1、Designation: G 46 94 (Reapproved 2005)Standard Guide forExamination and Evaluation of Pitting Corrosion1This standard is issued under the fixed designation G 46; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of last revi
2、sion. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers the selection of procedures that can beused in the identification and examination of pits and in theevaluati
3、on of pitting (See Terminology G15) corrosion todetermine the extent of its effect.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and
4、determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E3 Methods of Preparation of Metallographic SpecimensG1 Practice for Preparing, Cleaning, and Evaluating Cor-rosion Test SpecimensG15 Terminology Relating to Corrosion and CorrosionTestingG
5、16 Guide forApplying Statistics toAnalysis of CorrosionData2.2 National Association of Corrosion Engineers Standard:NACE RP-01-73 Collection and Identification of CorrosionProducts33. Significance and Use3.1 It is important to be able to determine the extent ofpitting, either in a service applicatio
6、n where it is necessary topredict the remaining life in a metal structure, or in laboratorytest programs that are used to select the most pitting-resistantmaterials for service.4. Identification and Examination of Pits4.1 Visual InspectionA visual examination of the cor-roded metal surface is usuall
7、y beneficial, and this is done underordinary light, with or without the use of a low-powermagnifying glass, to determine the extent of corrosion and theapparent location of pits. It is often advisable to photograph thecorroded surface at this point so that it can be compared withthe clean surface af
8、ter the removal of corrosion products.4.1.1 If the metal specimen has been exposed to an un-known environment, the composition of the corrosion productsmay be of value in determining the cause of corrosion. Followrecommended procedures in the removal of particulate corro-sion products and reserve th
9、em for future identification (seeNACE RP-01-73).4.1.2 To expose the pits fully, use recommended cleaningprocedures to remove the corrosion products and avoid solu-tions that attack the base metal excessively (see Practice G1).It may be advisable during cleaning to probe the pits with apointed tool t
10、o determine the extent of undercutting or subsur-face corrosion (Fig. 1). However, scrubbing with a stiff bristlebrush will often enlarge the pit openings sufficiently byremoval of corrosion products, or undercut metal to make thepits easier to evaluate.4.1.3 Examine the cleaned metal surface under
11、ordinarylight to determine the approximate size and distribution of pits.Follow this procedure by a more detailed examination througha microscope using low magnification (203).4.1.4 Determine the size, shape, and density of pits.4.1.4.1 Pits may have various sizes and shapes. A visualexamination of
12、the metal surface may show a round, elongated,or irregular opening, but it seldom provides an accurateindication of corrosion beneath the surface. Thus, it is oftennecessary to cross section the pit to see its actual shape and todetermine its true depth. Several variations in the cross-sectioned sha
13、pe of pits are shown in Fig. 1.4.1.4.2 It is a tedious job to determine pit density bycounting pits through a microscope eyepiece, but the task canbe made easier by the use of a plastic grid. Place the grid,containing 3 to 6-mm squares, on the metal surface. Count andrecord the number of pits in eac
14、h square, and move across thegrid in a systematic manner until all the surface has beencovered. This approach minimizes eyestrain because the eyescan be taken from the field of view without fear of losing thearea of interest.4.1.5 Metallographic ExaminationSelect and cut out arepresentative portion
15、of the metal surface containing the pitsand prepare a metallographic specimen in accordance with the1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals, and is the direct responsibility of Subcommittee G01.05 on LaboratoryCorrosion Tests.Current edition approved May
16、1, 2005. Published May 2005. Originallyapproved in 1976. Last previous edition approved in 1999 as G 46 94 (1999).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to th
17、e standards Document Summary page onthe ASTM website.3Insert in Materials Protection and Performance, Vol 12, June 1973, p. 65.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.recommended procedures given in Methods E3. Examinemicrosc
18、opically to determine whether there is a relationbetween pits and inclusions or microstructure, or whether thecavities are true pits or might have resulted from metal dropoutcaused by intergranular corrosion, dealloying, and so forth.4.2 Nondestructive InspectionA number of techniqueshave been devel
19、oped to assist in the detection of cracks orcavities in a metal surface without destroying the material (1).4These methods are less effective for locating and defining theshape of pits than some of those previously discussed, but theymerit consideration because they are often used in situ, andthus a
20、re more applicable to field applications.4.2.1 RadiographicRadiation, such as X rays, are passedthrough the object. The intensity of the emergent rays varieswith the thickness of the material. Imperfections may bedetected if they cause a change in the absorption of X rays.Detectors or films are used
21、 to provide an image of interiorimperfections. The metal thickness that can be inspected isdependent on the available energy output. Pores or pits must beas large as12 % of the metal thickness to be detected. Thistechnique has only slight application to pitting detection, but itmight be a useful mea
22、ns to compare specimens before and aftercorrosion to determine whether pitting has occurred andwhether it is associated with previous porosity. It may also beuseful to determine the extent of subsurface and undercuttingpitting (Fig. 1).4.2.2 Electromagnetic:4.2.2.1 Eddy currents can be used to detec
23、t defects orirregularities in the structure of electrically conducting mate-rials. When a specimen is exposed to a varying magnetic field,produced by connecting an alternating current to a coil, eddycurrents are induced in the specimen, and they in turn producea magnetic field of their own. Material
24、s with defects willproduce a magnetic field that is different from that of areference material without defects, and an appropriate detec-tion instrument is required to determine these differences.4.2.2.2 The induction of a magnetic field in ferromagneticmaterials is another approach that is used. Di
25、scontinuities thatare transverse to the direction of the magnetic field cause aleakage field to form above the surface of the part. Ferromag-netic particles are placed on the surface to detect the leakagefield and to outline the size and shape of the discontinuities.Rather small imperfections can be
26、 detected by this method.However, the method is limited by the required directionalityof defects to the magnetic field, by the possible need fordemagnetization of the material, and by the limited shape ofparts that can be examined.4.2.3 Sonics:4.2.3.1 In the use of ultrasonics, pulses of sound energ
27、y aretransmitted through a couplant, such as oil or water, onto themetal surface where waves are generated. The reflected echoesare converted to electrical signals that can be interpreted toshow the location of flaws or pits. Both contact and immersionmethods are used. The test has good sensitivity
28、and providesinstantaneous information about the size and location of flaws.However, reference standards are required for comparison, andtraining is needed to interpret the results properly.4.2.3.2 An alternative approach is to use acoustic emissionsin detecting flaws in metals. Imperfections, such a
29、s pits,generate high-frequency emissions under thermal or mechani-cal stress. The frequency of emission and the number ofoccurrences per unit time determine the presence of defects.4.2.4 PenetrantsDefects opening to the surface can bedetected by the application of a penetrating liquid that subse-que
30、ntly exudes from the surface after the excess penetrant has4The boldface numbers in parentheses refer to the list of references at the end ofthis practice.FIG. 1 Variations in the Cross-Sectional Shape of PitsG 46 94 (2005)2been removed. Defects are located by spraying the surface witha developer th
31、at reacts with a dye in the penetrant, or thepenetrant may contain a fluorescent material that is viewedunder black light. The size of the defect is shown by theintensity of the color and the rate of bleed-out. This techniqueprovides only an approximation of the depth and size of pits.4.2.5 None of
32、these nondestructive test methods providesatisfactory detailed information about pitting. They can beused to locate pits and to provide some information about thesize of pits, but they generally are not able to detect small pits,and confusion may arise in attempting to differentiate betweenpits and
33、other surface blemishes. Most of these methods weredeveloped to detect cracks or flaws in metals, but with morerefined development they may become more applicable topitting measurements.5. Extent of Pitting5.1 Mass LossMetal mass loss is not ordinarily recom-mended for use as a measure of the extent
34、 of pitting unlessgeneral corrosion is slight and pitting is fairly severe. Ifuniform corrosion is significant, the contribution of pitting tototal metal loss is small, and pitting damage cannot bedetermined accurately from mass loss. In any case, mass losscan only provide information about total me
35、tal loss due topitting but nothing about depth of penetration. However, massloss should not be neglected in every case because it may be ofvalue; for example, mass loss along with a visual comparisonof pitted surfaces may be adequate to evaluate the pittingresistance of alloys in laboratory tests.5.
36、2 Pit Depth Measurement:5.2.1 MetallographicPit depth can be determined by sec-tioning vertically through a pre-selected pit, mounting thecross-sectioned pit metallographically, and polishing the sur-face. The depth of the pit is measured on the flat, polishedsurface by the use of a microscope with
37、a calibrated eyepiece.The method is very accurate, but it requires good judgment inthe selection of the pit and good technique in cutting throughthe pit. Its limitations are that it is time consuming, the deepestpit may not have been selected, and the pit may not have beensectioned at the deepest po
38、int of penetration.5.2.2 Machining (2, 3):5.2.2.1 This method requires a sample that is fairly regularin shape, and it involves the destruction of the specimen.Measure the thickness of the specimen between two areas thathave not been affected by general corrosion. Select a portion ofthe surface on o
39、ne side of the specimen that is relativelyunaffected; then machine the opposite surface where the pitsare located on a precision lathe, grinder, or mill until all signsof corrosion have disappeared. (Some difficulty from gallingand smearing may be encountered with soft metals, and pitsmay be obliter
40、ated.) Measure the thickness of the specimenbetween the unaffected surface and subtract from the originalthickness to give the maximum depth of pitting. Repeat thisprocedure on the unmachined surface unless the thickness hasbeen reduced by 50% or more during the machining of the firstside.5.2.2.2 Th
41、is method is equally suitable for determining thenumber of pits with specific depths. Count the visible pits; thenmachine away the surface of the metal in measured stages andcount the number of visible pits remaining at each stage.Subtract the number of pits at each stage from the count at theprevio
42、us stage to obtain the number of pits at each depth of cut.5.2.3 Micrometer or Depth Gage:5.2.3.1 This method is based on the use of a pointed needleattached to a micrometer or calibrated depth gage to penetratethe pit cavity. Zero the instrument on an unaffected area at thelip of the pit. Insert th
43、e needle in the pit until it reaches the basewhere a new measurement is taken. The distance traveled bythe needle is the depth of the pit. It is best to use constant-tension instruments to minimize metal penetration at the baseof the pit. It can be advantageous to use a stereomicroscope inconjunctio
44、n with this technique so that the pit can be magnifiedto ensure that the needle point is at the bottom of the pit. Themethod is limited to pits that have a sufficiently large openingto accommodate the needle without obstruction; this eliminatesthose pits where undercutting or directional orientation
45、 hasoccurred.5.2.3.2 In a variation of this method, attach the probe to aspherometer and connect through a microammeter and batteryto the specimen (3, 4). When the probe touches the bottom ofthe pit, it completes the electrical circuit, and the probemovement is a measurement of pit depth. This metho
46、d islimited to very regularly shaped pits because contact with theside of the pit would give a false reading.5.2.4 MicroscopicalThis method is particularly valuablewhen pits are too narrow or difficult to penetrate with a probetype of instrument. The method is amenable to use as long aslight can be
47、focused on the base of the pit, which would not bepossible in the case of example (e)inFig. 1.5.2.4.1 Use a metallurgical microscope with a magnificationrange from 50 to 5003 and a calibrated fine-focus knob (forexample, 1 division = 0.001 mm). If the latter is not available,a dial micrometer can be
48、 attached to the microscope in such away that it will show movement of the stage relative to themicroscope body.5.2.4.2 Locate a single pit on the metal surface and centerunder the objective lens of the microscope at low magnification(for example, 503). Increase the objective lens magnificationuntil
49、 the pit area covers most of the field under view. Focus thespecimen surface at the lip of the pit, using first the coarse andthen the fine-focusing knobs of the microscope. Record theinitial reading from the fine-focusing knob. Refocus on thebottom of the pit with the fine-focusing knob and record thereading. The difference between the initial and the finalreadings on the fine-focusing knob is the pit depth.5.2.4.3 Repeat the steps in 5.2.4.2 to obtain additionalmeasurements or until satisfactory duplication has been ob-tained. The repeatability of pit depth measurements on
