Corrosion of metals and alloys - Stress corrosion testing - Part 1: General guidance on testing procedures (ISO 7539-1:1987)

Descsribes the general considerations which apply when designing and conducting tests to assess susceptibility of metals to stress corrosion. Particular methods of test are not treated in detail in this document. These are specified in the additional parts of ISO 7539.

Korrosion der Metalle und Legierungen - Prüfung der Spannungsrißkorrosion - Teil 1: Allgemeine Richtlinien für Prüfverfahren (ISO 7539-1:1987)

Dieser Teil von ISO 7539 beschreibt die allgemeinen Betrachtungen für die Planung und Durchführung von Prüfungen zur Beurteilung der Anfälligkeit von Metallen für Spannungsrißkorrosion. Anmerkung: Die einzelnen Verfahren sind in diesem Dokument nicht im Detail behandelt. Diese sind Gegenstand der weiteren Teile von ISO 7539.

Corrosion des métaux et alliages - Essais de corrosion sous contrainte - Partie 1: Guide général des méthodes d'essai (ISO 7539-1:1987)

La présente partie de l'ISO 7539 expose les considérations générales qui s'appliquent à la mise au point et à la réalisation des essais servant à évaluer la sensibilité à la corrosion sous contrainte.
NOTE -- Aucune méthode particulière n'est traitée en détail dans ce document. Voir pour cela les parties appropriées de l'ISO 7539.

Korozija kovin in zlitin – Ugotavljanje pokanja zaradi napetostne korozije - 1. del: Splošna navodila za postopke preskušanja (ISO 7539-1:1987)

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SIST EN ISO 7539-1:1999

Korozija kovin in zlitin – Ugotavljanje pokanja zaradi napetostne korozije - 1. del:

Splošna navodila za postopke preskušanja (ISO 7539-1:1987)

Corrosion of metals and alloys - Stress corrosion testing - Part 1: General guidance on

testing procedures (ISO 7539-1:1987)

Korrosion der Metalle und Legierungen - Prüfung der Spannungsrißkorrosion - Teil 1:

Allgemeine Richtlinien für Prüfverfahren (ISO 7539-1:1987)

Corrosion des métaux et alliages - Essais de corrosion sous contrainte - Partie 1: Guide

général des méthodes d'essai (ISO 7539-1:1987)
Ta slovenski standard je istoveten z: EN ISO 7539-1:1995
77.060 Korozija kovin Corrosion of metals
SIST EN ISO 7539-1:1999 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 7539-1:1999
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SIST EN ISO 7539-1:1999
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SIST EN ISO 7539-1:1999
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SIST EN ISO 7539-1:1999
First edition
1987-08- 15
Corrosion of metals and alloys - Stress corrosion
testing -
Part 1 :
General guidance on testing procedures
Essais de corrosion sous contrainte -
Corrosion des m&aux et alliages -
Partie 1: Guide g&&al des mhthodes d’essai
Reference number
ISO 7539-1 : 1987 (E)
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SIST EN ISO 7539-1:1999

ISO (the International Organization for Standardization) is a worldwide federation of

national Standards bodies (ISO member bedies). The work of preparing International

Standards is normally carried out through ISO technical committees. Esch 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, govern-

mental and non-governmental, in liaison with ISO, also take part in the work.

Draft International Standards adopted by the technical committees are circulated to

the member bodies for approval before their acceptance as International Standards by

the ISO Council. They are approved in accordance with ISO procedures requiring at

least 75 % approval by the member bodies voting.

International Standard ISO 7539-1 was prepared by Technical Committee ISO/TC 156,

Corrosion of metals and alloys.

Users should note that all International Standards undergo revision from time to time

and that any reference made herein to any other International Standard implies its

latest edition, unless otherwise stated.
0 International Organkation for Standardization, 1987 l
Printed in Switzerland
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SIST EN ISO 7539-1:1999
Corrosion of metals and alloys - Stress corrosion
testing -
Part 1 : .
General guidance on testing procedures
2.2 threshold stress (for stress corrosion) : The stress
0 lntroduction
above which stress corrosion Cracks initiate and grow, for the
specified test conditions.
This part of ISO 7539 gives general guidance on the selection,
use and interpretation of the significance of various test pro-
cedures that have been developed for the assessment of the
2.3 threshold stress intensity factor (for stress
resistance of metals and alloys to stress corrosion. These test
corrosion) : The stress intensity factor above which stress cor-
procedures are described in a series of additional Parts as
rosion Cracks will initiate, under conditions of high constraint to
follows :
plastic deformation, i.e. under plane strain predominant condi-
Part 2 : Preparation and use of bent-beam specimens.
2.4 test environment : Either a Service environment, or an
Part 3 : Preparation and use of U-bend specimens.
environment produced in the laboratory, to which the test
specimen is exposed and which is maintained constant or
Part 4 : Preparation and use of uniaxially loaded tension
varied in an agreed manner. In the case of stress corrosion the
environment is often quite specific (see clause 6).
Part 5 : Preparation and use of C-ring specimens.
2.5 Start of test : The time when the stress is applied or
Part 6 : Preparation and use of pre-cracked specimens.
when the specimen is exposed to the test environment,
whichever occurs later.
Part 7 : Slow strain rate testing.
2.6 Crack initiation time : The period from the statt of a
test to the time when a Crack is detectable by the means
1 Scope and field of application
2.7 time to failure : The period elapsing between the Start
This part of ISO 7539 describes the general considerations
of a test and the occurrence of failure, the criterion of failure
which apply when designing and conducting tests to assess
being the first appearance of cracking or the total Separation of
susceptibility of metals to stress corrosion.
the test piece, or some agreed intermediate condition.
NOTE - Particular methods of test are not treated in detail in this
document. These are described in the additional Parts of ISO 7539.
2.8 slow strain rate test : A test involving controlled
extension or bending of the test specimen at a strain rate
usually in the region 10 -3 to IO-7 s -1. The strain is increased
either continuously or in Steps, but not cyclically.
2 Definitions
2.9 average Crack velocity : The maximum depth of
2.1 stress corrosion : Synergistic attack on a metal caused
crack(s) due to stress corrosion, divided by the test time.
by the simultaneous action of a corrosive environment and
nominally static tensile stress which usually results in the for-

mation of Cracks. This process frequently results in a significant 2.10 orientation : The direction of applied tensile stress of a

reduction of the load-bearing properties of metallic structures. test specimen with respect to some specified direction in the

product from which it was prepared, e.g. the rolling direction in
the plate.
NOTE - See stress corrosion cracking (3.1).
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SIST EN ISO 7539-1:1999
Iso 7539-1: 1987 (El
3.5 In the following clauses attention is drawn particularly to
3 Background
the fact that the stress corrosion process tan be extremely sen-
sitive to small changes in exposure or test conditions. The user

3.1 From the definition of stress corrosion (2.1) it is apparent of materials is responsible for selecting the conditions under

that stress corrosion cracking is a particular case of stress
which stress corrosion tests are performed and the fact that

corrosion and in some circumstances attack may not result in some tests are described in this International Standard does not

the formation of Cracks. Although it is generally agreed that
imply that these tests are the most appropriate ones for any

cracking is the usual result, other manifestations such as given Situation. The justification for describing these tests in a

intergranular corrosion or elongated fissures, which are Standard is that they are in widespread use and have been pro-

enhanced by the presence of stress, have also to be recog-
ven as valid for specific or common equipment-environment
nized. Systems. However, the responsibility for interpretation of the
test results remains with the user of materials and it is in no way
diminished by the’existence of this Standard.
Whilst recognizing that these differentes exist, for the purpose
of this document, which is concerned with test methods, the
terms “stress corrosion” and “stress corrosion cracking” tan
3.6 In addition to specific Parts of this International Standard
be regarded as being synonymous as is usually the case in
to cover the most widely used methods, it is considered that
corrosion literature.
this more general document, concerned with the selection of
test details and the interpretation of results, is required. In
preparing this Part, use has been made of an earlier review of
As far as this International Standard is concerned, all
the subject, updated where appropriate.
phenonema involving metal dissolution or the action of
hydrogen introduced into the metal as the result of
simultaneous effects of a corrosive environment and a tensile
4 Selection of test method
stress are included except for embrittlement by liquid metal and
exfoliation corrosion.
4.1 Before embarking on a Programme of stress corrosion
testing, a decision has to be made regarding which type of test
NOTE - A distinction should be made between local dissolution due
is appropriate. Such a decision depends largely upon the pur-
to deteriorations and those phenomena caused by hydrogen. The two
pose of the test and the information required. Whilst some
types of phenomena may be superimposed but cannot be confused
tests attempt to reproduce Service conditions as closely as
with a phenomenon directly imputable to deliberate hydrogen loading.
possible and are of value to the plant engineer, others may be
designed to study a mechanistic aspect of failure; In the
former, for example restrictions of material, space, time, etc.,
3.2 There exists a wide diversity of methods used for assess-
may mean the use of a relatively simple test procedure whereas
ing the stress corrosion properties of metals. Esch has its own
in other circumstances more sophisticated testing techniques
particular advantage in certain situations.
may be essential. Thus, studies of Crack propagation rates may
involve the use of pre-cracked specimens, although these may
be inappropriate when considering, for example, the effects of
3.3 Itis important to realize that the word “test” has a special
surface finish. Although a number of sophisticated techniques
meaning in the context of stress corrosion resistance or suscep-
are available, the adoption of a simple test may prove of great
tibility. Whether or not a stress corrosion process occurs in a
value in some circumstances when more elaborate techniques
given case depends on both the exposure conditions and the
cannot be used.
propetties of the material. The word “susceptibility” to stress
corrosion does not describe a material property or quality that
tan be located on a universally applicable scale, since the Order
4.2 When selecting a test method of the pass/fail type, it is
of merit of a given set of alloys may vary with exposure con-
important to realize that this should not be so severe that it
leads to the condemnation of a material that would prove
adequate for a particular Service condition, nor should it be so
trifling as to encourage the use of a material in circumstances
3.4 Ideally, in Order to establish the risk of stress corrosion in
where rapid failure would ensue.
a given application, it is necessary to carry out Simulation >
testing under all likely Service exposure conditions. In practice
4.3 The aim of stress corrosion testing is usually to provide
this is difficult, if not impossible, and rarely achieved, but a
information more quickly than tan be obtained from Service ex-
number of “Standard tests” have been found as a result of
perience, but at the same time predictive of Service behaviour.
experience to provide reasonable guidance on likely Service
Among the most common approaches employed to achieve
behaviour for given specific applications. However, these
this are the use of higher stress, slow continuous straining, pre-
laboratory “Standard tests” are only appropriate to Service ton-
cracked specimens, higher concentration of species in test en-
ditions where experience has shown an appropriate relation-
vironment than in Service environment, increased temperature,
ship, however empirical, to exist. The fact that a given alloy
and electrochemical Stimulation. lt is important however, that
Passes or does not pass a test previously found useful in rela-
these methods be controlled in such a way that the details of
tion to another alloy may or may not be significant and a test
the failure mechanism are not changed.
that discriminates correctly between alloys used for a given
application will not necessarily provide safe guidance if the
exposure conditions are different. The use of a Standard test
4.4 If it is too difficult to reproduce the Service conditions
beyond the Point for which there is experience therefore
exactly, it may be useful to analyse the stress corrosion process
requires Validation.
in Order to determine as far as possible the main factors
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SIST EN ISO 7539-1:1999
ISO 75394 : 1987 (E)
5.2.5 Constant total strain tensile tests are sometimes pre-
operating at different stages. The stress corrosion test then
ferred to bend tests, thus simplifying both the application and
selected may involve only one step of the corrosion
calculation of the stress. However, the former require more
massive restraining frames than bend test specimens of similar
5 Stressing Systems
5.2.6 The use of restraining frames may be avoided by
employing internally stressed specimens containing residual
5.1 General
Stresses as the result of inhomogeneous deformation. The lat-
ter may be introduced by plastic bending, e.g. by producing a
Methods of loading test pieces, whether initially plain, notched
bulge in sheet or plate material, or by welding, but such tests
or pre-cracked, tan be conveniently grouped according to
involve Problems in systematic Variation of the initial stress,
whether they invoive
which usually achieves maximum values in the region of the
yield stress. Moreover, elastic spring-back, in introducing
a) a constant total strain (see 5.2);
residual Stresses by bulging plate or partially flattening tube,
may Cause Problems and where welding is involved the struc-

b) a constant load (see 5.3); tural modifications may raise difficulties, unless the test is

simulative of a practical Situation.
c) an applied slow strain rate (see 5.4).
5.2.7 Constant total strain specimens are sometimes loaded
In the case of pre-cracked specimens, threshold conditions are
by being placed initially into conventional testing machines or
defined in terms of a stress intensity value Klscc and tests may
similar devices and then, whilst being maintained in their strain-
also be conducted under constant strain intensity conditions.
ed condition, having a restraining frame attached. When the
load applied by the testing machine is removed, the specimen
Knowledge of the limitations of the various methods is at least
remains stressed by virtue of the restraint imposed by the
as important as the choice of method of stressing.
frame, the assumption being made that the strain in the
specimen remains constant as the restraint is transferred from
the testing machine to the frame. This implies a similar stiffness
5.2 Constant total strain tests
in the testing machine and frame, which is likely to be so only if
the frame is relatively massive compared with the specimens.
5.2.1 These form by far the most popular type of test as a
group, since bend tests in a variety of forms come into this
5.2.8 The stiffness of the stressing frame employed, may also
category. Furthermore, they simulate the fabrication Stresses
influence the time to failure of a specimen, quite apart from any
that are frequently associated with Service failures.
effect that it may have upon the initial stress level. Thus, in
most constant total strain tests and especially those upon duc-
5.2.2 Material in sheet form is frequently tested by bending;
tile materials, the initial elastic strain in the specimen is con-
plate material is tested under tension or as C-rings, with the lat-
verted in part to plastic strain as the Crack propagates.
ter also used for testing tubular products and other semi-
finished products of round Cross-section.
5.2.9 Once load relaxation has been initiated, the extent to
which it proceeds tan vary from specimen to specimen and this
5.2.3 Bend tests have the attraction of employing simple, and
may influence time to failure according to the number of Cracks
therefore frequently cheap, specimens and restraining jigs. The
or pits that develop. Marked load relaxation tan be observed on
Problems with such test methods are usually related to poor
a specimen with many Cracks or pits whereas little load relaxa-
reproducibility of the stress level, if indeed any quantitative
tion is observed when only a few Cracks are present. If only one
measure of this is available. Attemps to improve upon this
Crack develops, it will not need to grow to large dimensions
Situation have led to more sophisticated types of bend test, e.g.
before sudden, final failure occurs because the applied load re-
involving four-instead of three-Point loading, but the limitations
mains high, whereas the marked load relaxation associated
of simple bending theory, usually used to calculate the stress
with the presence of many stress corrosion Cracks means that
level, tan lead to errors in anticipated stress especially when
they must propagate much further before one of them
strains beyond the elastic limit are required. The use of strain
becomes large enough to create the stress conditions, at a
gauges for measuring surface Stresses may be useful in some
relatively small load, for sudden failure.
circumstances. The fabrication of Strip specimens to produce
“U” bends introduces significant amounts of plastic deforma-
tion which may influence cracking response.
5.2.10 The extent to which the number of Cracks present
influences the test results naturally depends upon the stress
corrosion System being studied, i.e. upon such properties as
5.2.4 Tubular material may be tested in the form of C-rings or
the fracture toughness of the material and even upon the ag-
O-rings, the former being stressed by partial opening or closing
gressiveness of the environment employed. The result also
of the gap and the latter by forced insertion of a plug that is
depends upon the stiffness of the restraining jig employed.
appropriately oversized for the bore. The C-ring has also been
Thus the stiffer the frame the less the elastic strain that is likely
found to be particularly useful for testing thick product forms,
to remain in the specimen after the propagation of a Luders
e.g. aluminium alloys in the short transverse direction.
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SIST EN ISO 7539-1:1999
ISO7539-1:1987 (EI

band. Therefore the time to failure for a given initial stress 5.3.6 Constant load tests involve an increasing stress situ-

ation as Cracks propagate, therefore Cracks once initiated are
varies according to whether the System is hard or soft; in some

cases Cracks may cease propagating so that failure is not less likely to stop propagating than in the case of constant total

achieved. strain tests at Stresses below the threshold stress. Thus, the
threshold stress value is likely to be lower in a particular System
when determined under constant load conditions than under
5.3 Constant load tests
constant deflection conditions.
These may simulate more closely stress corrosion failure
54 . Slow strain rate tests
from applied or working Stresses. Since the effective cross-
section of the test piece is reduced by Crack propagation, con-
5.4.1 The application of slow dynamic straining, which was
stant load tests involve an increasing stress Situation. Conse-
initially regarded as a rapid sorting test, is beginning to emerge
quently, such tests are more likely to lead to early failure or total
as one that has much more relevante to practice.
failure than are constant total strain tests.
In essence the method involves the application of a relatively
5.3.2 The relatively massive machinery usually required for
slow strain or deflection rate (e.g. IO-6 s-1) to a specimen,
dead-weight loading tests upon specimens of appreciable
under the appropriate environmental influence, until failure
Cross-section is sometimes circumvented by the use of a com-
Pression spring. The spring characteristics are Chosen to ensure
that the relaxation that occurs during testing does not
5.4.2 Stress corrosion Crack velocities usually fall in the range
significantly Change the load. In the same category are
of 10-3 to lO--6mms--1, which implies that failures in
modified proving rings used in the calibration of tensile testing
laboratory tests under constant total strain or constant load
machines. The axial load applied to a tensile specimen contain-
conditions for specimens of usual dimensions occur in a few
ed within the ring tan be determined from measurement of the
days. This is found to be so ff the System is one in which stress
Change in diameter of the calibrated ring.
corrosion Cracks are readily initiated, but it is also common
experience to find that test pieces do not fail in very extended
periods of testing, which are then terminated at some arbitrary
5.3.3 An alternative approach for minimizing the size of the
selected time. The consequences are that considerable scatter
loading System is to reduce the Cross-section of the specimen,
may be associated with replicate tests and the arbitrary ter-
e.g. by the use of very fine wire. However, it is dangerous to
mination of the test leaves an element of doubt concerning
reduce the Cross-section too far unless failure by stress corro-
what the outcome would have been if it had been allowed to
sion is confirmed by, say, metallography. This is because, in
continue for a longer time. Just as the use of pre-cracked
some stress corrosion environments, failure may result from
specimens assists in stress corrosion Crack initiation, so
pitting or other forms of attack with an attendant increase in
apparently may the application of slow dynamic strain, which
the effective stress to the ultimate tensile strength of the metal.
has the further advantage that the test is not terminated after
Other dangers are attendant on the use of very small section
some arbitrary time, since the conclusion is always achieved by
specimens (sec 7.2.2).
the specimen fracturing and the criterion of cracking is then
related to the mode of failure. Thus, in the form in which it is
5.3.4 The tost of testing specimens under constant load on
normally employed, the slow strain rate method frequently
individual testing machines tan be minimized by testing chains
results in failure within about 2 days, either by ductile fracture
of specimens on a Single machine. This practice also reduces
or by stress corrosion cracking, according to the susceptibility
the test chamber requirements. Chains of uniaxial tensile
towards the latter. The fact that the test concludes in this
specimens tan be connected with simple loading links, but this
positive manner in a relatively short period of time constitutes
approach is better suited to situations where failures are not
one of its main attractions.
anticipated since the failure of a Single specimen would in-
validate the remainder. Chains of more compliant pre-cracked
5.4.3 Early use of the test was in providing data whereby the
specimens tan be connected with loading links, which are
effects of such variables as alloy composition and structure, or
designed to progressively Unload specimens as Crack growth
inhibitive additions to cracking environments, could be
occurs in Order to avoid disturbance to the other specimens
compared and also for promoting stress corrosion cracking in
which would othervvise be inevitable in the event of a failure.
combinations of alloy and environment that could not be
Users must invalidate test procedures employing chains of
caused to fail in the laboratory under conditions of constant
specimens to ensure freedom from errors before proceeding
load or constant total strain. Thus, it constitutes a relatively
with their adoption.
severe type of test in the sense that it frequently promotes
stress corrosion failure in the laboratory where other modes of
5.3.5 The use of a tension specimen having a tapered gauge
stressing plain specimens do not promote cracking and in this
length, which is finding increasing popularity, has the obvious
respect it is in a similar category to tests on pre-cracked
attraction of providing a range of initial Stresses in a Single
specimens. In recent years an understanding of the implica-
specimen. However, caution should be exercised with their use
tions of dynamic strain testing has developed and it now
in, for example, determination of accurate threshold stress
appears that this type of test may have more relevante and
levels. The results may be influenced by such factors as number
significance than just that of an effective, rapid, sorting test. lt
of Cracks present, net section yielding, etc. lt may be more ap-
may be argued that laboratory tests involving the pulling of
propriate to use such specimens in “sorting” tests to be
specimens to failure at a slow strain rate show little relation to
followed by more limited conventional testing.
the reality of Service failures. In fact, in both constant total
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SIST EN ISO 7539-1:1999
ISO 75394 : 1987 (EI
Such effects are likely to be especially relevant in the case of
strain and constant load tests Crack propagation also occurs
stress corrosion which, as previously mentioned, occurs under
under conditions of slow dynamic strain to a greater or lesser
rather specific conditions often involving a balance between
degree depending upon the initial value of stress, the time at
active corrosion and passive behaviour.
which a Crack is initiated and various metallurgical Parameters
that govern creep in the specimen. Moreover, there is an in-
In view of the foregoing it is clear that the test temperature
creasing amount of evidente in some Systems which suggests

that the function of stress in stress corrosion is to promote a should be closely controlled and whenever possible this should

be selected to correspond to that expected in Service.
strain rate which, rather than the stress as such, is the import-
Although, as indicated in 4.3, increased temperature is
ant mechanical Parameter in Crack initiation or propagation. In
sometimes used to accelerate test results, clearly such an ap-
these cases the minimum creep rate for cracking is as much an
proach must be undertaken with caution.
engineering design Parameter as is the threshold stress or
stress intensity factor obtained from constant load tests on
plain or pre-cracked specimens.
6.3 Solution composition

5.4.4 The equipment required for slow strain testing is simply 6.3.1 Although it is inevitable that the environment remains

as one of the very important variables in stress corrosion
a device which permits a selection of strain rates whilst being
testing, some solutions have become particularly widely used
powetful enough to cope with the loads generated. Purpose-

built apparatus usually consists of a moderately stiff frame and for certain types of alloy. Boiling magnesium chloride solutions

for stainless steels and boiling nitrate solutions for carbon steels
a drive mechanism through a series of reduction gears that

allows a selection of crosshead Speeds in the range 10-3 to are two examples. Such solutions have been criticized for

several reasons, the main one being that they do not generally
IO-7 mm-s-1.
reproduce plant conditions. This may be of considerable im-
portance in that the relative cracking susceptibilities of a range
Plain or pre-cracked specimens in tension may be used, but if
of alloys are not necessarily the same in different environments.
the Cross-section of these would need to be large or the loads
high, bend specimens may be used.
6.3.2 Nevertheless, tests in these commonly used solutions
tan serve a useful purpose, provided that their limitations are
5.4.5 lt is important to appreciate that the same strain rate
borne in mind and that adequate care is taken in preparing and
does not produce the same cracking response in all Systems
using the solutions. Whilst the relatively small differentes that
and that the rate has to be Chosen in relation to the particular
may be expected to occur between laboratories preparing a
System being studied.
Solution to the same specification would frequently not
influence stress corrosion test results, there are situations
where relatively small changes in environment tan promote
6 Environmental aspects
marked changes in cracking response. The possible Problems
associated with the use of nominally 42 % boiling MgCI, for
testing stainless steels may serve as an example. Since the
6.1 General
hydrate of MgCI, is hygroscopic, Solution preparation by
weighing may lead to appreciable differentes in boiling Point
Stress corrosion cracking is still considered to occur in rather
and hence in time to failure in a stress corrosion test; it is
specific alloy/environment combinations, e.g. austenitic
therefore preferable to prepare the Solution by adding water to
stainless steels in chloride solutions and mild steels in nitrate
the hydrate to achieve a particular boiling Point.
solutions. However, the list of such combinations continues to
grow with time and even instances of the cracking of materials
6.3.3 The effects of varying the pH of the environment in rela-
in high pu

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