1、Designation: B825 13Standard Test Method forCoulometric Reduction of Surface Films on Metallic TestSamples1This standard is issued under the fixed designation B825; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last r
2、evision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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(includin
3、g 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 thela
4、tter are used for testing components or systems containingelectrical contacts used in customer product environments.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
5、.)1.4 This test method has been demonstrated 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 pa
6、rentheses are for information only.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
7、 (MSDS) for this product/materialas provided by the manufacturer, to establish appropriatesafety and health practices, and determine the applicability ofregulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3B808 Test Method for Monitoring ofAtmospheric CorrosionChambers by Q
8、uartz Crystal MicrobalancesB809 Test Method for Porosity in Metallic Coatings byHumid Sulfur Vapor (“Flowers-of-Sulfur”)B810 Test Method for Calibration ofAtmospheric CorrosionTest Chambers by Change in Mass of Copper CouponsB827 Practice for Conducting Mixed Flowing Gas (MFG)Environmental TestsD119
9、3 Specification for Reagent Water3. Summary of Test Method3.1 In constant-current coulometry, a fixed reduction-current density is applied to the sample in an electrolyticallyconductive solution, and the resulting variations in potentialmeasured against a standard reference electrode in the samesolu
10、tionare followed as a function of time. Typically, withwell-behaved surface films, the voltage-time plot should showa number 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
11、,corresponds to the reduction of hydrogen ions in the solution(to form hydrogen gas), and represents a limit beyond which nohigher 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 pinpoi
12、nting thecorresponding inflection points on the main reduction curve (see 6.2.3).3.2 From the elapsed times at the various steps, 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 volt
13、age step in order to calculatethe number of coulombs of electrical charge required tocomplete the reduction process at that particular voltage.1This test method is under the jurisdiction of ASTM Committee B02 onNonferrous Metals and Alloys and is the direct responsibility of SubcommitteeB02.11 on El
14、ectrical Contact Test Methods.Current edition approved Aug. 1, 2013. Published August 2013. Originallyapproved in 1997. Last previous edition approved in 2008 as B825 02 (2008).DOI: 10.1520/B0825-13.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For
15、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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700,
16、West Conshohocken, PA 19428-2959. United States1Furthermore, 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
17、 has already been established 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
18、oxygen orsulfur (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 described as early as the late 1930s byMiley (3) and by Campbell and Thom
19、as (4). Importantobservations of the effects of changing experimental variableswere later reported by Albano (5) and by Lambert and Trevoy(6) in the 1950s. The details and recommendations in this testmethod are primarily from a recently published papers (1) and(2).4. Significance and Use4.1 The pres
20、ent trend in environmental testing of materialswith electrically conductive surfaces is to produce, underaccelerated laboratory conditions, corrosion and film-formingreactions that are similar to those that cause failures in serviceenvironments. In many of these procedures the parts under testare ex
21、posed for days or weeks to controlled quantities of bothwater vapor and pollutant gases, which may be present inextremely dilute concentrations.NOTE 2Descriptions of such tests can be found in Practice B827.4.2 Many of these environmental test methods requiremonitoring of the conditions within the c
22、hamber during the testin order to confirm that the intended environmentally relatedreactions are actually taking place. The most common type ofmonitor consists of copper, silver, or other thin metalliccoupons of a few square centimeters that are placed within thetest chamber and that react with the
23、corrosive environment inmuch the same way as the significant surfaces of the partsunder test.4.3 In practice, a minimum number of control coupons areplaced in each specified location (see Test Method B810)within the chamber for a specified exposure time, dependingupon the severity of the test enviro
24、nment. At the end of thistime interval, the metal samples are removed and analyzed bythe coulometric reduction procedure.4.4 Other corrosion film evaluation techniques for metalliccoupons are also available. The most common of these is massgain, which is nondestructive to the surface films, but is l
25、imitedto the determination of the total amount of additional massacquired by the metal as a result of the environmental attack.The most common is weighing using high performance mi-crobalances or for purposes of real-time monitoring, quartzcrystal microbalances (see Specification B808).NOTE 3Detaile
26、d instructions for conducting such weighings, as wellas coupon cleaning and surface preparation procedures, are included aspart of Test Method B810.NOTE 4Some surface analytical techniques (such as X-ray methods)can provide nondestructive identification of some compounds in the films,but such method
27、s, for example, 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 mad
28、e.This resolving 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 procedur
29、e can also be used intest development and in the evaluation of test samples that havebeen exposed at industrial or other application environments(7). However, for outdoor exposures, some constraints mayhave to be put on the amount and type of corrosion productsallowed, particularly those involving m
30、oisture condensationand 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
31、 reduction 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 tra
32、nsformed during the electrochemi-cal reduction process. Nondestructive evaluation methods,such as mass gain, can be carried out with the same samplesFIG. 1 Ideal Reduction Behavior of Oxide and Sulfide Films onCopper (from Ref 1)FIG. 2 Typical Reduction Behavior of Films on Copper from 72-hExposure
33、to the Humid Sulfur Vapor Test (see Test Method B809)B825 132that are to be tested coulometrically. However, such proceduresmust precede coulometric reduction.4.9 The conditions specified in this test method are intendedprimarily for tarnish films whose total nominal thickness is ofthe order of 102t
34、o 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 For reproducible results the following precautions shallbe taken in order to
35、avoid interferences.5.1.1 Remove dissolved oxygen gas from the electrolytesolution (see 8.1.3), and prevent it from reentering the solutionby 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 fresh electrolyte solution for each n
36、ew 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 adherentfilms (see 4.9).5.1.5 If the sample
37、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 Electrolytic Reduction Cell and Ancill
38、ary 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 examples of typical cell systems).6.1.2
39、 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).6.1.2.1 In-house electrodes must be c
40、hecked periodically bytesting them against a standard reference electrode (forexample, saturated calomel electrode) using a potentiometer orpH meter. The potential exhibited when measuring thesesilver/silver-chloride electrodes in 0.1-M potassium chloridesolution against a saturated calomel referenc
41、e should be 0.05 V(60.01 V) (8).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) Mixed Flowing Gas (with
42、20 ppb Cl2and200 ppb NO2)FIG. 4 Schematic of Reduction Cell with Storage Reservoir, forProcedure A (8.1.3.1)B825 1336.1.3 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-Elect
43、rodesPure platinum foil or wire shallbe used. 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 SampleThe
44、 upperpart of the hook or clip shall be attached to a wire (inserted intoa glass or plastic tube) for ultimate connection to the negativeoutput of the power supply. If the wire hook is to be immersedin the solution, it shall be made of the same metal as thesample. If a clip is used, it shall be heav
45、ily gold plated (3 mor more in thickness) and attached to a platinum wire hook forelectrical contact.6.2 Electronic EquipmentFor producing the constant ca-thodic current and measuring the resulting voltages as afunction of time comprises three basic functional moduleswhose recommended characteristic
46、s (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 limited applications (for example,very lar
47、ge 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 chartrecorder shall have variable speed cap
48、ability, 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 digitalrecording system, capable of da
49、ta 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 106, preferably higher.6.2.3 Differential Circuit, or Commercial Differential Volt-age Output 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, de
