ISO 18971:2026

International
Standard
ISO 18971
First edition
Corrosion of metals and alloys —
2026-01
Monitoring method for corrosion
states of stainless steel in industrial
cooling water
Corrosion des métaux et alliages — Méthode de surveillance
de l'état de corrosion de l'acier inoxydable dans les eaux de
refroidissement industrielles
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Monitoring principle . . 2
5 Monitoring system . 4
5.1 General .4
5.2 Monitoring probes (sensors) .5
5.3 Detecting instruments .6
5.4 Selection and setting of on-site corrosion monitoring points .6
5.5 Calibration of corrosion monitoring system .7
5.5.1 Calibration cycle .7
5.5.2 Calibration of detecting instruments .7
6 Methods and procedures of corrosion monitoring . 7
6.1 Methods of polarization resistance measurement .7
6.1.1 Sine wave excitation .7
6.1.2 Triangular wave excitation .7
6.2 Corrosion potential measurement .7
6.3 Monitoring and evaluation of corrosion states of stainless steel.7
6.3.1 Monitoring procedures .7
6.3.2 Corrosion states evaluation of stainless steel .8
7 Data acquisition and processing . 8
7.1 Data sampling cycle .8
7.2 Data records .8
7.2.1 Records of the electrochemical data .8
7.2.2 Records of the environmental data .9
7.3 Data analysis and processing .9
8 Management and operation attentions of the monitoring system . 9
8.1 Management of monitoring system .9
8.2 Operation attentions .9
9 Monitoring report . 10
9.1 General .10
9.2 Basic information of monitoring .10
9.3 Information of the monitoring system .10
9.4 Monitoring results and analysis .10
Annex A (informative) Corrosiveness monitoring and evaluation of industrial cooling water .11
Annex B (informative) Case study: Monitoring the corrosion of stainless steel .12
Bibliography . 14

iii
Foreword
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iv
Introduction
Cooling water is widely used in industrial production. In the industrial cooling water system, the use of
stainless steel tubes in heat exchangers is very common. It generally accounts for over 60 %, but some
countries have accounted for over 90 %. The corrosion of stainless steel in industrial cooling water is mainly
localized corrosion dominated by pitting, which is invisible, fast-acting and difficult to prevent. When
localized corrosion causes perforation in stainless steel, this can have disastrous consequences.
The polarization resistance method is widely used to monitor the uniform corrosion rate of metals. This
document describes the method of using polarization resistance and corrosion potential to monitor
the corrosion states of stainless steel in industrial cooling water. It can be used to monitor the corrosion
tendency of stainless steel. It can also be used to indicate the change of corrosiveness of cooling water (see
Annex A). Protective measures can be taken in timely manner to prevent the occurrence of accidents caused
by corrosion perforation of stainless steel tubes.

v
International Standard ISO 18971:2026(en)
Corrosion of metals and alloys — Monitoring method for
corrosion states of stainless steel in industrial cooling water
1 Scope
This document specifies the method and criteria for monitoring corrosion states of heat exchange tubes, heat
exchange plates and other components made of stainless steel in industrial cooling water by combination of
polarization resistance and corrosion potential.
This document is applicable to the monitoring of the corrosion states of stainless steel in industrial cooling
water or similar media, as well as the monitoring of the corrosiveness of industrial cooling water. The cooling
water is generally taken from natural water, tap water and reclaimed water. It is applicable to industrial
cooling water with pH value of 5,0 to 11,0, temperature of 0 °C to 60 °C, and conductivity higher than 20 μS/
cm.
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 8044, Corrosion of metals and alloys — Vocabulary
ISO 15510, Stainless steels — Chemical composition
ASTM G59 -97: 2020, Standard test method for conducting potentiodynamic polarization resistance
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8044, ISO 15510 and the following
apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
cooling water
water which is used to absorb and remove heat
[SOURCE: ISO 6107:2021, 3.140]
3.2
natural water
water that exists in nature, including surface water (e.g. water from rivers, lakes and seas) and groundwater
3.3
reclaimed water
wastewater that has been treated to meet specific water quality for intended beneficial use
[SOURCE: ISO 20670:2023, 3.81]

4 Monitoring principle
In general, when a stainless-steel electrode is anodized in industrial cooling water, it has the anodic
polarization curve A1 as shown in Figure 1. The potential range of E to E is the active dissolution region
eM p
(Z1), where the stainless-steel surface is in a corrosion state. The potential range of E to E is the pre-
p pp
passivation region (Z2), where the stainless-steel surface is in an unstable state. The potential range of E
pp
to E is the stable passivation region (Z3), where the stainless-steel surface is in a passivation state. The
tp
potential range higher than E is the transpassivation region (Z4), where the stainless-steel surface is also
tp
in a corrosion state (excluding those generated by new anodic reactions, e.g. water oxidation). When the
polarization current increases sharply at a low potential (E ), the potential range of the passivation region
b
(Z3) of the polarization curve is shortened to E to E , and localized corrosion such as pitting can occur on
pp b
the surface of the stainless steel when the potential is higher than E (Z5) (see ISO 11463 and ISO 15158).
b
Key
i current density
E electrode potential
A1 anodic polarization curve of stainless steel electrode in cooling water
A2 anodic polarization curve of stainless steel electrode in aggressive cooling water
C1 cathodic polarization curve of depolarizer reduction in cooling water
C2 cathodic polarization curve in cooling water with weak oxidation
C3 cathodic polarization curve in cooling water with too strong oxidation
E equilibrium potential of metal electrode in cooling water
eM
E equilibrium potential of cathode in cooling water
eC1
E equilibrium potential of cathode in cooling water with weak oxidation
eC2
E equilibrium potential of cathode in cooling water with too strong oxidation
eC3
E passivation potential
p
E the lowest potential of the passivation region
pp
E transpassivation potential
tp
E pitting potential
b
E corrosion potential of stainless steel in a passive state
corr1
E corrosion potentials of stainless steel in an active state
corr2
E corrosion potentials of stainless steel in a pitting state
corr3
E corrosion potentials of stainless steel in a transpassive state
corr4
Z1 active dissolution zone
Z2 pre-passivation zone
Z3 passivation zone
Z4 transpassivation zone
Z5 pitting zone
S1 passivation state
S2 active state
S3 pitting state
S4 transpassivation state
Figure 1 — Schematic diagram of different corrosion states of stainless steel
The corrosion state of stainless steel in industrial cooling water is related to the anodic polarization curve
of the stainless-steel electrode. It is also related to the cathodic polarization curve that corresponds to
the reduction reaction of depolarizers such as dissolved oxygen in the cooling water. The intersection of
the anodic and the cathodic polarization curves determines the corrosion potential and corrosion current
density of the corrosion system composed of these two reactions (see Figure 1). Changes in the anodic and

cathodic polarization curves, and thus in the corrosion state of stainless steel, occur due to changes in the
cooling water, such as:
— the concentration of dissolved oxygen, additives and aggressive substances;
— the pH value;
— the temperature.
The surface of stainless steel in industrial cooling water is typically in a passive state with high polarization
resistance and the corrosion potential is in the passivation region (see S1 in Figure 1).
When the corrosiveness of the cooling water is enhanced (e.g. when the concentration of aggressive chloride
ions or reducing substances increases or the cooling water gets warmer), the passive film on the surface
of stainless steel can be damaged. This results in a sharp drop in polarization resistance. Under these
conditions, the surface of stainless steel will be in the following three corrosion states:
a) When the oxygen concentration of the cooling water is reduced or the water is contaminated by reducing
substances, the corrosion potential shifts negatively to E which is lower than the passivation
corr2
potential E , and the stainless steel surface can be in an active state (see S2 in Figure 1).
p
b) When the concentration of aggressive ions such as chloride ions increases in the cooling water, the
passive film breakdown potential (pitting potential) E of the stainless steel shifts negatively and the
b
range of the passivation region is reduced. The stainless steel will be in the pitting state when the pitting
potential E is lower than the corrosion potential E (see S3 in Figure 1).
b corr3
c) When the oxidability of the cooling water is enhanced, the corrosion potential of the stainless steel
shifts positively. If the E appears on the anode polarization curve and the corrosion potential is higher
b
than E , the stainless steel is in a state of pitting. If the transpassivation potential E appears on the
b tp
anode polarization curve and the corrosion potential is higher than E , the stainless steel will be in an
tp
unstable transpassive state (see S4 in Figure 1).
The four states of stainless steel in cooling water show different polarization resistance and corrosion
potential. The stainless steel in State 1 has a large polarization resistance, while the polarization resistances
of the stainless steel in State 2, State 3 and State 4 are all small. However, the corrosion potentials
corresponding to the latter three states are significantly different. Therefore, the corrosion states of
stainless steel can be judged by monitoring the polarization resistance and corrosion potential. In most
cases, the state of stainless steel in cooling water is State 1 or State 3.
5 Monitoring system
5.1 General
The corrosion monitoring system, shown in Figure 2, includes mon
...