ASTM B764-04(2021)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B764 − 04 (Reapproved 2021)
Standard Test Method for
Simultaneous Thickness and Electrode Potential
Determination of Individual Layers in Multilayer Nickel
Deposit (STEP Test)
This standard is issued under the fixed designation B764; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Summary of Test Method
1.1 This test method closely estimates the thickness of
3.1 This procedure is a modification of the well-known
individual layers of a multilayer nickel electrodeposit and the
coulometric method of thickness testing (Test Method B504).
potential differences between the individual layers while being
It is also known as the anodic dissolution or electrochemical
2,3
anodically stripped at constant current density.
stripping method.
1.2 This test method does not cover deposit systems other
3.2 Coulometric thickness testing instruments are based on
than multilayer electroplated nickel deposits.
the anodic dissolution (stripping) of the deposit at constant
1.3 This standard does not purport to address all of the
current, while the time is measured to determine thickness.As
safety concerns, if any, associated with its use. It is the commonly practiced, the method employs a small cell that is
responsibility of the user of this standard to establish appro-
filled with an appropriate electrolyte, and the test specimen
priate safety, health, and environmental practices and deter- serves as the bottom of the cell. To the bottom of the cell is
mine the applicability of regulatory limitations prior to use.
attached a rubber or plastic gasket whose opening defines the
1.4 This international standard was developed in accor- measuring(stripping,anodic)area.Ifametalliccellisused,the
dance with internationally recognized principles on standard-
rubber gasket also electrically insulates the test specimen from
ization established in the Decision on Principles for the the cell.With the specimen as the anode and the cell or agitator
Development of International Standards, Guides and Recom-
tube as the cathode, a constant direct current is passed through
mendations issued by the World Trade Organization Technical the cell until the nickel layer is dissolved. A sudden change in
Barriers to Trade (TBT) Committee.
voltagebetweentheelectrodesoccurswhenadifferentmetallic
layer starts to dissolve.
2. Referenced Documents
3.3 Each different metal or species of the same metal
2.1 ASTM Standards:
requires a given voltage to keep the current constant while
B456 Specification for Electrodeposited Coatings of Copper
being stripped. As one nickel layer is dissolved away and the
Plus Nickel Plus Chromium and Nickel Plus Chromium
next layer becomes exposed, there will be a voltage change
B504 Test Method for Measurement of Thickness of Metal-
(assuming a constant current and difference in the electro-
lic Coatings by the Coulometric Method
chemical characteristics of the two nickel layers). The elapsed
D1193 Specification for Reagent Water
timeatwhichthisvoltagechangeoccurs(relativetothestartof
the test or previous voltage change) is a measure of the deposit
thickness.
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
3.4 At the same time, the amplitude of the voltage change
Test Methods.
canbeobserved.Thatis,theease(ordifficulty)withwhichone
Current edition approved April 1, 2021. Published May 2021. Originally
approved in 1986. Last previous edition approved in 2014 as B764 – 04(2014) .
layer can be dissolved or stripped with reference to another
DOI: 10.1520/B0764-04R21.
layer can be compared. The lower the voltage needed the more
For discussion of this test, see Harbulak, E. P., “Simultaneous Thickness and
active the metal or the greater the tendency to corrode
Electrochemical Potential Determination of Individual Layers in Multilayer Nickel
Deposits,” Plating and Surface Finishing, Vol 67, No. 2, February 1980, pp. 49–54. preferentially to a more noble metal adjacent to it.
3.5 Where the metallic layers are of such a similar nature
U.S. Patent 4,310,389. Assignee: The Chrysler Corp., Highland Park, MI
48203.
that change of the stripping voltage is small, there can be
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
problems in detecting this change if the voltage between the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
deplating cell (cathode) and the sample (anode) is measured.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. As the sample is dissolved anodically, cathodic processes are
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B764 − 04 (2021)
occurring on the deplating cell (cathode) surface that can also 5.2 Constant Current Source—This should supply a con-
give rise to voltage changes, due to alterations of the cathode stant current that can be varied between 0 and 50 mA (typical
surface, thus obscuring the anode voltage change. This diffi-
25 to 35 mA). A current of 30 mA corresponds to a stripping
culty can be avoided by measuring the potential of the
rate of 7.8 µm/min at 100 % current efficiency when used with
dissolving anodic sample with respect to an unpolarized third
a gasket providing 0.08 cm stripping area. (This is achieved
electrode (reference) placed in the cell. By recording this
with the solution stated in 5.1.) Most commercial coulometric
potential any difference in electrochemical activity between
thickness testers can be used as the current source.
layers is more readily detected. The equipment may be
5.3 Electrolyte Agitation Source—All commercial coulo-
calibrated against standards with known STEP values.
metric thickness testers incorporate a means to agitate the
3.6 The thickness of any specific nickel layer may be
solution. It is possible to purchase these types of units
calculated from the quantity of electricity used (current multi-
separately, if so desired, to be used externally in conjunction
plied by time), area dissolved, electrochemical equivalent of
with other power supplies.
nickel, anode efficiency, and density of the nickel layer.
5.4 Recorder—Any time-based recorder with an input im-
3.7 Commercial instruments using this principle are avail-
pedance of at least 1.0 MΩ and capable of running at
able. They are usually a combination coulometric and STEP
approximately 0.5 mm/s (3 cm/min) can be used.
instrument. Reference standards are available to calibrate the
instrument. The STEPTest, as is the Coulometric Test, is rapid
5.5 Deplating Cell—The cell may be similar in construction
and destructive to the coating.
to commercially available coulometric deplating cells. It is
usually a cup-shaped cell of either 316 stainless steel, copper-
4. Significance and Use
nickel alloy, or plastic that engages a round rubber or plastic
4.1 The ability of a multilayer nickel deposit to enhance
gasket to the work piece or sample. The opening through the
corrosion resistance is a function of the difference in the
cell and gasket allows contact of the electrolyte to the test
electrode potentials of the nickel layers (as measured individu-
specimen and defines the stripping area.
ally at a fixed current density in a given electrolyte versus a
NOTE 2—A coulometric deplating cell could be constructed of plastic
reference electrode) and the thicknesses of the layers. The
using a cylindrical stainless steel or copper-nickel alloy sheet cathode
potential differences must be sufficient to cause the bright
located in the larger upper area of the cup. The advantages of such a cell
nickel or top layer to corrode preferentially and sacrificially
are the prevention of whisker growth and the choking off of the small bore
with respect to the semi-bright nickel layer beneath it.
opening, and the ease of cathode removal for cleaning or replacement.
4.2 This test procedure allows the measurement of these
5.6 Reference Electrode—Either silver or platinum wire of
potential differences directly on an electroplated part rather
approximately 1.5 mm in diameter can be used. Silver is
than on separate foil specimens in such a way that time
probably the better choice due to its ability to form a
determines the thickness of each layer, while the potential
silver-silver chloride electrode when used in a chloride con-
difference between nickel layers is an indication of the corro-
taining electrolyte. The tip of the reference electrode should
sion resistance of the total nickel deposit.
extend so that the distance between the tip of the electrode and
4.3 The interpretation and evaluation of the results of this the bottom of the agitator tube is approximately 5 mm.
test should be by agreement between the purchaser and the
NOTE 3—It is necessary to condition the silver electrode before using in
manufacturer.
order to form the silver-silver chloride surface. This is easily done by
anodically treating approximately a 75-mm length of wire in 1 N
NOTE 1—This test may be used as a quality assurance test of the
hydrochloric acid solution for 10 to 15 s using 35-mAanodic current.This
multilayer nickel coatings applied in production. It should be understood
will form a gray film on the wire, which should always be present. Once
that due to many factors that influence the progress of corrosion during
the gray film is formed, it is not necessary to repeat the conditioning
actual use of the part, the performance of different multilayer nickel
treatmentunlessthefilmhasbeenremoved.Itmaybeadvisable,however,
deposits in the test cannot be taken as an absolute indicator of the relative
to recondition the electrode after a prolonged period of inactivity or when
corrosion resistance of these deposits in service.
the electrode has been allowed to remain dry for an extended period of
time. Drying off the electrode should be avoided by immersion in either
5. Apparatus
the hydrochloric acid conditioning solution, the step test solution, or
5.1 Composition of the Electrolyte :
distilled water when not in use.
Nickel Chloride (NiCl ·6H O) 300g/L
2 2 NOTE 4—A ceramic junction reference electrode that does not require
Sodium Chloride (NaCl) 50g/L
conditioning is available commercially.
Boric Acid (H BO)25g/L
3 3
A
pH 3.0
5.7 Millivolt Meter (optional)—When using a sensitive and
well-calibrated recorder, a millivolt meter is not necessary. If
A
The pH may be adjusted with diluted hydrochloric acid or sodium hydroxide, as
one is desired, however, any sensitive, high-input impedance
required, and is more critical than the composition of the electrolyte.
meter can be used.Astandard pH meter with a millivolt setting
Prepared in Purified Water—Type IV or better as specified
would be satisfactory.The meter should have a range from 0 to
in Specification D1193.
2000 mV. If a millivolt meter is used which has low-output
impedance facilities, it can be used in parallel to drive the
5 recorder and will serve as a buffer amplifier. Most laboratory
Electrolyte can be obtained commercially that meets the requirements of this
test. pH meters have such output terminals.
B764 − 04 (2021)
6. Procedure 6.10 Stop the test by turning off the agitator, constant
current source, recorder, and milliampere meter. Remove the
6.1 Set up equipment as recommended by the manufacturer.
electrode assembly, if necessary, and empty the cell of the
If necessary, turn on the recorder and the millivolt meter and
strippingsolution.Washthecellthreetimeswithpurifiedwater
allow them to warm up.
(Type IV or better as specified in Specification D1193) before
6.2 If chromium is present on the nickel surface, remove it
continuing to the next test.
with concentrated hydrochloric acid. Make sure the nickel
6.11 This test is based on a measured current-time relation-
surface is clean. Rinse well and dry off the surface.
ship necessary to remove a given amount of nickel from a
specific area.
NOTE 5—Chromium can be removed by using the coulometric deplat-
ingcellasisdoneonmanycommercialcoulometrictesters.Ifthisisdone,
Example: if the constant current source produces 30 mA, the
secure the cell and gasket to the test piece as in 6.3 and 6.4 but do not
recordertimebaseis30mm/min,andthedeplatingareais0.08
insert the electrode assembly. Fill the cell with a common test stripping
cm , it would take 19.2 s to deplate 2.5 µm of nickel.The chart
solution for chromium (Test Method B504) and hook up only the cell and
would travel 9.6 mm.Ageneral equation that may be used is as
test piece to the power supply. Apply the current until all the chromium
hasbeenremoved.Adenseblanketofbubblesonthesurfaceofthesample
follows:
indicates that all the chromium is removed. Remove the stripping solution
SL A I
~ !~ !~ !
fromthecellwithoutmovingordisturbingthesealofthegaskettothetest
5 T (1)
surface. Wash the cell three times with purified water (Type IV or better ~0.303! S
as specified in Specification D1193) and once with the step test solution.
where:
Proceed to 6.5.
SL = chart scan length, mm,
6.3 Positionthetestspecimeninasecurehorizontalposition
S = chart speed, mm/min,
sothatthechromium-strippednickelsurfaceisdirectlybeneath
I = cell current, mA,
the cell gasket.
A = deplating area, cm,
T = nickel thickness, µm, and
6.4 Lower the coulometric deplating cell assembly; secure
0.303 = constant calculated from the electrochemical
by sealing the gasket to the nickel surface. A flat test area of
equivalent and density of nickel.
approximately 10 mm in diameter is desirable but not required.
NOTE 7—Commercial units are available that will modify and may
The criterion is that there be no leakage of the electrolyte. If
simplify the above procedure.
leakage does occur, discontinue test and start a new one.
6.5 Fill the coulometric deplating cell to the appropriate
7. Factors Affecting the Accuracy of the Method
level with the step test solution making sure that no air is
7.1 Excessive Metal Build-Up in Coulometric Deplating
trapped within the solution.
Cell—Excessive buildup of deposited nickel or the formation
6.6 Lower the reference electrode assembly into the coulo-
of “whiskers” on the inside of the coulometric deplating cell
metric deplating cell, if necessary. The positioning of the
(cathode)
...