ASTM G31-72(2004)
(Practice)Standard Practice for Laboratory Immersion Corrosion Testing of Metals
Standard Practice for Laboratory Immersion Corrosion Testing of Metals
SIGNIFICANCE AND USE
Corrosion testing by its very nature precludes complete standardization. This practice, rather than a standardized procedure, is presented as a guide so that some of the pitfalls of such testing may be avoided.
Experience has shown that all metals and alloys do not respond alike to the many factors that affect corrosion and that “accelerated” corrosion tests give indicative results only, or may even be entirely misleading. It is impractical to propose an inflexible standard laboratory corrosion testing procedure for general use, except for material qualification tests where standardization is obviously required.
In designing any corrosion test, consideration must be given to the various factors discussed in this practice, because these factors have been found to affect greatly the results obtained.
SCOPE
1.1 This practice describes accepted procedures for and factors that influence laboratory immersion corrosion tests, particularly mass loss tests. These factors include specimen preparation, apparatus, test conditions, methods of cleaning specimens, evaluation of results, and calculation and reporting of corrosion rates. This practice also emphasizes the importance of recording all pertinent data and provides a checklist for reporting test data. Other ASTM procedures for laboratory corrosion tests are tabulated in the Appendix. (Warning-In many cases the corrosion product on the reactive metals titanium and zirconium is a hard and tightly bonded oxide that defies removal by chemical or ordinary mechanical means. In many such cases, corrosion rates are established by mass gain rather than mass loss.)
1.2 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:G31–72 (Reapproved 2004)
Standard Practice for
Laboratory Immersion Corrosion Testing of Metals
ThisstandardisissuedunderthefixeddesignationG31;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (ϵ) indicates an editorial change since the last revision or reapproval.
1. Scope G16 Guide forApplying Statistics toAnalysis of Corrosion
Data
1.1 This practice describes accepted procedures for and
G46 Guide for Examination and Evaluation of Pitting
factors that influence laboratory immersion corrosion tests,
Corrosion
particularly mass loss tests. These factors include specimen
preparation, apparatus, test conditions, methods of cleaning
3. Significance and Use
specimens, evaluation of results, and calculation and reporting
3.1 Corrosion testing by its very nature precludes complete
of corrosion rates. This practice also emphasizes the impor-
standardization. This practice, rather than a standardized pro-
tance of recording all pertinent data and provides a checklist
cedure, is presented as a guide so that some of the pitfalls of
for reporting test data. Other ASTM procedures for laboratory
such testing may be avoided.
corrosion tests are tabulated in the Appendix. (Warning—In
3.2 Experience has shown that all metals and alloys do not
many cases the corrosion product on the reactive metals
respond alike to the many factors that affect corrosion and that
titanium and zirconium is a hard and tightly bonded oxide that
“accelerated” corrosion tests give indicative results only, or
defies removal by chemical or ordinary mechanical means. In
mayevenbeentirelymisleading.Itisimpracticaltoproposean
many such cases, corrosion rates are established by mass gain
inflexible standard laboratory corrosion testing procedure for
rather than mass loss.)
general use, except for material qualification tests where
1.2 The values stated in SI units are to be regarded as the
standardization is obviously required.
standard. The values given in parentheses are for information
3.3 In designing any corrosion test, consideration must be
only.
given to the various factors discussed in this practice, because
1.3 This standard does not purport to address all of the
these factors have been found to affect greatly the results
safety concerns, if any, associated with its use. It is the
obtained.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Interferences
bility of regulatory limitations prior to use.
4.1 The methods and procedures described herein represent
the best current practices for conducting laboratory corrosion
2. Referenced Documents
3 tests as developed by corrosion specialists in the process
2.1 ASTM Standards:
industries. For proper interpretation of the results obtained, the
A262 Practices for Detecting Susceptibility to Intergranular
specific influence of certain variables must be considered.
Attack in Austenitic Stainless Steels
These include:
E8 Test Methods for Tension Testing of Metallic Materials
4.1.1 Metal specimens immersed in a specific hot liquid
G1 Practice for Preparing, Cleaning, and Evaluating Corro-
may not corrode at the same rate or in the same manner as in
sion Test Specimens
equipment where the metal acts as a heat transfer medium in
G4 Guide for Conducting Corrosion Tests in FieldApplica-
heating or cooling the liquid. If the influence of heat transfer
tions
effects is specifically of interest, specialized procedures (in
which the corrosion specimen serves as a heat transfer agent)
This practice is under the jurisdiction of ASTM Committee J01 on Corrosion 4
must be employed (1).
and is the direct responsibility of Subcommittee J01.01 on Working Group on
4.1.2 In laboratory tests, the velocity of the environment
Laboratory Immersion Tests.
Current edition approved May 1, 2004. Published May 2004. Originally
relative to the specimens will normally be determined by
approved in 1972. Last previous edition approved in 1998 as G31 – 72 (1998). DOI:
convection currents or the effects induced by aeration or
10.1520/G0031-72R04.
2 boiling or both. If the specific effects of high velocity are to be
This practice is based upon NACE Standard TM-01-69, “Test Method-
studied, special techniques must be employed to transfer the
Laboratory Corrosion Testing of Metals for the Process Industries,” with modifica-
tions to relate more directly to Practices G1 and G1 and Guide G4.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers in parentheses refer to the list of references at the end of
the ASTM website. this practice.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G31–72 (2004)
environment through tubular specimens or to move it rapidly 4.1.7.4 Dealloying or “parting” corrosion is a condition in
past the plane face of a corrosion coupon (2).Alternatively, the which one constituent is selectively removed from an alloy, as
coupon may be rotated through the environment, although it is in the dezincification of brass or the graphitization of cast iron.
then difficult to evaluate the velocity quantitatively because of Close attention and a more sophisticated evaluation than a
the stirring effects incurred. simple mass loss measurement are required to detect this
phenomenon.
4.1.3 The behavior of certain metals and alloys may be
4.1.7.5 Certain metals and alloys are subject to a highly
profoundly influenced by the presence of dissolved oxygen. If
localized type of attack called pitting corrosion.This cannot be
this is a factor to be considered in a specific test, the solution
evaluated by mass loss alone. The reporting of nonuniform
should be completely aerated or deaerated in accordance with
corrosion is discussed below. It should be appreciated that
8.7.
pitting is a statistical phenomenon and that the incidence of
4.1.4 In some cases, the rate of corrosion may be governed
pittingmaybedirectlyrelatedtotheareaofmetalexposed.For
by other minor constituents in the solution, in which case they
example, a small coupon is not as prone to exhibit pitting as a
will have to be continually or intermittently replenished by
large one and it is possible to miss the phenomenon altogether
changing the solution in the test.
in the corrosion testing of certain alloys, such as theAISIType
4.1.5 Corrosion products may have undesirable effects on a
300 series stainless steels in chloride contaminated environ-
chemical product. The amount of possible contamination can
ments.
beestimatedfromthelossinmassofthespecimen,withproper
4.1.7.6 All metals and alloys are subject to stress-corrosion
application of the expected relationships among (1) the area of
cracking under some circumstances. This cracking occurs
corroding surface, (2) the mass of the chemical product
under conditions of applied or residual tensile stress, and it
handled, and (3) the duration of contact of a unit of mass of the
may or may not be visible to the unaided eye or upon casual
chemical product with the corroding surface.
inspection. A metallographic examination may confirm the
4.1.6 Corrosionproductsfromthecouponmayinfluencethe
presence of stress-corrosion cracking. It is imperative to note
corrosion rate of the metal itself or of different metals exposed
that this usually occurs with no significant loss in mass of the
at the same time. For example, the accumulation of cupric ions
testcoupon,althoughcertainrefractorymetalsareanexception
in the testing of copper alloys in intermediate strengths of
to these observations. Generally, if cracking is observed on the
sulfuric acid will accelerate the corrosion of copper alloys, as
coupon, it can be taken as positive indication of susceptibility,
compared to the rates that would be obtained if the corrosion
whereas failure to effect this phenomenon simply means that it
products were continually removed. Cupric ions may also
did not occur under the duration and specific conditions of the
exhibit a passivating effect upon stainless steel coupons ex-
test. Separate and special techniques are employed for the
posed at the same time. In practice, only alloys of the same
specific evaluation of the susceptibility of metals and alloys to
general type should be exposed in the testing apparatus.
stress corrosion cracking (see Ref. (3)).
4.1.7 Coupon corrosion testing is predominantly designed
to investigate general corrosion. There are a number of other
5. Apparatus
special types of phenomena of which one must be aware in the
5.1 A versatile and convenient apparatus should be used,
design and interpretation of corrosion tests.
consisting of a kettle or flask of suitable size (usually 500 to
4.1.7.1 Galvanic corrosion may be investigated by special
5000 mL), a reflux condenser with atmospheric seal, a sparger
devices which couple one coupon to another in electrical
for controlling atmosphere or aeration, a thermowell and
contact. The behavior of the specimens in this galvanic couple
temperature-regulating device, a heating device (mantle, hot
are compared with that of insulated specimens exposed on the
plate, or bath), and a specimen support system. If agitation is
same holder and the galvanic effects noted. It should be
required, the apparatus can be modified to accept a suitable
observed, however, that galvanic corrosion can be greatly
stirring mechanism, such as a magnetic stirrer. A typical resin
affected by the area ratios of the respective metals, the distance
flask setup for this type test is shown in Fig. 1.
between the metals and the resistivity of the electrolyte. The
5.2 The suggested components can be modified, simplified,
coupling of corrosion coupons then yields only qualitative
or made more sophisticated to fit the needs of a particular
results, as a particular coupon reflects only the relationship
investigation. The suggested apparatus is basic and the appa-
between these two metals at the particular area ratio involved.
ratus is limited only by the judgment and ingenuity of the
4.1.7.2 Crevice corrosion or concentration cell corrosion
investigator.
may occur where the metal surface is partially blocked from
5.2.1 Aglass reaction kettle can be used where the configu-
the corroding liquid as under a spacer or supporting hook. It is
ration and size of the specimen will permit entry through the
necessary to evaluate this localized corrosion separately from
narrowkettleneck(forexample,45/50ground-glassjoint).For
the overall mass loss. solutions corrosive to glass, suitable metallic or plastic kettles
may be employed.
4.1.7.3 Selective corrosion at the grain boundaries (for
5.2.2 Insomecasesawide-mouthjarwithasuitableclosure
example, intergranular corrosion of sensitized austenitic stain-
less steels) will not be readily observable in mass loss is sufficient when simple immersion tests at ambient tempera-
tures are to be investigated.
measurementsunlesstheattackissevereenoughtocausegrain
dropping, and often requires microscopic examination of the
5.2.3 Open-beaker tests should not be used because of
coupons after exposure. evaporation and contamination.
G31–72 (2004)
7.3 The size and shape of specimens will vary with the
purpose of the test, nature of the materials, and apparatus used.
A large surface-to-mass ratio and a small ratio of edge area to
total area are desirable. These ratios can be achieved through
the use of square or circular specimens of minimum thickness.
Maskingmayalsobeusedtoachievethedesiredarearatiosbut
may cause crevice corrosion problems. Circular specimens
should preferably be cut from sheet and not bar stock, to
minimizetheexposedendgrain.Specialcoupons(forexample,
sections of welded tubing) may be employed for specific
purposes.
7.3.1 A circular specimen of about 38-mm (1.5-in.) diam-
eter is a convenient shape for laboratory corrosion tests. With
a thickness of approximately 3 mm (0.125-in.) and an 8-mm
5 7
( ⁄16-in.) or 11-mm ( ⁄16-in.) diameter hole for mounting, these
specimens will readily pass through a 45/50 ground-glass joint
of a distillation kettle. The total surface area of a circular
specimen is given by the following equation:
2 2
A 5p/2~D 2 d ! 1 tpD 1 tpd (1)
where:
t = thickness,
D = diameter of the specimen, and
d = diameter of the mounting hole.
NOTE 1—The flask can be used as a versatile and convenient apparatus
7.3.1.1 If the hole is completely covered by the mounting
to conduct simple immersion tests. Configuration of top to flask is such
support, the last term (tπd) in the equation is omitted.
that more sophisticated apparatus can be added as required by the specific
7.3.2 Strip coupons 50 by 25 by 1.6 or 3 mm (2 by 1 by ⁄16
testbeingconducted.A = thermowell,B = resinflask,C = specimenshung
or ⁄8 in.) may be preferred as corrosion specimens, particularly
on supporting device, D = air inlet, E = heating mantle, F = liquid inter-
if interface or liquid line effects are to be studied by the
face, G = opening in flask for additional apparatus that may be required,
laboratory tests (see Fig. 1), but the evaluation of such specific
and H = reflux condenser.
FIG. 1 Typical Resin Flask
effects are beyond the scope of this practice.
7.3.3 All specimens should be measured carefully to permit
accurate calculation of the exposed areas. A geometric area
5.2.4 In more complex tests, provisions might be needed for
calculation accurate to 61 % is usually adequate.
continuous flow or replenishment of the corrosive liquid, while
7.4 More uniform results may be expected if a substantial
simultaneously maintaining a controlled atmosphere.
layer of metal is removed from the specimens to eliminate
variationsinconditionoftheoriginalmetallicsurface.Thiscan
6. Sampling
be done by chemical treatment (pickling), electrolytic removal,
6.1 The bulk sampling of products is outside the scope of
orbygrindingwithacoarseabrasivepaperorclothsuchasNo.
this practice.
50, using care not to work harden the surface (see section 5.7).
At least 0.0025 mm (0.0001 in.) or 0.0155 to 0.0233 mg/mm
7. Test Specimen
(10 to 15 mg/in. ) should be removed. (If clad alloy specimens
7.1 In laboratory tests, uniform corrosion rates of duplicate are to be used, special attention must be given to ensure that
specimens are usually within 610 % under the same test excessive metal is not removed.) After final preparation of the
conditions. Occasional exceptions, in which a large differenc
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