ASTM_G_46_-_94_2013.pdf

Designation:G4694(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 revision.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 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 determine 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Preparing,Cleaning,and Evaluating Corro-sion Test SpecimensG15 Terminology Relating to Corrosion and Corrosion Test-ing(Withdrawn 2010)3G16 Guide for Applying Statistics to Analysis of CorrosionData2.2 National Association of Corrosion Engineers Standard:NACE RP-01-73 Collection and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 service 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 surface 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 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 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 apointed 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 surface 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 visualexamination 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-sectioned 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 pits 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 Corrosion 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,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 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 States1 4.1.5 Metallographic ExaminationSelect and cut out arepresentative portion of the metal su