ASTM G112-92(1997)
(Guide)Standard Guide for Conducting Exfoliation Corrosion Tests in Aluminum Alloys
Standard Guide for Conducting Exfoliation Corrosion Tests in Aluminum Alloys
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 G15 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 problems, 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)
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Designation: G 112 – 92 (Reapproved 1997)
Standard Guide for
Conducting Exfoliation Corrosion Tests in Aluminum Alloys
This standard is issued under the fixed designation G 112; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.1 panel—a flat, rectangular specimen normally taken
with the test surface parallel to the longitudinal and long-
1.1 This guide differs from the usual ASTM standard in that
transverse dimensions of fabricated product. For thin sheet and
it does not address a specific test. Rather, it is an introductory
extrusions, the thickness may be the full thickness of the part.
guide for new users of other standard exfoliation test methods,
3.1.2 sample—a portion of a large piece, or an entire piece
(see Terminology G 15 for definition of exfoliation).
out of a group of many pieces, that is submitted for evaluation
1.2 This guide covers aspects of specimen preparation,
and considered representative of the larger piece or population.
exposure, inspection, and evaluation for conducting exfoliation
For castings and forgings, this may be an extra portion or
tests on aluminum alloys in both laboratory accelerated envi-
prolongation, or in the case of small parts, an entire extra piece
ronments and in natural, outdoor atmospheres. The intent is to
taken from a specific lot.
clarify any gaps in existent test methods.
3.1.3 specimen—the actual test piece to be corrosion tested.
1.3 The values stated in SI units are to be regarded as the
Frequently this has a specific shape with prescribed dimen-
standard. The inch-pound units given in parentheses are for
sional tolerances and finishes.
information only.
3.1.4 test plane—the plane in the thickness of the sample
1.4 This standard does not purport to address all of the
that is being tested. Generally this is the fabricated surface or
safety concerns, if any, associated with its use. It is the
some specified interior plane. Interior planes typically used are:
responsibility of the user of this standard to establish appro-
(a) T/10 5 10 % of the thickness removed, (this is representa-
priate safety and health practices and determine the applica-
tive of a minimal machining cut to obtain a flat surface), (b)
bility of regulatory limitations prior to use.
T/4 5 quarter plane, 25 % of the thickness removed, and (c)
2. Referenced Documents
T/2 5 midplane, 50 % of the thickness removed.
2.1 ASTM Standards:
4. Significance and Use
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
4.1 Although there are ASTM test methods for exfoliation
rosion Test Specimens
testing, they concentrate on specific procedures for test meth-
G 15 Terminology Relating to Corrosion and Corrosion
odology itself. Existent test methods do not discuss material
Testing
variables that can affect performance. Likewise they do not
G 34 Test Method for Exfoliation Corrosion Susceptibility
address the need to establish the suitability of an accelerated
in 2XXX and 7XXX Series Aluminum Alloys (EXCO
test for alloys never previously tested nor the need to correlate
Test)
results of accelerated tests with tests in outdoor atmospheres
G 50 Practice for Conducting Atmospheric Corrosion Tests
and with end use performance.
on Metals
4.2 This guide is a compilation of the experience of inves-
G 66 Method for Visual Assessment of Exfoliation Corro-
tigators skilled in the art of conducting exfoliation tests and
sion Susceptibility of 5XXX Series Aluminum Alloys
2 assessing the degree and significance of the damage encoun-
(ASSET Test)
tered. The focus is on two general aspects: guides to techniques
G 85 Practice For Modified Salt Spray (Fog) Testing
that will enhance the likelihood of obtaining reliable informa-
G 92 Practice for Characterization of Atmospheric Test
tion, and tips and procedures to avoid pitfalls that could lead to
Sites
erroneous results and conclusions.
3. Terminology
4.3 The following three areas of testing are considered: the
test materials starting with the “as-received” sample up through
3.1 Definitions of Terms Specific to This Standard:
final specimen preparation, the corrosion test procedures in-
cluding choice of test, inspection periods, termination point,
and rating procedures, and analyses of results and methods for
This guide is under the jurisdiction of ASTM Committee G-1 on Corrosion of
reporting them.
Metalsand is the direct responsibility of Subcommittee G01.05on Laboratory
Corrosion Tests.
4.4 This guide is not intended as a specific corrosion test
Current edition approved Sept. 15, 1992. Published November 1992.
procedure by which to evaluate the resistance to exfoliation of
Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
G112
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, of these alloys can be retarded almost completely by storing the
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
6. Selection of an ASTM Test Method
particular specimen size, but when beginning a new investiga-
tion it is best to obtain considerably more material than the
6.1 Selection of the appropriate ASTM test method(s) to use
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
method known to be applicable to the most similar commercial
confirmatory retests or extensions to a specific program.
alloy is normally selected. The user is cautioned, 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
for doing this is to apply different fabrication procedures to the
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 G 66 Acidified Salt Solution Exfoliation
grain structure. Complex shaped parts, such as certain extru-
Test (ASSET) is used for 5xxx alloys containing 2.0 % or more
sions or die forgings, may have several categories of grain
magnesium. The round robin qualification tests for this test
structures and grain flow that do not necessarily follow the part
method were conducted on alloy 5086 (3.5 to 4.5 % Mg) and
geometry. Grain structure of such parts must be determined by
on 5456 (4.7 to 5.5 % Mg). (2) However, Test Method G 66
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 Test Method G 34 Exfoliation Corrosion (EXCO) Test is
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
metallurgy alloys, a 96 h period being prescribed for the 2xxx
the long, rod shaped grains found in extruded or rolled rod and
alloys and a 48 h period for the 7xxx alloys.
bar with a symmetrical cross section, for example, circle,
6.3.1 For the 2xxx alloys, the round robin qualification tests
square, hex, or a rectangle with the width not more than twice
were conducted on alloys 2024 and 2124 in the T351 and T851
the thickness. An equiaxed grain structure is the least suscep-
tempers. The appropriateness of the method has not been fully
tible to exfoliation, especially if the grain size is large. Often
established for all other 2xxx alloys. It has been reported as
the recrystallized surface layer on products such as extrusions,
being too aggressive and nonrepresentative of performance in
forgings, or sheet will not exfoliate, even though it corrodes
outdoor atmospheres for alloys 2219, 2419 and 2519 in the
intergranularly.
T851 tempers (3) and for various Al-Li alloys in both as-
5.4 Sample Temper—When a large sample is obtained as a
quenched and artificially aged tempers (1).
stock item for use over a long time period, the extra material
6.3.2 For the 7xxx alloys the round robin qualification tests
should be stored in a stable temper and at a low enough
were conducted on alloy 7075 in the T651, T7651, and T7351
temperature so that no further precipitation will occur to alter
the starting condition of the metal. The unaged W temper of
7XXX alloys is not stable and will continue to age harden at
The boldface numbers given in parentheses refer to a list of references at the
room temperature. Room temperature storage of such material end of the text.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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G112
tempers and alloy 7178 in the T651 and T7651 tempers. known service experience.
Experience has shown that the EXCO method can be used for 7.2 When there is no actual service experience, then expo-
7050 and 7150 alloys in the T651, T6151, T7451, T7651, and sure in a severe outdoor atmosphere known to produce
T7751 tempers, but the test is somewhat more aggressive on exfoliation corrosion is a useful approximation of the condi-
these alloys (4). This method also was evaluated with copper tions a part will encounter in service. The most frequently used
free alloys such as 7021-T6 and 7146-T6, but generally an environments are seacoast sites and highly industrialized urban
abbreviated exposure period of 16 to 24 h was used. locations. Selection of the particular environment to use can
6.3.3 Exposure of the powder metallurgy alloys 7090 and best be based on the intended end use. If there is no prior
7091-T6 specimens to EXCO results in rapid dissolution and experience with the particular alloy being tested, then outdoor
powdering of the specimen, due to continuous drop of the tests should be started as soon as possible to establish a
extremely fine grains. Four years of exposure of the same parts baseline for eventual comparison.
to seacoast atmosphere resulted only in mild general corrosion 7.3 Seacoast atmospheres are representative of the more
and no exfoliation (5). extreme conditions most parts can encounter in service. How-
6.4 Annex A2 of Practice G 85 Modified ASTM Acetic Acid ever, it is noteworthy that “Seacoast Atmospheric Conditions”
Salt Intermittent Spray Test, (MASTMAASIS) was developed prevail only in the immediate vicinity of the seashore. Gener-
using alloys 2024, 2124, 7075, and 7178. This method usually ally “seacoast” conditions no longer exist after 0.4 Km (0.25
is run in the wet bottom condition (some solution and high mile) distance from the shoreline.
humidity always present). A dry bottom condition (no solution 7.3.1 Significant differences have been noted in tests con-
present and gradually falling humidity during the purge and ducted at the two beach sites at Kure Beach, NC which are
non-spraying periods) has been recommended for 2xxx 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 A notable example of this effect is observed at the U.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 have gradually sites are within sight of each other and have been photographed
evolved. Consequently some cabinet to cabinet variability in in one pict
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