ASTM B764-94
(Test Method)Standard Test Method for Simultaneous Thickness and Electrochemical Potential Determination of Individual Layers in Multilayer Nickel Deposit (STEP Test)
Standard Test Method for Simultaneous Thickness and Electrochemical Potential Determination of Individual Layers in Multilayer Nickel Deposit (STEP Test)
SCOPE
1.1 This test method closely estimates the thickness of individual layers of a multilayer nickel deposit and the electrochemical potential differences between the individual layers while being anodically stripped at constant current density.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: B 764 – 94
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Simultaneous Thickness and Electrochemical Potential
Determination of Individual Layers in Multilayer Nickel
Deposit (STEP Test)
This standard is issued under the fixed designation B 764; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope tube as the cathode, a constant direct current is passed through
the cell until the nickel layer is dissolved. A sudden change in
1.1 This test method closely estimates the thickness of
voltage between the electrodes occurs when a different metallic
individual layers of a multilayer nickel deposit and the elec-
layer starts to dissolve.
trochemical potential differences between the individual layers
2,3
3.3 Each different metal or species of the same metal
while being anodically stripped at constant current density.
requires a given voltage (electrochemical potential) to keep the
1.2 This standard does not purport to address all of the
current constant while being stripped. As one nickel layer is
safety concerns, if any, associated with its use. It is the
dissolved away and the next layer becomes exposed, there will
responsibility of the user of this standard to establish appro-
be a voltage change (assuming a constant current and differ-
priate safety and health practices and determine the applica-
ence in the electrochemical characteristics of the two nickel
bility of regulatory limitations prior to use.
layers). The elapsed time at which this voltage change occurs
2. Referenced Documents (relative to the start of the test or previous voltage change) is
a measure of the deposit thickness.
2.1 ASTM Standards:
3.4 At the same time, the amplitude of the voltage change
B 504 Test Method for Measurement of Thickness of Me-
can be observed. That is, the ease (or difficulty) with which one
tallic Coatings by the Coulometric Method
layer can be dissolved or stripped with reference to another
D 1193 Specification for Reagent Water
layer can be compared. The lower the voltage needed the more
3. Summary of Test Method
active the metal or the greater the tendency to corrode
preferentially to a more noble metal adjacent to it.
3.1 This procedure is a modification of the well-known
3.5 Where the metallic layers are of such a similar nature
coulometric method of thickness testing (Test Method B 504).
that change of the stripping voltage is small, there can be
It is also known as the anodic dissolution or electrochemical
problems in detecting this change if the voltage between the
stripping method.
deplating cell (cathode) and the sample (anode) is measured.
3.2 Coulometric thickness testing instruments are based on
As the sample is dissolved anodically, cathodic processes are
the anodic dissolution (stripping) of the deposit at constant
occurring on the deplating cell (cathode) surface that can also
current, while the time is measured to determine thickness. As
give rise to voltage changes, due to alterations of the cathode
commonly practiced, the method employs a small cell that is
surface, thus obscuring the anode voltage change. This diffi-
filled with an appropriate electrolyte, and the test specimen
culty can be avoided by measuring the potential of the
serves as the bottom of the cell. To the bottom of the cell is
dissolving anodic sample with respect to an unpolarized third
attached a rubber or plastic gasket whose opening defines the
electrode (reference) placed in the cell. By recording this
measuring (stripping, anodic) area. If a metallic cell is used, the
potential any difference in electrochemical activity between
rubber gasket also electrically insulates the test specimen from
layers is more readily detected.
the cell. With the specimen as the anode and the cell or agitator
3.6 The thickness of any specific nickel layer may be
calculated from the quantity of electricity used (current multi-
This method is under the jurisdiction of ASTM Committee B-8 on Metallic and
plied by time), the area dissolved, the electrochemical equiva-
Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
lent of nickel, the anode efficiency, and the density of the nickel
General Test Methods.
layer.
Current edition approved Jan 15, 1994. Published April 1994. Originally
published as B 764 – 86. Last previous edition B 764 – 86.
For discussion of this test, see Harbulak, E. P., “Simultaneous Thickness and
4. Significance and Use
Electrochemical Potential Determination of Individual Layers in Multilayer Nickel
4.1 The ability of a multilayer nickel deposit to enhance
Deposits,” Plating and Surface Finishing, Vol 67, No. 2, February 1980, pp. 49–54.
U.S. Patent 4,310,389. Assignee: The Chrysler Corp., Highland Park, MI
corrosion resistance is a function of the electrochemical
48203.
potential differences between the layers (as measured individu-
Annual Book of ASTM Standards, Vol 02.05.
5 ally at a fixed current density in a given electrolyte versus a
Annual Book of ASTM Standards, Vol 11.01.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
B 764
reference electrode) and the thicknesses of the layers. The 5.3 Electrolyte Agitation Source—All commercial coulom-
potential differences must be sufficient to cause the bright etric thickness testers incorporate a means to agitate the
nickel or top layer to corrode preferentially and sacrificially solution. It is possible to purchase these types of units
with respect to the semibright nickel layer beneath it. separately, if so desired, to be used externally in conjunction
4.2 This test procedure allows the measurement of these with other power supplies.
potential differences directly on an electroplated part rather 5.4 Recorder—Any time-based recorder with an input im-
than on separate foil specimens in such a way that time pedance of at least 1.0 MV and capable of running at
determines the thickness of each layer, while the potential approximately 0.5 mm/s (3 cm/min) can be used.
difference between nickel layers is an indication of the corro- 5.5 Deplating Cell—The cell may be similar in construction
sion resistance of the total nickel deposit. to commercially available deplating cells. It is usually a
4.3 The interpretation and evaluation of the results of this cup-shaped cell of either 316 stainless steel, Monel, or plastic
test should be by agreement between the purchaser and the that engages a round rubber or plastic gasket to the work piece
manufacturer. or sample. The opening through the cell and gasket allows
contact of the electrolyte to the test specimen and defines the
NOTE 1—This test may be used to help predetermine the corrosion
stripping area.
resistance of the multilayer nickel coatings applied in production that are
subjected to other corrosion media. It should be understood that due to
NOTE 2—A deplating cell could be constructed of plastic using a
many factors that influence the progress of corrosion during actual use of
cylindrical stainless steel or Monel sheet cathode located in the larger
the part, the performance of different multilayer nickel deposits in the test
upper area of the cup. The advantages of such a cell are the prevention of
cannot be taken as an absolute indicator of the relative corrosion resistance
whisker growth and the choking off of the small bore opening, and the
of these deposits in service.
ease of cathode removal for cleaning or replacement.
5.6 Reference Electrode Assembly—The general configura-
5. Apparatus
tion of one commercially available measuring system is shown
5.1 Composition of the Electrolyte:
in Fig. 1. The assembly is a “T”-shaped housing containing the
Nickel Chloride (NiCl ·6H O) 300g/L
2 2
reference electrode. The housing assembly includes an upper
Sodium Chloride (NaCl) 50g/L
Boric Acid (H BO ) 25g/L connector providing a port for the connection to the agitator
3 3
A
pH 3.0
pump. The upper position also contains a threaded contact to
A
The pH may be adjusted with diluted hydrochloric acid or sodium hydroxide, as the reference electrode. The lower portion of the assembly
required, and is more critical than the composition of the electrolyte.
contains an adjustable glass or plastic tube which surrounds the
Prepared in Purified Water—Type IV or better as specified
reference electrode so that when inserted into the deplating
in Specification D 1193.
cell, the reference electrode is immersed in the electrolyte. The
5.2 Constant Current Source—This should supply a con-
tip of the reference electrode should extend into the glass tip so
stant current that can be varied between 0 and 50 mA (typical
that the distance between the tip of the electrode and the
25 to 35 mA). A current of 30 mA corresponds to a stripping
bottom of the glass agitator tube is approximately 5.0 mm.
rate of 7.8 μm/min at 100 % current efficiency when used with
a gasket providing 0.08 cm stripping area. (This is achieved
with the solution stated in 5.1.) Most commercial coulometric
Monel is a trademark for a large group of corrosion-resistant alloys of
thickness testers can be used with slight modification as the
predominantly nickel and copper. Available from Huntington Alloy Products Div.,
current source. The International Nickel Co., Inc., Huntington, WV 25720.
FIG. 1 T-Shaped Reference Electrode Assembly
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
B 764
removed. A dense blanket of bubbles on the surface of the sample
5.7 Reference Electrode—Either silver or platinum wire of
indicates that all the chromium is removed. Remove the stripping solution
approximately 1.5 mm in diameter can be used. Silver is
from the cell without moving or disturbing the seal of the gasket to the test
probably the better choice due to its ability to form a
surface. Wash the cell three times with purified water (Type IV or better
silver-silver chloride electrode when used in a chloride con-
as specified in Specification D 1193) and once with the step test solution.
taining electrolyte (see Appendix X1).
Proceed to 6.5.
NOTE 3—It is necessary to condition the silver electrode before using in
6.3 Position the test specimen in a secure horizontal position
order to form the silver-silver chloride surface. This is easily done by
so that the chromium-stripped nickel surface is directly beneath
anodically treating approximately a 75-mm length of wire in an 1 N
the cell gasket.
hydrochloric acid solution for 10 to 15 s using a 34.7-mA anodic current.
6.4 Lower the deplating cell assembly; secure by sealing the
This will form a gray film on the wire which should always be present.
gasket to the nickel surface. A flat test area of approximately 10
Once the gray film is formed, it is not necessary to repeat the conditioning
treatment unless the film has been removed. It may be advisable, however, mm in diameter is desirable but not required. The criterion is
to recondition the electrode after a prolonged period of inactivity or when
that there be no leakage of the electrolyte. If leakage does
the electrode has been allowed to remain dry for an extended period of
occur, discontinue test and start a new one.
time. Drying off the electrode should be avoided by immersion in either
6.5 Fill the deplating cell to the appropriate level with the
the hydrochloric acid conditioning solution, the step test solution, or
step test solution making sure that no air is trapped within the
distilled water when not in use.
solution.
NOTE 4—A ceramic junction reference electrode that does not require
6.6 Lower the reference electrode assembly into the deplat-
conditioning is available commercially.
ing cell, if necessary.
5.8 Millivolt Meter (optional)—When using a sensitive and
well-calibrated recorder, a millivolt meter is not necessary. If
NOTE 6—The insertion depth of the electrolyte agitation tube which
one is desired, however, any sensitive, high-input impedance
includes the reference electrode is important and should always be the
same. The positioning of the reference electrode should be such that the
meter can be used. A standard pH meter with a millivolt setting
distance from the end of the electrode to the test specimen is reproducible
would be satisfactory. The meter should have a range from 0 to
to within 1 mm and be held constant throughout the test. It should be
2000 mV. If a millivolt meter is used which has low-output
remembered that the difference of potential rather than the absolute
impedance facilities, it can be used to drive the recorder and
potential is the important measurement.
will serve as a buffer amplifier. Most laboratory pH meters
6.7 Check all electrical connections. Make sure all connec-
have such output terminals (see Fig. 2).
tions are secure and that no corrosion exists at the contact
6. Procedure points and that all contact points are secure.
6.8 Start the recorder (turn on milliampere meter, if used).
6.1 Set up equipment as recommended by the manufacturer
The recorder must be calibrated in order to determine the
or as shown in Fig. 1. If necessary, turn on the recorder and the
thickness of the nickel layers. This may be accomplished by
millivolt meter and allow them to warm up.
using commercially available thickness standards or by apply-
6.2 If chromium is present on the nickel surface, remove it
ing Faraday’s Law. The latter requires information about the
with concentrated hydrochloric acid. Make sure the nickel
current, corroding area, electrochemical equivalent of nickel,
surface is clean. Rinse well and dry off the surface.
density of nickel, efficiency, and the time base of the recorder.
NOTE 5—Chromium can be removed by using the deplating cell as is
Example: if the constant current source produces 30 mA, the
done on many commercial coulometric testers. If this is done, secure the
recorder time base is 30 mm/min, and the deplating area is 0.08
cell and gasket to the test piece as in 6.3 and 6.4 but do not insert the
cm , it would take 19.2 s to deplate 2.5 μm of nickel. The chart
electrode assembly. Fill the cell with a common test stripping solution for
would travel 9.6 mm. A general equation that may be used is as
chromium (Test Metho
...
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