ISO 4680:2022

INTERNATIONAL ISO
STANDARD 4680
First edition
2022-12
Corrosion of metals and alloys —
Uniaxial constant-load test method
for evaluating susceptibility of
metals and alloys to stress corrosion
cracking in high-purity water at high
temperatures
Corrosion des métaux et alliages — Méthode d'essai sous charge
uniaxiale pour l'évaluation de la sensibilité des métaux et des alliages
à la fissuration par corrosion sous contrainte dans l'eau de haute
pureté à hautes températures
Reference number
ISO 4680:2022(E)
© ISO 2022

---------------------- Page: 1 ----------------------
ISO 4680:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
  © ISO 2022 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 4680:2022(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative reference .1
3 Terms and definitions . 1
4 Principle . 2
5 Specimens. 2
5.1 General . 2
5.1.1 General requirements . 2
5.1.2 Categories and shapes of specimens. 2
5.1.3 Shapes and dimensions of grips/clevises . 2
5.1.4 Dimensions of specimen shoulder and radius . 2
5.1.5 Machined notch . 3
5.1.6 Dimensions of gauge section. 3
5.1.7 Serial loading of multiple specimens. 3
5.2 Preparation of specimens . 4
5.2.1 Orientation of specimen sampling . 4
5.2.2 Methods of specimen sampling . 4
5.2.3 Surface finishing . 5
5.2.4 Mechanical grinding and polishing. 5
5.2.5 Electrolytic or chemical polishing . 5
5.2.6 Marking . . 5
6 Test apparatus .7
6.1 Test set-up . 7
6.2 Loading mechanism . 7
6.3 Autoclave . 8
6.4 Circulating loop . . . 8
7 Experimental procedure .9
7.1 Test environment . 9
7.1.1 General . 9
7.1.2 Corrosion potential . 10
7.1.3 Measurement parameters . 10
7.2 Specimen number and applied stress . 10
7.3 Testing procedures . 10
7.3.1 General . 10
7.3.2 Test solution adjustment . 11
7.3.3 Temperature control . 11
7.3.4 Loading . 11
7.3.5 Unloading . 11
7.3.6 Completion of test. 11
7.4 Addition of crevice . 11
7.4.1 General . 11
7.4.2 Materials of crevice formation . 11
7.4.3 Procedure of crevice addition .12
7.5 Electrical resistance measurement methods . .12
7.5.1 General .12
7.5.2 Measurement requirements. 13
8 Assessment of results .13
8.1 Time to failure. 13
8.2 Validity of failure position . 13
8.3 Other evaluation indexes . .13
8.4 Observation of crack morphology . 13
8.5 Detection of crack initiation using electrical resistance measurement methods . 14
iii
© ISO 2022 – All rights reserved

---------------------- Page: 3 ----------------------
ISO 4680:2022(E)
9 Test report .14
Annex A (informative) A comparison of specimen orientations in standards .15
Annex B (normative) Measuring items of test solution .17
Annex C (informative) Water chemistry conditions and measurement items for simulated
BWR and PWR primary water environment tests .18
Annex D (informative) Procedures to interrupt and resume testing .20
Annex E (informative) Electrical resistance measurement methods .22
Annex F (informative) Test report items .25
Bibliography .27
iv
  © ISO 2022 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 4680:2022(E)
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.
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 documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys.
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
v
© ISO 2022 – All rights reserved

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 4680:2022(E)
Corrosion of metals and alloys — Uniaxial constant-load
test method for evaluating susceptibility of metals and
alloys to stress corrosion cracking in high-purity water at
high temperatures
WARNING — This document can involve hazardous materials, operations and equipment. It is
the responsibility of the user of this document to consult and establish appropriate safety and
health practices and to determine the applicability of regulatory limitations prior to use.
1 Scope
The document specifies a method for undertaking uniaxial constant load testing of the susceptibility
of a metal, or an alloy, to stress corrosion cracking (SCC) in high-purity water environments at high
temperature (above the boiling point of water at normal pressures) and pressure. The test method is
particularly applicable to simulated primary water environments of light water reactors (LWRs).
The test method enables assessment of the relative resistance to SCC of a material in different
environments and the comparative resistance of different materials (using the same environment,
specimen dimensions and loading).
The terms “metal” and “alloy”, as used in the document, include weld metals and weld heat affected
zones.
2 Normative reference
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 7539-1, Corrosion of metals and alloys — Stress corrosion testing — Part 1: General guidance on testing
procedures
ISO 7539-4, Corrosion of metals and alloys — Stress corrosion testing — Part 4: Preparation and use of
uniaxially loaded tension specimens
ISO 8044, Corrosion of metals and alloys — Vocabulary
ISO 3785, Metallic materials — Designation of test specimen axes in relation to product texture
ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7539-1, ISO 8044, ISO 3785
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/
1
© ISO 2022 – All rights reserved

---------------------- Page: 6 ----------------------
ISO 4680:2022(E)
3.1
test time
period between the start and the end of a test, with the criterion for the end being the initiation or
failure of all test pieces, or the passage of an agreed test duration.
Note 1 to entry: The start of a test is when a specimen(s) is first exposed to the specified water chemistry,
temperature and load.
4 Principle
The test consists of subjecting a specimen to uniaxial constant load in a specified and well-controlled
environment for a set time period. The susceptibility to SCC is evaluated based on the time to initiation
(for tests with in-situ monitoring), the time to failure, the extent of cracking (especially if no failure
occurs) and/or the test time (when no cracking is observed). Because the test times can rarely be
as long as the desired component lifetime, the test is carried out under accelerated test conditions
of temperature, or test solution. The accelerating factor introduced shall not induce a change in
crack initiation mechanism; the crack morphology shall be the same as expected or observed in the
engineering application. The accelerating factor adopted should be appropriate for the intended
application to evaluate the SCC susceptibility, but the accelerated nature of the laboratory initiation test
can produce a different quantitative effect of materials, environments or stresses. For example, for a
simulated boiling water reactor (BWR) water environment, the test may be accelerated by the addition
of sodium sulfate (Na SO ) or by the use of a crevice. On the other hand, for a simulated pressurized
2 4
water reactor (PWR) primary water environment, the test may be accelerated by an increase in test
temperature.
5 Specimens
5.1 General
5.1.1 General requirements
Shapes and dimensions of specimens shall be designed so as to ensure that SCC initiates in a gauge
section or at a machined notch. Since SCC susceptibility is affected by microstructure and surface
finishing, specimen orientation shall be categorized and deformation shall be minimised during
specimen preparation.
5.1.2 Categories and shapes of specimens
The specimen type and geometry shall be based on existing test standards such as ISO 7539-4 or
ISO 6892-1. Specimen types such as rod-shaped or plate-shaped tensile test specimens are recommended.
Tubular or annular specimens may be tested, if appropriate.
5.1.3 Shapes and dimensions of grips/clevises
Grips/clevises shall match the linkage of the testing machine, which is often a threaded connection. For
tensile specimens, load is often applied using pin or threaded connections.
When using pin loading, failure can occur at the pin hole, particularly in high strength materials, and
the specimen shall be designed with this in mind. In general, the ligament surrounding the pin hole
should be about twice the area of the gauge section, and the pin hole diameter should be at least as large
as the thickest dimension of the gauge section.
5.1.4 Dimensions of specimen shoulder and radius
Failure can also occur by stress concentration in the radius transition from the gauge section to the
grip section of tensile specimens, and a transition radius that is substantially larger than the gauge
section is recommended. Undercut at the final transition to the gauge section shall also be avoided. The
2
  © ISO 2022 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 4680:2022(E)
shapes in ISO 6892-1 provide excellent examples, but the underlying criterion is that failure should not
occur in the radius nor predominantly in the adjacent gauge section. The following recommendations
provide guidance for tensile loaded specimens.
a) For rod-shaped specimens, the radius of the shoulders (r) should be at least twice the diameter (d)
of the parallel section.
b) For plate-shaped specimens, the radius of the shoulders (r) should be equal to or greater than the
width of the parallel section (w).
5.1.5 Machined notch
Specimens with notches created mechanically in the parallel section may be used. The stress
concentration and multiaxial stress create a higher stress than the nominal stress calculated using the
minimum cross-sectional area of the bottom of the notch.
If the bottom of the notch is in the elastic region, the stress at the bottom of the notch should be
evaluated by multiplying the nominal stress by the stress concentration factor K for the shape of the
t
notch.
If the bottom of the notch is the plastic region, the stress distribution at the bottom of the notch changes
due to plastic deformation, so it is desirable to evaluate the stress using elasto-plastic analysis such as
finite element analysis.
5.1.6 Dimensions of gauge section
Dimension tolerances for preparing the gauge section of the specimen shall conform to ISO 6892-1. The
diameter, thickness and width of the machined gauge section are usually constant, and shall be uniform
over the entire length of the gauge section, specified in ISO 6892-1.
However, the gauge section of the specimen may be tapered towards the centre within tolerances
specified in ISO 6892-1. By tapering the gauge section of the specimen, it is possible to increase the
number of cracks at effective positions in the specimen’s gauge section and to reduce breakages at the
shoulder sections of the specimen.
Specimens having tapered gauge lengths may be employed for the purpose of obtaining a range of
initial stresses.
When a specimen shape other than those specified in ISO 6892-1 is necessary, the specimen shape
should ideally have a gauge length of 10 mm or more and a width of 3 mm or more in the parallel section.
With agreement among the parties involved in the testing, specimens that are smaller than those
described above may be used. Without careful consideration, it is possible that the results of the SCC
test can be greatly influenced by the cross-sectional area of the specimen and the area exposed to the
environment.
Caution has to be exercised in the case of miniature specimens, as machining becomes difficult when
the cross-sectional area of the specimen decreases, and elevated susceptibility to stress concentration
can result from pitting, general corrosion, bending, etc. The net section stress also rises rapidly as
cracks form. When using miniature specimens, it is recommended that the diameter and thickness of
the gauge section of the specimen be at least 6 times the grain size.
In general, larger specimens are preferred in terms of the reproducibility and relevance of the test
results.
5.1.7 Serial loading of multiple specimens
The constant load test may be performed by connecting multiple specimens in series. In this case, a
load-catch jig designed to prevent unloading after fracture of one specimen shall be used, or the time
3
© ISO 2022 – All rights reserved

---------------------- Page: 8 ----------------------
ISO 4680:2022(E)
of unloading be detected. Load-catch mechanisms need to consider whether shock re-loading to the
remaining specimens can occur after the failure of one specimen.
5.2 Preparation of specimens
5.2.1 Orientation of specimen sampling
The orientation of the specimen should account for the orientation of cracks of concern in the application
of interest. Often, specimens are machined so that the longitudinal direction of specimens matches
the rolling direction. The sampling direction and location of the specimens should be determined by
agreement among the parties involved in the testing.
The test specimen axes in relation to product texture should be designated by an X-Y-Z orthogonal
coordinate system specified in ISO 3785. The letter X denotes the direction of principal deformation
(maximum grain flow in the product). The letter Y denotes the direction of least deformation. The
letter Z denotes the direction normal to the X-Y plane. The anticipated direction of crack extension for
notched specimens should be designated using a hyphenated code wherein the letter(s) preceding the
hyphen represent the longitudinal direction of the specimens and the letter(s) following the hyphen
represent the anticipated direction of crack extension. For unnotched specimens, surfaces normal
to the anticipated direction of crack development are specified in plate-shaped specimens and are
not specified in rod-shaped specimens. The specified surface in plate-shaped specimens should
be designated using a hyphenated code wherein the letter(s) preceding the hyphen represent the
longitudinal direction of the specimens and the letter(s) following the hyphen represent the direction
normal to the surface.
The orientation of specimens sampled from test products is designed according to ISO 3785. The
orientations of unnotched plate-shaped specimens for sheet, plate, rectangular bar, cylinder and tube,
are shown in Figure 1. When the specimen direction is aligned with the product's characteristic grain-
flow directions, a single letter for each case is used to denote the direction perpendicular to the crack
plane and the direction of intended crack extension, as shown in Figures 1 a), b) and c). When the
specimen orientation directions lie midway between the product's characteristic grain-flow directions,
two letters shall be used to denote the normal to the crack plane or the crack propagation direction, as
shown in Figure 1 d).
Designations by an L-T-S orthogonal coordinate system for rectangular sections and by an L-R-C
[1]
orthogonal coordinate system for cylindrical sections specified in ASTM E399-20a are described
together in Figure 1. The designations of specimen sampling orientations are compared to ISO 3785,
[1] [2]
ASTM E399-20a and ASTM A370-20 in Annex A.
[1] [2]
NOTE For rectangular sections, the definition of orientations in ASTM E399-20a and ASTM A370-20
corresponds to that in ISO. For cylindrical sections, the definition of orientations in ASTM does not always
correspond to that in ISO because that in the ASTM standards is a geometry-based system.
When there is no grain-flow direction as in a casting, specimen location and crack plane orientation
shall be defined on a part drawing and the test result shall carry no orientation designation.
For plate-shaped specimens, the most relevant orientation is generally when the test specimen is
parallel to the rolling surface of the plate.
In the case of a use of cold or warm rolling to accelerate tests for material comparisons, the orientation
of the specimen for the rolling direction also should account for the orientation of cracks.
5.2.2 Methods of specimen sampling
Specimens can be extracted by sawing, electrical discharge machining, cutting, or grinding, provided
that damage to the material is minimised and constrained to the surface.
4
  © ISO 2022 – All rights reserved

---------------------- Page: 9 ----------------------
ISO 4680:2022(E)
5.2.3 Surface finishing
Significant effects of surface conditions on the SCC initiation time are well known, and thus surface finish
needs to be carefully considered and well controlled. Concerns include physical defects, near-surface
microstructural changes (including plastic deformation and possible formation of nanocrystalline
layer), residual stress, increased hardness, and chemical contamination.
The increase of stress due to surface asperity usually has more of an effect on the SCC of high strength
materials than low strength materials.
Unless it is necessary to evaluate the as-supplied or as-fabricated surface, the material should be tested
with a reproducible surface finish by mechanical grinding or polishing, electropolishing or chemical
polishing.
Prior to testing the specimen shall be degreased using a solvent such as acetone and washing using
ethanol and high-purity water.
5.2.4 Mechanical grinding and polishing
Unless a manufacturing surface is being evaluated, the surface of a specimen should be carefully
prepared and controlled. A common surface preparation is the sequential use of coarser-to-finer
abrasives, and using cooling fluid is preferred, as described in ISO 3366 and ISO 21948. The final finish,
grinding or polishing should be agreed between the parties but as a default, a final finish of P600 is
recommended. The final grinding or polishing should be undertaken preferably in the longitudinal
direction of the specimen. H
...