ASTM G41-90(2018)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: G41 − 90 (Reapproved 2018)
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 2. Referenced Documents
1.1 This practice covers procedures for testing metals for 2.1 ASTM Standards:
embrittlement and cracking susceptibility when exposed under D1141Practice for the Preparation of Substitute Ocean
stress to a hot salt environment. This practice can be used for Water
testing all metals for which service conditions dictate the need D1193Specification for Reagent Water
for such information. The test procedures described herein are G1Practice for Preparing, Cleaning, and Evaluating Corro-
generallyapplicabletoallmetalalloys;requiredadjustmentsin sion Test Specimens
environmental variables (temperature, stress) to characterize a G30 Practice for Making and Using U-Bend Stress-
givenmaterialssystemshouldbemade.Thispracticedescribes Corrosion Test Specimens
the environmental conditions and degree of control required, G38 Practice for Making and Using C-Ring Stress-
and suggests means for obtaining this desired control. Corrosion Test Specimens
G39Practice for Preparation and Use of Bent-Beam Stress-
1.2 This practice can be used both for alloy screening for
Corrosion Test Specimens
determination of relative susceptibility to embrittlement and
G49Practice for Preparation and Use of Direct Tension
cracking, and for the determination of time-temperature-stress
Stress-Corrosion Test Specimens
threshold levels for onset of embrittlement and cracking.
However, certain specimen types are more suitable for each of
3. Summary of Practice
these two types of characterizations.
3.1 The hot salt test consists of exposing a stressed, salt-
NOTE1—Thispracticerelatessolelytotheperformanceoftheexposure
coated test specimen to elevated temperature for various
test. No detailed description concerning preparation and analysis of
predetermined lengths of time, depending on the alloy, stress
specimen types is offered. However, the optimum sample design may be
level, temperature, and selected damage criterion (that is,
one that uses the same type of stress encountered in service loading
situations. Standards describing principal types of stress corrosion embrittlement, cracking, or rupture, or a combination thereof).
specimens, their preparation, and analysis, include Practices G30, G38,
Exposures are normally carried out in laboratory ovens or
and G39.
furnaces with associated loading equipment for stressing of
1.3 This standard does not purport to address all of the
specimens.
safety concerns, if any, associated with its use. It is the
3.2 The ovens are provided with facilities to circulate air at
responsibility of the user of this standard to establish appro-
various flow rates and ambient pressure. However, for certain
priate safety, health, and environmental practices and deter-
specific applications, airflow and pressure may be adjusted to
mine the applicability of regulatory limitations prior to use.
obtain information on material behavior in simulated service
(For more specific safety hazard statements see Section 8.)
environments. Exposure temperatures and stress levels are
1.4 This international standard was developed in accor-
generally selected on the basis of mechanical property data for
dance with internationally recognized principles on standard-
a given alloy, or of expected service conditions, or both.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
4. Significance and Use
mendations issued by the World Trade Organization Technical
4.1 The hot salt test as applied to metals is utilized as a
Barriers to Trade (TBT) Committee.
secondary design consideration indicator, as cracking has been
shown to occur in laboratory tests simulating possible service
This practice is under the jurisdiction ofASTM Committee G01 on Corrosion
of Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-
tally Assisted Cracking. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Oct. 1, 2018. Published November 2018. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approvedin1974.Lastpreviouseditionapprovedin2013asG41–90(2013).DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/G0041-90R18. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G41 − 90 (2018)
conditions. Although limited evidence exists linking this phe- 5.4 Details regarding general surface preparation and use of
nomenon to actual service failures, cracking under stress in a bent-beam stress-corrosion specimens are outlined in Practice
hotsaltenvironmentshouldberecognizedasapotentialdesign G39. Procedures for making and using direct tension stress-
controlling factor. corrosion specimens is described in Practice G49. However,
because of the highly localized nature of onset of attack at the
4.2 The hot salt test is not to be misconstrued as being
surface in hot salt exposure testing, it is desirable to charac-
related to the stress corrosion cracking of materials in other
terize as fully as possible the surface condition of the material.
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
4.3 Because hot salt cracking under stress is considered a
regards the material surface. Both magnitude and algebraic
secondary design consideration and service failures have not
sign (tension or compression) of residual stress should be
been attributed solely to this phenomenon, manufacturing
determined and reported if possible. Chemical milling can be
processes will be optimized or alloying changes will be made
employed in final surface preparation in order to avoid extra-
onlyafterconsiderationisgiventoprimarydesignfactorssuch
neous surface effects. However, care should be taken to ensure
as creep resistance of a given high temperature alloy. The
thatproperchemicalmillingtechniquesareemployed,andthat
usefulnessofthetestliesratherinlimitingmaximumoperating hydrogenuptakedoesnotoccurduringthesurfacepreparation.
temperatures and stress levels or categorizing different alloys
as to susceptibility, or both, if it is found that hot salt damage
6. Apparatus
may accelerate failure by creep, fatigue, or rupture.
6.1 Apparatus for Salt Coating—A conventional air brush
4.4 Finally, the test does not lend itself to the utilization of should be used for spraying the specimens to accomplish the
pre-cracked specimens because cracking reinitiates at any salt-coating procedure. This will generally provide a thin
salt-metal-air interface, resulting generally in many small uniform salt deposition of the desired density.
crackswhichextendindependently.Forthisreason,specimens
6.2 Apparatus for Conducting Exposure Test:
that are recommended for utilization in routine testing are of
6.2.1 Apparatus required for conducting the exposure test
the smooth specimen category.
depends on the selection of the specimen type to be used. If a
constant-deflection type specimen is utilized for which no
5. Interferences
external loading requirement exists, conventional laboratory
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-
stitial concentration levels, chemical contaminants, and ther-
momechanical processing should be included in the documen-
See “A Stress Corrosion Test for Structural Sheet Materials,” Materials
tation (see Section 12). Research and Standards, Vol 5, No. 1, January 1965, pp. 18–22.
G41 − 90 (2018)
NOTE2—Whenusingvertical-tubefurnacescaremustbetakentoavoid
9.2.2 When utilizing constant-deflection type specimens for
a chimney effect through the furnace, which could result in excessive
the determination of behavior trends or relative material
airflow and uneven temperature distribution along the specimen length.
susceptibilities,specimengeometryshouldbelimitedsuchthat
Sealing at both ends will allow control of air flow and improve
control of the applied stress level can be maintained within
temperature distribution within the furnace.
610% of the desired stress level.
7. Reagents and Materials
9.3 Humidity Control—For routine testing, active control of
7.1 Reagent grade salts shall be used when preparing
humidity is not considered mandatory. Most testing is accom-
solutions from which the salt coating is derived. Sodium plished using ambient laboratory air. However, daily monitor-
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
accordance with Specification D1141. humidity of the air at ambient conditions.
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 invest
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