ISO 26146:2025
(Main)Corrosion of metals and alloys — Method for metallographic examination of samples after exposure to high-temperature corrosive environments
Corrosion of metals and alloys — Method for metallographic examination of samples after exposure to high-temperature corrosive environments
This document specifies the examination of samples that have been exposed to corrosive environments at high temperatures. This document specifies the classification, identification and thickness measurement of the corrosion layer that forms during exposure to corrosive environments at high temperatures.
Corrosion des métaux et alliages — Méthode d'étude métallographique d'échantillons soumis à des environnements corrosifs à haute température
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Standards Content (Sample)
International
Standard
ISO 26146
Second edition
Corrosion of metals and alloys —
2025-06
Method for metallographic
examination of samples after
exposure to high-temperature
corrosive environments
Corrosion des métaux et alliages — Méthode d'étude
métallographique d'échantillons soumis à des environnements
corrosifs à haute température
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 2
4.1 Symbols .2
4.2 Abbreviated terms used as subscripts .2
5 Requirements . 3
6 Test method . 3
6.1 Test pieces . .3
6.2 Procedure .4
6.2.1 Examination prior to exposure .4
6.2.2 Preparation of cross-sections .4
6.2.3 Classification of corrosion layers .6
6.2.4 Identification of corrosion layers .7
6.2.5 Thickness measurement of corrosion layers .7
6.3 Complementary techniques .9
7 Test report .10
iii
Foreword
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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 document should be noted. This document was drafted in accordance with the editorial rules of the
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This document was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys.
This second edition cancels and replaces the first edition (ISO 26146:2012), which has been technically
revised.
The main changes are as follows:
— the term "defect" has been added to Clause 3;
— the use of a digital microscope has been added to Clause 5;
— 6.2.2 has been expanded to include the processes for mounting, grinding and polishing;
— 6.2.5.3, on the evaluation of the defects of corrosion layers, has been added.
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
Introduction
This document specifies a method for examining samples that have been exposed to corrosive conditions
at high temperatures in most industrial environments. However, there are several technical improvements
that are added for the following reasons:
— Analysing the defects in the corrosion layer after exposure to high temperatures is an important part of
metallographic examination.
— Image technology has developed rapidly, which means digital microscope and image analysis technologies
can be applied when inspecting corrosion in metals at high temperatures.
— More detailed and easy-to-implement procedures are necessary to facilitate testing.
v
International Standard ISO 26146:2025(en)
Corrosion of metals and alloys — Method for metallographic
examination of samples after exposure to high-temperature
corrosive environments
1 Scope
This document specifies the examination of samples that have been exposed to corrosive environments at
high temperatures.
This document specifies the classification, identification and thickness measurement of the corrosion layer
that forms during exposure to corrosive environments at high temperatures.
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 3611, Geometrical product specifications (GPS) — Dimensional measuring equipment — Design and
metrological characteristics of micrometers for external measurements
ISO 8044, Corrosion of metals and alloys — Vocabulary
ISO 13385-1, Geometrical product specifications (GPS) — Dimensional measuring equipment — Part 1: Design
and metrological characteristics of callipers
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8044 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
original metal surface
superficial plane of sample to be tested before coating or exposure to the corrosive environment
Note 1 to entry: See 6.2.1.
3.2
interdiffusion zone
region around the original interface between coating and substrate that, during exposure, has changed
composition through diffusion processes between the coating and the substrate
3.3
deposit
chemically active liquid or solid substances that are placed in contact with or are deposited on the test piece
before and/or during exposure
EXAMPLE Salts, fly ashes, chars and molten metals.
3.4
outward growing corrosion scale
corrosion product that develops externally from the original metal or coating surface
3.5
inward growing corrosion scale
corrosion product that develops internally from the original metal or coating surface
3.6
external scale
total of outward and inward growing continuous corrosion scales
3.7
internal corrosion
corrosion products that form beneath any external scale
Note 1 to entry: Internal corrosion usually appears as discrete particles.
3.8
grain boundary corrosion
corrosion product that grows along boundaries between metal grains as a form of internal corrosion
3.9
de-alloyed zone
region beneath the corrosion scale(s) that exhibits a decrease in the concentration of scale-forming alloy
elements
Note 1 to entry: This can be manifested as the dissolution of precipitates originally present in the microstructure.
3.10
metal loss
distance between the original test piece surface and the boundary with unaffected alloy
3.11
remaining unaffected metal
uncorroded metallic parts of sample
3.12
defects
voids or cracks between or within corrosion layers, which can cause the corrosive medium to penetrate
easily or peel off
4 Symbols and abbreviated terms
4.1 Symbols
x thickness of individual layer
t substrate thickness
4.2 Abbreviated terms used as subscripts
The individual layers of x are identified by the following subscripts:
0 original metal surface
ml metal loss compared to original dimensions (metal loss)
rm remaining unaffected metal
5 Requirements
The measurement accuracy shall have an uncertainty at least at the 95 % confidence limit of ±5 µm or 5 %
of the measured material loss, whichever is the lesser for all errors, i.e. in terms of calibration, misalignment
(both in the vertical and horizontal directions) and measurement.
Measurements shall be carried out under a metallographic optical microscope with an X-Y moving stage,
and, preferably, a set of standard magnifications, e.g. ×100, ×400, and ×1 000. These vary depending upon the
extent of the attack. A digital microscope with an automatic X-Y moving stage is preferred. The measurement
system shall be to a precision of ±1 µm.
Image analysis software is an option to perform, for example, image stitching, measurement and analysis.
The measurement system shall be fully calibrated to include orthogonality, traceable to certified length
standards at intervals no greater than 12 months. The system shall be checked against secondary standards
at the beginning and end of each series of measurements.
6 Test method
6.1 Test pieces
The size(s) and shape(s) of the test pieces adopted are governed by the type and form of material received
from the various suppliers, e.g. wrought products (bar, rod, plate, strip) or cast products (sticks of varying
shapes).
Three basic forms of test piece are considered appropriate, i.e. rod, disc or block. These simple geometries
are easier to measure and hence less prone to errors.
Machine tolerances should be higher than ±0,05 mm. However, on larger test pieces, this can be relaxed.
Reference marks are not usually made on test pieces exposed to corrosion conditions at high temperatures in
the laboratory. However, for special cases or other exposures (e.g. probes in a plant), test pieces can contain
reference positions that are sufficient to identify a specific point within the measurement plane, both pre-
and post-exposure. The reference marks serve as a datum, thus ensuring that repeat measurements are
made at identical positions on the test piece. This can be achieved by u
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