ASTM G41-90(2006)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: G41 − 90 (Reapproved2006)
Standard Practice for
Determining Cracking Susceptibility of Metals Exposed
Under Stress to a Hot Salt Environment
ThisstandardisissuedunderthefixeddesignationG41;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope D1141Practice for the Preparation of Substitute Ocean
Water
1.1 This practice covers procedures for testing metals for
D1193Specification for Reagent Water
embrittlement and cracking susceptibility when exposed under
G1Practice for Preparing, Cleaning, and Evaluating Corro-
stress to a hot salt environment. This practice can be used for
sion Test Specimens
testing all metals for which service conditions dictate the need
G30 Practice for Making and Using U-Bend Stress-
for such information. The test procedures described herein are
Corrosion Test Specimens
generallyapplicabletoallmetalalloys;requiredadjustmentsin
G38 Practice for Making and Using C-Ring Stress-
environmental variables (temperature, stress) to characterize a
Corrosion Test Specimens
givenmaterialssystemshouldbemade.Thispracticedescribes
G39Practice for Preparation and Use of Bent-Beam Stress-
the environmental conditions and degree of control required,
Corrosion Test Specimens
and suggests means for obtaining this desired control.
G49Practice for Preparation and Use of Direct Tension
1.2 This practice can be used both for alloy screening for
Stress-Corrosion Test Specimens
determination of relative susceptibility to embrittlement and
cracking, and for the determination of time-temperature-stress
3. Summary of Practice
threshold levels for onset of embrittlement and cracking.
3.1 The hot salt test consists of exposing a stressed, salt-
However, certain specimen types are more suitable for each of
coated test specimen to elevated temperature for various
these two types of characterizations.
predetermined lengths of time, depending on the alloy, stress
NOTE1—Thispracticerelatessolelytotheperformanceoftheexposure
level, temperature, and selected damage criterion (that is,
test. No detailed description concerning preparation and analysis of
embrittlement, cracking, or rupture, or a combination thereof).
specimen types is offered. However, the optimum sample design may be
Exposures are normally carried out in laboratory ovens or
one that uses the same type of stress encountered in service loading
situations. Standards describing principal types of stress corrosion
furnaces with associated loading equipment for stressing of
specimens, their preparation, and analysis, include Practices G30, G38,
specimens.
and G39.
3.2 The ovens are provided with facilities to circulate air at
1.3 This standard does not purport to address all of the
various flow rates and ambient pressure. However, for certain
safety concerns, if any, associated with its use. It is the
specific applications, airflow and pressure may be adjusted to
responsibility of the user of this standard to establish appro-
obtain information on material behavior in simulated service
priate safety and health practices and determine the applica-
environments. Exposure temperatures and stress levels are
bility of regulatory limitations prior to use. (For more specific
generally selected on the basis of mechanical property data for
safety hazard statements see Section 8.)
a given alloy, or of expected service conditions, or both.
2. Referenced Documents
2 4. Significance and Use
2.1 ASTM Standards:
4.1 The hot salt test as applied to metals is utilized as a
secondary design consideration indicator, as cracking has been
This practice is under the jurisdiction ofASTM Committee G01 on Corrosion
shown to occur in laboratory tests simulating possible service
of Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-
tally Assisted Cracking. conditions. Although limited evidence exists linking this phe-
Current edition approved Nov. 1, 2006. Published December 2006. Originally
nomenon to actual service failures, cracking under stress in a
approvedin1974.Lastpreviouseditionapprovedin2000asG41–90(2000).DOI:
hotsaltenvironmentshouldberecognizedasapotentialdesign
10.1520/G0041-90R06.
controlling factor.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
4.2 The hot salt test is not to be misconstrued as being
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. related to the stress corrosion cracking of materials in other
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G41 − 90 (2006)
environments. It is considered solely as a test in an environ- If an as-received surface condition is to be investigated, efforts
ment that might be encountered in service. should be made to ascertain the state of residual stress as
regards the material surface. Both magnitude and algebraic
4.3 Because hot salt cracking under stress is considered a
sign (tension or compression) of residual stress should be
secondary design consideration and service failures have not
determined and reported if possible. Chemical milling can be
been attributed solely to this phenomenon, manufacturing
employed in final surface preparation in order to avoid extra-
processes will be optimized or alloying changes will be made
neous surface effects. However, care should be taken to ensure
onlyafterconsiderationisgiventoprimarydesignfactorssuch
thatproperchemicalmillingtechniquesareemployed,andthat
as creep resistance of a given high temperature alloy. The
hydrogenuptakedoesnotoccurduringthesurfacepreparation.
usefulnessofthetestliesratherinlimitingmaximumoperating
temperatures and stress levels or categorizing different alloys
6. Apparatus
as to susceptibility, or both, if it is found that hot salt damage
6.1 Apparatus for Salt Coating—A conventional air brush
may accelerate failure by creep, fatigue, or rupture.
should be used for spraying the specimens to accomplish the
4.4 Finally, the test does not lend itself to the utilization of
salt-coating procedure. This will generally provide a thin
pre-cracked specimens because cracking reinitiates at any
uniform salt deposition of the desired density.
salt-metal-air interface, resulting generally in many small
6.2 Apparatus for Conducting Exposure Test:
crackswhichextendindependently.Forthisreason,specimens
6.2.1 Apparatus required for conducting the exposure test
that are recommended for utilization in routine testing are of
depends on the selection of the specimen type to be used. If a
the smooth specimen category.
constant-deflection type specimen is utilized for which no
external loading requirement exists, conventional laboratory
5. Interferences
ovens are suitable for conducting the exposure test. Provision
5.1 Hot salt cracking under stress is often considered a
for controlling or monitoring inlet air humidity is recom-
hydrogen-relatedphenomenon,andthesourceofhydrogenisa
mended.
corrosionreactioninvolvingmoisture,availableeitherfromthe
6.2.1.1 Specimen Holders, suitable for applying stress to
hydratedsalt,trappedasfluidinclusionsinnonhydratedsalt,or
constant-deflectiontypespecimensshouldbemadeofthesame
from humidity in the test atmosphere if absent in the salt
or a similar alloy as the material to be tested in order to avoid
crystals. Because of this fact, considerable variation in test
galvanic effects. The requirement for the use of a fixture to
results can be obtained, simply from the method of salt
apply stress can be avoided when testing sheet materials by
deposition on the test specimen, even when effective controls
utilizing a self-stressed specimen design.
on other test variables are realized. Efforts should be made to
6.2.1.2 Racks,suitableforsupportingspecimensintheoven
standardize the salt deposition techniques and to control or
and for transferring specimens should be made of the same or
monitor humidity in order to achieve desired test validity.
a similar alloy as the material to be tested. Open circuit
5.2 The effects of cycling time at temperature to achieve a
conditions should be maintained, although galvanic effects are
given total cumulative exposure have been shown to have a
considered to be highly localized on the surface.
significanteffectontestresults,withshortercycledurationand
6.2.2 If a constant-deflection type specimen is utilized, care
greater cycle frequency generally resulting in less damage for
must be taken to either avoid or take into account differences
the same cumulative exposure time. For this reason, selection
in thermal expansion between test specimen and test fixture.
between continuous and cyclic exposure, duration, and fre-
Thermal expansion differences can substantially change the
quencyofcycling,andheatingandcoolingratesmustbemade
stresslevelappliedatambienttemperaturewhenspecimensare
with the end purpose of the test in mind.
heated to the test temperature.
6.2.3 If a constant-load type specimen is to be utilized,
5.3 Variationsinheattoheatorproductforms,orboth,have
provision must be made to combine both heating and loading
been shown to have a significant effect on damage thresholds
equipment. Vertical-tube resistance-wound furnaces can be
determinedfromexperimentaltesting.Thiseffectmaybemore
utilized with dead-weight loading or conventional creep frame
pronounced than is observed in more conventional stress
equipment for low and high loading conditions, respectively
corrosion testing of the aqueous type. For this reason, it is
(Note2).Directinductionorresistanceheatingofthespecimen
important to obtain and document to the fullest extent possible
itself is not recommended.
allcertifiedanalysesandtestsassociatedwiththematerialtobe
tested and associated fabrication and treatment histories. Inter-
NOTE2—Whenusingvertical-tubefurnacescaremustbetakentoavoid
stitial concentration levels, chemical contaminants, and ther-
a chimney effect through the furnace, which could result in excessive
airflow and uneven temperature distribution along the specimen length.
momechanical processing should be included in the documen-
Sealing at both ends will allow control of air flow and improve
tation (see Section 12).
temperature distribution within the furnace.
5.4 Details regarding general surface preparation and use of
7. Reagents and Materials
bent-beam stress-corrosion specimens are outlined in Practice
G39. Procedures for making and using direct tension stress-
7.1 Reagent grade salts shall be used when preparing
corrosion specimens is described in Practice G49. However, solutions from which the salt coating is derived. Sodium
because of the highly localized nature of onset of attack at the
surface in hot salt exposure testing, it is desirable to charac-
See “A Stress Corrosion Test for Structural Sheet Materials,” Materials
terize as fully as possible the surface condition of the material. Research and Standards, Vol 5, No. 1, January 1965, pp. 18–22.
G41 − 90 (2006)
chloride (NaCl) should be used for routine testing. Other salts ing and recording of humidity should be made and humidity
that may be encountered in service can be used for specialized considered as a potential cause of data scatter. In tests for
applications. Synthetic sea water (Note 3), should be used for ascertaining the effects of humidity on cracking behavior,
characterizing alloys for use in marine environments. moisture levels can be adjusted by mixing various ratios of
saturated and dry air to oven or furnace air inlet. Sampling of
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 D1141.
7.2 Purity of Water—Unless otherwise indicated, references 9.4 Airflow—Care must be taken to prevent airflow veloci-
to water shall be understood to mean Type IV water prepared ties beyond that achieved in recirculating ovens (30 to 120
in accordance with Specification D1193. m/min (100 to 400 ft/min)).Variations in this factor have been
shown to produce differences in test results. If airflow is an
8. Hazards
experimental variable to be investigated, it should be con-
trolled and monitored.
8.1 Shatterproof glasses with side shields should be worn
when handling and examining stressed samples. Generally the
10. Procedure
requiredsafetyequipmentissimilartothatusedforconducting
routine mechanical tests.
10.1 Cleaning of Specimens—Before salt coating, thor-
oughly clean the specimens to remove all identification mark-
8.2 Appropriate heat-resistant equipment, for example,
ings, grease, oil, or other hydrocarbon contaminants. Speci-
gloves, may be required when exposing test samples to high
mens may be cleaned in a variety of cleaning media, but end
temperatures.
the cleaning procedure with a hot and cold water rinse. Do not
9. Calibration and Standardization clean the specimens with chlorinated hydrocarbons such as
trichloroethylene because these compounds can chemisorb,
9.1 When conducting elevated temperature exposure tests,
and decompose after heating, which will affect exposure test
determination of the temperature profile within the oven or
results. Information contained in Practice G1 on clearing
furnaceshouldbemade,includingtemperaturesamplingalong
methods may be utilized where appropriate.
the width, depth, and height of the hot zone to ensure that
temperatures within all locations of specimen exposure are
10.2 Salt Coating of Specimens:
within prescribed limits. Deviation from the desired test
10.2.1 Salt coat the specimens in such a manner as to
temperature should not be more than 62% of the absolute
provide many small separate particles. This is best accom-
temperature. plishedbypreparingasaltsolutionforsprayingthespecimens.
9.1.1 Temperature control of the exposure test shall be
The concentration of the salt solution should provide a reason-
accomplished by determining true specimen temperature. This able salt deposit for each spray-drying cycle. The 3.5% salt
can be done by means of affixing a thermocouple of appropri-
solution has been shown to produce very satisfactory results
atesensitivityforthetemperaturerangetobeinvestigatedonto and, because of its widespread use in other tests, is arbitrarily
a control specimen either by spotwelding or mechanical
selected as a baseline for the test described herein.
fastening. In either insta
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