1、Designation: G 44 99 (Reapproved 2005)Standard Practice forExposure of Metals and Alloys by Alternate Immersion inNeutral 3.5 % Sodium Chloride Solution1This standard is issued under the fixed designation G 44; the number immediately following the designation indicates the year of originaladoption o
2、r, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope
3、1.1 This practice covers procedures for making alternateimmersion stress corrosion tests in 3.5 % sodium chloride(NaCl) (Note 1). It is primarily for tests of aluminum alloys(Test Method G47) and ferrous alloys, but may be used forother metals exhibiting susceptibility to chloride ions. It setsforth
4、 the environmental conditions of the test and the means forcontrolling them.NOTE 1Alternate immersion stress corrosion exposures are some-times made in substitute ocean water (without heavy metals) prepared inaccordance with Specification D 1141. The general requirements of thispresent practice are
5、also applicable to such exposures except that thereagents used, the solution concentration, and the solution pH should be asspecified in Specification D 1141.1.2 This practice can be used for both stressed and un-stressed corrosion specimens. Historically, it has been used forstress-corrosion cracki
6、ng testing, but is often used for otherforms of corrosion, such as uniform, pitting, intergranular, andgalvanic.1.3 This practice is intended for alloy development and forapplications where the alternate immersion test is to serve as acontrol test on the quality of successive lots of the samemateria
7、l. Therefore, strict test conditions are stipulated formaximum assurance that variations in results are attributable tovariations in the material being tested.1.4 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.5 This standar
8、d 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
9、 Standards:2D 1141 Practice for Preparation of Substitute Ocean WaterD 1193 Specification for Reagent WaterE 3 Methods of Preparation of Metallographic SpecimensG1 Practice for Preparing, Cleaning, and Evaluating Cor-rosion Test SpecimensG 16 Guide forApplying Statistics toAnalysis of CorrosionDataG
10、47 Test Method for Determining Susceptibility to Stress-Corrosion Cracking of 2XXX and 7XXXAluminumAlloyProducts3. Summary of Practice3.1 The alternate immersion test utilizes a 1-h cycle thatincludes a 10-min period in an aqueous solution of 3.5 %sodium chloride (NaCl) followed by a 50-min period o
11、ut of thesolution, during which the specimens are allowed to dry. This1-h cycle is continued 24 h/day for the total number of daysrecommended for the particular alloy being tested. Typically,aluminum and ferrous alloys are exposed from 20 to 90 days orlonger, depending upon the resistance of the all
12、oy to corrosionby saltwater.4. Significance and Use4.1 The 3.5 % NaCl alternate immersion procedure is ageneral, all-purpose procedure that produces valid comparisonsfor most metals, particularly when specimens are exposed athigh levels of applied stress or stress intensity.4.2 While the alternate i
13、mmersion test is an accelerated testand is considered to be representative of certain naturalconditions, it is not intended to predict performance in special-ized chemical environments in which a different mode ofcracking may be operative. For example, it does not predict theperformance of aluminum
14、alloys in highly acidic environments1This recommended practice is under the jurisdiction of ASTM Committee G01on Corrosion of Metals, and is the direct responsibility of Subcommittee G01.06 onEnvironmentally Assisted Cracking.Current edition approved May 1, 2005. Published May 2005. Originallyapprov
15、ed in 1975. Last previous edition approved in 1999 as G 44 99.2For 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
16、.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.such as heated inhibited red fuming nitric acid (IRFNA). Forsuch cases, the results of the alternate immersion test are ofdoubtful significance until a relationship has been establishe
17、dbetween it and anticipated service environments.4.3 While this practice is applicable in some degree to allmetals, it is not equally discriminative of all alloys, even withinthe same metal system. Consequently, information should beestablished to allow comparisons of performances of the alloyof int
18、erest in the alternate immersion test and in naturalenvironments.NOTE 2The alternate immersion concept can be useful for exposureof corrosion specimens in other solutions because the procedure andapparatus provide a controlled set of conditions. Details of this are beyondthe scope of this practice.5
19、. Interferences5.1 A disadvantage of the 3.5 % NaCl alternate immersiontest for stres-corrosion cracking tests of certain high-strengthaluminum alloys is the severe pitting that develops in thespecimens. Such pitting can interfere with the initiation ofstress-corrosion cracks and may cause mechanica
20、l failures thatcomplicate the interpretation of the stress-corrosion test results.This is particularly a problem with copper-bearing aluminumalloys when tested with specimens of small cross section.Thorough metallographic examination of the specimens isnecessary for proper diagnosis of the cause of
21、failure andseparation of stress corrosion failures from those caused bymechanical overload.5.2 An advantage of the substitute ocean water (Note 1)isthat it causes less pitting corrosion of aluminum alloys than the3.5 % NaCl solution.6. Apparatus6.1 Method of CyclingAny suitable mechanism may beused
22、to accomplish the immersion portion of the cycle pro-vided that: (1) it achieves the specified rate of immersion andremoval, and (2) the apparatus is constructed of suitable inertmaterials. The usual methods of immersion are:6.1.1 Specimens placed on a movable rack that is periodi-cally lowered into
23、 a stationary tank containing the solution.6.1.2 Specimens placed on a hexagonal Ferris wheel ar-rangement which rotates every 10 min through 60 and,thereby, passes the specimens through a stationary tank ofsolution. Use of a Ferris wheel continuously rotating at a rateof 1 revolution per hour is no
24、t recommended for very largespecimens for which the rate of immersion would be slowerthan that specified in 6.2.6.1.3 Specimens placed in a stationary tray open to theatmosphere and having the solution moved by air pressure,nonmetallic pump, or gravity drain from a reservoir to the tray.6.2 Rate of
25、ImmersionThe rate of immersion and removalof the specimens from the solution should be as rapid aspossible without jarring them. For purposes of standardization,an arbitrary limit shall be adopted such that no more than 2 minelapse from the time the first portion of any specimen iscovered (or uncove
26、red) until it is fully covered (or uncovered)by solution.6.3 Materials of Construction:6.3.1 Materials of construction that come in contact with thesalt solution shall be such that they are not affected by thecorrodent to an extent that they can cause contamination of thesolution and change its corr
27、osiveness.6.3.2 Use of inert plastics or glass is recommended wherefeasible.6.3.3 Metallic materials of construction should be selectedfrom alloys that are recommended for marine use and of thesame general type as the metals being tested. Preferably, allmetal parts should be protected with a suitabl
28、e corrosion-resistant coating that also satisfies paragraph 6.3.1.6.4 Specimen Holders:6.4.1 Specimen holders should be designed to electricallyinsulate the specimens from each other and from any other baremetal. When this is not possible, as in the case of certainstressing bolts or jigs, the bare m
29、etal contacting the specimenshould be isolated from the corrodent by a suitable coating.Protective coatings should be of a type that will not leachinhibiting or accelerating ions or protective oils over thenoncoated portions of the specimen. Coatings containingchromates are to be particularly avoide
30、d.NOTE 3Coatings that have been satisfactorily used by several labo-ratories are described in Appendix X1.6.4.2 The shape and form of specimen supports and holdersshould be such that:6.4.2.1 They avoid, as much as possible, any interference offree contact of the specimen with the salt solution;6.4.2
31、.2 They do not obstruct air flow over the specimen,thereby retarding the drying rate;6.4.2.3 They do not retain a pool of solution in contact withthe specimen after the immersion period; and6.4.2.4 Drainage from one specimen does not directly con-tact any other specimen.7. Reagents7.1 Reagent grade
32、sodium chloride (NaCl) shall be usedconforming to the specifications of the Committee on Analyti-cal Reagents of the American Chemical Society, where suchspecifications are applicable (see Note 1).37.2 The solution shall be prepared using distilled or deion-ized water conforming to the purity requir
33、ements of Specifi-cation D 1193, Type IV reagent water except that for thispractice the values for chloride and sodium shall be disre-garded.8. Solution Conditions8.1 ConcentrationThe salt solution shall be prepared bydissolving 3.5 6 0.1 parts by weight of NaCl in 96.5 parts ofwater.8.2 Solution pH
34、:3Reagent Chemicals, American Chemical Society Specifications, AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacop
35、eiaand National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,MD.G 44 99 (2005)28.2.1 The pH of the salt solution, when freshly prepared,shall be within the range from 6.4 to 7.2. Only diluted, reagentgrade hydrochloric acid (HCl) or reagent grade sodium hydrox-ide (NaOH) shall be
36、 used to adjust the pH.8.2.2 Experience has shown that periodic adjustment of pHis not necessary when the minimum volume of solution is metand the solution is replaced at the specified interval. For a newtesting facility, however, it is recommended that daily pHmeasurements be made for one week to v
37、erify stable operationand adjustments made if required. If the solution is used longerthan the recommended interval (8.6), a pH measurement andany necessary adjustment should be made at least weekly.8.3 TemperatureA freshly prepared solution should beallowed to come to within 3C of the specified roo
38、m tempera-ture before being used (9.1.1). Thereafter, no control isrequired on the solution temperature per se. Instead, the roomair temperature is controlled and the solution is allowed toreach temperature equilibrium.8.4 Minimum VolumeThe volume of the test solutionshould be large enough to avoid
39、any appreciable change in itscorrosiveness either through exhaustion of corrosive constitu-ents, or the accumulation of corrosion products or otherconstituents that might significantly affect further corrosion.An arbitrary minimum ratio between the volume of testsolution and area of specimen (includ
40、ing any uncoated acces-sories) of 32 mL/cm2(200 mL/in.2) of specimen area isrecommended.8.5 Replenishment of Water Lost by EvaporationEvaporation losses should be made up by frequent, at leastdaily, additions of water of the required purity (7.2). Evapora-tion losses must not be replenished with the
41、 salt solution. Thesimplest and recommended procedure is to initially fill thesolution to a liquid level line and refill to that line daily.Automatic constant liquid level devices may be used, but arenot required. An alternative method is to check the solutionwith a hydrometer and add the necessary
42、amount of water tobring the salt concentration to 3.5 %.8.6 Replacement of SolutionFresh solution shall be pre-pared weekly. At such time, the portions of the apparatus thatcontact the solution should be cleansed by flushing with water.More frequent replacement of solution may be required forcertain
43、 steels if severe rusting occurs.9. Air ConditionsNOTE 4Other than temperature and humidity, the gaseous and par-ticulate makeup of the laboratory atmosphere is beyond the scope of thisrecommended practice. However, testers are cautioned not to exposespecimens in the vicinity of obvious fumes or air
44、borne contamination, andto consider the possibility of such contamination if unusual results areobtained.9.1 Temperature:9.1.1 The air temperature shall be maintained at 27 6 1C(80 6 2F) throughout the entire test cycle.NOTE 5When a large Ferris wheel apparatus is used such that thespecimens travel
45、through several feet of height, care must be taken toensure that the air temperatures at all height levels are within theprescribed limits.9.2 Relative HumidityThe percent relative humidity of theair shall be controlled at 45 6 10 % throughout the entire testcycle.NOTE 6Care should be taken to avoid
46、 overnight and weekendchanges in the operation of the laboratory heating, ventilating, and airconditioning equipment which could result in systematic excursionsoutside the temperature and humidity control ranges. Occasional excur-sions related to power outage, mechanical failure, or weather conditio
47、nsshall be recorded.9.3 Air Circulation:9.3.1 Air circulation is recognized to be an importantconsideration because it affects both the rate at which speci-mens dry and the loss of water by evaporation. Optimumconditions for air circulation have not been established, but therecommendations described
48、 in 9.3.2 should be followed.9.3.2 The most important consideration is to achieve themoderate specimen drying conditions stipulated in 11.2. Be-cause various testing facilities use different immersion appa-ratus and room sizes, individual experimentation is required toachieve adequate circulation. A
49、 mild circulation of air isrecommended with two precautionary considerations:9.3.2.1 Drying by forced air blasts on the specimens is notrecommended because of difficulty in maintaining uniformdrying of large groups of specimens.9.3.2.2 Stagnant air conditions should be avoided.10. Calibration and Standardization10.1 When a new test facility is established, calibrationstress corrosion tests should be conducted to determine how theresults obtained compare with published data on well estab-lished alloys. Such tests are best made on products of simplegeometry with unifo
