ISO 26146:2012
(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
ISO 26146:2012 covers examination of samples that have previously been exposed to corrosive environments at high temperatures. ISO 26146:2012 covers the classification, identification, and thickness measurement of any corrosion layer forming during the exposure.
Corrosion des métaux et alliages — Méthode d'étude métallographique d'échantillons soumis à des environnements corrosifs à haute température
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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.
Reference marks are not usually made on test pieces exposed in the laboratory. However, for special
cases or other exposures, e.g. probes in plant, test pieces may contain reference positions 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 use of a reference mark and reference groove. These reference marks
and grooves vary with test piece geometry. Figure 2 shows the reference mark and groove for a block
sample; similar marks and grooves may be placed on other sample geometries.
4 © ISO 2012 – All rights reserved
Key
1 reference mark
2 reference groove
Figure 2 — Example of reference mark on block sample
6.2 Procedure
6.2.1 Examination prior to exposure
The dimensions of the uncorroded sample shall be measured prior to exposure. The dimensions of the
test piece shall be measured with a precision of ±0,02 mm by means of the measuring instruments
specified in ISO 3611 and ISO 13385-1. Measurements should be taken at a minimum of eight equidistant
positions across the intended section for the post-test measurements.
This forms the basis of the definition of the original metal surface.
6.2.2 Preparation of cross-sections
Dimensional changes of the test piece after exposure shall be determined by preparing a metallographic
cross-section. Particular care should be taken in preparation of this cross-section to ensure that the
test piece is mounted and polished parallel to the intended plane of the cross-section and all corrosion
products and deposits are retained. The procedure recommended is as follows.
In order to retain the corrosion product and deposit, the test piece shall be protected with an appropriate
coating. A deposited metallic coating or thermoplastic resin may be suitable depending on the system
to be investigated.
Specimen supports may be included in the mount to ensure that the section surface remains planar. The
artefacts described below may fulfil this purpose.
The test piece may be sectioned before or after mounting. Rod and tube test pieces should be sectioned to
a circular cross-section normal to the primary axis. Disc test pieces should be cut parallel to the primary
axis. Block test pieces should be cut parallel to the longest side. Sectioning details are shown in Figure 3.
If there is a requirement to reveal a cross-section at a specific plane relative to the surface by a reference
mark, the cut should be made at a sufficient distance from that plane to allow for removal of material
during grinding and polishing (see Figure 3).
Key
1 sectioning cutting
2 reference groove
Figure 3 — Sectioning procedure for test pieces with reference mark
In order to verify that the test piece has been mounted and polished parallel to the intended plane of the
cross-section, one or more reference artefacts of known dimensions should be included in the mount.
Flat test pieces (disc and square geometries) may be mounted together with a metal sheet of constant
and known thickness (±1 µm) clamped parallel to the test piece. Rod-shaped test pieces may use a
similar metal sheet of known thickness that has been bent through a known angle (90° is convenient)
and the test piece held in the corner. Alternatively, a series of spheres may be positioned around the test
piece in the mount. An illustration of the use of comparison artefacts to check the alignment of polishi
...
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