1、Designation: B 825 02Standard Test Method forCoulometric Reduction of Surface Films on Metallic TestSamples1This standard is issued under the fixed designation B 825; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last
2、 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 test method covers procedures and equipment fordetermining the relative buildup of corrosion and tarnish films(inclu
3、ding oxides) on metal surfaces by the constant-currentcoulometric technique, also known as the cathodic reductionmethod.1.2 This test method is designed primarily to determine therelative quantities of tarnish films on control coupons thatresult from gaseous environmental tests, particularly when th
4、elatter are used for testing components or systems containingelectrical contacts.1.3 This test method may also be used to evaluate testsamples that have been exposed to indoor industrial locationsor other specific application environments. (See 4.6 for limi-tations.)1.4 This test method has been dem
5、onstrated to be applicableparticularly to copper and silver test samples (see (1).2Othermetals require further study to prove their applicability withinthe scope of this test method.1.5 The values stated in SI units are the preferred units. Thevalues provided in parentheses are for information only.
6、1.6 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 become familiarwith all hazards including those identified in the appropriateMaterial Safety Data Sheet for this product/material as pro-v
7、ided by the manufacturer, to establish appropriate safety andhealth practices, and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:B 809 Test Method for Porosity in Metallic Coatings byHumid Sulfur Vapor (“Flowers-of-Sulfur”)3B 810 Test Met
8、hod for Calibration of Atmospheric Corro-sion Test Chambers by Change in Mass of Copper Cou-pons4B 827 Practice for Conducting Mixed Flowing Gas (MFG)Environmental Tests5D 1193 Specification for Reagent Water63. Summary of Test Method3.1 In constant-current coulometry, a fixed reduction-current dens
9、ity is applied to the sample in an electrolyticallyconductive solution, and the resulting variations in potentialmeasured against a standard reference electrode in the samesolutionare followed as a function of time. Typically, withwell-behaved surface films, the voltage-time plot should showa number
10、 of horizontal portions, or steps, each correspondingto a specific reduction potential or voltage (Fig. 1). The finalpotential step, which is always present with all substances,corresponds to the reduction of hydrogen ions in the solution(to form hydrogen gas), and represents a limit beyond which no
11、higher potential reduction process can occur.NOTE 1As shown in Figs. 1 and 2, a differential circuit is recom-mended to help in resolving the individual voltage steps by pinpointing thecorresponding inflection points on the main reduction curve (see 6.2.3).3.2 From the elapsed times at the various s
12、teps, conclusionscan often be drawn regarding the corrosion processes that havetaken place to produce the surface films. Also, calculations canbe made from the time at each voltage step in order to calculatethe number of coulombs of electrical charge required tocomplete the reduction process at that
13、 particular voltage.Furthermore, since the reduction of any particular chemicalcompound takes place at a characteristic reduction potential orvoltage range, this voltage can be used to indicate the presenceof a compound or compounds whose characteristic reductionpotential has already been establishe
14、d under the conditions ofthe test. Under ideal conditions it may also be possible todetermine the number of reducible compounds present in thetarnish film.3.3 For the purpose of this test method, tarnish films shall bedefined as the corrosion products of the reactions of oxygen or1This test method i
15、s under the jurisdiction of ASTM Committee B02 onNonferrous Metals and Alloys and is the direct responsibility of SubcommitteeB02.11 on Electrical Contact Test Methods.Current edition approved Oct. 10, 2002. Published December 2002. Originallypublished as B 825 - 97. Last previous edition B 825 - 97
16、.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3Annual Book of ASTM Standards, Vol 02.05.4Annual Book of ASTM Standards, Vol 02.04.5Annual Book of ASTM Standards, Vol 03.04.6Annual Book of ASTM Standards, Vol 11.01.1Copyright ASTM International, 100
17、Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.sulfur (or of other reactive gases or vapors) with the metallicsurface that adhere to the surface and do not protrude signifi-cantly from it.3.4 The basic techniques for the reduction of films oncopper and silver were de
18、scribed as early as the late 1930s byMiley (2) and by Campbell and Thomas (3). Importantobservations of the effects of changing experimental variableswere later reported by Albano (4) and by Lambert and Trevoy(5) in the 1950s. The details and recommendations in this testmethod are primarily from a r
19、ecently published paper (1) onthe practical development over the past fifteen years ofcoulometric reduction for monitoring environmental tests.4. Significance and Use4.1 The present trend in environmental testing of materialswith electrically conductive surfaces is to produce, underaccelerated labor
20、atory conditions, corrosion and film-formingreactions that are similar to those that cause failures in serviceenvironments. In many of these procedures the parts under testare exposed for days or weeks to controlled quantities of bothwater vapor and pollutant gases, which may be present inextremely
21、dilute concentrations.NOTE 2Descriptions of such tests can be found in Practice B 827.4.2 Many of these environmental test methods requiremonitoring of the conditions within the chamber during the testin order to confirm that the intended environmentally relatedreactions are actually taking place. T
22、he most common type ofmonitor consists of copper, silver, or other metallic couponsthat are placed within the test chamber and that react with thecorrosive environment in much the same way as the significantsurfaces of the parts under test.4.3 In practice, a minimum number of control coupons areplac
23、ed in each specified location (see Test Method B 810)within the chamber for a specified exposure time, dependingupon the severity of the test environment. At the end of thistime interval, the metal samples are removed and analyzed bythe coulometric reduction procedure.4.4 Other corrosion film evalua
24、tion techniques for metalliccoupons are also available. The most common of these is massgain, which is nondestructive to the surface films, but is limitedto the determination of the total amount of additional massacquired by the metal as a result of the environmental attack.NOTE 3Detailed instructio
25、ns for conducting such weighings, as wellas coupon cleaning and surface preparation procedures, are included aspart of Test Method B 810.NOTE 4Some surface analytical techniques (such as X-ray methods)can provide nondestructive identification of some compounds in the films,but such methods, for exam
26、ple, X-ray diffraction, can miss amorphouscompounds and compounds present in quantities less than 5 % of thetarnish film volume.4.5 With the coulometric technique, it is possible to resolvethe complex total film into a number of individual components(Fig. 1) so that comparisons can be made. This res
27、olving powerprovides a fingerprint capability for identifying significantdeviations from intended test conditions, and a comparison ofthe corrosive characteristics of different environmental cham-bers and of different test runs within the same chamber.4.6 The coulometric reduction procedure can also
28、 be used intest development and in the evaluation of test samples that havebeen exposed at industrial or other application environments(6). However, for outdoor exposures, some constraints mayhave to be put on the amount and type of corrosion productsallowed, particularly those involving moisture co
29、ndensationand the possible loss of films due to flaking (also see 4.9 and8.3.2).4.7 In laboratory environmental testing, the coulometric-reduction procedure is of greatest utility after repeated charac-terizations of a given corrosive environment have been made toestablish a characteristic reduction
30、 curve for that environment.These multiple runs should come from both the use of multiplespecimens within a given test exposure as well as from severalconsecutive test runs with the same test conditions.4.8 The coulometric-reduction procedure is destructive inthat the tarnish films are transformed d
31、uring the electrochemi-cal reduction process. Nondestructive evaluation methods,such as mass gain, can be carried out with the same samplesthat are to be tested coulometrically. However, such proceduresmust precede coulometric reduction.4.9 The conditions specified in this test method are intendedpr
32、imarily for tarnish films whose total nominal thickness is ofthe order of 102to 103nm (103to 104). Environmentallyproduced films that are much thicker than 103nm are oftenpoorly adherent and are more likely to undergo loosening orflaking upon placement in the electrolyte solution.5. Interferences5.1
33、 For reproducible results the following precautions shallbe taken in order to avoid interferences.5.1.1 Remove dissolved oxygen gas from the electrolytesolution (see 8.1.3), and prevent it from reentering the solutionFIG. 1 Ideal Reduction Behavior of Oxide and Sulfide Films onCopper (from Ref 1)NOT
34、E 1No chlorine is present in test environment.FIG. 2 Typical Reduction Behavior of Films on Copper from 72-hExposure to the Humid Sulfur Vapor Test (see Test MethodB 809)B825022by keeping the cell closed, with an inert gas flowing over thesolution during the reduction (see 8.3.2 and 8.3.3).5.1.2 Use
35、 fresh electrolyte solution for each new coupon inorder to avoid contamination from the reduction of previouscoupons (see 8.3.5).5.1.3 Do not apply masking finishes or other nonmetalliccoatings to the coupons, prior to environmental exposure.5.1.4 Do not use this test method to analyze poorly adhere
36、ntfilms (see 4.9).5.1.5 If the sample had been exposed to environments thatwere likely to deposit soluble particulates (in addition to theunderlying insoluble overall films), care must be taken toremove most of the particulates prior to coulometric reduction(see 8.3.2 for procedure).6. Apparatus6.1
37、Electrolytic Reduction Cell and Ancillary Equipment:6.1.1 Reduction Cell, preferably of glass, with a totalinternal volume of at least 600 mL. The cell shall be enclosed,but should have a sufficient number of entry ports or tubes toaccommodate the required ancillary equipment (see Figs. 4 and5 for e
38、xamples of typical cell systems).6.1.2 Reference ElectrodeA silver/silver-chloride refer-ence is preferred since much of the data in the technicalliterature have been obtained with this type of electrode. It canbe obtained commercially or made in-house from pure silverstrip or wire (see Appendix X1)
39、.6.1.2.1 In-house electrodes must be checked periodically bytesting them against a standard reference electrode (for ex-ample, saturated calomel electrode) using a potentiometer orpH meter. The potential exhibited when measuring thesesilver/silver-chloride electrodes in 0.1-M potassium chloridesolut
40、ion against a saturated calomel reference should be 0.05 V(60.01 V) (7).6.1.3 Inert-Gas Purging TubeThe end that is in theelectrolyte should be fitted with fritted glass or drawn to a finetip (for example, 0.5-mm inner diameter or less).6.1.4 Counter-ElectrodesPure platinum foil or wire shallbe used
41、. The number of counter-electrodes may vary from 2 to4 and shall be positioned symmetrically around the sample.The area of the counter-electrodes preferably should be equalto or greater than the sample area.6.1.5 Wire Hook or Clip for Holding the SampleTheupper part of the hook or clip shall be atta
42、ched to a wire(inserted into a glass or plastic tube) for ultimate connection tothe negative output of the power supply. If the wire hook is tobe immersed in the solution, it shall be made of the same metalas the sample. If a clip is used, it shall be heavily gold plated(3 m or more in thickness) an
43、d attached to a platinum wirehook for electrical contact.NOTE 1The vertical lines correspond to major peaks in the differential curve (not shown) and delineate the main reducible film types from thisenvironment.FIG. 3 Typical Reduction Curve of Copper from 48-h Exposure to High Sulfide (100 ppb H2S)
44、 Mixed Flowing Gas (with 20 ppb Cl2and200 ppb NO2)FIG. 4 Schematic of Reduction Cell with Storage Reservoir, forProcedure A (8.1.3.1)B8250236.2 Electronic EquipmentFor producing the constant ca-thodic current and measuring the resulting voltages as afunction of time comprises three basic functional
45、moduleswhose recommended characteristics (for routine tarnish-filmanalysis) are listed as follows:6.2.1 Constant Current Power Supply, such as, apotentiostat/galvanostat, capable of supplying a constant directcurrent, and adjustable from 0.02 to 2 mA with a precision of61 %. However, for certain lim
46、ited applications (for example,very large area samples), currents greater than 20 mA mightconceivably be required, see 8.2.1.6.2.2 Strip Chart or Digital Recorder, or BothFor astrip-chart recorder, two pens are preferred, one pen for voltageand the other for a voltage-time derivative curve. The char
47、trecorder shall have variable speed capability, from 10 mm/h to100 mm/min, and full-scale voltage ranges from 0.5 to 2 V. Aresolution of the order of 0.5 % (namely, 10 mV with 2-V fullscale), though not essential, is helpful in data evaluation, and isobtained easily with any 250-mm chart recorder. A
48、 digitalrecording system, capable of data storage and graphic repre-sentation can be used instead of, or in conjunction with, thestrip chart recorder system. Both systems shall have inputimpedance of at least 106V, preferably higher.6.2.3 Differential Circuit, or Commercial Differential Volt-age Out
49、put ApparatusIf a digital recording system is used inconjunction with, or to replace, an analog recording system, thefollowing method can be used to create a differential curve.After the reduction is recorded completely, each data point,except for the first and last, must be analyzed. For a givenpoint, X, determine the slope to the previous point, Xp, and thesubsequent point, Xs. Knowing the time interval, T, betweeneach reading, the required slopes are as follows:Sp5 X Xp!/TSs5 Xs X!/T (1)An approximation of the slope at X is then found by takingthe average of the
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