ASTM G44-99(2005)
(Practice)Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution
Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution
SIGNIFICANCE AND USE
The 3.5 % NaCl alternate immersion procedure is a general, all-purpose procedure that produces valid comparisons for most metals, particularly when specimens are exposed at high levels of applied stress or stress intensity.
While the alternate immersion test is an accelerated test and is considered to be representative of certain natural conditions, it is not intended to predict performance in specialized chemical environments in which a different mode of cracking may be operative. For example, it does not predict the performance of aluminum alloys in highly acidic environments such as heated inhibited red fuming nitric acid (IRFNA). For such cases, the results of the alternate immersion test are of doubtful significance until a relationship has been established between it and anticipated service environments.
While this practice is applicable in some degree to all metals, it is not equally discriminative of all alloys, even within the same metal system. Consequently, information should be established to allow comparisons of performances of the alloy of interest in the alternate immersion test and in natural environments.
Note 2—The alternate immersion concept can be useful for exposure of corrosion specimens in other solutions because the procedure and apparatus provide a controlled set of conditions. Details of this are beyond the scope of this practice.
SCOPE
1.1 This practice covers procedures for making alternate immersion stress corrosion tests in 3.5 % sodium chloride (NaCl) (). It is primarily for tests of aluminum alloys (Test Method G 47) and ferrous alloys, but may be used for other metals exhibiting susceptibility to chloride ions. It sets forth the environmental conditions of the test and the means for controlling them.Note 1
Alternate immersion stress corrosion exposures are sometimes made in substitute ocean water (without heavy metals) prepared in accordance with Specification D 1141. The general requirements of this present practice are also applicable to such exposures except that the reagents used, the solution concentration, and the solution pH should be as specified in Specification D 1141.
1.2 This practice can be used for both stressed and unstressed corrosion specimens. Historically, it has been used for stress-corrosion cracking testing, but is often used for other forms of corrosion, such as uniform, pitting, intergranular, and galvanic.
1.3 This practice is intended for alloy development and for applications where the alternate immersion test is to serve as a control test on the quality of successive lots of the same material. Therefore, strict test conditions are stipulated for maximum assurance that variations in results are attributable to variations in the material being tested.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
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: G44 − 99(Reapproved 2005)
Standard Practice for
Exposure of Metals and Alloys by Alternate Immersion in
Neutral 3.5 % Sodium Chloride Solution
ThisstandardisissuedunderthefixeddesignationG44;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.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This practice covers procedures for making alternate
immersion stress corrosion tests in 3.5 % sodium chloride D1141 Practice for the Preparation of Substitute Ocean
Water
(NaCl) (Note 1). It is primarily for tests of aluminum alloys
(Test Method G47) and ferrous alloys, but may be used for D1193 Specification for Reagent Water
E3 Guide for Preparation of Metallographic Specimens
other metals exhibiting susceptibility to chloride ions. It sets
forththeenvironmentalconditionsofthetestandthemeansfor G1 Practice for Preparing, Cleaning, and Evaluating Corro-
sion Test Specimens
controlling them.
G16 Guide for Applying Statistics to Analysis of Corrosion
NOTE 1—Alternate immersion stress corrosion exposures are some-
Data
times made in substitute ocean water (without heavy metals) prepared in
G47 Test Method for Determining Susceptibility to Stress-
accordance with Specification D1141. The general requirements of this
present practice are also applicable to such exposures except that the Corrosion Cracking of 2XXX and 7XXX Aluminum
reagentsused,thesolutionconcentration,andthesolutionpHshouldbeas
Alloy Products
specified in Specification D1141.
3. Summary of Practice
1.2 This practice can be used for both stressed and un-
stressed corrosion specimens. Historically, it has been used for
3.1 The alternate immersion test utilizes a 1-h cycle that
stress-corrosion cracking testing, but is often used for other
includes a 10-min period in an aqueous solution of 3.5 %
forms of corrosion, such as uniform, pitting, intergranular, and
sodium chloride (NaCl) followed by a 50-min period out of the
galvanic.
solution, during which the specimens are allowed to dry. This
1-h cycle is continued 24 h/day for the total number of days
1.3 This practice is intended for alloy development and for
recommended for the particular alloy being tested. Typically,
applications where the alternate immersion test is to serve as a
aluminumandferrousalloysareexposedfrom20to90daysor
control test on the quality of successive lots of the same
longer, depending upon the resistance of the alloy to corrosion
material. Therefore, strict test conditions are stipulated for
by saltwater.
maximumassurancethatvariationsinresultsareattributableto
variations in the material being tested.
4. Significance and Use
1.4 The values stated in SI units are to be regarded as the
4.1 The 3.5 % NaCl alternate immersion procedure is a
standard. The values given in parentheses are for information
general,all-purposeprocedurethatproducesvalidcomparisons
only.
for most metals, particularly when specimens are exposed at
1.5 This standard does not purport to address all of the
high levels of applied stress or stress intensity.
safety concerns, if any, associated with its use. It is the
4.2 While the alternate immersion test is an accelerated test
responsibility of the user of this standard to establish appro-
and is considered to be representative of certain natural
priate safety and health practices and determine the applica-
conditions, it is not intended to predict performance in special-
bility of regulatory limitations prior to use.
ized chemical environments in which a different mode of
cracking may be operative. For example, it does not predict the
performance of aluminum alloys in highly acidic environments
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion
of Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-
tally Assisted Cracking. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2005. Published May 2005. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1975. Last previous edition approved in 1999 as G44 – 99. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/G0044-99R05. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G44 − 99 (2005)
such as heated inhibited red fuming nitric acid (IRFNA). For 6.3 Materials of Construction:
such cases, the results of the alternate immersion test are of 6.3.1 Materialsofconstructionthatcomeincontactwiththe
doubtful significance until a relationship has been established salt solution shall be such that they are not affected by the
between it and anticipated service environments. corrodent to an extent that they can cause contamination of the
solution and change its corrosiveness.
4.3 While this practice is applicable in some degree to all
6.3.2 Use of inert plastics or glass is recommended where
metals,itisnotequallydiscriminativeofallalloys,evenwithin
feasible.
the same metal system. Consequently, information should be
6.3.3 Metallic materials of construction should be selected
established to allow comparisons of performances of the alloy
from alloys that are recommended for marine use and of the
of interest in the alternate immersion test and in natural
same general type as the metals being tested. Preferably, all
environments.
metal parts should be protected with a suitable corrosion-
NOTE2—Thealternateimmersionconceptcanbeusefulforexposureof
resistant coating that also satisfies paragraph 6.3.1.
corrosion specimens in other solutions because the procedure and appa-
6.4 Specimen Holders:
ratus provide a controlled set of conditions. Details of this are beyond the
scope of this practice.
6.4.1 Specimen holders should be designed to electrically
insulatethespecimensfromeachotherandfromanyotherbare
5. Interferences
metal. When this is not possible, as in the case of certain
stressing bolts or jigs, the bare metal contacting the specimen
5.1 A disadvantage of the 3.5 % NaCl alternate immersion
should be isolated from the corrodent by a suitable coating.
test for stres-corrosion cracking tests of certain high-strength
Protective coatings should be of a type that will not leach
aluminum alloys is the severe pitting that develops in the
inhibiting or accelerating ions or protective oils over the
specimens. Such pitting can interfere with the initiation of
noncoated portions of the specimen. Coatings containing
stress-corrosion cracks and may cause mechanical failures that
chromates are to be particularly avoided.
complicatetheinterpretationofthestress-corrosiontestresults.
This is particularly a problem with copper-bearing aluminum
NOTE 3—Coatings that have been satisfactorily used by several labo-
ratories are described in Appendix X1.
alloys when tested with specimens of small cross section.
Thorough metallographic examination of the specimens is
6.4.2 The shape and form of specimen supports and holders
necessary for proper diagnosis of the cause of failure and
should be such that:
separation of stress corrosion failures from those caused by
6.4.2.1 They avoid, as much as possible, any interference of
mechanical overload.
free contact of the specimen with the salt solution;
6.4.2.2 They do not obstruct air flow over the specimen,
5.2 An advantage of the substitute ocean water (Note 1)is
thereby retarding the drying rate;
that it causes less pitting corrosion of aluminum alloys than the
6.4.2.3 They do not retain a pool of solution in contact with
3.5 % NaCl solution.
the specimen after the immersion period; and
6.4.2.4 Drainage from one specimen does not directly con-
6. Apparatus
tact any other specimen.
6.1 Method of Cycling—Any suitable mechanism may be
used to accomplish the immersion portion of the cycle pro-
7. Reagents
vided that: (1) it achieves the specified rate of immersion and
7.1 Reagent grade sodium chloride (NaCl) shall be used
removal, and (2) the apparatus is constructed of suitable inert
conforming to the specifications of the Committee onAnalyti-
materials. The usual methods of immersion are:
cal Reagents of the American Chemical Society, where such
6.1.1 Specimens placed on a movable rack that is periodi-
specifications are applicable (see Note 1).
cally lowered into a stationary tank containing the solution.
6.1.2 Specimens placed on a hexagonal Ferris wheel ar-
7.2 The solution shall be prepared using distilled or deion-
rangement which rotates every 10 min through 60° and,
ized water conforming to the purity requirements of Specifi-
thereby, passes the specimens through a stationary tank of
cation D1193, Type IV reagent water except that for this
solution. Use of a Ferris wheel continuously rotating at a rate
practice the values for chloride and sodium shall be disre-
of 1 revolution per hour is not recommended for very large
garded.
specimens for which the rate of immersion would be slower
than that specified in 6.2. 8. Solution Conditions
6.1.3 Specimens placed in a stationary tray open to the
8.1 Concentration—The salt solution shall be prepared by
atmosphere and having the solution moved by air pressure,
dissolving 3.5 6 0.1 parts by weight of NaCl in 96.5 parts of
nonmetallic pump, or gravity drain from a reservoir to the tray.
water.
6.2 Rate of Immersion—The rate of immersion and removal
of the specimens from the solution should be as rapid as
possible without jarring them. For purposes of standardization,
Reagent Chemicals, American Chemical Society Specifications, American
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
anarbitrarylimitshallbeadoptedsuchthatnomorethan2min
listed by the American Chemical Society, see Analar Standards for Laboratory
elapse from the time the first portion of any specimen is
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
covered (or uncovered) until it is fully covered (or uncovered)
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
by solution. MD.
G44 − 99 (2005)
8.2 Solution pH: 9.2 Relative Humidity—The percent relative humidity of the
8.2.1 The pH of the salt solution, when freshly prepared, air shall be controlled at 45 6 10 % throughout the entire test
shall be within the range from 6.4 to 7.2. Only diluted, reagent cycle.
gradehydrochloricacid(HCl)orreagentgradesodiumhydrox-
NOTE6—Careshouldbetakentoavoidovernightandweekendchanges
ide (NaOH) shall be used to adjust the pH.
in the operation of the laboratory heating, ventilating, and air conditioning
8.2.2 Experience has shown that periodic adjustment of pH
equipment which could result in systematic excursions outside the
temperatureandhumiditycontrolranges.Occasionalexcursionsrelatedto
is not necessary when the minimum volume of solution is met
poweroutage,mechanicalfailure,orweatherconditionsshallberecorded.
and the solution is replaced at the specified interval. For a new
9.3 Air Circulation:
testing facility, however, it is recommended that daily pH
measurements be made for one week to verify stable operation 9.3.1 Air circulation is recognized to be an important
and adjustments made if required. If the solution is used longer consideration because it affects both the rate at which speci-
than the recommended interval (8.6), a pH measurement and mens dry and the loss of water by evaporation. Optimum
any necessary adjustment should be made at least weekly. conditions for air circulation have not been established, but the
recommendations described in 9.3.2 should be followed.
8.3 Temperature—A freshly prepared solution should be
9.3.2 The most important consideration is to achieve the
allowed to come to within 3°C of the specified room tempera-
moderate specimen drying conditions stipulated in 11.2. Be-
ture before being used (9.1.1). Thereafter, no control is
cause various testing facilities use different immersion appa-
required on the solution temperature per se. Instead, the room
ratus and room sizes, individual experimentation is required to
air temperature is controlled and the solution is allowed to
achieve adequate circulation. A mild circulation of air is
reach temperature equilibrium.
recommended with two precautionary considerations:
8.4 Minimum Volume—The volume of the test solution
9.3.2.1 Drying by forced air blasts on the specimens is not
should be large enough to avoid any appreciable change in its
recommended because of difficulty in maintaining uniform
corrosiveness either through exhaustion of corrosive constitu-
drying of large groups of specimens.
ents, or the accumulation of corrosion products or other
9.3.2.2 Stagnant air conditions should be avoided.
constituents that might significantly affect further corrosion.
10. Calibration and Standardization
An arbitrary minimum ratio between the volume of test
solution and area of specimen (including any uncoated acces-
10.1 When a new test facility is established, calibration
2 2
sories) of 32 mL/cm (200 mL/in. ) of specimen area is
stresscorrosiontestsshouldbeconductedtodeterminehowthe
recommended.
results obtained compare with published data on well estab-
lished alloys. Such tests are best made on products of simple
8.5 Replenishment of Water Lost by Evaporation—
geometry with uniform grain structure, such as rolled bar stock
Evaporation losses should be made up by frequent, at least
or plate.
daily, additions of water of the required purity (7.2). Evapora-
tion losses must not be replenished with the salt solution. The
10.2 It is recommended that each testing facility maintain a
simplest and recommended procedure is to initially fill the
supply of a control lot of stress corrosion-susceptible material
solution to a liquid level line and refill to that line daily.
that can be exposed periodically to demonstrate the reproduc-
Automatic constant liquid level devices may be used, but are
ibility of its test procedure. Preferably this should be material
not required. An alternative method is to check the solution
for which a history of performance in natural environments is
with a hydrometer and add the necessary amount of water to
available.
bring the salt concentration to 3.5 %.
11. Procedure
8.6 Replacement of Solution—Fresh solution shall be pre-
pared weekly. At such time, the portions of the apparatus that 11.1 Period of Cycle—Totallyimmersespecimensinthesalt
contact the solution should be c
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