ASTM G46-94(2005)
(Guide)Standard Guide for Examination and Evaluation of Pitting Corrosion
Standard Guide for Examination and Evaluation of Pitting Corrosion
SIGNIFICANCE AND USE
It is important to be able to determine the extent of pitting, either in a service application where it is necessary to predict the remaining life in a metal structure, or in laboratory test programs that are used to select the most pitting-resistant materials for service.
SCOPE
1.1 This guide covers the selection of procedures that can be used in the identification and examination of pits and in the evaluation of pitting (See Terminology G 15) corrosion to determine the extent of its effect.
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.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: G46 − 94(Reapproved 2005)
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 4. Identification and Examination of Pits
4.1 VisualInspection—Avisualexaminationofthecorroded
1.1 Thisguidecoverstheselectionofproceduresthatcanbe
used in the identification and examination of pits and in the metal surface is usually beneficial, and this is done under
evaluation of pitting (See Terminology G15) corrosion to ordinary light, with or without the use of a low-power
determine the extent of its effect. magnifying glass, to determine the extent of corrosion and the
apparentlocationofpits.Itisoftenadvisabletophotographthe
1.2 This standard does not purport to address all of the
corroded surface at this point so that it can be compared with
safety concerns, if any, associated with its use. It is the
the clean surface after the removal of corrosion products.
responsibility of the user of this standard to establish appro-
4.1.1 If the metal specimen has been exposed to an un-
priate safety and health practices and determine the applica-
knownenvironment,thecompositionofthecorrosionproducts
bility of regulatory limitations prior to use.
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
2.2 National Association of Corrosion Engineers Standard:
removal of corrosion products, or undercut metal to make the
NACE RP-01-73Collection and Identification of Corrosion pits easier to evaluate.
Products
4.1.3 Examine the cleaned metal surface under ordinary
lighttodeterminetheapproximatesizeanddistributionofpits.
3. Significance and Use
Follow this procedure by a more detailed examination through
a microscope using low magnification (20×).
3.1 It is important to be able to determine the extent of
4.1.4 Determine the size, shape, and density of pits.
pitting, either in a service application where it is necessary to
4.1.4.1 Pits may have various sizes and shapes. A visual
predict the remaining life in a metal structure, or in laboratory
examinationofthemetalsurfacemayshowaround,elongated,
test programs that are used to select the most pitting-resistant
or irregular opening, but it seldom provides an accurate
materials for service.
indication of corrosion beneath the surface. Thus, it is often
necessary to cross section the pit to see its actual shape and to
determine its true depth. Several variations in the cross-
This guide is under the jurisdiction ofASTM Committee G01 on Corrosion of
sectioned shape of pits are shown in Fig. 1.
Metals and is the direct responsibility of Subcommittee G01.05 on Laboratory
4.1.4.2 It is a tedious job to determine pit density by
Corrosion Tests.
Current edition approved May 1, 2005. Published May 2005. Originally
counting pits through a microscope eyepiece, but the task can
approvedin1976.Lastpreviouseditionapprovedin1999asG46–94(1999).DOI:
be made easier by the use of a plastic grid. Place the grid,
10.1520/G0046-94R05.
containing3to6-mmsquares,onthemetalsurface.Countand
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
record the number of pits in each square, and move across the
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
grid in a systematic manner until all the surface has been
the ASTM website.
covered. This approach minimizes eyestrain because the eyes
The last approved version of this historical standard is referenced on
can be taken from the field of view without fear of losing the
www.astm.org.
Insert in Materials Protection and Performance, Vol 12, June 1973, p. 65. area of interest.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G46 − 94 (2005)
FIG. 1 Variations in the Cross-Sectional Shape of Pits
4.1.5 Metallographic Examination—Select and cut out a 4.2.2 Electromagnetic:
representative portion of the metal surface containing the pits
4.2.2.1 Eddy currents can be used to detect defects or
and prepare a metallographic specimen in accordance with the
irregularities in the structure of electrically conducting mate-
recommended procedures given in Methods E3. Examine
rials.When a specimen is exposed to a varying magnetic field,
microscopically to determine whether there is a relation
produced by connecting an alternating current to a coil, eddy
between pits and inclusions or microstructure, or whether the
currents are induced in the specimen, and they in turn produce
cavitiesaretruepitsormighthaveresultedfrommetaldropout
a magnetic field of their own. Materials with defects will
caused by intergranular corrosion, dealloying, and so forth.
produce a magnetic field that is different from that of a
4.2 Nondestructive Inspection—A number of techniques
reference material without defects, and an appropriate detec-
have been developed to assist in the detection of cracks or
tion instrument is required to determine these differences.
cavitiesinametalsurfacewithoutdestroyingthematerial (1).
4.2.2.2 The induction of a magnetic field in ferromagnetic
These methods are less effective for locating and defining the
materials is another approach that is used. Discontinuities that
shapeofpitsthansomeofthosepreviouslydiscussed,butthey
are transverse to the direction of the magnetic field cause a
merit consideration because they are often used in situ, and
leakage field to form above the surface of the part. Ferromag-
thus are more applicable to field applications.
netic particles are placed on the surface to detect the leakage
4.2.1 Radiographic—Radiation, such as X rays, are passed
field and to outline the size and shape of the discontinuities.
through the object. The intensity of the emergent rays varies
Rather small imperfections can be detected by this method.
with the thickness of the material. Imperfections may be
However, the method is limited by the required directionality
detected if they cause a change in the absorption of X rays.
of defects to the magnetic field, by the possible need for
Detectors or films are used to provide an image of interior
demagnetization of the material, and by the limited shape of
imperfections. The metal thickness that can be inspected is
parts that can be examined.
dependentontheavailableenergyoutput.Poresorpitsmustbe
1 4.2.3 Sonics:
as large as ⁄2 % of the metal thickness to be detected. This
4.2.3.1 In the use of ultrasonics, pulses of sound energy are
technique has only slight application to pitting detection, but it
transmitted through a couplant, such as oil or water, onto the
mightbeausefulmeanstocomparespecimensbeforeandafter
metalsurfacewherewavesaregenerated.Thereflectedechoes
corrosion to determine whether pitting has occurred and
whether it is associated with previous porosity. It may also be are converted to electrical signals that can be interpreted to
useful to determine the extent of subsurface and undercutting
showthelocationofflawsorpits.Bothcontactandimmersion
pitting (Fig. 1). methods are used. The test has good sensitivity and provides
instantaneous information about the size and location of flaws.
5 However,referencestandardsarerequiredforcomparison,and
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this practice. training is needed to interpret the results properly.
G46 − 94 (2005)
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.
4.2.5 None of these nondestructive test methods provide
5.2.3.1 This method is based on the use of a pointed needle
satisfactory detailed information about pitting. They can be
attached to a micrometer or calibrated depth gage to penetrate
used to locate pits and to provide some information about the
the pit cavity. Zero the instrument on an unaffected area at the
size of pits, but they generally are not able to detect small pits,
lipofthepit.Inserttheneedleinthepituntilitreachesthebase
and confusion may arise in attempting to differentiate between
where a new measurement is taken. The distance traveled by
pits and other surface blemishes. Most of these methods were
the needle is the depth of the pit. It is best to use constant-
developed to detect cracks or flaws in metals, but with more
tension instruments to minimize metal penetration at the base
refined development they may become more applicable to
of the pit. It can be advantageous to use a stereomicroscope in
pitting measurements.
conjunctionwiththistechniquesothatthepitcanbemagnified
to ensure that the needle point is at the bottom of the pit. The
5. Extent of Pitting
method is limited to pits that have a sufficiently large opening
5.1 Mass Loss—Metal mass loss is not ordinarily recom-
toaccommodatetheneedlewithoutobstruction;thiseliminates
mended for use as a measure of the extent of pitting unless
those pits where undercutting or directional orientation has
general corrosion is slight and pitting is fairly severe. If
occurred.
uniform corrosion is significant, the contribution of pitting to
5.2.3.2 In a variation of this method, attach the probe to a
total metal loss is small, and pitting damage cannot be
spherometer and connect through a microammeter and battery
determined accurately from mass loss. In any case, mass loss
to the specimen (3, 4). When the probe touches the bottom of
can only provide information about total metal loss due to
the pit, it completes the electrical circuit, and the probe
pitting but nothing about depth of penetration. However, mass
movement is a measurement of pit depth. This method is
lossshouldnotbeneglectedineverycasebecauseitmaybeof
limited to very regularly shaped pits because contact with the
value; for example, mass loss along with a visual comparison
of pitted surfaces may be adequate to evaluate the pitting side of the pit would give a false reading.
resistance of alloys in laboratory tests.
5.2.4 Microscopical—This method is particularly valuable
when pits are too narrow or difficult to penetrate with a probe
5.2 Pit Depth Measurement:
type of instrument. The method is amenable to use as long as
5.2.1 Metallographic—Pit depth can be determined by sec-
tioning vertically through a pre-selected pit, mounting the lightcanbefocusedonthebaseofthepit,whichwouldnotbe
cross-sectioned pit metallographically, and polishing the sur- possible in the case of example (e)in Fig. 1.
face. The depth of the pit is measured on the flat, polished
5.2.4.1 Useametallurgicalmicroscopewithamagnification
surface by the use of a microscope with a calibrated eyepiece.
range from 50 to 500× and a calibrated fine-focus knob (for
The method is very accurate, but it requires good judgment in
example, 1 division=0.001 mm). If the latter is not available,
the selection of the pit and good technique in cutting through
a dial micrometer can be attached to the microscope in such a
thepit.Itslimitationsarethatitistimeconsuming,thedeepest
way that it will show movement of the stage relative to the
pit may not have been selected, and the pit may not have been
microscope body.
sectioned at the deepest point of penetration.
5.2.4.2 Locate a single pit on the metal surface and center
5.2.2 Machining (2, 3):
undertheobjectivelensofthemicroscopeatlowmagnification
5.2.2.1 This method requires a sample that is fairly regular
(for example, 50×). Increase the objective lens magnification
in shape, and it involves the destruction of the specimen.
untilthepitareacoversmostofthefieldunderview.Focusthe
Measure the thickness of the specimen between two areas that
specimen surface at the lip of the pit, using first the coarse and
havenotbeenaffectedbygeneralcorrosion.Selectaportionof
then the fine-focusing knobs of the microscope. Record the
the surface on one side of the specimen that is relatively
initial reading from the fine-focusing knob. Refocus on the
unaffected; then machine the opposite surface where the pits
bottom of the pit with the fine-focusing knob and record the
are located on a precision lathe, grinder, or mill until all signs
reading. The difference between the initial and the final
of corrosion have disappeared. (Some difficulty from
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.