ASTM G64-99(2021)

Standard Classification of Resistance to Stress-Corrosion Cracking of Heat-Treatable Aluminum Alloys

SIGNIFICANCE AND USE
4.1 This classification involves alphabetical ratings intended only to provide a qualitative guide for materials selection. The ratings are based primarily on the results of standard corrosion tests.  
4.2 Interpretations of the SCC ratings in terms of typical problem areas including service experience are given in Table 1. Practical experience has shown that SCC problems with aluminum alloys generally have involved situations where the direction and magnitude of the tensile stresses resulting from manufacturing or use, or both, of the material were not recognized. (A) The sum of all stresses including those from service loads (applied), heat treatment, straightening, forming, and so forth.  
4.3 A list of the SCC ratings for the heat-treatable aluminum alloy products is given in Table 2. Revisions to the table will be required as new materials become available and additional test results are accumulated. (A) The ratings apply to standard mill products in the types of tempers indicated, including stress-relieved tempers, and could be invalidated in some cases by application of nonstandard thermal treatments or mechanical deformation at room temperature by the user.(B) Test direction refers to orientation of the stressing direction relative to the directional grain structure typical of wrought materials, which in the case of extrusions and forgings may not be predictable from the geometrical cross section of the product.
L — Longitudinal: parallel to direction of principal metal extension during manufacture of the product.
LT—Long Transverse: perpendicular to direction of principal metal extension. In products whose grain structure clearly shows directionality (width-to-thickness ratio greater than two) it is that perpendicular direction parallel to the major grain dimension.
ST—Short Transverse: perpendicular to direction of principal metal extension and parallel to minor dimension of grains in products with significant grain directionality.(C) Sections wit...
SCOPE
1.1 This classification covers alphabetical ratings of the relative resistance to SCC of various mill product forms of the wrought 2XXX, 6XXX, and 7XXX series heat-treated aluminum alloys and the procedure for determining the ratings.  
1.2 The ratings do not apply to metal in which the metallurgical structure has been altered by welding, forming, or other fabrication processes.  
1.3 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 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.

General Information

• Status: Published
• Publication Date: 31-Jul-2021
• ICS: 77.120.10 - Aluminium and aluminium alloys
• Technical Committee: G01 - Corrosion of Metals
• Drafting Committee: G01.06 - Environmentally Assisted Cracking

Relations

• Refers: ASTM G47-98(2019) - Standard Test Method for Determining Susceptibility to Stress-Corrosion Cracking of 2XXX and 7XXX Aluminum Alloy Products
• Effective Date: 15-Jun-2019
• Refers: ASTM G47-98(2011) - Standard Test Method for Determining Susceptibility to Stress-Corrosion Cracking of 2XXX and 7XXX Aluminum Alloy Products
• Effective Date: 01-Sep-2011
• Refers: ASTM G44-99(2005) - Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution
• Effective Date: 01-May-2005
• Refers: ASTM G47-98(2004) - Standard Test Method for Determining Susceptibility to Stress-Corrosion Cracking of 2XXX and 7XXX Aluminum Alloy Products
• Effective Date: 01-May-2004
• Refers: ASTM G44-99 - Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution
• Effective Date: 10-Dec-1999
• Refers: ASTM G47-98 - Standard Test Method for Determining Susceptibility to Stress-Corrosion Cracking of 2XXX and 7XXX Aluminum Alloy Products
• Effective Date: 10-Apr-1998

Overview

ASTM G64-99(2021): Standard Classification of Resistance to Stress-Corrosion Cracking of Heat-Treatable Aluminum Alloys provides a widely recognized framework for evaluating the relative resistance to stress-corrosion cracking (SCC) in wrought, heat-treatable aluminum alloys, specifically from the 2XXX, 6XXX, and 7XXX series. Developed by ASTM International, this standard introduces a qualitative, alphabetical rating system based on laboratory test results and service experiences, serving as an essential guide for engineers and materials specialists focused on optimal alloy selection for corrosion-critical applications.

Key Topics

• Stress-Corrosion Cracking (SCC): SCC is a deterioration phenomenon in aluminum alloys, arising from the combined action of tensile stress and a corrosive environment. The standard defines SCC in the context of aluminum, noting its typical intergranular cracking path.
• Alphabetical Ratings: The standard uses ratings from A (very high resistance) to D (low resistance) to qualitatively rank the SCC performance of different alloy product forms, tempers, and grain orientations.
  • A: Very high resistance - SCC not anticipated if sustained tensile stresses remain below 75% of specified minimum yield strength.
  • B: High resistance - SCC not anticipated if stresses are below 50% of minimum yield strength.
  • C: Intermediate resistance - SCC unlikely if stresses are under 25% of yield strength, primarily for short transverse direction in select products.
  • D: Low resistance - SCC failures have occurred or are anticipated under any sustained tensile stress.
• Influencing Factors: The document emphasizes that stresses introduced during manufacturing, fabrication, or service-such as heat treatment, forming, and loading-play a significant role in SCC susceptibility.
• Testing Methodology: Ratings are primarily determined through alternate immersion laboratory tests in a 3.5% sodium chloride solution, as described in ASTM G44, and using practices outlined in ASTM G47.

Applications

This standard has broad, practical relevance in industries where heat-treatable aluminum alloys are specified under demanding service environments, such as:

• Aerospace Structures: For aircraft frames, fuselage panels, and components, material selection guided by ASTM G64 ratings can reduce the risk of in-service SCC failures.
• Transportation and Marine: Rail cars, ship superstructures, and automotive components benefit from the qualitative SCC resistance classifications, promoting extended service life and lower maintenance costs.
• Structural Engineering: Civil infrastructure applications, including bridges and building facades, often rely on standards-based material selection to prevent unexpected corrosion-induced damage.
• Materials Procurement and Specification: Procurement professionals and design engineers use these ratings to compare alloy options and document compliance with client and regulatory requirements.

Related Standards

For comprehensive corrosion control and materials selection decisions, ASTM G64 should be used in conjunction with:

• ASTM G44: Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution
• ASTM G47: Test Method for Determining Susceptibility to Stress-Corrosion Cracking of 2XXX and 7XXX Aluminum Alloy Products
• MIL-HANDBOOK-5: Metallic Materials and Elements for Aerospace Vehicle Structures
• MIL-STD-1568: Materials and Processes for Corrosion Prevention and Control in Aerospace Systems

These standards provide detailed test procedures and design references to reinforce SCC management strategies in engineering applications.

Practical Value

By referencing ASTM G64-99(2021), decision-makers gain a qualitative, experience-backed framework for assessing and classifying the SCC resistance of commercially available heat-treatable aluminum alloys. While not designed for direct use in mathematical material selection models, the ratings guide alloy choice to mitigate SCC risk across multiple industries. Users are cautioned, however, to consider the impact of unique environments, stress conditions, and any post-standard processing, as these factors may alter SCC performance outside the scope of this classification. For the most robust results, ASTM G64 ratings should be integrated into a broader materials engineering and corrosion prevention strategy.

Standards Content (Sample)

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: G64 −99 (Reapproved 2021)
Standard Classification of
Resistance to Stress-Corrosion Cracking of Heat-Treatable
Aluminum Alloys
ThisstandardisissuedunderthefixeddesignationG64;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Stress-corrosion behavior is an important characteristic to be considered when optimizing the
choice of material for an engineering structure. Unfortunately, there is no generally accepted scale for
measuring it, and stress-corrosion tendencies are difficult to define because of the complex
interdependence of the material, tensile stress, environment, and time. Conventional test-dependent
types of laboratory stress-corrosion data have only very limited applicability in mathematical models
used for materials selection.
This standard is intended to provide a qualitative classification of the relative resistance to
stress-corrosion cracking (SCC) of high-strength aluminum alloys to assist in the selection of
materials. The classification is based on a combination of service experience and a widely accepted
laboratory corrosion test.
It is cautioned, however, that any such generalized classification of alloys can involve an
oversimplification in regard to their behavior in unusual environments. Moreover, the quantitative
prediction of the service performance of a material in a specific situation is outside the scope of this
standard.
1. Scope 1.5 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 This classification covers alphabetical ratings of the
ization established in the Decision on Principles for the
relative resistance to SCC of various mill product forms of the
Development of International Standards, Guides and Recom-
wrought 2XXX, 6XXX, and 7XXX series heat-treated alumi-
mendations issued by the World Trade Organization Technical
num alloys and the procedure for determining the ratings.
Barriers to Trade (TBT) Committee.
1.2 The ratings do not apply to metal in which the metal-
2. Referenced Documents
lurgical structure has been altered by welding, forming, or
other fabrication processes.
2.1 ASTM Standards:
G44 PracticeforExposureofMetalsandAlloysbyAlternate
1.3 The values stated in SI units are to be regarded as
Immersion in Neutral 3.5 % Sodium Chloride Solution
standard. The values given in parentheses after SI units are
G47 Test Method for Determining Susceptibility to Stress-
provided for information only and are not considered standard.
Corrosion Cracking of 2XXX and 7XXX Aluminum
1.4 This standard does not purport to address all of the
Alloy Products
safety concerns, if any, associated with its use. It is the
2.2 Other Documents:
responsibility of the user of this standard to establish appro-
MIL-HANDBOOK-5 Metallic Materials and Elements for
priate safety, health, and environmental practices and deter- 3
Aerospace Vehicle Structures
mine the applicability of regulatory limitations prior to use.
MIL-STD-1568 Materials and Processes for Corrosion Pre-
vention and Control in Aerospace Systems
1 2
This classification is under the jurisdiction of ASTM Committee G01 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Corrosion of Metals and is the direct responsibility of Subcommittee G01.06 on contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Environmentally Assisted Cracking. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Aug. 1, 2021. Published August 2021. Originally the ASTM website.
approved in 1980. Last previous edition approved in 2013 as G64 – 99 (2013). DOI: Available from DLA Document Services, Building 4/D, 700 Robbins Ave.,
10.1520/G0064-99R21. Philadelphia, PA 19111-5094, http://quicksearch.dla.mil.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G64 − 99 (2021)
TABLE 1 Practical Interpretation of Ratings for Resistance to SCC
NOTE 1—The stress levels mentioned below and the test stresses mentioned in 6.2 are not to be interpreted as “threshold” stresses, and are not
recommended for design. Other documents, such as MIL-HANDBOOK-5, MIL-STD-1568, NASC SD-24, and MSFC-SPEC-522A, should be consulted
for design recommendations.
Rating Interpretation
A
A Very high. SCC not anticipated in general applications if the total sustained tensile stress is less than 75 % of the minimum specified yield strength for
the alloy, heat treatment, product form, and orientation.
A
B High. SCC not anticipated if the total sustained tensile stress is less than 50 % of the minimum specified yield strength.
A
C Intermediate. SCC not anticipated if the total sustained tensile stress is less than 25 % of the minimum specified yield strength. This rating is designated
for the short transverse direction in improved products used primarily for high resistance to exfoliation corrosion in relatively thin structures where appre-
ciable short transverse stresses are unlikely.
A
D Low. SCC failures have occurred in service or would be anticipated if there is any sustained tensile stress in the designated test direction. This rating cur-
rently is designated only for the short transverse direction in certain materials.
A
The sum of all stresses including those from service loads (applied), heat treatment, straightening, forming, and so forth.
MSFC-SPEC-522A Design Criteria for Controlling Stress weights and confidence factors can be devised on the basis of
4 5
Corrosion Cracking experience and judgment of the materials engineer.
5. Basis of Classification
3. Terminology
5.1 The stress-corrosion ratings for new or additional ma-
3.1 Definitions:
terials shall be based on laboratory tests of standard smooth
3.1.1 lot—an identifiable quantity of material of the same
specimens for susceptibility at specified stress levels. The
mill form, alloy, temper, section, and size (or thickness, in the
3.5 % NaCl alternate immersion test (Practice G44) was
case of sheet and plate) traceable to a heat treat lot or lots, and
chosen for the laboratory test because it is widely used for
subjected to inspection at one time.
aluminum alloys and is capable of detecting materials that
3.1.2 stress-corrosion cracking (SCC)—a cracking process
would be likely to be susceptible to SCC in natural environ-
that requires the simultaneous action of a corrodent and
ments.
sustained tensile stress. SCC in aluminum alloy products
5.2 Other types of tests using precracked specimens or
historically has been observed to follow an intergranular path
dynamic loading have promise as alternative or supplemen-
leading to the ultimate fracture. Thus, for the purpose of this
tary methods, but they presently require better understanding
standard, a fractured test specimen that reveals only pitting
and standardization.
corrosion or pitting plus transgranular cracking shall not be
considered as an SCC failure (Test Method G47).
6. Test Method
4. Significance and Use 6.1 To rate a new material and test direction, stress-
corrosion tests shall be performed on at least ten random lots.
4.1 Thisclassificationinvolvesalphabeticalratingsintended
The highest rating assigned shall be that for which the test
only to provide a qualitative guide for materials selection. The
results show 90 % conformance at the 95 % confidence level
ratings are based primarily on the results of standard corrosion
when tested at the following stresses:
tests.
A—Equal to or greater than 75 % of the specified minimum
4.2 Interpretations of the SCC ratings in terms of typical
yield strength.
problem areas including service experience are given in Table
B—Equal to or greater than 50 % of the specified minimum
1. P
...