ISO 22410:2020

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
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ISO/FDIS 22410:2020(E)
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©
NATIONAL REGULATIONS. ISO 2020

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ISO/FDIS 22410:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2020
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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
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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
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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
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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
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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 .
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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)
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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).
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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.
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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.
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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
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
−1
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
...