ISO 19691:2026

• Abstract:

  This document specifies a method for measuring the corrosion loss of each part of structures using unpainted steel such as weathering steel by using the attached coupons. This document also provides a prediction method for corrosion loss over a long period from the measurement results and a method for determining the applicability of existing and new constructing weathering steel structures.

• Status: Published
• Publication Date: 20-Oct-2026
• ICS: 77.060 - Corrosion of metals
• ICS: 91.080.13 - Steel structures
• Technical Committee: ISO/TC 156 - Corrosion of metals and alloys
• Drafting Committee: ISO/TC 156 - Corrosion of metals and alloys
• Current Stage: 6060 - International Standard published
• Start Date: 21-Oct-2026
• Completion Date: 21-Oct-2026

ISO 19691:2026 is an ISO standard on measuring corrosion loss in weathering steel structures by attaching coupons to the structure. It specifies a field exposure method, a way to calculate corrosion loss and predict long-term loss from the results, and a method for judging whether existing or newly planned weathering steel structures are suitable for the environment.

What does ISO 19691:2026 specify?

ISO 19691:2026 specifies how to measure corrosion loss at different parts of unpainted steel structures, including weathering steel, using attached coupons. It also covers how to turn those measurements into a long-term corrosion prediction and how to use the results when deciding whether a weathering steel structure is suitable.

The document is organized into scope, normative references, terms and definitions, object, exposure test method, evaluation methods, electrochemical measurement of rust-layer protectivity, and test report. Annex A gives the prediction method and the guidance for selecting coupon positions and judging suitability.

The method is intended for structures where corrosion varies with the local microclimate of each part, such as exposure to rain, orientation, and contamination. It is limited to general corrosion on uncoated steel surfaces, which matters because the coupon results are used to estimate metal loss rather than localized damage.

What are the key requirements of ISO 19691:2026?

Coupon preparation and form

ISO 19691:2026 requires coupons with a standard square shape, standard thickness, chamfered edges, and the same steel material as the structure being assessed. The coupon number is marked on the adhesive side, and the exposed faces are mechanically finished. In practice, this keeps the coupon mass measurement reliable and makes the result representative of the structure material.

Coupon attachment and safety

ISO 19691:2026 uses double-sided adhesive tape as the standard attachment method, with the surface cleaned before mounting and the tape larger than the coupon. If a longer exposure is used, the attachment should be checked at least yearly. The standard also allows a wire restraint for safety so a loose coupon does not fall and cause damage, provided the wire does not affect corrosion and is checked for degradation.

Where the coupons are placed

ISO 19691:2026 says the attachment location should represent the part of the structure being evaluated, including positions with typical exposure and positions with more severe corrosion stress. Annex A then distinguishes between partial evaluation, overall evaluation, and newly constructed structures. This matters because the selected position controls whether the result represents one critical area or the whole structure.

Exposure period and prediction basis

ISO 19691:2026 requires at least three exposure periods for corrosion-loss measurement, and it says the exposure test should run for more than 3 years for good regression accuracy. It also notes that 5, 8 and 10 years are other exposure periods, and that 3 years is the minimum for atmospheres in corrosivity category C5. In practice, the long-term prediction depends on having enough time points to fit a regression.

Recovery, cleaning and loss calculation

ISO 19691:2026 requires the recovered coupon to be documented by photograph, then cleaned by removing the tape, removing corrosion products, and measuring mass loss. The document points to ISO 8407 for rust removal and gives the calculation basis for corrosion rate from mass loss, area and exposure time. This is the core measurement step, because it turns the exposed coupon into a numerical corrosion loss.

Rust-layer characterisation and reporting

ISO 19691:2026 says electrochemical measurement of ion transfer resistance should be used to characterize the protective rust layer on weathering steel, with ISO 22410 as the referenced method. The test report must cover the structure, the test plan and timing, the measured results, and relevant environmental conditions such as rainfall, temperature, humidity and contamination deposition. That report set supports later engineering decisions and traceability.

What terms does ISO 19691:2026 define?

• Weathering steel - a low-maintenance steel widely used for large structures, which forms a protective and adhesive rust layer when exposed to the atmosphere.
• Coupon - the small steel test piece attached to the structure to measure corrosion loss at a specific location.
• Corrosion loss - the measured metal loss obtained from the exposure test after cleaning and weighing the coupon.
• Corrosion rate - corrosion loss expressed per unit area and time, used to compare different exposure periods.
• Corrosivity category - the atmospheric corrosion class used to describe how severe the environment is for the steel.
• Protective rust layer - the rust layer on weathering steel that helps reduce further corrosion.
• General corrosion - the corrosion form covered by this method, as opposed to localized attack.

Who uses ISO 19691:2026?

ISO 19691:2026 is used by corrosion engineers, bridge and structural engineers, asset managers, maintenance planners, and inspection teams working with weathering steel. It is relevant where owners need to decide where to place coupons, how long to expose them, how to interpret the results, and whether a structure can remain in service or be built in a given environment.

Which standards are used with ISO 19691:2026?

ISO 19691:2026 cites ISO 8407 for removal of corrosion products from test specimens. It cites ISO 9223 for atmospheric corrosivity classification and estimation, ISO 9224 for guiding values for corrosivity categories, and ISO 22410 for electrochemical measurement of ion transfer resistance on weathering steel.

What does the ISO 19691:2026 document contain?

ISO 19691:2026 includes figures for coupon shape, coupon attachment, a safety-wire arrangement, and example documentation of a recovered coupon. It also includes the calculation basis for corrosion rate in clause 6 and the reporting checklist in clause 8.

Annex A contains the long-term prediction method and the practical guidance for choosing coupon positions. It also covers how to estimate corrosion for newly constructed structures and how to judge suitability either from required performance or from the corrosion environment.

International
Standard
ISO 19691
First edition
Measurement method of corrosion
2026-10
for weathering steel structures
Méthode de mesure de la corrosion des structures en acier
autopatinable
Reference number
© ISO 2026
All rights reserved.
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Object . 1
5 Exposure test method . 1
5.1 Shape of coupon .1
5.2 Installation .2
5.3 Installation part .4
5.4 Exposure period .4
6 Evaluation methods . 4
6.1 General .4
6.2 Documentation of coupon appearance .4
6.3 Method of removing corrosion product on coupon surface .5
6.4 Calculation of corrosion loss .5
6.5 Prediction method.5
7 Electrochemical measurement of ion transfer resistance to characterize the protective
rust layer on weathering steel . 5
8 Test report . 5
Annex A (informative) Prediction method for long term corrosion loss and determining the
applicability of weathering steel structures . 7
Bibliography .10

iii
Foreword
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iv
Introduction
Weathering steels are environmentally friendly steels that are used in structures without painting. For
example, weathering steel is often used in bridges in mountainous areas in the world. In structural steels,
reduction of the cross section of members due to corrosion can cause deformation and buckling and thus
fracture. For this reason, it is important to understand the corrosion behaviour of each part of the structural
members.
Corrosion depends on the microclimate of each part of a member. For example, corrosion is affected by
whether the member is directly exposed to rainfall, whether its surface is horizontal or vertical, how much
de-icing salt is applied, etc.
In the outdoor exposure test using a normal rack, the corrosion behaviour of the individual parts of
complicated structures cannot be evaluated. Although evaluation is possible by using a simulation or the
actual structure, coring samples cannot be evaluated by weight loss. A method using a depth gauge and
ultrasonic waves is also possible but has low accuracy for thickness reduction.
To solve these problems, a unique exposure test method, in which small steel coupons are attached to the
target parts of members of the actual structure, has been established. This method makes it possible to
measure the corrosion loss on only one side of the members of the structure. It is useful for making decisions
regarding the timing of maintenance, or whether a new structure should be constructed at the site.
This document is useful and effective for the security of structures worldwide.

v
International Standard ISO 19691:2026(en)
Measurement method of corrosion for weathering steel
structures
1 Scope
This document specifies a method for measuring the corrosion loss of each part of structures using
unpainted steel such as weathering steel by using the attached coupons.
This document also provides a prediction method for corrosion loss over a long period from the measurement
results and a method for determining the applicability of existing and new constructing weathering steel
structures.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
weathering steel
low-maintenance steel widely used for large structures
Note 1 to entry: Weathering steel typically contains Cu and Ni. It is widely used for large structures like
bridges, buildings and towers.
Note 2 to entry: This type of steel is low maintenance because a protective and adhesive rust layer forms
when exposed to the atmosphere.
4 Object
For this test, the object of measurement of corrosion loss is large structures with specific microclimates
for either different elements or different surfaces or both. The material of the object structures is uncoated
steel, such as weathering steel, to which a corrosion protective covering is not applied and is limited to cases
that take the form of general corrosion.
5 Exposure test method
5.1 Shape of coupon
The coupon structure shall be as follows:
a) The standard shape of the coupon is square with a one-side length of 50 mm as shown in Figure 1.
However, square coupons with a one-side length of 40 mm to 150 mm may also be used, provided it is
possible to secure the accuracy of mass measurements.

b) The standard thickness of the coupon is 2 mm. Thicknesses in the range of 1,5 mm to 3 mm should be
used.
c) The edges of the coupon shall be chamfered.
d) The material of the coupon shall be the same as the steel material of the structure for which the
corrosion loss is being evaluated. To evaluate corrosion of each part of the structure, the test coupons
should be made of the same alloys of the structure material.
e) The coupon number shall be marked on the adhesive side of the coupon. Mechanical finishing of both
the front and back of the exposed side should be performed.
Key
1 test piece
2 chamfer
3 exposed side
4 back side
Figure 1 — Standard shape and specifications of test piece
5.2 Installation
The coupon shall be attached as follows:
a) The standard coupon attachment method is shown in Figure 2. Dust shall be removed from the surface
to which the coupon is to be attached, and the coupon shall be attached using double-sided adhesive
tape as the standard method.
b) The double-sided tape shall have sufficient adhesion and durability so that the coupon does not detach
during the exposure period. If an exposure period of 5 or more years is applicable, the panels should be
checked at least every year. Make use of a fall prevention aid, list item d).
c) The double-sided tape shall be set so that it is larger than the dimensions of the coupon. The tape should
protrude from the outer periphery of the coupon by approximately 1 mm.
d) Examples of the condition of coupon installation are shown in Figure 2 and Figure 3. As shown
in Figure 3, to prevent the test pieces from falling and damaging pedestrians, cars, etc., a method
is u
...