ISO 26146:2012

INTERNATIONAL ISO
STANDARD 26146
First edition
2012-12-15
Corrosion of metals and alloys —
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 2012
© ISO 2012
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ii © ISO 2012 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 3
4.1 Symbols . 3
4.2 Abbreviated terms used as subscripts . 3
5 Requirements . 4
6 Test method . 4
6.1 Test pieces . 4
6.2 Procedure . 5
6.3 Complementary techniques .10
6.4 Test report .10
Bibliography .12
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International
Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
ISO 26146 was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys.
iv © ISO 2012 – All rights reserved

INTERNATIONAL STANDARD ISO 26146:2012(E)
Corrosion of metals and alloys — Method for
metallographic examination of samples after exposure to
high-temperature corrosive environments
1 Scope
This International Standard covers examination of samples that have previously been exposed to
corrosive environments at high temperatures.
This International Standard covers the classification, identification, and thickness measurement of any
corrosion layer forming during the exposure.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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: Micrometers for
external measurements — Design and metrological characteristics
ISO 13385-1, Geometrical product specifications (GPS) — Dimensional measuring equipment — Part 1:
Callipers; Design and metrological characteristics
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
NOTE Figure 1 provides a diagrammatic representation of appropriate parameters.
3.1
original metal surface
surface of the metallic sample before coating or exposure to the corrosive environment
3.2
original coating surface
surface of the coating after final processing treatment but before exposure to the corrosive environment
3.3
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
3.4
deposit
chemically active liquid or solid species 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.
3.5
deposit thickness
distance, normal to the test piece surface, between the outer extent of corrosion product and the surface
of the deposit layer
3.6
coating
layer of material applied to a metal surface to provide protection
3.7
coating thickness
distance, normal to the test piece surface, between the original metal surface and the surface of the coating
3.8
outward growing corrosion scale
corrosion scale that grows outwards from the original metal or coating surface
3.9
inward growing corrosion scale
corrosion scale that grows inwards from the original metal or coating surface
3.10
external scale
total of outward and inward growing continuous corrosion scales
3.11
internal corrosion
corrosion products that form beneath any external scale
Note 1 to entry: These usually appear as discrete particles.
3.12
grain boundary corrosion
corrosion product that grows along metal grain boundaries as a particular form of internal corrosion
3.13
de-alloyed zone
zone beneath the corrosion scale(s) that exhibits a decrease in the concentration of scale-forming
alloy elements
Note 1 to entry: This may be manifested as the dissolution of precipitates originally present in the microstructure.
3.14
metal loss
distance between the original test piece surface and the boundary with unaffected alloy
3.15
remaining sound metal
section of metal that has not been affected by corrosion
2 © ISO 2012 – All rights reserved

a) Uncoated b) Coated
Key
1 substrate
2 de-alloyed zone
3 internal corrosion
4 external scale
5 deposit
6 interdiffusion zone
7 coating
Figure 1 — Identification of individual layers to be measured in exposed test pieces
4 Symbols and abbreviated terms
4.1 Symbols
x thickness of individual layer
t metal thickness
4.2 Abbreviated terms used as subscripts
The identification of individual layers is through the use of the following subscripts:
0 initial
og outward growing part of the external corrosion scale
ig inward growing part of the external corrosion scale
tot total thickness of external scale (x = x + x )
tot og ig
in internal corrosion
gb grain boundary corrosion
d deposit
c coating
da de-alloyed
ml metal loss compared to original dimensions
rm remaining sound metal
5 Requirements
The minimum requirement for measurement accuracy is to achieve an uncertainty at the 95 % confidence
limit of ±5 µm or 5 % of the measured material loss, whichever is the lesser for all errors, i.e. calibration,
misalignment, both in the vertical and horizontal directions, and measurement.
Measurements shall be carried out under an optical microscope with an X-Y moving stage or with
graticule and cross wires. The precision of the measurement system shall be to a precision of ±1 µm.
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 the 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 preferably be better than ±0,05 mm; however, on larger test pieces, this
requirement may be relaxed.
Refere
...