1、Designation: G 50 76 (Reapproved 2003)Standard Practice forConducting Atmospheric Corrosion Tests on Metals1This standard is issued under the fixed designation G 50; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of last
2、revision. 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 practice defines conditions for exposure of metalsand alloys to the weather. It sets forth the general proceduresthat
3、 should be followed in any atmospheric test. It is presentedas an aid in conducting atmospheric corrosion tests so thatsome of the pitfalls of such testing may be avoided. As such, itis concerned mainly with panel exposures to obtain data forcomparison purposes.1.2 This standard does not purport to
4、address all of thesafety concerns, if any, associated with its use. It is theresponsibility of whoever uses this standard to consult andestablish appropriate safety and health practices and deter-mine the applicability of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:
5、A 380 Practice for Cleaning, Descaling and Passivation ofStainless Steel Parts, Equipment, and Systems2D 2010/D 2010M Test Methods for Evaluation of TotalSulfation Activity in the Atmosphere by the Lead DioxideTechnique3G 1 Practice for Preparing, Cleaning, and Evaluating Cor-rosion Test Specimens4G
6、 33 Practice for Recording Data from Atmospheric Cor-rosion Tests of Metallic-Coated Steel Specimens4G 46 Guide for Examination and Evaluation of PittingCorrosion43. Significance and Use3.1 The procedures described herein can be used to evaluatethe corrosion resistance of metals when exposed to the
7、weather,as well as to evaluate the relative corrosivity of the atmosphereat specific locations. Because of the variability and complexityof weather effects and the industrial and natural factorsinfluencing the atmospheric corrosivity of a test site, a multi-year exposure period should be considered
8、to minimize theirinfluence. Also, as corrosivity may vary at a site from season toseason, exposures should be made either at the same time ofthe year to minimize variability or these differences should beestablished by multiple exposures.3.2 Control specimens should always be employed inweathering t
9、ests. The control specimens should be from amaterial having established weathering characteristics. A sub-stantial amount of corrosion data shall have been accumulatedfor the control specimens. It is also good practice to retainsamples of all materials exposed so that possible effects oflong-term ag
10、ing can be measured.4. Test Sites4.1 Test sites should be chosen at a number of locationsrepresentative of the atmospheric environments where themetals or alloys are likely to be used. If such information is notavailable, the selection should include sites typical of indus-trial, rural, and marine a
11、tmospheres.4.2 Exposure racks should be located in cleared, well-drained areas such that the exposed specimens will be sub-jected to the full effects of the atmosphere at the location of thetest site. Shadows of trees, buildings, or structures should notfall on the specimens, and local contamination
12、 of the atmo-sphere should be avoided, unless the specific influences of suchconditions are intended to be assessed.4.3 In special cases, the exposure racks may be partiallysheltered to allow accumulation of corrosive materials from theair but at the same time prevent washing by rain. If shelteringi
13、s used, its purpose and configuration should be described indetail.4.4 If local pollution effects are to be investigated, thesamples should be exposed at different distances from thesource and at different elevations. Where it is particularlyimportant to obtain corrosion rates involving a micro-envi
14、ronment, samples should be mounted directly on thestructure involved. Suitable attachment must be devised foreach case.5. Exposure Racks and Frames5.1 Test racks and frames should be constructed of amaterial that will remain intact for the entire proposed periodof exposure. Galvanized pipe has been
15、found adequate for rackconstruction in most environments (Note 1). Type 304 stainlesssteel is adequate as a frame material for all environments. Formarine exposures, alloy 400 UNS No. N04400 or Type 3161This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals, and is the d
16、irect responsibility of Subcommittee G01.04 on AtmosphericCorrosion.Current edition approved Aug. 27, 1976. Published March 1977.2Annual Book of ASTM Standards, Vol 01.03.3Annual Book of ASTM Standards, Vol 11.03.4Annual Book of ASTM Standards, Vol 03.02.1Copyright ASTM International, 100 Barr Harbo
17、r Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.stainless steel has also been successfully used. Aluminum(5052 and 6061-T6) and copper frames also have givensatisfactory service in a wide range of environments. Careshould be observed in the use of copper frames, as corrosionpro
18、ducts splashed during rainfall might affect the corrosion ofother metals such as aluminum or magnesium.NOTE 1If galvanized pipe is field-threaded, thread areas must beprotected to ensure joint integrity for long exposure periods. In severeenvironments, additional coatings may be required to provide
19、corrosion-free service.5.2 Racks and frames also may be constructed of wood.Insulators may be attached to wooden frames with aluminumbronze, alloy 400, or stainless steel screws. In such a case, nowood sections should be used with dimensions less than 2 by4 in. (50 by 100 mm), and at least two coats
20、 of an exteriorgrade paint or enamel over a suitable primer must be applied.Periodic maintenance will be required on all wood construc-tion.5.3 Solid, glazed, electrical insulator knobs should be usedto hold the specimens on the frames, using stainless steel, alloy400, aluminum, nylon, or bronze bol
21、ts and nuts. Specimensshall be mounted in the grooves of these insulators. In selectingfasteners for use on specific frame materials, care should betaken to avoid unfavorable galvanic relationships.5.4 A suitable frame for mounting the insulators is shown inFig. 1. This frame will accommodate 70 sta
22、ndard 4 by 6-in.(100 by 150-mm) specimens; other sizes can be mounted byrearranging the insulators in the holes provided. It is acceptableto slot the holes in the frames in such a manner that themounting frames are adjustable for specimens of other sizes.This is a convenience when it is not possible
23、 to preparespecimens of a preplanned size, and it is often helpful in fittingthe specimens snugly into the frames.5.5 The racks should be designed to give exposure to aslarge an area of the underside of the specimens as possible.Structural members of the rack should not be located directlyunder the
24、specimens where they would shelter the underside ofthe specimens.5.6 As most published data on atmospheric corrosion ofmetals are based on an exposure angle of 30 deg from thehorizontal, facing south, it is recommended that this angle beused. Racks should be designed so that the lowest specimensare
25、at least 30 in. (760 mm) above the ground.NOTE 2Maximum exposure to the sun may be obtained by exposingspecimens facing south (for the northern hemisphere) at an angle equal tothe latitude of the test site. Exposure at this angle will yield the lowestcorrosion rates for most materials. Although thes
26、e corrosion rates willchange at other angles of exposure, the order of merit established for eachmaterial will be the same.NOTE 3In special instances, it may be desirable to orient racks andframes in the direction of a specific corrodent source, for example, theocean, for marine environments. Also,
27、this practice should not beconstrued as prohibiting special orientation of test frames for specific testpurposes, but it is strongly suggested that in such cases testing also bedone in accordance with this practice so that a basis point for comparisonwith available data is determined. Any special or
28、ientation or preferentialsource of corrosion should be specifically identified in the exposure sitedescription.5.7 A rack of the design and dimensions shown in Fig. 2 willgive the correct exposure angle and can support the specimenframe described in 5.4.5.8 The ground under the racks should be kept
29、free ofweeds, bushes, and debris. Organic herbicides, defoliants, orpesticides should not be used for this purpose.6. Test Specimens6.1 When the material to be tested is in sheet form, aspecimen size of 4 by 6 in. (100 by 150 mm) is appropriate.Specimens may be larger, for example, 4 by 8 in. (100 b
30、y 200mm), to suit a particular test; however, the specimens prefer-ably should not be smaller than 4 by 6 in.6.2 To assure adequate rigidity of the specimens on the rack,a minimum thickness of 0.030 in. (0.75 mm) is suggested. Itmay be difficult to accommodate thicknesses greater than 0.250in. (6.25
31、 mm) in the insulator grooves. (Special deep-throatedinsulators can be obtained to accommodate thicker specimens,or the edges of thicker specimens can be machined to fitstandard insulators.)6.3 When it is desired to test samples of odd shapes, such asbolts, nuts, pipes, angles, assemblies, and struc
32、tures, etc., ameans of supporting them in the test racks must be devised. Itis important that the specimens be electrically insulated fromtheir respective supports and from each other to preventunintentional galvanic corrosion. However, if desired, galvaniccouples of dissimilar metals can be exposed
33、 on these frames.Efforts should be made to minimize crevices between speci-mens and support materials.6.4 The total number of test specimens required should bedetermined from a knowledge of the duration of the test and theplanned removals of the specimens for intermediate evalua-tions. Usually it sh
34、ould not be necessary to remove specimensprior to completing one years exposure, unless specific dataare required for corrosion occurring during earlier stages ofexposure. For reliable results, sufficient specimens should beused for multiple removals at each exposure period. Triplicatespecimens for
35、each examination period will usually satisfy thisrequirement. A suggested suitable removal schedule is 1, 2, 4,8, and 16 years. Removal schedules for tests of differentperiods of total exposure should be adjusted accordingly.6.5 Included with each series of test specimens should be anappropriate num
36、ber of control specimens, as defined in 3.2.7. Preparation of Test Specimens7.1 Specimens should be identified in a manner that willendure for the life of the test. A good method is the use of aseries of edge notches or drilled holes in the body of thespecimen arranged according to some desired code
37、. Anothermethod is to attach a stainless steel tag by means of aninsulated cord and a suitably located hole. Numbers stampedon the back of the specimen and further protected by coveringwith a good grade of electrical tape is a suitable technique forshort-term exposure tests. For materials that do no
38、t exhibitsignificant atmospheric corrosion (copper, aluminum, stainlesssteels, etc.), it is sufficient to stamp the identification on theface of the panel.7.2 Oil, grease, and dirt should be removed by degreasingwith a solvent cleaner or scrubbing, or both, to removeG 50 76 (2003)2insoluble soils (s
39、ee Practice G 1). Any mill scale or rust shouldbe removed from all ferrous specimens unless it is specificallydesired to perform the test with the mill scale intact. Picklingwith inhibited acid as well as blasting with sand or grit areacceptable descaling methods. If acid pickling is used, caremust
40、be taken to stop the pickling action as soon as the millscale and rust have been removed. Stainless steels should bepickled in accordance with Practice A 380 to ensure surfacesfree of iron contamination.7.3 Specimens should be weighed to at least the nearest0.01 g before exposure. More corrosion-res
41、istant materials arefrequently weighed to the nearest 0.1 mg. When deemedappropriate, the specimens should be photographed to take intoaccount pre-exposure surface defects. Records should be keptof the weight, dimensions, and appearance of each specimen atthe beginning of the test. Data to be record
42、ed prior to exposureare explicitly outlined in Practice G 33. Changes in thephysical appearance and any corrosion losses of the specimensdue to weathering can then be determined. If information onchanges in mechanical properties is desired, initial measure-ments should be determined on materials in
43、test (see also 3.2).For materials that change their mechanical properties on agingFIG. 1 Suitable Test FrameG 50 76 (2003)3at the temperature of the test site, mechanical properties shallbe determined on separate specimens stored at that temperaturebut protected from corrosion.8. Procedure8.1 Mount
44、the specimens on the racks so that they aresupported by the insulators and do not make electrical contactwith each other or with the supporting racks.8.2 Atmospheric factors such as time of wetness of thespecimens, temperature of the specimens, and the concentra-tion of atmospheric contaminants such
45、 as sulfur dioxide andchlorides in the local environment have a great influence on thecorrosion rate of many metals, particularly in the early stagesof exposure. Therefore, if possible, expose the test materials inan environment similar to that to which they will be subjectedin actual use.8.3 Period
46、ic observations of weather factors and a means fordetermining atmospheric variables may be incorporated in thetest.5Also see Test Method D 2010.8.4 Make periodic evaluations as to the conditions of boththe top (skyward) and the bottom (groundward) surfaces,noting whether or not any oxide coating is
47、tightly adhering orif it continues to flake off as it oxidizes. Also record the colorof the oxide coating, uniformity, and texture. Make suchevaluations annually, whether specimens are removed or not.Data to be recorded during exposure are also outlined inPractice G 33.8.5 Establish a removal schedu
48、le (see 6.4).9. Evaluation of Test Specimens9.1 Remove specimens from exposure at the scheduledintervals.9.2 Following recording of data in accordance with PracticeG 33, clean all surfaces of specimens in accordance withPractice G 1 and reweigh specimens to the nearest 0.01 g (or tothe same precisio
49、n as originally weighed). For certain tests, itmay be of interest to preserve corrosion products for laboratoryanalysis.9.3 When appropriate, determine the mass loss of eachspecimen and convert the results to a corrosion rate (PracticeG 1).9.4 Where the corrosion that occurs is highly localized (asin pitting) and the loss in mass is low, mass loss results can bemisleading. In such cases, other means of evaluation arenecessary; for example, the tensile strengths of the exposedspecimens can be determined after cleaning and compared withthe tensile strengths of unexposed specimens cut
