1、Designation: B 764 04Standard Test Method forSimultaneous Thickness and Electrode PotentialDetermination of Individual Layers in Multilayer NickelDeposit (STEP Test)1This standard is issued under the fixed designation B 764; the number immediately following the designation indicates the year oforigi
2、nal adoption or, in the case of revision, 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 closely estimates the thickness ofindividual l
3、ayers of a multilayer nickel electrodeposit and thepotential differences between the individual layers while beinganodically stripped at constant current density.2,31.2 This test method does not cover deposit systems otherthan multilayer electroplated nickel deposits.1.3 This standard does not purpo
4、rt 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 Standards:4B 45
5、6 Specification for Electrodeposited Coatings of Cop-per Plus Nickel Plus Chromium and Nickel Plus Chro-miumB 504 Test Method for Measurement of Thickness of Me-tallic Coatings by the Coulometric MethodD 1193 Specification for Reagent Water3. Summary of Test Method3.1 This procedure is a modificatio
6、n of the well-knowncoulometric method of thickness testing (Test Method B 504).It is also known as the anodic dissolution or electrochemicalstripping method.3.2 Coulometric thickness testing instruments are based onthe anodic dissolution (stripping) of the deposit at constantcurrent, while the time
7、is measured to determine thickness. Ascommonly practiced, the method employs a small cell that isfilled with an appropriate electrolyte, and the test specimenserves as the bottom of the cell. To the bottom of the cell isattached a rubber or plastic gasket whose opening defines themeasuring (strippin
8、g, anodic) area. If a metallic cell is used, therubber gasket also electrically insulates the test specimen fromthe cell. With the specimen as the anode and the cell or agitatortube as the cathode, a constant direct current is passed throughthe cell until the nickel layer is dissolved. A sudden chan
9、ge involtage between the electrodes occurs when a different metalliclayer starts to dissolve.3.3 Each different metal or species of the same metalrequires a given voltage to keep the current constant whilebeing stripped. As one nickel layer is dissolved away and thenext layer becomes exposed, there
10、will be a voltage change(assuming a constant current and difference in the electro-chemical characteristics of the two nickel layers). The elapsedtime at which this voltage change occurs (relative to the start ofthe test or previous voltage change) is a measure of the depositthickness.3.4 At the sam
11、e time, the amplitude of the voltage changecan be observed. That is, the ease (or difficulty) with which onelayer can be dissolved or stripped with reference to anotherlayer can be compared. The lower the voltage needed the moreactive the metal or the greater the tendency to corrodepreferentially to
12、 a more noble metal adjacent to it.3.5 Where the metallic layers are of such a similar naturethat change of the stripping voltage is small, there can beproblems in detecting this change if the voltage between thedeplating cell (cathode) and the sample (anode) is measured.As the sample is dissolved a
13、nodically, cathodic processes areoccurring on the deplating cell (cathode) surface that can alsogive rise to voltage changes, due to alterations of the cathodesurface, thus obscuring the anode voltage change. This diffi-culty can be avoided by measuring the potential of thedissolving anodic sample w
14、ith respect to an unpolarized thirdelectrode (reference) placed in the cell. By recording thispotential any difference in electrochemical activity between1This method is under the jurisdiction of ASTM Committee B08 on Metallic andInorganic Coatings and is the direct responsibility of Subcommittee B0
15、8.10 on TestMethods.Current edition approved April 1, 2004. Published April 2004. Originallyapproved in 1986. Last previous edition approved in 2003 as B 764 94 (2003).2For discussion of this test, see Harbulak, E. P., “Simultaneous Thickness andElectrochemical Potential Determination of Individual
16、Layers in Multilayer NickelDeposits,” Plating and Surface Finishing, Vol 67, No. 2, February 1980, pp. 4954.3U.S. Patent 4,310,389. Assignee: The Chrysler Corp., Highland Park, MI48203.4For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceast
17、m.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 C700, West Conshohocken, PA 19428-2959, United States.layers is more readily detected. The equipment may becalibrat
18、ed against standards with known STEP values.3.6 The thickness of any specific nickel layer may becalculated from the quantity of electricity used (current multi-plied by time), area dissolved, electrochemical equivalent ofnickel, anode efficiency, and density of the nickel layer.3.7 Commercial instr
19、uments using this principle are avail-able. They are usually a combination coulometric and STEPinstrument. Reference standards are available to calibrate theinstrument. The STEP Test, as is the Coulometric Test, is rapidand destructive to the coating.4. Significance and Use4.1 The ability of a multi
20、layer nickel deposit to enhancecorrosion resistance is a function of the difference in theelectrode potentials of the nickel layers (as measured individu-ally at a fixed current density in a given electrolyte versus areference electrode) and the thicknesses of the layers. Thepotential differences mu
21、st be sufficient to cause the brightnickel or top layer to corrode preferentially and sacrificiallywith respect to the semi-bright nickel layer beneath it.4.2 This test procedure allows the measurement of thesepotential differences directly on an electroplated part ratherthan on separate foil specim
22、ens in such a way that timedetermines the thickness of each layer, while the potentialdifference between nickel layers is an indication of the corro-sion resistance of the total nickel deposit.4.3 The interpretation and evaluation of the results of thistest should be by agreement between the purchas
23、er and themanufacturer.NOTE 1This test may be used as a quality assurance test of themultilayer nickel coatings applied in production. It should be understoodthat due to many factors that influence the progress of corrosion duringactual use of the part, the performance of different multilayer nickel
24、deposits in the test cannot be taken as an absolute indicator of the relativecorrosion resistance of these deposits in service.5. Apparatus5.1 Composition of the Electrolyte5:Nickel Chloride (NiCl26H2O) 300g/LSodium Chloride (NaCl) 50g/LBoric Acid (H3BO3) 25g/LpH 3.0AAThe pH may be adjusted with dil
25、uted hydrochloric acid or sodium hydroxide, asrequired, and is more critical than the composition of the electrolyte.Prepared in Purified WaterType IV or better as specifiedin Specification D 1193.5.2 Constant Current SourceThis should supply a con-stant current that can be varied between 0 and 50 m
26、A (typical25 to 35 mA). A current of 30 mA corresponds to a strippingrate of 7.8 m/min at 100 % current efficiency when used witha gasket providing 0.08 cm2stripping area. (This is achievedwith the solution stated in 5.1.) Most commercial coulometricthickness testers can be used as the current sourc
27、e.5.3 Electrolyte Agitation SourceAll commercial coulom-etric thickness testers incorporate a means to agitate thesolution. It is possible to purchase these types of unitsseparately, if so desired, to be used externally in conjunctionwith other power supplies.5.4 RecorderAny time-based recorder with
28、 an input im-pedance of at least 1.0 MV and capable of running atapproximately 0.5 mm/s (3 cm/min) can be used.5.5 Deplating CellThe cell may be similar in constructionto commercially available coulometric deplating cells. It isusually a cup-shaped cell of either 316 stainless steel, copper-nickel a
29、lloy, or plastic that engages a round rubber or plasticgasket to the work piece or sample. The opening through thecell and gasket allows contact of the electrolyte to the testspecimen and defines the stripping area.NOTE 2A coulometric deplating cell could be constructed of plasticusing a cylindrical
30、 stainless steel or copper-nickel alloy sheet cathodelocated in the larger upper area of the cup. The advantages of such a cellare the prevention of whisker growth and the choking off of the small boreopening, and the ease of cathode removal for cleaning or replacement.5.6 Reference ElectrodeEither
31、silver or platinum wire ofapproximately 1.5 mm in diameter can be used. Silver isprobably the better choice due to its ability to form asilver-silver chloride electrode when used in a chloride con-taining electrolyte. The tip of the reference electrode shouldextend so that the distance between the t
32、ip of the electrode andthe bottom of the agitator tube is approximately 5 mm.NOTE 3It is necessary to condition the silver electrode before using inorder to form the silver-silver chloride surface. This is easily done byanodically treating approximately a 75-mm length of wire in 1 Nhydrochloric acid
33、 solution for 10 to 15 s using 35-mA anodic current. Thiswill form a gray film on the wire, which should always be present. Oncethe gray film is formed, it is not necessary to repeat the conditioningtreatment unless the film has been removed. It may be advisable, however,to recondition the electrode
34、 after a prolonged period of inactivity or whenthe electrode has been allowed to remain dry for an extended period oftime. Drying off the electrode should be avoided by immersion in eitherthe hydrochloric acid conditioning solution, the step test solution, ordistilled water when not in use.NOTE 4A c
35、eramic junction reference electrode that does not requireconditioning is available commercially.5.7 Millivolt Meter (optional)When using a sensitive andwell-calibrated recorder, a millivolt meter is not necessary. Ifone is desired, however, any sensitive, high-input impedancemeter can be used. A sta
36、ndard pH meter with a millivolt settingwould be satisfactory. The meter should have a range from 0 to2000 mV. If a millivolt meter is used which has low-outputimpedance facilities, it can be used in parallel to drive therecorder and will serve as a buffer amplifier. Most laboratorypH meters have suc
37、h output terminals.6. Procedure6.1 Set up equipment as recommended by the manufacturer.If necessary, turn on the recorder and the millivolt meter andallow them to warm up.6.2 If chromium is present on the nickel surface, remove itwith concentrated hydrochloric acid. Make sure the nickelsurface is cl
38、ean. Rinse well and dry off the surface.5Electrolyte can be obtained commercially that meets the requirements of thistest.B 764 042NOTE 5Chromium can be removed by using the coulometric deplat-ing cell as is done on many commercial coulometric testers. If this is done,secure the cell and gasket to t
39、he test piece as in 6.3 and 6.4 but do notinsert the electrode assembly. Fill the cell with a common test strippingsolution for chromium (Test Method B 504) and hook up only the cell andtest piece to the power supply. Apply the current until all the chromiumhas been removed. A dense blanket of bubbl
40、es on the surface of the sampleindicates that all the chromium is removed. Remove the stripping solutionfrom the cell without moving or disturbing the seal of the gasket to the testsurface. Wash the cell three times with purified water (Type IV or betteras specified in Specification D 1193) and once
41、 with the step test solution.Proceed to 6.5.6.3 Position the test specimen in a secure horizontal positionso that the chromium-stripped nickel surface is directly beneaththe cell gasket.6.4 Lower the coulometric deplating cell assembly; secureby sealing the gasket to the nickel surface. A flat test
42、area ofapproximately 10 mm in diameter is desirable but not required.The criterion is that there be no leakage of the electrolyte. Ifleakage does occur, discontinue test and start a new one.6.5 Fill the coulometric deplating cell to the appropriatelevel with the step test solution making sure that n
43、o air istrapped within the solution.6.6 Lower the reference electrode assembly into the coulo-metric deplating cell, if necessary. The positioning of thereference electrode should be such that the distance from theend of the electrode to the test specimen is reproducible towithin 1 mm and be held co
44、nstant throughout the test.NOTE 6The insertion depth of the electrolyte agitation tube whichincludes the reference electrode is important and should always be thesame. The difference of potential rather than the absolute potential is theimportant measurement.6.7 Check all electrical connections. Mak
45、e sure all connec-tions are secure and that no corrosion exists at the contactpoints and that all contact points are secure.6.8 Start the recorder (turn on milliampere meter, if used).The recorder must be calibrated in order to determine thethickness of the nickel layers. This may be accomplished by
46、using commercially available thickness standards or by apply-ing Faradays Law. The latter requires information about thecurrent, corroding area, electrochemical equivalent of nickel,density of nickel, efficiency, and the time base of the recorder(see 6.11).6.9 Turn on the constant current source and
47、 agitator, whichin turn will start the deplating reaction. Continue recordinguntil the surface underlying the nickel is reached. This endpoint can be recognized graphically by a sudden change involtage. If the basis metal is zinc, iron, or steel, the voltage willdecrease; if it is copper or brass, t
48、he voltage will increase.6.10 Stop the test by turning off the agitator, constantcurrent source, recorder, and milliampere meter. Remove theelectrode assembly, if necessary, and empty the cell of thestripping solution. Wash the cell three times with purified water(Type IV or better as specified in S
49、pecification D 1193) beforecontinuing to the next test.6.11 This test is based on a measured current-time relation-ship necessary to remove a given amount of nickel from aspecific area.Example: if the constant current source produces 30 mA, therecorder time base is 30 mm/min, and the deplating area is 0.08cm2, it would take 19.2 s to deplate 2.5 m of nickel. The chartwould travel 9.6 mm. A general equation that may be used is asfollows:SL! A! I!0.303! S5 T (1)where:SL = chart scan length, mm,S = chart speed, mm/min,I = cell current, mA,A = deplating area, cm,T =
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