ASTM G34-99

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Designation: G 34 – 99
Standard Test Method for
Exfoliation Corrosion Susceptibility in 2XXX and 7XXX
Series Aluminum Alloys (EXCO Test)
This standard is issued under the fixed designation G 34; 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.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 4. Summary of Test Method
1.1 This test method describes a procedure for constant 4.1 This test method provides an accelerated exfoliation
immersion exfoliation corrosion (EXCO) testing of high- corrosion test for 2XXX and 7XXX series aluminum alloys
strength 2XXX and 7XXX series aluminum alloys. that involves the continuous immersion of test materials in a
solution containing 4 M sodium chloride, 0.5 M potassium
NOTE 1—This test method is useful for research and development
nitrate, and 0.1 M nitric acid at 25 6 3°C (776 5°F). The
purposes and should not be construed as a quality assurance test.
susceptibility to exfoliation is determined by visual examina-
1.2 This test method applies to all wrought products such as
tion, with performance ratings established by reference to
sheet, plate, extrusions, and forgings produced from conven-
standard photographs.
tional ingot metallurgy process.
1.3 This test method can be used with any form of specimen
5. Significance and Use
or part that can be immersed in the test solution.
5.1 This test method is primarily used for research and
1.4 This standard does not purport to address all of the
development and should not be construed as a method for
safety concerns, if any, associated with its use. It is the
quality acceptance.
responsibility of the user of this standard to establish appro-
5.2 Use of this test method provides a useful prediction of
priate safety and health practices and determine the applica-
the exfoliation corrosion behavior of these alloys in various
bility of regulatory limitations prior to use.
types of outdoor service, especially in marine and industrial
environments. The test solution is very corrosive and repre-
2. Referenced Documents
sents the more severe types of environmental service, exclud-
2.1 ASTM Standards:
ing, of course, unusual chemicals not likely to be encountered
D 1193 Specification for Reagent Water
in natural environments.
E 3 Methods of Preparation of Metallographic Specimens
5.3 The exfoliation ratings were arbitrarily chosen to illus-
G 15 Terminology Relating to Corrosion and Corrosion
trate a wide range in resistance to exfoliation in this test.
Testing
However, it remains to be determined whether correlations can
G 112 Guide for Conducting Exfoliation Corrosion Tests in
be established between EXCO test ratings and realistic service
Aluminum Alloys
conditions for a given alloy. It is an ongoing activity of the
Task Group on Exfoliation Corrosion of Aluminum Alloys
3. Terminology
(G01.05.02.08) to maintain outdoor exposure tests for this
3.1 Definitions:
purpose. For example, it has been reported that samples of
3.1.1 exfoliation—corrosion that proceeds laterally from the
Al-Zn-Mg-Cu alloys rated EA or P in a 48-h EXCO test did not
sites of initiation along planes parallel to the surface, generally
develop more than a slight amount of incipient exfoliation
at grain boundaries, forming corrosion products that force
metal away from the body of the material giving rise to a
layered appearance (Terminology G 15).
Ketcham, S. J., and Jeffrey, P. W., “Exfoliation Corrosion Testing of 7178 and
7075 Aluminum Alloys” (Report of ASTM G01.05 Interlaboratory Testing Program
in Cooperation with the Aluminum Association); and Sprowls, D. O., Walsh, J. D.,
and Shumaker, M. B., “Simplified Exfoliation Testing of Aluminum Alloys,”
This test method is under the jurisdiction of ASTM Committee G01 on
Localized Corrosion—Cause of Metal Failure, ASTM STP 516, ASTM, 1972.
Corrosion of Metals and is the direct responsibility of Subcommittee G01.05 on 6
Sprowls, D. O., Summerson, T. J., and Loftin, F. E., “Exfoliation Corrosion
Laboratory Corrosion Tests.
Testing of 7075 and 7178 Aluminum Alloys—Interim Report on Atmospheric
Current edition approved Aug. 10, 1999. Published October 1999. Originally
Exposure Tests” (Report of ASTM G01.05.02 Interlaboratory Testing Program in
published as G34 – 72. Last previous edition G 34 – 97.
Cooperation with the Aluminum Association); and Lifka, B. W. and Sprowls, D. O.,
Annual Book of ASTM Standards, Vol 11.01.
“Relationship of Accelerated Test Methods for Exfoliation Resistance in 7XXX
Annual Book of ASTM Standards, Vol 03.01.
Series Aluminum Alloys With Exposure to a Seacoast Atmosphere,” Corrosion in
Annual Book of ASTM Standards, Vol 03.02.
Natural Environments, ASTM STP 558, ASTM, 1974.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
G34
(EA) during six- to nine-year exposures to seacoast atmo- cations. It is assumed that the test specimens are removed from
spheres, whereas, ED rated materials in most cases developed representative samples of materials.
severe exfoliation within a year in the seacoast atmosphere. It
9. Test Specimens
is anticipated that additional comparisons will become avail-
9.1 Specimens may be of any practical size or shape.
able as the outdoor tests are extended.
Nevertheless, for the results to be of most significance a
6. Apparatus
specimen size of at least 50 by 100 mm (2 by 4 in.), or the
6.1 Any suitable glass, plastic, or similarly inert container equivalent, is recommended.
can be used to contain the solution and specimens during the 9.2 The edges of sawed specimens need not be machined,
but specimens obtained by blanking or shearing shall have
period of test. Depending upon the shape and size of the
specimens, rods or racks of glass, plastic, or any inert sub- edges dressed by machining or filling to a depth equal to the
thickness of the specimen to remove cold-worked metal.
stance shall be used to support the specimen above the bottom
of the container. The container should be fitted with a loose- 9.3 Remove the cladding of alclad sheet by machining the
test surface; remove or mask the cladding on the back side
fitting cover to reduce evaporation.
(non-test surface) also.
7. Reagents
9.4 When removing test specimens from extrusions and
7.1 Purity of Reagents—The test solution shall be prepared
forgings, take care to avoid specimen locations underneath
with reagent grade sodium chloride (NaCl), potassium nitrate
flanges, ribs, etc., where the grain structure is usually variable.
(KNO ), and nitric acid (HNO ).
3 3
10. Standardization
7.2 Purity of Water—Distilled or deionized water conform-
10.1 To provide an indication when some inadvertent de-
ing to Specification D 1193, Type IV, shall be used to prepare
the test solution. viation from the correct test conditions occurs, it is necessary
to expose to the test at regular intervals a control specimen of
7.3 A test solution of the following composition shall be
used: a material with known resistance. This control should exhibit
the same degree of exfoliation each time it is included in the
NaCl (4.0 M)
KNO (0.5 M)
3 test.
HNO (0.1 M)
10.2 The control may be any material of the alloy type
Dissolve 234 g of NaCl, 50 g of KNO in water, and add 6.3 included in the scope of this standard, preferably one with an
mL of concentrated HNO (70 weight %). Dilute to 1 L. This intermediate degree of susceptibility (Fig. 1 and Fig. 3).
solution has an apparent pH of 0.4.
11. Procedure
7.4 The solution shall be maintained at a temperature of 25
11.1 Degrease the specimens with a suitable solvent.
6 3°C (77 6 5°F).
11.2 Mask the back surfaces of the specimens to minimize
8. Sampling
corrosion of non-test areas. Protective coatings must have good
8.1 Sampling procedures are not considered applicable to adherence to avoid crevice corrosion beneath the coating; also,
this test method, as they are often covered by product specifi- they should not contain leachable ions or protective oils that
FIG. 1 Example of Exfoliation Rating EB (Moderate); Notable Layering and Penetration into the Metal
G34
Exfoliation resulting from rapid lateral attack of selective boundaries or strata forming wedges of corrosion product that force layers of metal upward giving rise to a
layered appearance (Keller’s Etch; 1003).
FIG. 2 Four Degrees of Severity of Exfoliation Corrosion
will influence the corrosion of the test surface. surface of the specimen. Do not concurrently immerse in the
11.3 Use the solution in sufficient quantity to provide a same container alloys containing less than 0.25 % copper with
volume-to-metal surface area ratio of 10 to 30 mL/cm (65 to those containing greater amounts of copper.
200 mL/in. ). Include all exposed metal area in the determina-
11.6 The following maximum periods of exposure are
tion of total surface area.
recommended for testing the alloy types indicated:
11.4 Use fresh solution at the start of each test. Do not
2XXX Series 96 h
change the solution even though the pH increases during the 7XXX Series 48 h
test. It is normal for the pH to increase from the initial apparent
The length of time to develop exfoliation in material of a
value of 0.4 to about 3 during the first several hours depending
given alloy and temper may vary with the mill pr
...