ASTM G46-94(2018)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: G46 − 94 (Reapproved 2018)
Standard Guide for
Examination and Evaluation of Pitting Corrosion
ThisstandardisissuedunderthefixeddesignationG46;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Significance and Use
3.1 It is important to be able to determine the extent of
1.1 Thisguidecoverstheselectionofproceduresthatcanbe
pitting, either in a service application where it is necessary to
used in the identification and examination of pits and in the
predict the remaining life in a metal structure, or in laboratory
evaluation of pitting (See Terminology G15) corrosion to
test programs that are used to select the most pitting-resistant
determine the extent of its effect.
materials for service.
1.2 This standard does not purport to address all of the
4. Identification and Examination of Pits
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4.1 VisualInspection—Avisualexaminationofthecorroded
priate safety, health, and environmental practices and deter-
metal surface is usually beneficial, and this is done under
mine the applicability of regulatory limitations prior to use.
ordinary light, with or without the use of a low-power
1.3 This international standard was developed in accor- magnifying glass, to determine the extent of corrosion and the
dance with internationally recognized principles on standard- apparentlocationofpits.Itisoftenadvisabletophotographthe
corroded surface at this point so that it can be compared with
ization established in the Decision on Principles for the
the clean surface after the removal of corrosion products.
Development of International Standards, Guides and Recom-
4.1.1 If the metal specimen has been exposed to an un-
mendations issued by the World Trade Organization Technical
knownenvironment,thecompositionofthecorrosionproducts
Barriers to Trade (TBT) Committee.
may be of value in determining the cause of corrosion. Follow
recommended procedures in the removal of particulate corro-
2. Referenced Documents
sion products and reserve them for future identification (see
2.1 ASTM Standards:
NACE RP-01-73).
E3Guide for Preparation of Metallographic Specimens
4.1.2 To expose the pits fully, use recommended cleaning
G1Practice for Preparing, Cleaning, and Evaluating Corro-
procedures to remove the corrosion products and avoid solu-
sion Test Specimens
tions that attack the base metal excessively (see Practice G1).
G15TerminologyRelatingtoCorrosionandCorrosionTest-
It may be advisable during cleaning to probe the pits with a
ing (Withdrawn 2010)
pointed tool to determine the extent of undercutting or subsur-
G16Guide for Applying Statistics to Analysis of Corrosion
face corrosion (Fig. 1). However, scrubbing with a stiff bristle
Data
brush will often enlarge the pit openings sufficiently by
removal of corrosion products, or undercut metal to make the
2.2 National Association of Corrosion Engineers Standard:
pits easier to evaluate.
NACE RP-01-73Collection and Identification of Corrosion
4.1.3 Examine the cleaned metal surface under ordinary
Products
lighttodeterminetheapproximatesizeanddistributionofpits.
Follow this procedure by a more detailed examination through
a microscope using low magnification (20×).
This guide is under the jurisdiction ofASTM Committee G01 on Corrosion of
4.1.4 Determine the size, shape, and density of pits.
Metals and is the direct responsibility of Subcommittee G01.05 on Laboratory
4.1.4.1 Pits may have various sizes and shapes. A visual
Corrosion Tests.
Current edition approved Oct. 1, 2018. Published November 2018. Originally
examinationofthemetalsurfacemayshowaround,elongated,
approvedin1976.Lastpreviouseditionapprovedin2013asG46–94(2013).DOI:
or irregular opening, but it seldom provides an accurate
10.1520/G0046-94R18.
indication of corrosion beneath the surface. Thus, it is often
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
necessary to cross section the pit to see its actual shape and to
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
determine its true depth. Several variations in the cross-
the ASTM website.
sectioned shape of pits are shown in Fig. 1.
The last approved version of this historical standard is referenced on
4.1.4.2 It is a tedious job to determine pit density by
www.astm.org.
Insert in Materials Protection and Performance, Vol 12, June 1973, p. 65. counting pits through a microscope eyepiece, but the task can
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G46 − 94 (2018)
FIG. 1 Variations in the Cross-Sectional Shape of Pits
be made easier by the use of a plastic grid. Place the grid, mightbeausefulmeanstocomparespecimensbeforeandafter
containing3to6-mmsquares,onthemetalsurface.Countand corrosion to determine whether pitting has occurred and
record the number of pits in each square, and move across the whether it is associated with previous porosity. It may also be
grid in a systematic manner until all the surface has been useful to determine the extent of subsurface and undercutting
covered. This approach minimizes eyestrain because the eyes pitting (Fig. 1).
can be taken from the field of view without fear of losing the
4.2.2 Electromagnetic:
area of interest.
4.2.2.1 Eddy currents can be used to detect defects or
4.1.5 Metallographic Examination—Select and cut out a
irregularities in the structure of electrically conducting mate-
representative portion of the metal surface containing the pits
rials.When a specimen is exposed to a varying magnetic field,
and prepare a metallographic specimen in accordance with the
produced by connecting an alternating current to a coil, eddy
recommended procedures given in Methods E3. Examine
currents are induced in the specimen, and they in turn produce
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
cavitiesaretruepitsormighthaveresultedfrommetaldropout
reference material without defects, and an appropriate detec-
caused by intergranular corrosion, dealloying, and so forth.
tion instrument is required to determine these differences.
4.2.2.2 The induction of a magnetic field in ferromagnetic
4.2 Nondestructive Inspection—A number of techniques
have been developed to assist in the detection of cracks or materials is another approach that is used. Discontinuities that
are transverse to the direction of the magnetic field cause a
cavitiesinametalsurfacewithoutdestroyingthematerial (1).
These methods are less effective for locating and defining the leakage field to form above the surface of the part. Ferromag-
netic particles are placed on the surface to detect the leakage
shapeofpitsthansomeofthosepreviouslydiscussed,butthey
merit consideration because they are often used in situ, and field and to outline the size and shape of the discontinuities.
Rather small imperfections can be detected by this method.
thus are more applicable to field applications.
4.2.1 Radiographic—Radiation, such as X rays, are passed However, the method is limited by the required directionality
of defects to the magnetic field, by the possible need for
through the object. The intensity of the emergent rays varies
demagnetization of the material, and by the limited shape of
with the thickness of the material. Imperfections may be
parts that can be examined.
detected if they cause a change in the absorption of X rays.
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
metalsurfacewherewavesaregenerated.Thereflectedechoes
technique has only slight application to pitting detection, but it
are converted to electrical signals that can be interpreted to
showthelocationofflawsorpits.Bothcontactandimmersion
methods are used. The test has good sensitivity and provides
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this practice. instantaneous information about the size and location of flaws.
G46 − 94 (2018)
However,referencestandardsarerequiredforcomparison,and of corrosion have disappeared. (Some difficulty from galling
training is needed to interpret the results properly. and smearing may be encountered with soft metals, and pits
4.2.3.2 An alternative approach is to use acoustic emissions
may be obliterated.) Measure the thickness of the specimen
in detecting flaws in metals. Imperfections, such as pits, between the unaffected surface and subtract from the original
generate high-frequency emissions under thermal or mechani-
thickness to give the maximum depth of pitting. Repeat this
cal stress. The frequency of emission and the number of
procedure on the unmachined surface unless the thickness has
occurrences per unit time determine the presence of defects.
beenreducedby50%ormoreduringthemachiningofthefirst
4.2.4 Penetrants—Defects opening to the surface can be
side.
detected by the application of a penetrating liquid that subse-
5.2.2.2 This method is equally suitable for determining the
quently exudes from the surface after the excess penetrant has
numberofpitswithspecificdepths.Countthevisiblepits;then
beenremoved.Defectsarelocatedbysprayingthesurfacewith
machine away the surface of the metal in measured stages and
a developer that reacts with a dye in the penetrant, or the
count the number of visible pits remaining at each stage.
penetrant may contain a fluorescent material that is viewed
Subtract the number of pits at each stage from the count at the
under black light. The size of the defect is shown by the
previousstagetoobtainthenumberofpitsateachdepthofcut.
intensity of the color and the rate of bleed-out. This technique
5.2.3 Micrometer or Depth Gage:
provides only an approximation of the depth and size of pits.
5.2.3.1 This method is based on the use of a pointed needle
4.2.5 None of these nondestructive test methods provide
attached to a micrometer or calibrated depth gage to penetrate
satisfactory detailed information about pitting. They can be
the pit cavity. Zero the instrument on an unaffected area at the
used to locate pits and to provide some information about the
lipofthepit.Inserttheneedleinthepituntilitreachesthebase
size of pits, but they generally are not able to detect small pits,
where a new measurement is taken. The distance traveled by
and confusion may arise in attempting to differentiate between
the needle is the depth of the pit. It is best to use constant-
pits and other surface blemishes. Most of these methods were
tension instruments to minimize metal penetration at the base
developed to detect cracks or flaws in metals, but with more
of the pit. It can be advantageous to use a stereomicroscope in
refined development they may become more applicable to
conjunctionwiththistechniquesothatthepitcanbemagnified
pitting measurements.
to ensure that the needle point is at the bottom of the pit. The
method is limited to pits that have a sufficiently large opening
5. Extent of Pitting
toaccommodatetheneedlewithoutobstruction;thiseliminates
5.1 Mass Loss—Metal mass loss is not ordinarily recom-
those pits where undercutting or directional orientation has
mended for use as a measure of the extent of pitting unless
occurred.
general corrosion is slight and pitting is fairly severe. If
5.2.3.2 In a variation of this method, attach the probe to a
uniform corrosion is significant, the contribution of pitting to
spherometer and connect through a microammeter and battery
total metal loss is small, and pitting damage cannot be
to the specimen (3, 4). When the probe touches the bottom of
determined accurately from mass loss. In any case, mass loss
the pit, it completes the electrical circuit, and the probe
can only provide information about total metal loss due to
movement is a measurement of pit depth. This method is
pitting but nothing about depth of penetration. However, mass
limited to very regularly shaped pits because contact with the
lossshouldnotbeneglectedineverycasebecauseitmaybeof
side of the pit would give a false reading.
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:
lightcanbefocusedonthebaseofthepit,whichwouldnotbe
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 pre-selected pit, mounting the
5.2.4.1 Useametallurgicalmicroscopewithamagnification
cross-sectioned pit metallographically, and polishing the sur-
range from 50 to 500× and a calibrated fine-focus knob (for
face. The depth of the pit is measured on the flat, polished
example, 1 division=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.
thepit.Itslimitationsarethatitistimeconsuming,thedeepest
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, 50×). Increase the objective lens magnification
5.2.2 Machining (2, 3):
5.2.2.1 This method requires a sample that is fairly regular untilthepitareacoversmostofthefieldunderview.Focusthe
specimen surface at the lip of the pit, using first the coarse and
in shape, and it involves the destruction of the specimen.
Measure the thickness of the specimen between two areas that then the fine-focusing knobs of the microscope. Record th
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