ASTM G46-1994(2013) Standard Guide for Examination and Evaluation of Pitting Corrosion《点状腐蚀的检查和评估标准指南》.pdf

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1、Designation: G46 94 (Reapproved 2013)Standard Guide forExamination and Evaluation of Pitting Corrosion1This standard is issued under the fixed designation G46; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of last revisi

2、on.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () 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 theevaluation of p

3、itting (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 determi

4、ne the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E3 Guide for Preparation of Metallographic SpecimensG1 Practice for Preparing, Cleaning, and Evaluating Corro-sion Test SpecimensG15 Terminology Relating to Corrosion and Corrosion Test-ing (Withd

5、rawn 2010)3G16 Guide for Applying Statistics to Analysis of CorrosionData2.2 National Association of Corrosion Engineers Standard:NACE RP-01-73 Collection and Identification of CorrosionProducts43. Significance and Use3.1 It is important to be able to determine the extent ofpitting, either in a serv

6、ice application 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 InspectionAvisual examination of the corrodedmetal surfac

7、e is usually 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

8、 surface after 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

9、 reserve them 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 apoi

10、nted tool to 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 sur

11、face under ordinarylight to determine the approximate size and distribution of pits.Follow this procedure by a more detailed examination througha microscope using low magnification (20).4.1.4 Determine the size, shape, and density of pits.4.1.4.1 Pits may have various sizes and shapes. A visualexami

12、nation of 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-sec

13、tioned shape 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 p

14、its in each 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.1This guide is under the jurisdiction of ASTM Committee G01 on Co

15、rrosion ofMetals and is the direct responsibility of Subcommittee G01.05 on LaboratoryCorrosion Tests.Current edition approved May 1, 2013. Published July 2013. Originally approvedin 1976. Last previous edition approved in 2005 as G46 94 (2005). DOI:10.1520/G0046-94R13.2For referenced ASTM standards

16、, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.4Insert

17、 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 States14.1.5 Metallographic ExaminationSelect and cut out arepresentative portion of the metal surface containing the pitsand

18、prepare a metallographic specimen in accordance with therecommended procedures given in Methods E3. Examinemicroscopically 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 in

19、tergranular corrosion, dealloying, and so forth.4.2 Nondestructive InspectionA number of techniqueshave been developed to assist in the detection of cracks orcavities in a metal surface without destroying the material (1).5These methods are less effective for locating and defining theshape of pits t

20、han some of those previously discussed, but theymerit consideration because they are often used in situ, andthus are 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 mat

21、erial. Imperfections may bedetected if they cause a change in the absorption of X rays.Detectors or films are used 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 thi

22、ckness to be detected. Thistechnique has only slight application to pitting detection, but itmight be a useful means 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 exte

23、nt of subsurface and undercuttingpitting (Fig. 1).4.2.2 Electromagnetic:4.2.2.1 Eddy currents can be used to detect 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

24、 a coil, eddycurrents are induced in the specimen, and they in turn producea magnetic field of their own. Materials 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 di

25、fferences.4.2.2.2 The induction of a magnetic field in ferromagneticmaterials is another approach that is used. Discontinuities 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

26、detect the leakagefield and to outline the size and shape of the discontinuities.Rather small imperfections can be 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 th

27、e limited shape ofparts that can be examined.4.2.3 Sonics:4.2.3.1 In the use of ultrasonics, pulses of sound energy 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 interprete

28、d toshow the location of flaws or pits. Both contact and immersionmethods are used. The test has good sensitivity and providesinstantaneous information about the size and location of flaws.However, reference standards are required for comparison, andtraining is needed to interpret the results proper

29、ly.5The boldface numbers in parentheses refer to the list of references at the end ofthis practice.FIG. 1 Variations in the Cross-Sectional Shape of PitsG46 94 (2013)24.2.3.2 An alternative approach is to use acoustic emissionsin detecting flaws in metals. Imperfections, such as pits,generate high-f

30、requency 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-quently exudes from the s

31、urface after the excess penetrant hasbeen removed. Defects are located by spraying the surface witha developer that 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

32、 the rate of bleed-out. This techniqueprovides only an approximation of the depth and size of pits.4.2.5 None of 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 ge

33、nerally are not able to detect small pits,and confusion may arise in attempting to differentiate betweenpits and 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

34、. Extent of Pitting5.1 Mass LossMetal mass loss is not ordinarily recom-mended for use as a measure of the extent 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

35、 cannot bedetermined accurately from mass loss. In any case, mass losscan only provide information about total metal 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 visua

36、l comparisonof pitted surfaces may be adequate to evaluate the pittingresistance of alloys in laboratory tests.5.2 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 polis

37、hing the sur-face. The depth of the pit is measured on the flat, polishedsurface by the use of a microscope with 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 ti

38、me consuming, the deepestpit may not have been selected, and the pit may not have beensectioned at the deepest point 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 th

39、e specimen between two areas thathave not been affected by general corrosion. Select a portion ofthe surface on one 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 d

40、isappeared. (Some difficulty from gallingand smearing may be encountered with soft metals, and pitsmay be obliterated.) 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 unmach

41、ined surface unless the thickness hasbeen reduced by 50% or more during the machining of the firstside.5.2.2.2 This 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 num

42、ber of visible pits remaining at each stage.Subtract the number of pits at each stage from the count at theprevious 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

43、 depth gage to penetratethe pit cavity. Zero the instrument on an unaffected area at thelip of the pit. Insert the 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 t

44、o minimize metal penetration at the baseof the pit. It can be advantageous to use a stereomicroscope inconjunction 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 ac

45、commodate the needle without obstruction; this eliminatesthose pits where undercutting or directional orientation 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 botto

46、m ofthe pit, it completes the electrical circuit, and the probemovement is a measurement of pit depth. This method 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 narro

47、w or difficult to penetrate with a probetype of instrument. The method is amenable to use as long aslight can be 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 500 and a calibra

48、ted fine-focus knob (forexample, 1 division = 0.001 mm). If the latter is not available,a dial micrometer can be 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 objecti

49、ve lens of the microscope at low magnification(for example, 50). Increase the objective lens magnificationuntil 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.G46 94 (2013)35.2.4.3 Repeat the steps in 5.2.4.2 to