ASTM G112-92(2009)
(Guide)Standard Guide for Conducting Exfoliation Corrosion Tests in Aluminum Alloys
Standard Guide for Conducting Exfoliation Corrosion Tests in Aluminum Alloys
SIGNIFICANCE AND USE
Although there are ASTM test methods for exfoliation testing, they concentrate on specific procedures for test methodology itself. Existent test methods do not discuss material variables that can affect performance. Likewise they do not address the need to establish the suitability of an accelerated test for alloys never previously tested nor the need to correlate results of accelerated tests with tests in outdoor atmospheres and with end use performance.
This guide is a compilation of the experience of investigators skilled in the art of conducting exfoliation tests and assessing the degree and significance of the damage encountered. The focus is on two general aspects: guides to techniques that will enhance the likelihood of obtaining reliable information, and tips and procedures to avoid pitfalls that could lead to erroneous results and conclusions.
The following three areas of testing are considered: the test materials starting with the “as-received” sample up through final specimen preparation, the corrosion test procedures including choice of test, inspection periods, termination point, and rating procedures, and analyses of results and methods for reporting them.
This guide is not intended as a specific corrosion test procedure by which to evaluate the resistance to exfoliation of an aluminum alloy product.
This guide is not intended as a basis for specifications, nor as a guide for material lot acceptance.
SCOPE
1.1 This guide differs from the usual ASTM standard in that it does not address a specific test. Rather, it is an introductory guide for new users of other standard exfoliation test methods, (see Terminology G 15 for definition of exfoliation).
1.2 This guide covers aspects of specimen preparation, exposure, inspection, and evaluation for conducting exfoliation tests on aluminum alloys in both laboratory accelerated environments and in natural, outdoor atmospheres. The intent is to clarify any gaps in existent test methods.
1.3 The values stated in SI units are to be regarded as the standard. The inch-pound units given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: G112 − 92 (Reapproved2009)
Standard Guide for
Conducting Exfoliation Corrosion Tests in Aluminum Alloys
This standard is issued under the fixed designation G112; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope G85Practice for Modified Salt Spray (Fog) Testing
G92Practice for Characterization ofAtmospheric Test Sites
1.1 ThisguidediffersfromtheusualASTMstandardinthat
2.2 ASTM Adjuncts:
it does not address a specific test. Rather, it is an introductory
Illustrations of Appearance Classifications (6 glossy pho-
guide for new users of other standard exfoliation test methods,
tos)
(see Terminology G15 for definition of exfoliation).
1.2 This guide covers aspects of specimen preparation, 3. Terminology
exposure,inspection,andevaluationforconductingexfoliation
3.1 Definitions of Terms Specific to This Standard:
tests on aluminum alloys in both laboratory accelerated envi-
3.1.1 panel—a flat, rectangular specimen normally taken
ronments and in natural, outdoor atmospheres. The intent is to
with the test surface parallel to the longitudinal and long-
clarify any gaps in existent test methods.
transversedimensionsoffabricatedproduct.Forthinsheetand
1.3 The values stated in SI units are to be regarded as the extrusions, the thickness may be the full thickness of the part.
standard. The inch-pound units given in parentheses are for
3.1.2 sample—a portion of a large piece, or an entire piece
information only.
out of a group of many pieces, that is submitted for evaluation
1.4 This standard does not purport to address all of the andconsideredrepresentativeofthelargerpieceorpopulation.
safety concerns, if any, associated with its use. It is the For castings and forgings, this may be an extra portion or
responsibility of the user of this standard to establish appro- prolongation,orinthecaseofsmallparts,anentireextrapiece
priate safety and health practices and determine the applica- taken from a specific lot.
bility of regulatory limitations prior to use.
3.1.3 specimen—the actual test piece to be corrosion tested.
Frequently this has a specific shape with prescribed dimen-
2. Referenced Documents
sional tolerances and finishes.
2.1 ASTM Standards:
3.1.4 test plane—the plane in the thickness of the sample
G1Practice for Preparing, Cleaning, and Evaluating Corro-
that is being tested. Generally this is the fabricated surface or
sion Test Specimens
somespecifiedinteriorplane.Interiorplanestypicallyusedare:
G15TerminologyRelatingtoCorrosionandCorrosionTest-
(a) T/10=10% of the thickness removed, (this is representa-
ing (Withdrawn 2010)
tive of a minimal machining cut to obtain a flat surface), (b)
G34TestMethodforExfoliationCorrosionSusceptibilityin
T/4=quarter plane, 25% of the thickness removed, and (c)
2XXX and 7XXX SeriesAluminumAlloys (EXCO Test)
T/2=midplane, 50% of the thickness removed.
G50Practice for Conducting Atmospheric Corrosion Tests
on Metals
4. Significance and Use
G66Test Method for Visual Assessment of Exfoliation
4.1 Although there are ASTM test methods for exfoliation
Corrosion Susceptibility of 5XXX Series Aluminum Al-
testing, they concentrate on specific procedures for test meth-
loys (ASSET Test)
odology itself. Existent test methods do not discuss material
variables that can affect performance. Likewise they do not
This guide is under the jurisdiction ofASTM Committee G01 on Corrosion of
address the need to establish the suitability of an accelerated
Metals and is the direct responsibility of Subcommittee G01.05 on Laboratory
test for alloys never previously tested nor the need to correlate
Corrosion Tests.
results of accelerated tests with tests in outdoor atmospheres
Current edition approved May 1, 2009. Published May 2009. Originally
approved in 1992. Last previous edition approved in 2003 as G112–92(2003). DOI:
and with end use performance.
10.1520/G0112-92R09.
2 4.2 This guide is a compilation of the experience of inves-
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
tigators skilled in the art of conducting exfoliation tests and
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
The last approved version of this historical standard is referenced on Available from ASTM International Headquarters. Order Adjunct No.
www.astm.org. ADJG003402. Original adjunct produced in 1980.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G112 − 92 (2009)
assessing the degree and significance of the damage encoun- thelong,rodshapedgrainsfoundinextrudedorrolledrodand
tered.Thefocusisontwogeneralaspects:guidestotechniques bar with a symmetrical cross section, for example, circle,
that will enhance the likelihood of obtaining reliable informa- square, hex, or a rectangle with the width not more than twice
tion,andtipsandprocedurestoavoidpitfallsthatcouldleadto the thickness. An equiaxed grain structure is the least suscep-
erroneous results and conclusions. tible to exfoliation, especially if the grain size is large. Often
the recrystallized surface layer on products such as extrusions,
4.3 The following three areas of testing are considered: the
forgings, or sheet will not exfoliate, even though it corrodes
testmaterialsstartingwiththe“as-received”sampleupthrough
intergranularly.
final specimen preparation, the corrosion test procedures in-
cluding choice of test, inspection periods, termination point,
5.4 Sample Temper—When a large sample is obtained as a
and rating procedures, and analyses of results and methods for
stock item for use over a long time period, the extra material
reporting them.
should be stored in a stable temper and at a low enough
temperature so that no further precipitation will occur to alter
4.4 This guide is not intended as a specific corrosion test
the starting condition of the metal. The unaged W temper of
procedure by which to evaluate the resistance to exfoliation of
7XXX alloys is not stable and will continue to age harden at
an aluminum alloy product.
room temperature. Room temperature storage of such material
4.5 This guide is not intended as a basis for specifications,
should be limited to a couple of months at most. Natural aging
nor as a guide for material lot acceptance.
ofthesealloyscanberetardedalmostcompletelybystoringthe
material in a freezer at−40°C (−40°F) or colder. This factor is
5. Material
of even more importance in determination of mechanical
5.1 Sample Size—Most exfoliation tests do not require any
properties than the investigation of corrosion resistance.
particular specimen size, but when beginning a new investiga-
tion it is best to obtain considerably more material than the
6. Selection of an ASTM Test Method
minimum amount needed. About 50 to 100% overage is
6.1 SelectionoftheappropriateASTMtestmethod(s)touse
recommended. This avoids the need of procuring a second
will depend primarily on the type of alloy and on the end use
sample, that may have a different response, to complete any
environment. When testing a new alloy or temper, a test
confirmatory retests or extensions to a specific program.
methodknowntobeapplicabletothemostsimilarcommercial
5.2 Sample Reproducibility—The specific location of
alloyisnormallyselected.Theuseriscautioned,however,that
samples in a mill product, and the number of samples to take
even small changes in alloy chemistry, or changes in process-
are beyond the scope of this guide. When testing large
ing method (for example, rapid solidification processes) can
production items, a typical procedure is to test at both ends
markedly effect resistance of an alloy and the appropriateness
(front and rear), and to test at the side and at the mid-width if
of a test method. Normally exfoliation tests are conducted on
the product is 0.6 m (2 ft) or more in width. Thick products
ingot metallurgy alloys, that tend to have the elongated grain
should be tested at various planes through the thickness.
structure prone to exfoliate. The known alloy applicability of
5.2.1 In addition, some assessment should be made of the
the ASTM test methods are listed below. Included are some
uniformity of a large sample, or of numerous small samples.
observed instances where a test method was found to be
Typical quick check methods would be to measure electrical
inappropriate, or at least produced results different than those
conductivity or hardness. If the material variability has a
observed on the initial qualification alloys.
pattern, for example, a difference between front and rear of a
6.1.1 It is advisable to initially employ more than one
long extrusion, then this should be noted and the specimens
laboratory test method and determine whether they agree; or if
segregated accordingly. If the variability is random, then
not, which method is the most discriminating. One procedure
multiple test specimens should be randomized.
fordoingthisistoapplydifferentfabricationprocedurestothe
5.3 Sample Microstructure—The directionality of the grain
metal that are known to generally affect resistance to exfolia-
structure of aluminum alloys will markedly affect the suscep-
tion and determine which of the test methods best detects
tibility to exfoliation. When a product shape and alloy are
differences in the corresponding resistance to exfoliation.
being tested for the first time, it is advisable to macroetch full
Fabrication variables that often affect resistance to exfoliation
thickness by longitudinal and by transverse slices to establish
are variable quench cooling rates, slow quenches being ad-
the directionality and uniformity of the grain structure. Test
verse; and variable amounts of aging, underaged, or peak aged
panels are normally positioned such that the test surface is
conditions generally being more susceptible than overaged
parallel to the plane in the product with the most elongated
conditions. (1)
grain structure. Complex shaped parts, such as certain extru-
6.2 Test Method G66 Acidified Salt Solution Exfoliation
sions or die forgings, may have several categories of grain
Test (ASSET) is used for 5XXX alloys containing 2.0% or
structuresandgrainflowthatdonotnecessarilyfollowthepart
more magnesium. The round robin qualification tests for this
geometry. Grain structure of such parts must be determined by
test method were conducted on alloys 5086 (3.5 to 4.5% Mg)
macroetching or from prior experience.
5.3.1 For a given temper condition, unrecrystallized, pan-
cake shaped grains, that are long and wide but relatively thin,
are the most susceptible. Pancake shaped recrystallized grains,
The boldface numbers in parentheses refer to a list of references at the end of
as in sheet, are the next most susceptible. This is followed by this standard.
G112 − 92 (2009)
and 5456 (4.7 to 5.5% Mg). (2) However, Test Method G66 intergranular corrosion that could be confused with exfoliation
(ASSET) gives problem free exfoliation indications with all corrosion unless specimens are examined metallographically.
5XXX alloys.
6.5 Annex A3 of Practice G85 Seawater Acetic Acid Test
6.3 TestMethodG34ExfoliationCorrosion(EXCO)Testis
(SWAAT) was developed using the same 5XXX, 2XXX, and
intended for use with high strength 2XXX and 7XXX ingot
7XXX alloys as mentioned above for the ASSET and EXCO
metallurgyalloys,a96hperiodbeingprescribedforthe2XXX
methods (6).
alloys and a 48 h period for the 7XXX alloys.
6.6 Practice G85AnnexA4 (SALT/SO Spray Testing) was
6.3.1 For the 2XXX alloys, the round robin qualification
developed using the same, 2XXX and 7XXX alloys as men-
tests were conducted on alloys 2024 and 2124 in theT351 and
tioned above for the EXCO method (7).
T851tempers.Theappropriatenessofthemethodhasnotbeen
fully established for all other 2XXX alloys. It has been 6.7 Both the methods in Annex A3 and Annex A4 of
reported as being too aggressive and nonrepresentative of
Practice G85 result in more gelatinous corrosion products than
performanceinoutdooratmospheresforalloys2219,2419and
doesAnnexA2.This tends to increase pitting corrosion on the
2519 in the T851 tempers (3) and for various Al-Li alloys in
specimens.AnnexmethodsA2,A3,andA4inPracticeG85are
both as-quenched and artificially aged tempers (1).
not equivalent, and the user should determine which method
best suits the alloys and applications under investigation.
6.3.2 For the 7XXX alloys the round robin qualification
tests were conducted on alloy 7075 in the T651, T7651, and
T7351tempersandalloy7178intheT651andT7651tempers. 7. Baseline Experience
Experience has shown that the EXCO method can be used for
7.1 The best check on the appropriateness of an accelerated
7050 and 7150 alloys in the T651, T6151, T7451, T7651, and
test is to determine whether the results it produces agree with
T7751 tempers, but the test is somewhat more aggressive on
known service experience.
these alloys (4). This method also was evaluated with copper
free alloys such as 7021-T6 and 7146-T6, but generally an
7.2 When there is no actual service experience, then expo-
abbreviated exposure period of 16 to 24 h was used. sure in a severe outdoor atmosphere known to produce
6.3.3 Exposure of the powder metallurgy alloys 7090 and exfoliation corrosion is a useful approximation of the condi-
tionsapartwillencounterinservice.Themostfrequentlyused
7091-T6 specimens to EXCO results in rapid dissolution and
powdering of the specimen, due to continuous drop of the environmentsareseacoastsitesandhighlyindustrializedurban
locations. Selection of the particular environment to use can
extremelyfinegrains.Fouryearsofexposureofthesameparts
to seacoast atmosphere resulted only in mild general corrosion best be based on the intended end use. If there is no prior
experience with the particular alloy being tested, then outdoor
and no exfoliation (5).
tests should be started as soon as possible to establish a
6.4 AnnexA2ofPracticeG85ModifiedASTMAceticAcid
baseline for eventual comparison.
Salt Intermittent Spray Test, (MASTMAASIS) was developed
using alloys 2024, 2124, 7075, and 7178. This method usually 7.3 Seacoast atmospheres are representative of the more
is run in the wet bottom condition (some solution and high extreme conditions most parts can encounter in service. How-
humidity always present).Adry bottom condition (no solution ever, it is noteworthy that “SeacoastAtmospheric Conditions”
present and gradually falling humidity duri
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