ASTM G41-90(2000)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:G41–90(Reapproved 2000)
Standard Practice for
Determining Cracking Susceptibility of Metals Exposed
Under Stress to a Hot Salt Environment
ThisstandardisissuedunderthefixeddesignationG 41;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
rosion Test Specimens
1.1 This practice covers procedures for testing metals for
G 30 Practice for Making and Using U-Bend Stress-
embrittlement and cracking susceptibility when exposed under
Corrosion Test Specimens
stress to a hot salt environment. This practice can be used for
G 38 Practice for Making and Using C-Ring Stress-
testing all metals for which service conditions dictate the need
Corrosion Test Specimens
for such information. The test procedures described herein are
G 39 PracticeforPreparationandUseofBent-BeamStress-
generallyapplicabletoallmetalalloys;requiredadjustmentsin
Corrosion Test Specimens
environmental variables (temperature, stress) to characterize a
G 49 Practice for Preparation and Use of Direct Tension
givenmaterialssystemshouldbemade.Thispracticedescribes
Stress-Corrosion Test Specimens
the environmental conditions and degree of control required,
and suggests means for obtaining this desired control.
3. Summary of Practice
1.2 This practice can be used both for alloy screening for
3.1 The hot salt test consists of exposing a stressed, salt-
determination of relative susceptibility to embrittlement and
coated test specimen to elevated temperature for various
cracking, and for the determination of time-temperature-stress
predetermined lengths of time, depending on the alloy, stress
threshold levels for onset of embrittlement and cracking.
level, temperature, and selected damage criterion (that is,
However, certain specimen types are more suitable for each of
embrittlement, cracking, or rupture, or a combination thereof).
these two types of characterizations.
Exposures are normally carried out in laboratory ovens or
NOTE 1—Thispracticerelatessolelytotheperformanceoftheexposure
furnaces with associated loading equipment for stressing of
test. No detailed description concerning preparation and analysis of
specimens.
specimen types is offered. However, the optimum sample design may be
3.2 The ovens are provided with facilities to circulate air at
one that uses the same type of stress encountered in service loading
various flow rates and ambient pressure. However, for certain
situations. Standards describing principal types of stress corrosion speci-
specific applications, airflow and pressure may be adjusted to
mens, their preparation, and analysis, include Practices G 30, G 38, and
G 39. obtain information on material behavior in simulated service
environments. Exposure temperatures and stress levels are
1.3 This standard does not purport to address all of the
generally selected on the basis of mechanical property data for
safety concerns, if any, associated with its use. It is the
a given alloy, or of expected service conditions, or both.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Significance and Use
bility of regulatory limitations prior to use. (For more specific
4.1 The hot salt test as applied to metals is utilized as a
safety hazard statements see Section 8.)
secondary design consideration indicator, as cracking has been
2. Referenced Documents shown to occur in laboratory tests simulating possible service
conditions. Although limited evidence exists linking this phe-
2.1 ASTM Standards:
nomenon to actual service failures, cracking under stress in a
D 1141 Practice for the Preparation of Substitute Ocean
hotsaltenvironmentshouldberecognizedasapotentialdesign
Water
controlling factor.
D 1193 Specification for Reagent Water
4.2 The hot salt test is not to be misconstrued as being
related to the stress corrosion cracking of materials in other
This practice is under the jurisdiction of ASTM Committee G-1 on Corrosion environments. It is considered solely as a test in an environ-
of Metals and is the direct responsibility of Subcommittee G01.06 on Stress
ment that might be encountered in service.
Corrosion Cracking and Corrosion Fatigue.
Current edition approved March 30, 1990. Published May 1990. Originally
published as G 41 – 74. Last previous edition G 41 – 85.
Annual Book of ASTM Standards, Vol 11.02.
3 4
Annual Book of ASTM Standards, Vol 11.01. Annual Book of ASTM Standards, Vol 03.02.
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G41
4.3 Because hot salt cracking under stress is considered a determined and reported if possible. Chemical milling can be
secondary design consideration and service failures have not employed in final surface preparation in order to avoid extra-
been attributed solely to this phenomenon, manufacturing neous surface effects. However, care should be taken to ensure
processes will be optimized or alloying changes will be made that proper chemical milling techniques are employed, and that
only after consideration is given to primary design factors such hydrogen uptake does not occur during the surface preparation.
as creep resistance of a given high temperature alloy. The
6. Apparatus
usefulnessofthetestliesratherinlimitingmaximumoperating
6.1 Apparatus for Salt Coating—A conventional air brush
temperatures and stress levels or categorizing different alloys
should be used for spraying the specimens to accomplish the
as to susceptibility, or both, if it is found that hot salt damage
salt-coating procedure. This will generally provide a thin
may accelerate failure by creep, fatigue, or rupture.
uniform salt deposition of the desired density.
4.4 Finally, the test does not lend itself to the utilization of
6.2 Apparatus for Conducting Exposure Test:
pre-cracked specimens because cracking reinitiates at any
6.2.1 Apparatus required for conducting the exposure test
salt-metal-air interface, resulting generally in many small
depends on the selection of the specimen type to be used. If a
cracks which extend independently. For this reason, specimens
constant-deflection type specimen is utilized for which no
that are recommended for utilization in routine testing are of
external loading requirement exists, conventional laboratory
the smooth specimen category.
ovens are suitable for conducting the exposure test. Provision
5. Interferences
for controlling or monitoring inlet air humidity is recom-
5.1 Hot salt cracking under stress is often considered a mended.
6.2.1.1 Specimen Holders suitable for applying stress to
hydrogen-related phenomenon, and the source of hydrogen is a
corrosionreactioninvolvingmoisture,availableeitherfromthe constant-deflectiontypespecimensshouldbemadeofthesame
or a similar alloy as the material to be tested in order to avoid
hydratedsalt,trappedasfluidinclusionsinnonhydratedsalt,or
from humidity in the test atmosphere if absent in the salt galvanic effects. The requirement for the use of a fixture to
apply stress can be avoided when testing sheet materials by
crystals. Because of this fact, considerable variation in test
results can be obtained, simply from the method of salt utilizing a self-stressed specimen design.
6.2.1.2 Racks suitable for supporting specimens in the oven
deposition on the test specimen, even when effective controls
on other test variables are realized. Efforts should be made to and for transferring specimens should be made of the same or
standardize the salt deposition techniques and to control or a similar alloy as the material to be tested. Opencircuit
monitor humidity in order to achieve desired test validity. conditions should be maintained, although galvanic effects are
5.2 The effects of cycling time at temperature to achieve a considered to be highly localized on the surface.
given total cumulative exposure have been shown to have a 6.2.2 If a constant-deflection type specimen is utilized, care
significanteffectontestresults,withshortercycledurationand must be taken to either avoid or take into account differences
greater cycle frequency generally resulting in less damage for in thermal expansion between test specimen and test fixture.
the same cumulative exposure time. For this reason, selection Thermal expansion differences can substantially change the
stresslevelappliedatambienttemperaturewhenspecimensare
between continuous and cyclic exposure, duration, and fre-
quency of cycling, and heating and cooling rates must be made heated to the test temperature.
6.2.3 If a constant-load type specimen is to be utilized,
with the end purpose of the test in mind.
5.3 Variationsinheattoheatorproductforms,orboth,have provision must be made to combine both heating and loading
equipment. Vertical-tube resistance-wound furnaces can be
been shown to have a significant effect on damage thresholds
determined from experimental testing.This effect may be more utilized with dead-weight loading or conventional creep frame
equipment for low and high loading conditions, respectively
pronounced than is observed in more conventional stress
corrosion testing of the aqueous type. For this reason, it is (Note2).Directinductionorresistanceheatingofthespecimen
itself is not recommended.
important to obtain and document to the fullest extent possible
allcertifiedanalysesandtestsassociatedwiththematerialtobe
NOTE 2—When using vertical-tube furnaces care must be taken to
tested and associated fabrication and treatment histories. Inter-
avoid a chimney effect through the furnace, which could result in
stitial concentration levels, chemical contaminants, and ther-
excessive airflow and uneven temperature distribution along the specimen
momechanical processing should be included in the documen-
length. Sealing at both ends will allow control of air flow and improve
temperature distribution within the furnace.
tation (see Section 12).
5.4 Details regarding general surface preparation and use of
7. Reagents and Materials
bent-beam stress-corrosion specimens are outlined in Practice
7.1 Reagent grade salts shall be used when preparing
G 39. Procedures for making and using direct tension stress-
solutions from which the salt coating is derived. Sodium
corrosion specimens is described in Practice G 49. However,
chloride (NaCl) should be used for routine testing. Other salts
because of the highly localized nature of onset of attack at the
that may be encountered in service can be used for specialized
surface in hot salt exposure testing, it is desirable to charac-
applications. Synthetic sea water (Note 3), should be used for
terize as fully as possible the surface condition of the material.
characterizing alloys for use in marine environments.
If an as-received surface condition is to be investigated, efforts
should be made to ascertain the state of residual stress as
regards the material surface. Both magnitude and algebraic
See “A Stress Corrosion Test for Structural Sheet Materials,” Materials
sign (tension or compression) of residual stress should be Research and Standards, Vol 5, No. 1, January 1965, pp. 18–22.
G41
NOTE 3—If tests are to be conducted on specimens with salt deposits
dew point at oven or furnace inlet will allow determination of
derived from substitute ocean water, solutions should be prepared in
humidity of the air at ambient conditions.
accordance with Specification D 1141.
9.4 Airflow—Care must be taken to prevent airflow veloci-
ties beyond that achieved in recirculating ovens (30 to 120
7.2 Purity of Water—Unless otherwise indicated, references
m/min (100 to 400 ft/min)). Variations in this factor have been
to water shall be understood to mean Type IV water prepared
shown to produce differences in test results. If airflow is an
in accordance with Specification D 1193.
experimental variable to be investigated, it should be con-
trolled and monitored.
8. Hazards
8.1 Shatterproof glasses with side shields should be worn
10. Procedure
when handling and examining stressed samples. Generally the
10.1 Cleaning of Specimens—Before salt coating, thor-
requiredsafetyequipmentissimilartothatusedforconducting
oughly clean the specimens to remove all identification mark-
routine mechanical tests.
ings, grease, oil, or other hydrocarbon contaminants. Speci-
8.2 Appropriate heat-resistant equipment, for example,
mens may be cleaned in a variety of cleaning media, but end
gloves, may be required when exposing test samples to high
the cleaning procedure with a hot and cold water rinse. Do not
temperatures.
clean the specimens with chlorinated hydrocarbons such as
trichloroethylene because these compounds can chemisorb,
9. Calibration and Standardization
and decompose after heating, which will affect exposure test
9.1 When conducting elevated temperature exposure tests,
results. Information contained in Practice G 1 on clearing
determination of the temperature profile within the oven or
methods may be utilized where appropriate.
furnace should be made, including temperature sampling along
10.2 Salt Coating of Specimens:
the width, depth, and height of the hot zone to ensure that
10.2.1 Salt coat the specimens in such a manner as to
temperatures within all locations of specimen exposure are
provide many small separate particles. This is best accom-
within prescribed limits. Deviation from the desired test
plished by preparing a salt solution for spraying the specimens.
temperature should not be more than 62 % of the absolute
The concentration of the salt solution should provide a reason-
temperature.
able salt deposit for each spray-drying cycle. The 3.5 % salt
9.1.1 Temperature control of the exposure test shall be
solution has been shown to produce very satisfactory results
accomplished by determining true specimen temperature. This
and, because of its widespread use in other tests, is arbitrarily
can be done by means of affixing a thermocouple of appropri-
selected as a baseline for the test described herein.
ate sensitivity for the temperature range to be investigated onto
10.2.2 Spray the specimens with the prepared solution by a
a control specimen either by spotwelding or mechanical
means that provides atomization of the solution and uniform
fastening. In either instance it must be determined that the
coverage of the test specimen. A conventional air brush will
technique of thermocouple fastening does not introduce any
provide satisfactory results. Spray specimen
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