ASTM G112-92(2003)
(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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Designation:G112–92(Reapproved 2003)
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope G85 Practice For Modified Salt Spray (Fog) Testing
G92 Practice for Characterization of Atmospheric Test
1.1 ThisguidediffersfromtheusualASTMstandardinthat
Sites
it does not address a specific test. Rather, it is an introductory
guide for new users of other standard exfoliation test methods,
3. Terminology
(see TerminologyG15 for definition of exfoliation).
3.1 Definitions of Terms Specific to This Standard:
1.2 This guide covers aspects of specimen preparation,
3.1.1 panel—a flat, rectangular specimen normally taken
exposure,inspection,andevaluationforconductingexfoliation
with the test surface parallel to the longitudinal and long-
tests on aluminum alloys in both laboratory accelerated envi-
transversedimensionsoffabricatedproduct.Forthinsheetand
ronments and in natural, outdoor atmospheres. The intent is to
extrusions, the thickness may be the full thickness of the part.
clarify any gaps in existent test methods.
3.1.2 sample—a portion of a large piece, or an entire piece
1.3 The values stated in SI units are to be regarded as the
out of a group of many pieces, that is submitted for evaluation
standard. The inch-pound units given in parentheses are for
andconsideredrepresentativeofthelargerpieceorpopulation.
information only.
For castings and forgings, this may be an extra portion or
1.4 This standard does not purport to address all of the
prolongation,orinthecaseofsmallparts,anentireextrapiece
safety concerns, if any, associated with its use. It is the
taken from a specific lot.
responsibility of the user of this standard to establish appro-
3.1.3 specimen—the actual test piece to be corrosion tested.
priate safety and health practices and determine the applica-
Frequently this has a specific shape with prescribed dimen-
bility of regulatory limitations prior to use.
sional tolerances and finishes.
2. Referenced Documents 3.1.4 test plane—the plane in the thickness of the sample
that is being tested. Generally this is the fabricated surface or
2.1 ASTM Standards:
somespecifiedinteriorplane.Interiorplanestypicallyusedare:
G1 Practice for Preparing, Cleaning, and Evaluating Cor-
(a) T/10=10% of the thickness removed, (this is representa-
rosion Test Specimens
tive of a minimal machining cut to obtain a flat surface), (b)
G15 Terminology Relating to Corrosion and Corrosion
T/4=quarter plane, 25% of the thickness removed, and (c)
Testing
T/2=midplane, 50% of the thickness removed.
G34 Test Method for Exfoliation Corrosion Susceptibility
in 2XXX and 7XXX Series Aluminum Alloys (EXCO
4. Significance and Use
Test)
4.1 Although there are ASTM test methods for exfoliation
G50 Practice for ConductingAtmospheric Corrosion Tests
2 testing, they concentrate on specific procedures for test meth-
on Metals
odology itself. Existent test methods do not discuss material
G66 Method for Visual Assessment of Exfoliation Corro-
variables that can affect performance. Likewise they do not
sion Susceptibility of 5XXX Series Aluminum Alloys
2 address the need to establish the suitability of an accelerated
(ASSET Test)
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 under the jurisdiction ofASTM Committee G01 on Corrosion of
Metals and is the direct responsibility of Subcommittee G01.05 on Laboratory
4.2 This guide is a compilation of the experience of inves-
Corrosion Tests.
tigators skilled in the art of conducting exfoliation tests and
Current edition approved October 1, 2003. Published October 2003. Originally
assessing the degree and significance of the damage encoun-
approved in 1992. Last previous edition approved in 1997 as G 112–92 (1997).
Annual Book of ASTM Standards, Vol 03.02. tered.Thefocusisontwogeneralaspects:guidestotechniques
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G112–92 (2003)
that will enhance the likelihood of obtaining reliable informa- tible to exfoliation, especially if the grain size is large. Often
tion,andtipsandprocedurestoavoidpitfallsthatcouldleadto the recrystallized surface layer on products such as extrusions,
erroneous results and conclusions. forgings, or sheet will not exfoliate, even though it corrodes
4.3 The following three areas of testing are considered: the intergranularly.
testmaterialsstartingwiththe“as-received”sampleupthrough 5.4 Sample Temper—When a large sample is obtained as a
final specimen preparation, the corrosion test procedures in- stock item for use over a long time period, the extra material
cluding choice of test, inspection periods, termination point, should be stored in a stable temper and at a low enough
and rating procedures, and analyses of results and methods for temperature so that no further precipitation will occur to alter
reporting them. the starting condition of the metal. The unaged W temper of
4.4 This guide is not intended as a specific corrosion test 7XXX alloys is not stable and will continue to age harden at
procedure by which to evaluate the resistance to exfoliation of room temperature. Room temperature storage of such material
an aluminum alloy product. should be limited to a couple of months at most. Natural aging
4.5 This guide is not intended as a basis for specifications, ofthesealloyscanberetardedalmostcompletelybystoringthe
nor as a guide for material lot acceptance. material in a freezer at−40°C (−40°F) or colder. This factor is
of even more importance in determination of mechanical
5. Material
properties than the investigation of corrosion resistance.
5.1 Sample Size—Most exfoliation tests do not require any
particular specimen size, but when beginning a new investiga-
6. Selection of an ASTM Test Method
tion it is best to obtain considerably more material than the
6.1 SelectionoftheappropriateASTMtestmethod(s)touse
minimum amount needed. About 50 to 100% overage is
will depend primarily on the type of alloy and on the end use
recommended. This avoids the need of procuring a second
environment. When testing a new alloy or temper, a test
sample, that may have a different response, to complete any
methodknowntobeapplicabletothemostsimilarcommercial
confirmatory retests or extensions to a specific program.
alloyisnormallyselected.Theuseriscautioned,however,that
5.2 Sample Reproducibility—The specific location of
even small changes in alloy chemistry, or changes in process-
samples in a mill product, and the number of samples to take
ing method (for example, rapid solidification processes) can
are beyond the scope of this guide. When testing large
markedly effect resistance of an alloy and the appropriateness
production items, a typical procedure is to test at both ends
of a test method. Normally exfoliation tests are conducted on
(front and rear), and to test at the side and at the mid-width if
ingot metallurgy alloys, that tend to have the elongated grain
the product is 0.6 m (2 ft) or more in width. Thick products
structure prone to exfoliate. The known alloy applicability of
should be tested at various planes through the thickness.
the ASTM test methods are listed below. Included are some
5.2.1 In addition, some assessment should be made of the
observed instances where a test method was found to be
uniformity of a large sample, or of numerous small samples.
inappropriate, or at least produced results different than those
Typical quick check methods would be to measure electrical
observed on the initial qualification alloys.
conductivity or hardness. If the material variability has a
6.1.1 It is advisable to initially employ more than one
pattern, for example, a difference between front and rear of a
laboratory test method and determine whether they agree; or if
long extrusion, then this should be noted and the specimens
not, which method is the most discriminating. One procedure
segregated accordingly. If the variability is random, then
fordoingthisistoapplydifferentfabricationprocedurestothe
multiple test specimens should be randomized.
metal that are known to generally affect resistance to exfolia-
5.3 Sample Microstructure—The directionality of the grain
tion and determine which of the test methods best detects
structure of aluminum alloys will markedly affect the suscep-
differences in the corresponding resistance to exfoliation.
tibility to exfoliation. When a product shape and alloy are
Fabrication variables that often affect resistance to exfoliation
being tested for the first time, it is advisable to macroetch full
are variable quench cooling rates, slow quenches being ad-
thickness by longitudinal and by transverse slices to establish
verse; and variable amounts of aging, underaged, or peak aged
the directionality and uniformity of the grain structure. Test
conditions generally being more susceptible than overaged
panels are normally positioned such that the test surface is
conditions. (1)
parallel to the plane in the product with the most elongated
6.2 Test Method G66 Acidified Salt Solution Exfoliation
grain structure. Complex shaped parts, such as certain extru-
Test (ASSET) is used for 5XXX alloys containing 2.0% or
sions or die forgings, may have several categories of grain
more magnesium. The round robin qualification tests for this
structuresandgrainflowthatdonotnecessarilyfollowthepart
test method were conducted on alloys 5086 (3.5 to 4.5% Mg)
geometry. Grain structure of such parts must be determined by
and 5456 (4.7 to 5.5% Mg). (2) However, Test MethodG66
macroetching or from prior experience.
(ASSET) gives problem free exfoliation indications with all
5.3.1 For a given temper condition, unrecrystallized, pan-
5XXX alloys.
cake shaped grains, that are long and wide but relatively thin,
6.3 TestMethodG34ExfoliationCorrosion(EXCO)Testis
are the most susceptible. Pancake shaped recrystallized grains,
intended for use with high strength 2XXX and 7XXX ingot
as in sheet, are the next most susceptible. This is followed by
thelong,rodshapedgrainsfoundinextrudedorrolledrodand
bar with a symmetrical cross section, for example, circle,
square, hex, or a rectangle with the width not more than twice
The boldface numbers given in parentheses refer to a list of references at the
the thickness. An equiaxed grain structure is the least suscep- end of the text.
G112–92 (2003)
metallurgyalloys,a96hperiodbeingprescribedforthe2XXX 6.6 PracticeG85AnnexA4(SALT/SO SprayTesting)was
alloys and a 48 h period for the 7XXX alloys. developed using the same, 2XXX and 7XXX alloys as men-
tioned above for the EXCO method (7).
6.3.1 For the 2XXX alloys, the round robin qualification
6.7 Both the methods in Annex A3 and Annex A4 of
tests were conducted on alloys 2024 and 2124 in theT351 and
PracticeG85resultinmoregelatinouscorrosionproductsthan
T851tempers.Theappropriatenessofthemethodhasnotbeen
doesAnnexA2. This tends to increase pitting corrosion on the
fully established for all other 2XXX alloys. It has been
specimens. Annex methods A2, A3, and A4 in PracticeG85
reported as being too aggressive and nonrepresentative of
are not equivalent, and the user should determine which
performanceinoutdooratmospheresforalloys2219,2419and
method best suits the alloys and applications under investiga-
2519 in the T851 tempers (3) and for various Al-Li alloys in
tion.
both as-quenched and artificially aged tempers (1).
6.3.2 For the 7XXX alloys the round robin qualification
7. Baseline Experience
tests were conducted on alloy 7075 in the T651, T7651, and
7.1 The best check on the appropriateness of an accelerated
T7351tempersandalloy7178intheT651andT7651tempers.
test is to determine whether the results it produces agree with
Experience has shown that the EXCO method can be used for
known service experience.
7050 and 7150 alloys in the T651, T6151, T7451, T7651, and
7.2 When there is no actual service experience, then expo-
T7751 tempers, but the test is somewhat more aggressive on
sure in a severe outdoor atmosphere known to produce
these alloys (4). This method also was evaluated with copper
exfoliation corrosion is a useful approximation of the condi-
free alloys such as 7021-T6 and 7146-T6, but generally an
tionsapartwillencounterinservice.Themostfrequentlyused
abbreviated exposure period of 16 to 24 h was used.
environmentsareseacoastsitesandhighlyindustrializedurban
6.3.3 Exposure of the powder metallurgy alloys 7090 and
locations. Selection of the particular environment to use can
7091-T6 specimens to EXCO results in rapid dissolution and
best be based on the intended end use. If there is no prior
powdering of the specimen, due to continuous drop of the
experience with the particular alloy being tested, then outdoor
extremelyfinegrains.Fouryearsofexposureofthesameparts
tests should be started as soon as possible to establish a
to seacoast atmosphere resulted only in mild general corrosion
baseline for eventual comparison.
and no exfoliation (5).
7.3 Seacoast atmospheres are representative of the more
6.4 AnnexA2ofPracticeG85ModifiedASTMAceticAcid
extreme conditions most parts can encounter in service. How-
Salt Intermittent Spray Test, (MASTMAASIS) was developed
ever, it is noteworthy that “SeacoastAtmospheric Conditions”
using alloys 2024, 2124, 7075, and 7178. This method usually
prevail only in the immediate vicinity of the seashore. Gener-
is run in the wet bottom condition (some solution and high
ally “seacoast” conditions no longer exist after 0.4 Km (0.25
humidity always present).Adry bottom condition (no solution
mile) distance from the shoreline.
present and gradually falling humidity during the purge and
7.3.1 Significant differences have been noted in tests con-
non-spraying periods) has been recommended for 2XXX
ducted at the two beach sites at Kure Beach, NC which are
alloys.
located 25 and 250 m (80 and 800 ft) from the shoreline (8).
6.4.1 The test cabinets used to conduct the MASTMAASIS
7.3.2 AnotableexampleofthiseffectisobservedattheU.S.
test, and the salt fog tests subsequently described in 6.5 and
Army’s exposure sites at Fort Sherman, at the Caribbean
6.6, are produced by several suppliers. The fog delivery
entrance to the Panama Canal. The Breakwater and Coastal
systems and cabinet geometry can differ and hav
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