ASTM G46-94(1999)
(Guide)Standard Guide for Examination and Evaluation of Pitting Corrosion
Standard Guide for Examination and Evaluation of Pitting Corrosion
SCOPE
1.1 This guide is intended to assist in the selection of procedures that can be used in the identification and examination of pits and in the evaluation of pitting (See Terminology G15) corrosion to determine the extent of its effect.
1.2 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:G46–94 (Reapproved 1999)
Standard Guide for
Examination and Evaluation of Pitting Corrosion
ThisstandardisissuedunderthefixeddesignationG 46;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope apparentlocationofpits.Itisoftenadvisabletophotographthe
corroded surface at this point so that it can be compared with
1.1 This guide is intended to assist in the selection of
the clean surface after the removal of corrosion products.
procedures that can be used in the identification and examina-
4.1.1 If the metal specimen has been exposed to an un-
tion of pits and in the evaluation of pitting (See Terminology
known environment, the composition of the corrosion products
G 15) corrosion to determine the extent of its effect.
may be of value in determining the cause of corrosion. Follow
1.2 This standard does not purport to address all of the
recommended procedures in the removal of particulate corro-
safety concerns, if any, associated with its use. It is the
sion products and reserve them for future identification (see
responsibility of the user of this standard to establish appro-
NACE Standard RP-01-73).
priate safety and health practices and determine the applica-
4.1.2 To expose the pits fully, use recommended cleaning
bility of regulatory limitations prior to use.
procedures to remove the corrosion products and avoid solu-
2. Referenced Documents tions that attack the base metal excessively (see Practice G 1).
It may be advisable during cleaning to probe the pits with a
2.1 ASTM Standards:
pointed tool to determine the extent of undercutting or subsur-
E 3 Methods of Preparation of Metallographic Specimens
face corrosion (Fig. 1). However, scrubbing with a stiff bristle
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
brush will often enlarge the pit openings sufficiently by
rosion Test Specimens
removal of corrosion products, or undercut metal to make the
G 15 Terminology Relating to Corrosion and Corrosion
pits easier to evaluate.
Testing
4.1.3 Examine the cleaned metal surface under ordinary
G 16 Guide forApplying Statistics toAnalysis of Corrosion
light to determine the approximate size and distribution of pits.
Data
Follow this procedure by a more detailed examination through
2.2 NationalAssociationofCorrosionEngineersStandard:
a microscope using low magnification (203).
RP-01-73 Collection and Identification of Corrosion Prod-
4.1.4 Determine the size, shape, and density of pits.
ucts
4.1.4.1 Pits may have various sizes and shapes. A visual
3. Significance and Use
examinationofthemetalsurfacemayshowaround,elongated,
or irregular opening, but it seldom provides an accurate
3.1 It is important to be able to determine the extent of
indication of corrosion beneath the surface. Thus, it is often
pitting, either in a service application where it is necessary to
necessary to cross section the pit to see its actual shape and to
predict the remaining life in a metal structure, or in laboratory
determine its true depth. Several variations in the cross-
test programs that are used to select the most pitting-resistant
sectioned shape of pits are shown in Fig. 1.
materials for service.
4.1.4.2 It is a tedious job to determine pit density by
4. Identification and Examination of Pits
counting pits through a microscope eyepiece, but the task can
be made easier by the use of a plastic grid. Place the grid,
4.1 Visual Inspection—A visual examination of the cor-
containing 3 to 6-mm squares, on the metal surface. Count and
rodedmetalsurfaceisusuallybeneficial,andthisisdoneunder
record the number of pits in each square, and move across the
ordinary light, with or without the use of a low-power
grid in a systematic manner until all the surface has been
magnifying glass, to determine the extent of corrosion and the
covered. This approach minimizes eyestrain because the eyes
can be taken from the field of view without fear of losing the
This practice is under the jurisdiction of ASTM Committee G-1 on Corrosion
area of interest.
of Metals, and is the direct responsibility of Subcommittee G01.05 on Laboratory
4.1.5 Metallographic Examination—Select and cut out a
Corrosion Tests.
representative portion of the metal surface containing the pits
Current edition approved Feb. 15, 1994. Published April 1994. Originally
published as G 46 – 76. Last previous edition G 46 – 92. and prepare a metallographic specimen in accordance with the
Annual Book of ASTM Standards, Vol 03.01.
recommended procedures given in Methods E 3. Examine
Annual Book of ASTM Standards, Vol 03.02.
Insert in Materials Protection and Performance, Vol 12, June 1973, p. 65.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G46
FIG. 1 Variations in the Cross-Sectional Shape of Pits
microscopically to determine whether there is a relation a magnetic field of their own. Materials with defects will
between pits and inclusions or microstructure, or whether the produce a magnetic field that is different from that of a
cavities are true pits or might have resulted from metal dropout reference material without defects, and an appropriate detec-
caused by intergranular corrosion, dealloying, etc. tion instrument is required to determine these differences.
4.2 Nondestructive Inspection—A number of techniques
4.2.2.2 The induction of a magnetic field in ferromagnetic
have been developed to assist in the detection of cracks or
materials is another approach that is used. Discontinuities that
cavities in a metal surface without destroying the material (1).
are transverse to the direction of the magnetic field cause a
These methods are less effective for locating and defining the
leakage field to form above the surface of the part. Ferromag-
shape of pits than some of those previously discussed, but they
netic particles are placed on the surface to detect the leakage
merit consideration because they are often used in situ, and
field and to outline the size and shape of the discontinuities.
thus are more applicable to field applications.
Rather small imperfections can be detected by this method.
4.2.1 Radiographic—Radiation, such as X rays, are passed
However, the method is limited by the required directionality
through the object. The intensity of the emergent rays varies
of defects to the magnetic field, by the possible need for
with the thickness of the material. Imperfections may be
demagnetization of the material, and by the limited shape of
detected if they cause a change in the absorption of X rays.
parts that can be examined.
Detectors or films are used to provide an image of interior
4.2.3 Sonics:
imperfections. The metal thickness that can be inspected is
4.2.3.1 In the use of ultrasonics, pulses of sound energy are
dependentontheavailableenergyoutput.Poresorpitsmustbe
transmitted through a couplant, such as oil or water, onto the
as large as ⁄2 % of the metal thickness to be detected. This
metal surface where waves are generated. The reflected echoes
technique has only slight application to pitting detection, but it
are converted to electrical signals that can be interpreted to
mightbeausefulmeanstocomparespecimensbeforeandafter
show the location of flaws or pits. Both contact and immersion
corrosion to determine whether pitting has occurred and
methods are used. The test has good sensitivity and provides
whether it is associated with previous porosity. It may also be
instantaneous information about the size and location of flaws.
useful to determine the extent of subsurface and undercutting
However, reference standards are required for comparison, and
pitting (Fig. 1).
training is needed to interpret the results properly.
4.2.2 Electromagnetic:
4.2.3.2 An alternative approach is to use acoustic emissions
4.2.2.1 Eddy currents can be used to detect defects or
in detecting flaws in metals. Imperfections, such as pits,
irregularities in the structure of electrically conducting mate-
generate high-frequency emissions under thermal or mechani-
rials. When a specimen is exposed to a varying magnetic field,
cal stress. The frequency of emission and the number of
produced by connecting an alternating current to a coil, eddy
occurrences per unit time determine the presence of defects.
currents are induced in the specimen, and they in turn produce
4.2.4 Penetrants—Defects opening to the surface can be
detected by the application of a penetrating liquid that subse-
quently exudes from the surface after the excess penetrant has
The boldface numbers in parentheses refer to the list of references at the end of
this practice. beenremoved.Defectsarelocatedbysprayingthesurfacewith
G46
a developer that reacts with a dye in the penetrant, or the Subtract the number of pits at each stage from the count at the
penetrant may contain a fluorescent material that is viewed previousstagetoobtainthenumberofpitsateachdepthofcut.
under black light. The size of the defect is shown by the
5.2.3 Micrometer or Depth Gage:
intensity of the color and the rate of bleed-out. This technique
5.2.3.1 This method is based on the use of a pointed needle
provides only an approximation of the depth and size of pits.
attached to a micrometer or calibrated depth gage to penetrate
4.2.5 None of these nondestructive test methods provide
the pit cavity. Zero the instrument on an unaffected area at the
satisfactory detailed information about pitting. They can be
lipofthepit.Inserttheneedleinthepituntilitreachesthebase
used to locate pits and to provide some information about the
where a new measurement is taken. The distance traveled by
size of pits, but they generally are not able to detect small pits,
the needle is the depth of the pit. It is best to use constant-
and confusion may arise in attempting to differentiate between
tension instruments to minimize metal penetration at the base
pits and other surface blemishes. Most of these methods were
of the pit. It can be advantageous to use a stereomicroscope in
developed to detect cracks or flaws in metals, but with more
conjunctionwiththistechniquesothatthepitcanbemagnified
refined development they may become more applicable to
to ensure that the needle point is at the bottom of the pit. The
pitting measurements.
method is limited to pits that have a sufficiently large opening
toaccommodatetheneedlewithoutobstruction;thiseliminates
5. Extent of Pitting
those pits where undercutting or directional orientation has
5.1 Mass Loss—Metal mass loss is not ordinarily recom-
occurred.
mended for use as a measure of the extent of pitting unless
5.2.3.2 In a variation of this method, attach the probe to a
general corrosion is slight and pitting is fairly severe. If
spherometer and connect through a microammeter and battery
uniform corrosion is significant, the contribution of pitting to
to the specimen (3, 4). When the probe touches the bottom of
total metal loss is small, and pitting damage cannot be
the pit, it completes the electrical circuit, and the probe
determined accurately from mass loss. In any case, mass loss
movement is a measurement of pit depth. This method is
can only provide information about total metal loss due to
limited to very regularly shaped pits because contact with the
pitting but nothing about depth of penetration. However, mass
side of the pit would give a false reading.
loss should not be neglected in every case because it may be of
value; for example, mass loss along with a visual comparison 5.2.4 Microscopical—This method is particularly valuable
of pitted surfaces may be adequate to evaluate the pitting
when pits are too narrow or difficult to penetrate with a probe
resistance of alloys in laboratory tests.
type of instrument. The method is amenable to use as long as
5.2 Pit Depth Measurement:
light can be focused on the base of the pit, which would not be
5.2.1 Metallographic—Pit depth can be determined by sec-
possible in the case of example (e) in Fig. 1.
tioning vertically through a preselected pit, mounting the
5.2.4.1 Useametallurgicalmicroscopewithamagnification
cross-sectioned pit metallographically, and polishing the sur-
range from 50 to 500X and a calibrated fine-focus knob (for
face. The depth of the pit is measured on the flat, polished
example, 1 division 5 0.001 mm). If the latter is not available,
surface by the use of a microscope with a calibrated eyepiece.
a dial micrometer can be attached to the microscope in such a
The method is very accurate, but it requires good judgment in
way that it will show movement of the stage relative to the
the selection of the pit and good technique in cutting through
microscope body.
the pit. Its limitations are that it is time consuming, the deepest
5.2.4.2 Locate a single pit on the metal surface and center
pit may not have been selected, and the pit may not have been
undertheobjectivelensofthemicroscopeatlowmagnification
sectioned at the deepest point of penetration.
(for example, 50X). Increase the objective lens magnification
5.2.2 Machining (2, 3):
until the pit area covers most of the field under view. Focus the
5.2.2.1 This method requires a sample that is fairly regular
specimen surface at the lip of the pit, using first the coarse and
in shape, and it involves the destruction of the specimen.
then the fine-focusing knobs of the microscope. Record the
Measure the thickness of the specimen between two areas that
initial reading from the fine-focusing knob. Refocus on the
have not been affected by general corrosion. Select a portion of
bottom of the pit with the fine-focusing knob and record the
the surface on one side of the specimen that is relatively
reading. The difference between the initial and the final
unaffected; then machine the opposite surface where the pits
readings on the fine-focusing knob is the pit depth.
are located on a precision lathe, grinder, or mill until all signs
5.2.4.3 Repeat the steps in 5.2.4.2 to obtain additional
of corrosion have disappeared. (Some difficulty from galling
measurements or until satisfactory duplication has been ob-
and smearing may be encountered with soft metals, and pits
tained.The repeatability of pit depth measurements on a single
may be obliterated.) Measure the thickness of the specimen
pit at four magnifications is shown in Annex A1.
between the unaffected surface and subtract from the original
5.2.4.4 A variation of the microscopical technique employs
thickness to give the maximum depth of pitting. Repeat this
procedure on the unmachined surface unless
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