acier autopatinable

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Published
Publication Date
22-Jun-2020
Current Stage
6060 - International Standard published
Start Date
23-Jun-2020
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INTERNATIONAL ISO
STANDARD 22410
First edition
2020-06
Corrosion of metals and alloys —
Electrochemical measurement of ion
transfer resistance to characterize
the protective rust layer on
weathering steel
Corrosion des métaux et alliages — Mesurage électrochimique de la
résistance au transfert d'ions pour caractériser la couche de rouille
protectrice sur l'acier autopatinable
Reference number
ISO 22410:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 22410:2020(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: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 22410:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Corrosion resistance by weathering steel .................................................................................................................................. 2

4.1 Formation of rust .................................................................................................................................................................................. 2

4.2 Typical types of rust by cross-sectional view ............................................................................................................... 3

5 Measurement of ion transfer resistance ...................................................................................................................................... 4

5.1 General ........................................................................................................................................................................................................... 4

5.2 Electrochemical impedance and harmonic current measurements ......................................................... 4

6 Measurements in the field .......................................................................................................................................................................... 6

7 Evaluation of the corrosion protective property of the rust layer .................................................................... 8

8 Practical method of measurement for ion transfer resistance and thickness .....................................9

9 Test report ................................................................................................................................................................................................................10

10 Application of the method of measurement .........................................................................................................................11

Annex A (informative) Example of a frequency range suitable for measuring the ion

transfer resistance ......... ..................................................................................................................................................................................12

Bibliography .............................................................................................................................................................................................................................13

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 22410:2020(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.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 22410:2020(E)
Introduction

Weathering steel containing Cu, Ni, P, etc., is widely used for bridges, buildings, towers and other

structures because of its maintenance-free characteristics resulting from the formation of a protective

and adhesive rust layer when exposed to the atmosphere.

This feature makes it possible for weathering steel to be used without any surface treatment. It

requires, however, several years of exposure to form the protective rust on untreated surfaces. During

this period, quite complicated reactions occur within the rust layer affected by local environmental

[1][2]

factors . Therefore, there is a strong need for a quantitative and non-destructive monitoring method

[3]

for the assessment of the protective rust layer condition on weathering steel structures .

© ISO 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 22410:2020(E)
Corrosion of metals and alloys — Electrochemical
measurement of ion transfer resistance to characterize the
protective rust layer on weathering steel
1 Scope

This document specifies a method for the electrochemical measurement of ion transfer resistance of

the rust layer formed on weathering steel alloys in order to assess their protective properties against

[3] [4][5][6]

corrosion thereafter . This method uses an electrochemical AC impedance measurement

[7][8]

together with harmonic analysis, to identify the ion transfer resistance, and a rust thickness

measurement to characterize the stability of the protective rust layer in terms of corrosion protection

under used 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 2178, Non-magnetic coatings on magnetic substrates — Measurement of coating thickness —

Magnetic method

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 following terms and definitions 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
ion transfer resistance

resistance against an ionic current migrated by the voltage difference through media

Note 1 to entry: In this document, “media” means the rust layer and electrolyte.
3.2
protective rust layer

layer formed on weathering steel (3.3) after long-time exposure to the atmosphere that enables

corrosion resistance
EXAMPLE Patina on Cu alloys.

Note 1 to entry: The protective rust layer usually consists of a double-layered structure comprising a dense

protective rust layer (a-Rust) underneath a crystalline FeOOH rust layer.
3.3
weathering steel

low-alloyed steel containing Cu, Ni, P, etc. that allows for a protective rust layer (3.2)

© ISO 2020 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO 22410:2020(E)
3.4
harmonic current
current with a frequency that is a multiple of the primary current

Note 1 to entry: A higher-order harmonic current means a nonlinear component of frequency current response

under regular frequency voltage to a system.
4 Corrosion resistance by weathering steel
4.1 Formation of rust

Generally, weathering steel exhibits a good corrosion resistance after long-time exposure to the

atmosphere, as shown in Figure 1. This is because it forms a protective rust layer adhered to the substrate

steel. However, the formation of the protective rust layer depends on the atmospheric environment and

the time elapsed. Sometimes, severe corrosion can occur because there is no protective rust layer, in

particular when chloride ion hinders the formation of such rust.
Key
X elapsed time, in years A initial rust
Y corrosion depth, in mm B protective rust
C unprotective rust
Figure 1 — General effect of atmospheric corrosion on weathering steel over time

The protective rust usually consists of a double-layered structure, as shown in Figure 2. It can be

seen whether or not the rust has grown by examining a cross-sectional view of the whole rust under a

microscope, as shown in Figure 2. The expected features of the corrosion resistant rust are:

a) an outer layer consisting mainly of crystalline FeOOH;

b) an inner layer consisting mainly of amorphous or very fine crystalline iron oxyhydroxides with an

enrichment of Cu, Ni, P, etc.

In initial rust and protective rust, the outer bright rust layer consists of crystalline FeOOH, whereas the

inner dark rust layer has amorphous rust (very fine crystal).
2 © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO 22410:2020(E)
Key
A initial rust 1 resin
B protective rust 2 FeOOH
C unprotective (anomalous) rust 3 a-Rust
4 metal (steel)
5 thick Fe O
3 4

NOTE 1 “a-Rust” means amorphous or very fine crystalline iron oxyhydroxides with an enrichment of Cu,

Ni, P, etc.
NOTE 2 “Anomalous rust” means anomalously grown rust.

Figure 2 — Classification of typical rust layers based on cross-sectional observations made

using a polarized light microscope
4.2 Typical types of rust by cross-sectional view

Based on the cross-sectional microscopic observations of numerous rust layers on weathering steels

exposed to various locations for various periods (see Figure 2 and Reference [3]), the rust layers can

be typically classified into three types: initial rust (unknown), protective rust and anomalous rust

(unprotective).

a) Initial rust: Islands of thin amorphous (very fine crystals) rust are visible at the steel-rust interface

under a crystalline rust layer. The total thickness is usually less than 400 μm. The rust layer can

be observed either at the initial stage of service or after long-term exposure to a mildly corrosive

environment. The corrosion loss of the steel is very small.

b) Protective rust: An amorphous or very fine crystalline rust layer covers the whole surface area

under the crystalline rust layer (FeOOH). The total thickness is usually between 40 μm and 400 μm.

This is the ideal protective rust layer for weathering steel.

c) Anomalous rust: FeOOH exists both at the external surface and along crack surfaces. The rest of

the rust consists of crystalline Fe O (usually confirmed by X-ray analysis). The total thickness is

3 4

more than 400 μm. The rust contains numerous cracks, sometimes bearing sea salt. It tends to

form when the wetting period is excessive and/or accumulated salt is present.
© ISO 2020 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO 22410:2020(E)

The initial rust or protective rust can be identified by observing the cut samples under a microscope,

although this type of characterization is destructive. The anomalous rust can be identified by observing

a thickness measurement of rust of > 400 μm.

A non-destructive monitoring method to identify the rust layer on weathering steel is to take

electrochemical measurements of the ion transfer resistance.
5 Measurement of ion transfer resistance
5.1 General

The protective properties of rust can be determined by measuring the ion transfer resistance through

the dense and adhesive rust layer. This technique consists of using electrochemical impedance together

with harmonic current measurements to identify the ion transfer resistance from the rust layer and

its thickness measurement. In this case, the current is linear to potential, meaning there is almost no

higher-order harmonic current. Many existing steel structures made of weathering steel under various

atmospheres and periods are measured using a double-probe type of portable monitoring instrument.

The thickness of the rust is measured, usually by using an electromagnetic device, and the gathered

data provide a criterion map by which to characterize the property of the rust layer formed on the

weathering steel, and therefore to determine whether the existing rust layer is effective as a corrosion

protective barrier.
5.2 Electrochemical impedance and harmonic current measurements

In order to obtain the ion transfer resistance as a measure of the protective properties of the rust

layer, the electrochemical impedance and harmonic current measurements are taken in a laboratory

on various kinds of rusted low-alloyed steels exposed to the actual atmosphere over a long period. A

conventional three-electrodes measurement is adopted with an Ag-AgCl reference electrode. For a

standard impedance measurement, the system consists of a potentiostat and a frequency response

analyser, which are controlled by a computer. An aqueous solution of 0,1 M Na SO opened to the air is

2 4

usually used as an electrolyte because the impedance parameters of the rust layer exhibit little change

when this solution is used. This solution generally does not cause degradation or changes in the rust

layer in a relatively short time. All the measurements are performed at room temperature (about 25 °C),

which simulates the measurement practice as used in the field.

A typical impedance and second and tertiary harmonic current response for a weathering steel covered

with protective rust is shown in Figure 3, where three distinct regions of frequency exist.

4 © ISO 2020 – All rights reserved
---------------------- Page: 9 ----------------------
ISO 22410:2020(E)
Key
X frequency, in log hertz (Hz) 1 impedance region 1
−6 2

Y harmonic current, in × 10 amperes per square centimetre (A/cm ) 2 impedance region 2

Z impedance, in log ohms per square centimetre (Ω/cm ) 3 impedance region 3
θ phase shift, in degree

NOTE n = 2 and n = 3 represent the second and the third harmonic currents, respectively.

Figure 3 — Typical impedance and the higher-order harmonic current spectra for a dense

protective rust layer on weathering steel (0,1 M Na SO solution)
2 4

Region 1 is the frequency region of less than approximately 10 Hz, where the phase shift deviates

negatively from zero and the impedance is dependent on the frequency to some extent. This is called

the “pseudo-capacitive impedance” since the slope of log Z/log f is not equal to −1, which would be

exhibited in an ideal capacitor. The second and third harmonic currents are also observed, thereby

indicating corrosion resistance resulting from charge transfer reactions. The higher order harmonic

current is observed only when the current-potential relationship is nonlinear, e.g. the current-potential

relationship depicted by the Butler-Volmer equation, on which the charge transfer reaction is based.

Pseudo-capacitance in this region, of which the electrical connection should be theore

...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 22410
ISO/TC 156
Corrosion of metals and alloys —
Secretariat: SAC
Electrochemical measurement of ion
Voting begins on:
2020­03­06 transfer resistance to characterize
the protective rust layer on
Voting terminates on:
2020­05-01
weathering steel
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 22410:2020(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 22410:2020(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: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/FDIS 22410:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Corrosion resistance by weathering steel .................................................................................................................................. 2

4.1 Formation of rust .................................................................................................................................................................................. 2

4.2 Typical types of rust by cross-sectional view ............................................................................................................... 3

5 Measurement of ion transfer resistance ...................................................................................................................................... 4

5.1 General ........................................................................................................................................................................................................... 4

5.2 Electrochemical impedance and harmonic current measurements ......................................................... 4

6 Measurements in the field .......................................................................................................................................................................... 6

7 Evaluation of the corrosion protective property of the rust layer .................................................................... 8

8 Practical method of measurement for ion transfer resistance and thickness .....................................9

9 Test report ................................................................................................................................................................................................................10

10 Application of the method of measurement .........................................................................................................................11

Annex A (informative) Example of a frequency range suitable for measuring the ion

transfer resistance ......... ..................................................................................................................................................................................12

Bibliography .............................................................................................................................................................................................................................13

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/FDIS 22410:2020(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.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/FDIS 22410:2020(E)
Introduction

Weathering steel containing Cu, Ni, P, etc., is widely used for bridges, buildings, towers and other

structures because of its maintenance­free characteristics resulting from the formation of a protective

and adhesive rust layer when exposed to the atmosphere.

This feature makes it possible for weathering steel to be used without any surface treatment. It

requires, however, several years of exposure to form the protective rust on untreated surfaces. During

this period, quite complicated reactions occur within the rust layer affected by local environmental

[1][2]

factors . Therefore, there is a strong need for a quantitative and non-destructive monitoring method

[3]

for the assessment of the protective rust layer condition on weathering steel structures .

© ISO 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 22410:2020(E)
Corrosion of metals and alloys — Electrochemical
measurement of ion transfer resistance to characterize the
protective rust layer on weathering steel
1 Scope

This document specifies a method for the electrochemical measurement of ion transfer resistance of

the rust layer formed on weathering steel alloys in order to assess their protective properties against

[3] [4][5][6]

corrosion thereafter . This method uses an electrochemical AC impedance measurement

[7][8]

together with harmonic analysis, to identify the ion transfer resistance, and a rust thickness

measurement to characterize the stability of the protective rust layer in terms of corrosion protection

under used 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 2178, Non-magnetic coatings on magnetic substrates — Measurement of coating thickness —

Magnetic method

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 following terms and definitions 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
ion transfer resistance

resistance against an ionic current migrated by the voltage difference through media

Note 1 to entry: In this document, “media” means the rust layer and electrolyte.
3.2
protective rust layer

layer formed on weathering steel (3.3) after long-time exposure to the atmosphere that enables

corrosion resistance
EXAMPLE Patina on Cu alloys.

Note 1 to entry: The protective rust layer usually consists of a double-layered structure comprising a dense

protective rust layer (a-Rust) underneath a crystalline FeOOH rust layer.
3.3
weathering steel

low-alloyed steel containing Cu, Ni, P, etc. that allows for a protective rust layer (3.2)

© ISO 2020 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/FDIS 22410:2020(E)
3.4
harmonic current
current with a frequency that is a multiple of the primary current

Note 1 to entry: A higher-order harmonic current means a nonlinear component of frequency current response

under regular frequency voltage to a system.
4 Corrosion resistance by weathering steel
4.1 Formation of rust

Generally, weathering steel exhibits a good corrosion resistance after long-time exposure to the

atmosphere, as shown in Figure 1. This is because it forms a protective rust layer adhered to the substrate

steel. However, the formation of the protective rust layer depends on the atmospheric environment and

the time elapsed. Sometimes, severe corrosion can occur because there is no protective rust layer, in

particular when chloride ion hinders the formation of such rust.
Key
X elapsed time, in years A initial rust
Y corrosion depth, in mm B protective rust
C unprotective rust
Figure 1 — General effect of atmospheric corrosion on weathering steel over time

The protective rust usually consists of a double-layered structure, as shown in Figure 2. It can be

seen whether or not the rust has grown by examining a cross-sectional view of the whole rust under a

microscope, as shown in Figure 2. The expected features of the corrosion resistant rust are:

a) an outer layer consisting mainly of crystalline FeOOH;

b) an inner layer consisting mainly of amorphous or very fine crystalline iron oxyhydroxides with an

enrichment of Cu, Ni, P, etc.

In initial rust and protective rust, the outer bright rust layer consists of crystalline FeOOH, whereas the

inner dark rust layer has amorphous rust (very fine crystal).
2 © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/FDIS 22410:2020(E)
Key
A initial rust 1 resin
B protective rust 2 FeOOH
C unprotective (anomalous) rust 3 a­Rust
4 metal (steel)
5 thick Fe O
3 4

NOTE 1 “a-Rust” means amorphous or very fine crystalline iron oxyhydroxides with an enrichment of Cu,

Ni, P, etc.
NOTE 2 “Anomalous rust” means anomalously grown rust.

Figure 2 — Classification of typical rust layers based on cross-sectional observations made

using a polarized light microscope
4.2 Typical types of rust by cross-sectional view

Based on the cross-sectional microscopic observations of numerous rust layers on weathering steels

exposed to various locations for various periods (see Figure 2 and Reference [3]), the rust layers can

be typically classified into three types: initial rust (unknown), protective rust and anomalous rust

(unprotective).

a) Initial rust: Islands of thin amorphous (very fine crystals) rust are visible at the steel-rust interface

under a crystalline rust layer. The total thickness is usually less than 400 μm. The rust layer can

be observed either at the initial stage of service or after long-term exposure to a mildly corrosive

environment. The corrosion loss of the steel is very small.

b) Protective rust: An amorphous or very fine crystalline rust layer covers the whole surface area

under the crystalline rust layer (FeOOH). The total thickness is usually between 40 μm and 400 μm.

This is the ideal protective rust layer for weathering steel.

c) Anomalous rust: FeOOH exists both at the external surface and along crack surfaces. The rest of

the rust consists of crystalline Fe O (usually confirmed by X-ray analysis). The total thickness is

3 4

more than 400 μm. The rust contains numerous cracks, sometimes bearing sea salt. It tends to

form when the wetting period is excessive and/or accumulated salt is present.
© ISO 2020 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO/FDIS 22410:2020(E)

The initial rust or protective rust can be identified by observing the cut samples under a microscope,

although this type of characterization is destructive. The anomalous rust can be identified by observing

a thickness measurement of rust of > 400 μm.

A non-destructive monitoring method to identify the rust layer on weathering steel is to take

electrochemical measurements of the ion transfer resistance.
5 Measurement of ion transfer resistance
5.1 General

The protective properties of rust can be determined by measuring the ion transfer resistance through

the dense and adhesive rust layer. This technique consists of using electrochemical impedance together

with harmonic current measurements to identify the ion transfer resistance from the rust layer and

its thickness measurement. In this case, the current is linear to potential, meaning there is almost no

higher-order harmonic current. Many existing steel structures made of weathering steel under various

atmospheres and periods are measured using a double-probe type of portable monitoring instrument.

The thickness of the rust is measured, usually by using an electromagnetic device, and the gathered

data provide a criterion map by which to characterize the property of the rust layer formed on the

weathering steel, and therefore to determine whether the existing rust layer is effective as a corrosion

protective barrier.
5.2 Electrochemical impedance and harmonic current measurements

In order to obtain the ion transfer resistance as a measure of the protective properties of the rust

layer, the electrochemical impedance and harmonic current measurements are taken in a laboratory

on various kinds of rusted low-alloyed steels exposed to the actual atmosphere over a long period. A

conventional three­electrodes measurement is adopted with an Ag­AgCl reference electrode. For a

standard impedance measurement, the system consists of a potentiostat and a frequency response

analyser, which are controlled by a computer. An aqueous solution of 0,1 M Na SO opened to the air is

2 4

usually used as an electrolyte because the impedance parameters of the rust layer exhibit little change

when this solution is used. This solution generally does not cause degradation or changes in the rust

layer in a relatively short time. All the measurements are performed at room temperature (about 25 °C),

which simulating the measurement practice as used in the field.

A typical impedance and second and tertiary harmonic current response for a weathering steel covered

with protective rust is shown in Figure 3, where three distinct regions of frequency exist.

4 © ISO 2020 – All rights reserved
---------------------- Page: 9 ----------------------
ISO/FDIS 22410:2020(E)
Key
X frequency, in log hertz (Hz) 1 impedance region 1
−6 2

Y harmonic current, in × 10 amperes per square centimetre (A/cm ) 2 impedance region 2

Z impedance, in log ohms per square centimetre (Ω/cm ) 3 impedance region 3
θ phase shift, in degree

NOTE n = 2 and n = 3 represent the second and the third harmonic currents, respectively.

Figure 3 — Typical impedance and the higher-order harmonic current spectra for a dense

protective rust layer on weathering steel (0,1 M Na SO solution)
2 4

Region 1 is the frequency region of less than approximately 10 Hz, where the phase shift deviates

negatively from zero and the impedance is dependent on the frequency to some extent. This is called

the “pseudo-capacitive impedance” since the slope of log Z/log f is not equal to −1, which would be

exhibited in an ideal capacitor. The second and third harmonic currents are also observed, thereby

indicating corrosion resistance resulting from charge transfer reactions. The higher order harmonic

current is observed only when the current-potential relationship
...

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