1、Designation: B764 04 (Reapproved 2014)Standard Test Method forSimultaneous Thickness and Electrode PotentialDetermination of Individual Layers in Multilayer NickelDeposit (STEP Test)1This standard is issued under the fixed designation B764; the number immediately following the designation indicates
2、the year oforiginal adoption or, in the case of revision, the year of last revision. 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 closely estimates the thickness
3、 ofindividual layers 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
4、 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
5、Standards:4B456 Specification for Electrodeposited Coatings of CopperPlus Nickel Plus Chromium and Nickel Plus ChromiumB504 Test Method for Measurement of Thickness of Metal-lic Coatings by the Coulometric MethodD1193 Specification for Reagent Water3. Summary of Test Method3.1 This procedure is a mo
6、dification of the well-knowncoulometric method of thickness testing (Test Method B504).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 t
7、he time 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 (
8、stripping, 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 sud
9、den change 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
10、, there 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
11、 the same 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 corrodepreferent
12、ially to 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 dis
13、solved anodically, 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
14、sample with respect to an unpolarized thirdelectrode (reference) placed in the cell. By recording this1This method is under the jurisdiction ofASTM Committee B08 on Metallic andInorganic Coatings and is the direct responsibility of Subcommittee B08.10 on TestMethods.Current edition approved May 1, 2
15、014. Published May 2014. Originallyapproved in 1986. Last previous edition approved in 2009 as B764 04(2009) .DOI: 10.1520/B0764-04R14.2For discussion of this test, see Harbulak, E. P., “Simultaneous Thickness andElectrochemical Potential Determination of Individual Layers in Multilayer NickelDeposi
16、ts,” 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 serviceastm.org. For Annual Book of ASTMSta
17、ndards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1potential any difference in electrochemical activity betweenlayers is more readily detected. Th
18、e equipment may becalibrated 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
19、layer.3.7 Commercial instruments 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 Use
20、4.1 The ability of a multilayer 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. T
21、hepotential differences must 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 ratherth
22、an on separate foil specimens 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 agr
23、eement between the purchaser 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 d
24、ifferent multilayer nickeldeposits 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)2g/LpH 3.0AAThe pH
25、may be adjusted with diluted 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 D1193.5.2 Constant Current SourceThis should supply a con-stant current that can be va
26、ried between 0 and 50 mA (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 us
27、ed as the current source.5.3 Electrolyte Agitation SourceAll commercial coulo-metric 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 t
28、ime-based recorder with an input im-pedance of at least 1.0 M 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
29、steel, copper-nickel alloy, 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 plas
30、ticusing a cylindrical 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 Refe
31、rence ElectrodeEither 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
32、distance between the tip 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 i
33、n 1 Nhydrochloric acid solution for 10 to 15 s using 35-mAanodic 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 reco
34、ndition the electrode 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
35、 not in use.NOTE 4A ceramic 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 impedancemet
36、er can be used.Astandard 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 laborator
37、ypH meters have such 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
38、nickelsurface is clean. Rinse well and dry off the surface.5Electrolyte can be obtained commercially that meets the requirements of thistest.B764 04 (2014)2NOTE 5Chromium can be removed by using the coulometric deplat-ing cell as is done on many commercial coulometric testers. If this is done,secure
39、 the cell and gasket to the 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 B504) and hook up only the cell andtest piece to the power supply. Apply the current until all the chromiumhas been removed.A
40、dense blanket of bubbles 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 Specific
41、ation D1193) and once 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
42、surface. A flat test 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 soluti
43、on making sure that no 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 towithi
44、n 1 mm and be held constant 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 electr
45、ical connections. Make 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 m
46、ay be accomplished byusing 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 consta
47、nt current source and 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
48、is copper or brass, the 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 bet
49、ter as specified in Specification D1193) 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.Ageneral 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 = depl
