1、Designation: G 112 92 (Reapproved 2003)Standard Guide forConducting Exfoliation Corrosion Tests in Aluminum Alloys1This standard is issued under the fixed designation G 112; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide differs from the usualASTM standard in thatit does not address a specific test. Rather, it is an introd
3、uctoryguide for new users of other standard exfoliation test methods,(see Terminology G15for definition of exfoliation).1.2 This guide covers aspects of specimen preparation,exposure, inspection, and evaluation for conducting exfoliationtests on aluminum alloys in both laboratory accelerated envi-ro
4、nments and in natural, outdoor atmospheres. The intent is toclarify any gaps in existent test methods.1.3 The values stated in SI units are to be regarded as thestandard. The inch-pound units given in parentheses are forinformation only.1.4 This standard does not purport to address all of thesafety
5、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:G1 Practice for Preparing, Cleaning,
6、 and Evaluating Cor-rosion Test Specimens2G15 Terminology Relating to Corrosion and CorrosionTesting2G34 Test Method for Exfoliation Corrosion Susceptibilityin 2XXX and 7XXX Series Aluminum Alloys (EXCOTest)2G50 Practice for Conducting Atmospheric Corrosion Testson Metals2G66 Method for Visual Asses
7、sment of Exfoliation Corro-sion Susceptibility of 5XXX Series Aluminum Alloys(ASSET Test)2G85 Practice For Modified Salt Spray (Fog) Testing2G92 Practice for Characterization of Atmospheric TestSites23. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 panela flat, rectangular spec
8、imen normally takenwith the test surface parallel to the longitudinal and long-transverse dimensions of fabricated product. For thin sheet andextrusions, the thickness may be the full thickness of the part.3.1.2 samplea portion of a large piece, or an entire pieceout of a group of many pieces, that
9、is submitted for evaluationand considered representative of the larger piece or population.For castings and forgings, this may be an extra portion orprolongation, or in the case of small parts, an entire extra piecetaken from a specific lot.3.1.3 specimenthe actual test piece to be corrosion tested.
10、Frequently this has a specific shape with prescribed dimen-sional tolerances and finishes.3.1.4 test planethe plane in the thickness of the samplethat is being tested. Generally this is the fabricated surface orsome specified interior plane. Interior planes typically used are:(a) T/10 = 10 % of the
11、thickness removed, (this is representa-tive of a minimal machining cut to obtain a flat surface), (b)T/4 = quarter plane, 25 % of the thickness removed, and (c)T/2 = midplane, 50 % of the thickness removed.4. Significance and Use4.1 Although there are ASTM test methods for exfoliationtesting, they c
12、oncentrate on specific procedures for test meth-odology itself. Existent test methods do not discuss materialvariables that can affect performance. Likewise they do notaddress the need to establish the suitability of an acceleratedtest for alloys never previously tested nor the need to correlateresu
13、lts of accelerated tests with tests in outdoor atmospheresand with end use performance.4.2 This guide is a compilation of the experience of inves-tigators skilled in the art of conducting exfoliation tests andassessing the degree and significance of the damage encoun-tered. The focus is on two gener
14、al aspects: guides to techniques1This 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 October 1, 2003. Published October 2003. Originallyapproved in 1992. Last pr
15、evious edition approved in 1997 as G 112 92 (1997).2Annual Book of ASTM Standards, Vol 03.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.that will enhance the likelihood of obtaining reliable informa-tion, and tips and procedures
16、 to avoid pitfalls that could lead toerroneous results and conclusions.4.3 The following three areas of testing are considered: thetest materials starting with the “as-received” sample up throughfinal specimen preparation, the corrosion test procedures in-cluding choice of test, inspection periods,
17、termination point,and rating procedures, and analyses of results and methods forreporting them.4.4 This guide is not intended as a specific corrosion testprocedure by which to evaluate the resistance to exfoliation ofan aluminum alloy product.4.5 This guide is not intended as a basis for specificati
18、ons,nor as a guide for material lot acceptance.5. Material5.1 Sample SizeMost exfoliation tests do not require anyparticular specimen size, but when beginning a new investiga-tion it is best to obtain considerably more material than theminimum amount needed. About 50 to 100 % overage isrecommended.
19、This avoids the need of procuring a secondsample, that may have a different response, to complete anyconfirmatory retests or extensions to a specific program.5.2 Sample ReproducibilityThe specific location ofsamples in a mill product, and the number of samples to takeare beyond the scope of this gui
20、de. When testing largeproduction items, a typical procedure is to test at both ends(front and rear), and to test at the side and at the mid-width ifthe product is 0.6 m (2 ft) or more in width. Thick productsshould be tested at various planes through the thickness.5.2.1 In addition, some assessment
21、should be made of theuniformity of a large sample, or of numerous small samples.Typical quick check methods would be to measure electricalconductivity or hardness. If the material variability has apattern, for example, a difference between front and rear of along extrusion, then this should be noted
22、 and the specimenssegregated accordingly. If the variability is random, thenmultiple test specimens should be randomized.5.3 Sample MicrostructureThe directionality of the grainstructure of aluminum alloys will markedly affect the suscep-tibility to exfoliation. When a product shape and alloy arebei
23、ng tested for the first time, it is advisable to macroetch fullthickness by longitudinal and by transverse slices to establishthe directionality and uniformity of the grain structure. Testpanels are normally positioned such that the test surface isparallel to the plane in the product with the most e
24、longatedgrain structure. Complex shaped parts, such as certain extru-sions or die forgings, may have several categories of grainstructures and grain flow that do not necessarily follow the partgeometry. Grain structure of such parts must be determined bymacroetching or from prior experience.5.3.1 Fo
25、r a given temper condition, unrecrystallized, pan-cake shaped grains, that are long and wide but relatively thin,are the most susceptible. Pancake shaped recrystallized grains,as in sheet, are the next most susceptible. This is followed bythe long, rod shaped grains found in extruded or rolled rod a
26、ndbar with a symmetrical cross section, for example, circle,square, hex, or a rectangle with the width not more than twicethe thickness. An equiaxed grain structure is the least suscep-tible to exfoliation, especially if the grain size is large. Oftenthe recrystallized surface layer on products such
27、 as extrusions,forgings, or sheet will not exfoliate, even though it corrodesintergranularly.5.4 Sample TemperWhen a large sample is obtained as astock item for use over a long time period, the extra materialshould be stored in a stable temper and at a low enoughtemperature so that no further precip
28、itation will occur to alterthe starting condition of the metal. The unaged W temper of7XXX alloys is not stable and will continue to age harden atroom temperature. Room temperature storage of such materialshould be limited to a couple of months at most. Natural agingof these alloys can be retarded a
29、lmost completely by storing thematerial in a freezer at 40C (40F) or colder. This factor isof even more importance in determination of mechanicalproperties than the investigation of corrosion resistance.6. Selection of an ASTM Test Method6.1 Selection of the appropriateASTM test method(s) to usewill
30、 depend primarily on the type of alloy and on the end useenvironment. When testing a new alloy or temper, a testmethod known to be applicable to the most similar commercialalloy is normally selected. The user is cautioned, however, thateven small changes in alloy chemistry, or changes in process-ing
31、 method (for example, rapid solidification processes) canmarkedly effect resistance of an alloy and the appropriatenessof a test method. Normally exfoliation tests are conducted oningot metallurgy alloys, that tend to have the elongated grainstructure prone to exfoliate. The known alloy applicabilit
32、y ofthe ASTM test methods are listed below. Included are someobserved instances where a test method was found to beinappropriate, or at least produced results different than thoseobserved on the initial qualification alloys.6.1.1 It is advisable to initially employ more than onelaboratory test metho
33、d and determine whether they agree; or ifnot, which method is the most discriminating. One procedurefor doing this is to apply different fabrication procedures to themetal that are known to generally affect resistance to exfolia-tion and determine which of the test methods best detectsdifferences in
34、 the corresponding resistance to exfoliation.Fabrication variables that often affect resistance to exfoliationare variable quench cooling rates, slow quenches being ad-verse; and variable amounts of aging, underaged, or peak agedconditions generally being more susceptible than overagedconditions. (1
35、)36.2 Test Method G66Acidified Salt Solution ExfoliationTest (ASSET) is used for 5XXX alloys containing 2.0 % ormore magnesium. The round robin qualification tests for thistest method were conducted on alloys 5086 (3.5 to 4.5 % Mg)and 5456 (4.7 to 5.5 % Mg). (2) However, Test Method G66(ASSET) gives
36、 problem free exfoliation indications with all5XXX alloys.6.3 Test Method G34Exfoliation Corrosion (EXCO)Test isintended for use with high strength 2XXX and 7XXX ingot3The boldface numbers given in parentheses refer to a list of references at theend of the text.G 112 92 (2003)2metallurgy alloys, a 9
37、6 h period being prescribed for the 2XXXalloys and a 48 h period for the 7XXX alloys.6.3.1 For the 2XXX alloys, the round robin qualificationtests were conducted on alloys 2024 and 2124 in the T351 andT851 tempers. The appropriateness of the method has not beenfully established for all other 2XXX al
38、loys. It has beenreported as being too aggressive and nonrepresentative ofperformance in outdoor atmospheres for alloys 2219, 2419 and2519 in the T851 tempers (3) and for various Al-Li alloys inboth as-quenched and artificially aged tempers (1).6.3.2 For the 7XXX alloys the round robin qualification
39、tests were conducted on alloy 7075 in the T651, T7651, andT7351 tempers and alloy 7178 in the T651 and T7651 tempers.Experience has shown that the EXCO method can be used for7050 and 7150 alloys in the T651, T6151, T7451, T7651, andT7751 tempers, but the test is somewhat more aggressive onthese allo
40、ys (4). This method also was evaluated with copperfree alloys such as 7021-T6 and 7146-T6, but generally anabbreviated exposure period of 16 to 24 h was used.6.3.3 Exposure of the powder metallurgy alloys 7090 and7091-T6 specimens to EXCO results in rapid dissolution andpowdering of the specimen, du
41、e to continuous drop of theextremely fine grains. Four years of exposure of the same partsto seacoast atmosphere resulted only in mild general corrosionand no exfoliation (5).6.4 AnnexA2 of Practice G85ModifiedASTMAceticAcidSalt Intermittent Spray Test, (MASTMAASIS) was developedusing alloys 2024, 2
42、124, 7075, and 7178. This method usuallyis run in the wet bottom condition (some solution and highhumidity always present). A dry bottom condition (no solutionpresent and gradually falling humidity during the purge andnon-spraying periods) has been recommended for 2XXXalloys.6.4.1 The test cabinets
43、used to conduct the MASTMAASIStest, and the salt fog tests subsequently described in 6.5 and6.6, are produced by several suppliers. The fog deliverysystems and cabinet geometry can differ and have graduallyevolved. Consequently some cabinet to cabinet variability intest results is inherent, due prim
44、arily to variation in spraytechniques and the relative humidity conditions during thenon-spray portions of the cycle.6.4.2 There is no record of the MASTMAASIS environ-ment being unrealistically aggressive, causing exfoliation of amaterial that did not subsequently exfoliate in the seacoast. Assuch
45、any occurrence of exfoliation in this test most likelyindicates susceptibility under some service conditions. Theconverse of this statement has not been observed.6.4.3 MASTMAASIS is not appropriate for 5XXX alloys,because it does not always detect exfoliation susceptibility inmaterials proven to be
46、susceptible by other test methods.6.4.4 MASTMAASIS has been used with some success on6XXX series alloys. However, in some cases it caused severeintergranular corrosion that could be confused with exfoliationcorrosion unless specimens are examined metallographically.6.5 Annex A3 of Practice G85Seawat
47、er Acetic Acid Test(SWAAT) was developed using the same 5XXX, 2XXX, and7XXX alloys as mentioned above for the ASSET and EXCOmethods (6).6.6 Practice G85AnnexA4 (SALT/SO2Spray Testing) wasdeveloped using the same, 2XXX and 7XXX alloys as men-tioned above for the EXCO method (7).6.7 Both the methods i
48、n Annex A3 and Annex A4 ofPractice G85result in more gelatinous corrosion products thandoes Annex A2. This tends to increase pitting corrosion on thespecimens. Annex methods A2, A3, and A4 in Practice G85are not equivalent, and the user should determine whichmethod best suits the alloys and applicat
49、ions under investiga-tion.7. Baseline Experience7.1 The best check on the appropriateness of an acceleratedtest is to determine whether the results it produces agree withknown service experience.7.2 When there is no actual service experience, then expo-sure in a severe outdoor atmosphere known to produceexfoliation corrosion is a useful approximation of the condi-tions a part will encounter in service. The most frequently usedenvironments are seacoast sites and highly industrialized urbanlocations. Selection of the particular environment to use canbest be based on the inten
