ASTM G78-95

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Designation: G 78 – 95
Standard Guide for
Crevice Corrosion Testing of Iron-Base and Nickel-Base
Stainless Alloys in Seawater and Other Chloride-Containing
Aqueous Environments
This standard is issued under the fixed designation G 78; 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.
INTRODUCTION
Crevice corrosion of iron-base and nickel-base stainless alloys can occur when an occlusion or
crevice limits access of the bulk environment to a localized area of the metal surface. Localized
environmental changes in this stagnant area can result in the formation of acidic/high chloride
conditions that may result in initiation and propagation of crevice corrosion of susceptible alloys.
In practice, crevices can generally be classified into two categories: (a) naturally occurring, that is,
those created by biofouling, sediment, debris, deposits, etc. and (b) man-made, that is, those created
during manufacturing, fabrication, assembly, or service. Crevice formers utilized in laboratory and
field studies can represent actual geometric conditions encountered in some service applications. Use
of such crevice formers in service-type environments are not considered accelerated test methods.
The geometry of a crevice can be described by the dimensions of crevice gap and crevice depth.
Crevice gap is identified as the width or space between the metal surface and the crevice former.
Crevice depth is the distance from the mouth to the center or base of the crevice.
1. Scope G 4 Guide for Conducting Corrosion Coupon Tests in Field
Applications
1.1 This guide provides information for conducting crevice-
G 15 Terminology Relating to Corrosion and Corrosion
corrosion tests and identifies factors that may affect results and
Testing
influence conclusions.
G 46 Guide for Examination and Evaluation of Pitting
1.2 These procedures can be used to identify conditions
Corrosion
most likely to result in crevice corrosion and provide a basis for
G 48 Test Methods for Pitting and Crevice Corrosion Re-
assessing the relative resistance of various alloys to crevice
sistance of Stainless Steels and Related Alloys by the Use
corrosion under certain specified conditions.
of Ferric Chloride Solution
1.3 The values stated in SI units are to be regarded as the
standard. The values given in parentheses are for information
3. Terminology
only.
3.1 Definitions of related terms can be found in Terminol-
1.4 This standard does not purport to address all of the
ogy G 15.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Significance and Use
priate safety and health practices and determine the applica-
4.1 This guide covers procedures for crevice-corrosion test-
bility of regulatory limitations prior to use. For a specific
ing of iron-base and nickel-base stainless alloys in seawater.
precautionary statement, see 7.1.1.
The guidance provided may also be applicable to crevicecor-
rosion testing in other chloride containing natural waters and
2. Referenced Documents
various laboratory prepared aqueous chloride environments.
2.1 ASTM Standards:
4.2 This guide describes the use of a variety of crevice
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
formers including the nonmetallic, segmented washer design
rosion Test Specimens
referred to as the multiple crevice assembly (MCA) as de-
scribed in 9.2.2.
This guide is under the jurisdiction of ASTM Committee G-1 on Corrosion of
4.3 In-service performance data provide the most reliable
Metals and is the direct responsibility of Subcommittee G01.09on Corrosion in
determination of whether a material would be satisfactory for
Natural Waters.
Current edition approved March 15, 1995. Published May 1995. Originally
a particular end use. Translation of laboratory data from a
published as G 78 – 83. Last previous edition G 78 – 89.
single test program to predict service performance under a
Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
G78
variety of conditions should be avoided. Terms, such as former should be determined and recorded.
immunity, superior resistance, etc., provide only a general and
6.3 Variations in the boldly exposed (crevice-free) to
relatively qualitative description of an alloy’s corrosion per-
shielded (crevice) area ratio of the test specimen may influence
formance. The limitations of such terms in describing resis-
crevice corrosion. All specimens in a test series should have the
tance to crevice corrosion should be recognized.
same nominal surface area. While no specific specimen dimen-
4.4 While the guidance provided is generally for the pur-
sions are recommended, test panels measuring up to 300 by
pose of evaluating sheet and plate materials, it is also appli-
300 mm (11.81 3 11.81 in.) have been used in seawater tests
cable for crevice-corrosion testing of other product forms, such
with both naturally occurring and man-made crevice formers.
as tubing and bars.
For laboratory studies, the actual size of the specimen may be
4.5 The presence or absence of crevice corrosion under one
limited by the dimensions of the test apparatus and this should
set of conditions is no guarantee that it will or will not occur
be taken into consideration in making comparisons.
under other conditions. Because of the many interrelated
6.3.1 A test program may be expanded to assess any effect
metallurgical, environmental, and geometric factors known to
of boldly exposed to shielded area ratio.
affect crevice corrosion, results from any given test may or
6.3.2 If crevice geometry aspects, such as crevice depth, are
may not be indicative of actual performance in service appli-
to be studied, the adoption of a constant boldly exposed to
cations where the conditions may be different from those of the
shielded area ratio is recommended to minimize the number of
test.
test variables.
5. Apparatus
6.4 When specimens are cut by shearing, it is recommended
5.1 Laboratory tests utilizing filtered, natural seawater, or
that the deformed material be removed by machining or
other chloride containing aqueous environments are frequently
grinding. Test pieces that are warped or otherwise distorted
conducted in tanks or troughs under low velocity (for example,
should not be used. The need to provide parallel surfaces
;0.5 m/s (1.64 ft/s) or less) or quiescent conditions. Contain-
between the crevice former and the test specimen is an
ers should be resistant to the test media.
important consideration in providing maximum consistency in
5.2 Fig. 1 shows a typical test apparatus for conducting
the application of the crevice former.
crevice-corrosion tests under controlled temperature conditions
6.5 Appropriate holes should be drilled (and deburred) in
with provisions for recirculation or refreshment of the aqueous
the test specimen to facilitate attachment of the crevice former.
environment, or both, at a constant level.
Punched holes are not recommended since the punching
5.3 The apparatus should be suitably sized to provide
process may contribute to specimen distortion or work hard-
complete immersion of the test panel. Vertical positioning of
ening, or both. The diameter of the holes should be large
the crevice-corrosion specimens facilitates visual inspection
enough to allow clearance of the fastener (and insulator)
without the need to remove them from the environments.
otherwise additional crevice sites may be introduced.
6. Test Specimens
6.6 Specimens should be identified by alloy and replication.
Mechanical stenciling or engraving are generally suitable,
6.1 Because of the number of variables which may affect the
provided that the coding is on surfaces away from the intended
test results, a minimum of three specimens are suggested for
each set of environmental, metallurgical, or geometric condi- crevice sites. Identification markings should be applied prior to
tions to be evaluated. If reproducibility is unsatisfactory, the final specimen cleaning before test. Marking the samples
additional specimens should be tested. may affect the test results. See the Identification of Test
6.2 Dimensions of both the test specimen and crevice Specimens section of Method G 4.
FIG. 1 Positioning of Crevice-Corrosion Test Specimens—Typical Arrangement in Controlled Environment Apparatus
G78
6.7 Depending on the test objectives, mill-produced sur- ene, PTFE-fluorocarbons, and acetal resin are a few of the
faces may be left intact or specimens may be prepared by commonly used nonmetallics.
providing a surface definable in terms of a given preparation 9.1.4 The properties of the nonmetallic crevice former must
process. be compatible with the physical and environmental demands of
6.7.1 Because of the possible variations between “as- the test.
produced” alloy surface finishes, the adoption of a given 9.1.5 Regardless of the material or type of crevice former,
surface finish is recommended if various alloys are to be contacting surfaces should be kept as flat as possible to
compared. This will tend to minimize the variability of crevice enhance reproducibility of crevice geometry.
geometry in contact areas. 9.2 Various Designs:
6.7.2 While some specific alloys may have proprietary 9.2.1 Fig. 2 shows the shapes of a few popular crevice
surface conditioning, some uncertainty may exist with regard former designs, such as coupons, strips, O-rings, blocks,
to the actual end use surface finish. It is recommended that continuous and segmented washers. In many cases, two crevice
more than one surface condition be examined to assess any formers are fastened to a flat specimen, that is, one on each
effect of surface finish on an individual alloy’s crevice corro- side.
sion behavior. 9.2.2 Multiple crevice assemblies (MCA) consist of two
6.7.3 Surface grinding with 120-grit SiC abrasive paper is a nonmetallic segmented washers, each having a number of
suitable method for preparing laboratory test specimens. Wet grooves and plateaus. The design shown in Figs. 3 and 4 is only
grinding is preferred to avoid any heating. Depending on the one of a number of variations of the multiple crevice assembly
surface roughness of the mill product, machining may be which are in use. Each plateau, in contact with the metal
required prior to final grinding. surface, provides a possible site for initiation of crevice
corrosion. Multiple crevice assemblies fabricated of acetal
7. Cleaning
resin have been shown to be suitable for seawater exposures.
Other nonmetallics, such as PTFE-fluorocarbon, have also
7.1 Pre-Test Cleaning:
been used (see 9.1.4).
7.1.1 Cleaning procedures shall be consistent with Practice
9.2.3 For metal-to-metal crevice-corrosion tests, flat wash-
G 1. Typically, this may include degreasing with a suitable
ers or coupons are often fastened to a larger test specimen. All
solvent, followed by vigorous brush scrubbing with pumice
components should be of the same material and prepared for
powder, followed by water rinse, clean solvent rinse, and air
exposure in the same manner.
drying.
9.2.3.1 Crevice testing with metal to metal components
NOTE 1—Caution: Solvent safety and compatibility with the test ma-
assembled with either nonmetal or metal fasteners (with
terial should be investigated and safe practices followed.
insulator) will necessarily result in the formation of secondary
7.1.2 For the most part, commercially produced stainless
crevice sites where the fastener contacts the metallic crevice
alloys and surface ground materials do not require a pre-
former. In some cases, the geometry of these secondary sites
exposure pickling treatment. The use of acid cleaning or
may be more severe than the intended primary crevice site.
pretreatments shall be considered only when the crevice-
9.3 Method of Attachment:
corrosion test is designed to provide guidance for a specific
9.3.1 Either metallic or nonmetallic fasteners, for example,
application.
nut- and bolt-type, can be used to secure the crevice formers to
7.1.3 Any use of chemical pretreatments shall be thoroughly 4
the test panel.
documented and appropriate safety measures followed.
9.3.2 Metallic fasteners are often preferable because of their
greater strength advantage over nonmetallics. Corrosion resis-
8. Mass Loss Determinations
tant alloys should be selected for the fastener material. Tita-
8.1 Mass loss data calculated from specimen weighing
nium, Alloy 625 (UNS No. N06625) and Alloy C-276 (UNS
before and after testing may provide some useful information
No. N10276) have proven corrosion resistance in marine
in specific cases. However, comparisons of alloy performance
environments and are frequently utilized for crevice-corrosion
based solely on mass loss may be misleading because highly
tests.
localized corrosion, which is typical of crevice corrosion, can
9.3.3 When metallic fasteners are used, they should be
often result in relatively small mass losses.
electrically insulated from the test specimen.
9.3.4 The use of a torque wrench is recommended to help
9. Crevice Formers
provide consistency in tightening. All crevice assemblies in a
9.1 General Comments:
given series should be tightened to the same torque, preferably
9.1.1 The severity of a crevice-corrosion test in a given
by the same individual in order to minimize variability.
environment can be influenced by the size and physical
9.3.4.1 A torque of 8.5 N·m (75 in.-lbs) on an acetal resin
properties of the crevice former.
MCA (using a ⁄4-20 metallic fastener) for example, will
9.1.2 Both metal-to-metal and nonmetal-to-metal crevice
components are frequently used in laboratory and field studies.
Delrin has been found satisfactory for this purpose.
9.1.3 Nonmetallic crevice formers often have the capacity
While it is recognized that rubber bands may be used in the 72 h ferric chloride
for greater elastic deformation and may produce tighter crev-
test method covered by Test Methods G 48, rubber bands are not recommended for
ices which are generally considered to more readily promote
long term tests. Potential crevice sites formed by rubber bands on specimen edges
crevice-corrosion initiation. Acrylic plastic, nylon, polyethyl- may not be desirous for tests beyond the scope of Test Methods G 48.
G78
NOTE 1—Various crevice former designs utilized in laboratory and field test crevice-corrosion studies. S
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