1、Designation: B 825 02 (Reapproved 2008)Standard 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
2、, the year 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 test method covers procedures and equipment fordetermining the relative buildup of corrosion and t
3、arnish films(including 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, pa
4、rticularly when thelatter 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 m
5、ethod 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 parentheses are for
6、 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 (MSDS) for this
7、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:3B 809 Test Method for Porosity in Metallic Coatings byHumid Sulfur Vapor (“Flowers-of
8、-Sulfur”)B 810 Test Method for Calibration of Atmospheric Corro-sion Test Chambers by Change in Mass of Copper Cou-ponsB 827 Practice for Conducting Mixed Flowing Gas (MFG)Environmental TestsD 1193 Specification for Reagent Water3. Summary of Test Method3.1 In constant-current coulometry, a fixed re
9、duction-current density 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
10、 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,corresponds to the reduction of hydrogen ions in the solution(to form hydrogen gas), and represents a l
11、imit 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 pinpointing thecorresponding inflection points on the main reduction curve (see 6.2.3).3.2 From the elapsed ti
12、mes 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 voltage step in order to calculatethe number of coulombs of electrical charge required tocomplete the reduc
13、tion process at that 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 alr
14、eady 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.1This test method is under the jurisdiction of ASTM Committee B02 onNonferrous Metals and Alloys and is the direct responsibi
15、lity of SubcommitteeB02.11 on Electrical Contact Test Methods.Current edition approved March 1, 2008. Published March 2008. Originallyapproved in 1997. Last previous edition approved in 2002 as B 825 - 02.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard
16、.3For 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.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box
17、 C700, West Conshohocken, PA 19428-2959, United States.3.3 For the purpose of this test method, tarnish films shall bedefined as the corrosion products of the reactions of oxygen orsulfur (or of other reactive gases or vapors) with the metallicsurface that adhere to the surface and do not protrude s
18、ignifi-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 (2) and by Campbell and Thomas (3). Importantobservations of the effects of changing experimental variableswere later reported by Albano (4) and by Lambert
19、and Trevoy(5) in the 1950s. The details and recommendations in this testmethod are primarily from a recently 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 environm
20、ental 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 exposed for days or wee
21、ks 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 B 827.4.2 Many of these environmental test methods requiremonitoring of the conditions within the chamber during the te
22、stin order to confirm that the intended environmentally relatedreactions are actually taking place. The 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 sig
23、nificantsurfaces of the parts under test.4.3 In practice, a minimum number of control coupons areplaced 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 s
24、amples 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 limitedto the determination of the total amount of a
25、dditional massacquired by the metal as a result of the environmental attack.NOTE 3Detailed instructions 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 method
26、s)can provide nondestructive identification of some compounds in the films,but such methods, 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 comple
27、x total film into a number of individual components(Fig. 1) so that comparisons can be made. This resolving powerprovides a fingerprint capability for identifying significantdeviations from intended test conditions, and a comparison ofthe corrosive characteristics of different environmental cham-ber
28、s and of different test runs within the same chamber.4.6 The coulometric reduction procedure can also 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
29、 be put on the amount and type of corrosion productsallowed, particularly those involving moisture 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 char
30、ac-terizations of a given corrosive environment have been made toestablish a characteristic 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 conditi
31、ons.4.8 The coulometric-reduction procedure is destructive inthat the tarnish films are transformed during 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 procedu
32、resmust 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 102to 103nm (103to 104). Environmentallyproduced films that are much thicker than 103nm are oftenpoorly adherent and are more
33、 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 avoid interferences.FIG. 1 Ideal Reduction Behavior of Oxide and Sulfide Films onCopper (from Ref 1)NOTE 1No chlorine is
34、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)B 825 02 (2008)25.1.1 Remove dissolved oxygen gas from the electrolytesolution (see 8.1.3), and prevent it from reentering the solutionby keeping th
35、e 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 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 nonmetalliccoati
36、ngs 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 had been exposed to environments thatwere likely to deposit soluble particulates (in addition to theunderlying insoluble overall films), care must be ta
37、ken toremove most of the particulates prior to coulometric reduction(see 8.3.2 for procedure).6. Apparatus6.1 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 su
38、fficient number of entry ports or tubes toaccommodate the required ancillary equipment (see Figs. 4 and5 for examples 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
39、 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 checked periodically bytesting them against a standard reference electrode (for ex-ample, saturated calomel electrode) using a potentiometer orpH meter.
40、The potential exhibited when measuring thesesilver/silver-chloride electrodes in 0.1-M potassium chloridesolution 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 fineti
41、p (for example, 0.5-mm inner diameter or less).6.1.4 Counter-ElectrodesPure 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
42、 the sample area.6.1.5 Wire Hook or Clip for Holding the SampleTheupper part of the hook or clip shall be attached 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 toNOTE 1The vertical lines correspond to major pe
43、aks 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 20 ppb Cl2and200 ppb NO2)FIG. 4 Schematic of Reduction Cell with Storage Reserv
44、oir, forProcedure A (8.1.3.1)B 825 02 (2008)3be 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) and attached to a platinum wirehook for electrical contact.6.2 Electronic EquipmentFor producing the
45、 constant ca-thodic current and measuring the resulting voltages as afunction of time comprises three basic functional moduleswhose recommended characteristics (for routine tarnish-filmanalysis) are listed as follows:6.2.1 Constant Current Power Supply, such as, apotentiostat/galvanostat, capable of
46、 supplying a constant directcurrent, and adjustable from 0.02 to 2 mA with a precision of61 %. However, for certain limited 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
47、-chart recorder, two pens are preferred, one pen for voltageand the other for a voltage-time derivative curve. The chartrecorder 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 f
48、ullscale), though not essential, is helpful in data evaluation, and isobtained easily with any 250-mm chart recorder. A 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
49、 inputimpedance of at least 106V, 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, determine the slope to the previous point, Xp, and thesubsequent point, Xs. Knowing the time interval, T, betweeneach reading, the required slopes
