ASTM B764-04(2009)
(Test Method)Standard Test Method for Simultaneous Thickness and Electrode Potential Determination of Individual Layers in Multilayer Nickel Deposit (STEP Test)
Standard Test Method for Simultaneous Thickness and Electrode Potential Determination of Individual Layers in Multilayer Nickel Deposit (STEP Test)
SIGNIFICANCE AND USE
The ability of a multilayer nickel deposit to enhance corrosion resistance is a function of the difference in the electrode potentials of the nickel layers (as measured individually at a fixed current density in a given electrolyte versus a reference electrode) and the thicknesses of the layers. The potential differences must be sufficient to cause the bright nickel or top layer to corrode preferentially and sacrificially with respect to the semi-bright nickel layer beneath it.
This test procedure allows the measurement of these potential differences directly on an electroplated part rather than on separate foil specimens in such a way that time determines the thickness of each layer, while the potential difference between nickel layers is an indication of the corrosion resistance of the total nickel deposit.
The interpretation and evaluation of the results of this test should be by agreement between the purchaser and the manufacturer.
Note 1—This test may be used as a quality assurance test of the multilayer nickel coatings applied in production. It should be understood that due to many factors that influence the progress of corrosion during actual use of the part, the performance of different multilayer nickel deposits in the test cannot be taken as an absolute indicator of the relative corrosion resistance of these deposits in service.
SCOPE
1.1 This test method closely estimates the thickness of individual layers of a multilayer nickel electrodeposit and the potential differences between the individual layers while being anodically stripped at constant current density. ,
1.2 This test method does not cover deposit systems other than multilayer electroplated nickel deposits.
1.3 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: B764 − 04(Reapproved 2009)
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 3.2 Coulometric thickness testing instruments are based on
the anodic dissolution (stripping) of the deposit at constant
1.1 This test method closely estimates the thickness of
current, while the time is measured to determine thickness.As
individual layers of a multilayer nickel electrodeposit and the
commonly practiced, the method employs a small cell that is
potential differences between the individual layers while being
2,3 filled with an appropriate electrolyte, and the test specimen
anodically stripped at constant current density.
serves as the bottom of the cell. To the bottom of the cell is
1.2 This test method does not cover deposit systems other
attached a rubber or plastic gasket whose opening defines the
than multilayer electroplated nickel deposits.
measuring(stripping,anodic)area.Ifametalliccellisused,the
1.3 This standard does not purport to address all of the
rubber gasket also electrically insulates the test specimen from
safety concerns, if any, associated with its use. It is the the cell.With the specimen as the anode and the cell or agitator
responsibility of the user of this standard to establish appro-
tube as the cathode, a constant direct current is passed through
priate safety and health practices and determine the applica- the cell until the nickel layer is dissolved. A sudden change in
bility of regulatory limitations prior to use.
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
3. Summary of Test Method
thickness.
3.1 This procedure is a modification of the well-known
3.4 At the same time, the amplitude of the voltage change
coulometric method of thickness testing (Test Method B504).
canbeobserved.Thatis,theease(ordifficulty)withwhichone
It is also known as the anodic dissolution or electrochemical
layer can be dissolved or stripped with reference to another
stripping method.
layer can be compared. The lower the voltage needed the more
active the metal or the greater the tendency to corrode
ThismethodisunderthejurisdictionofASTMCommitteeB08onMetallicand
preferentially to a more noble metal adjacent to it.
Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on Test
Methods.
3.5 Where the metallic layers are of such a similar nature
Current edition approved Sept. 1, 2009. Published December 2009. Originally
that change of the stripping voltage is small, there can be
approved in 1986. Last previous edition approved in 2004 as B764 – 04 . DOI:
problems in detecting this change if the voltage between the
10.1520/B0764-04R09.
For discussion of this test, see Harbulak, E. P., “Simultaneous Thickness and
deplating cell (cathode) and the sample (anode) is measured.
Electrochemical Potential Determination of Individual Layers in Multilayer Nickel
As the sample is dissolved anodically, cathodic processes are
Deposits,” Plating and Surface Finishing, Vol 67, No. 2, February 1980, pp. 49–54.
occurring on the deplating cell (cathode) surface that can also
give rise to voltage changes, due to alterations of the cathode
U.S. Patent 4,310,389. Assignee: The Chrysler Corp., Highland Park, MI
48203.
surface, thus obscuring the anode voltage change. This diffi-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
culty can be avoided by measuring the potential of the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
dissolving anodic sample with respect to an unpolarized third
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. electrode (reference) placed in the cell. By recording this
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B764 − 04 (2009)
potential any difference in electrochemical activity between with the solution stated in 5.1.) Most commercial coulometric
layers is more readily detected. The equipment may be thickness testers can be used as the current source.
calibrated against standards with known STEP values.
5.3 Electrolyte Agitation Source—All commercial coulo-
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
gasket to the work piece or sample. The opening through the
4.1 The ability of a multilayer nickel deposit to enhance
cell and gasket allows contact of the electrolyte to the test
corrosion resistance is a function of the difference in the
specimen and defines the stripping area.
electrode potentials of the nickel layers (as measured individu-
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
located in the larger upper area of the cup. The advantages of such a cell
potential differences must be sufficient to cause the bright
are the prevention of whisker growth and the choking off of the small bore
nickel or top layer to corrode preferentially and sacrificially
opening, and the ease of cathode removal for cleaning or replacement.
with respect to the semi-bright nickel layer beneath it.
5.6 Reference Electrode—Either silver or platinum wire of
4.2 This test procedure allows the measurement of these
approximately 1.5 mm in diameter can be used. Silver is
potential differences directly on an electroplated part rather
probably the better choice due to its ability to form a
than on separate foil specimens in such a way that time
silver-silver chloride electrode when used in a chloride con-
determines the thickness of each layer, while the potential
taining electrolyte. The tip of the reference electrode should
difference between nickel layers is an indication of the corro-
extend so that the distance between the tip of the electrode and
sion resistance of the total nickel deposit.
the bottom of the agitator tube is approximately 5 mm.
4.3 The interpretation and evaluation of the results of this
NOTE 3—It is necessary to condition the silver electrode before using in
test should be by agreement between the purchaser and the
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
manufacturer.
hydrochloric acid solution for 10 to 15 s using 35-mAanodic current.This
NOTE 1—This test may be used as a quality assurance test of the
will form a gray film on the wire, which should always be present. Once
multilayer nickel coatings applied in production. It should be understood
the gray film is formed, it is not necessary to repeat the conditioning
that due to many factors that influence the progress of corrosion during
treatmentunlessthefilmhasbeenremoved.Itmaybeadvisable,however,
actual use of the part, the performance of different multilayer nickel
to recondition the electrode after a prolonged period of inactivity or when
deposits in the test cannot be taken as an absolute indicator of the relative
the electrode has been allowed to remain dry for an extended period of
corrosion resistance of these deposits in service.
time. Drying off the electrode should be avoided by immersion in either
the hydrochloric acid conditioning solution, the step test solution, or
5. Apparatus distilled water when not in use.
NOTE 4—A ceramic junction reference electrode that does not require
5.1 Composition of the Electrolyte :
conditioning is available commercially.
Nickel Chloride (NiCl ·6H O) 300g/L
2 2
5.7 Millivolt Meter (optional)—When using a sensitive and
Sodium Chloride (NaCl) 50g/L
well-calibrated recorder, a millivolt meter is not necessary. If
Boric Acid (H BO)25g/L
3 3
A
pH 3.0 one is desired, however, any sensitive, high-input impedance
meter can be used.Astandard pH meter with a millivolt setting
A
The pH may be adjusted with diluted hydrochloric acid or sodium hydroxide, as
would be satisfactory.The meter should have a range from 0 to
required, and is more critical than the composition of the electrolyte.
2000 mV. If a millivolt meter is used which has low-output
Prepared in Purified Water—Type IV or better as specified
impedance facilities, it can be used in parallel to drive the
in Specification D1193.
recorder and will serve as a buffer amplifier. Most laboratory
pH meters have such output terminals.
5.2 Constant Current Source—This should supply a con-
stant current that can be varied between 0 and 50 mA (typical
6. Procedure
25 to 35 mA). A current of 30 mA corresponds to a stripping
6.1 Set up equipment as recommended by the manufacturer.
rate of 7.8 µm/min at 100 % current efficiency when used with
If necessary, turn on the recorder and the millivolt meter and
a gasket providing 0.08 cm stripping area. (This is achieved
allow them to warm up.
6.2 If chromium is present on the nickel surface, remove it
with concentrated hydrochloric acid. Make sure the nickel
Electrolyte can be obtained commercially that meets the requirements of this
test. surface is clean. Rinse well and dry off the surface.
B764 − 04 (2009)
NOTE 5—Chromium can be removed by using the coulometric deplat-
Example: if the constant current source produces 30 mA, the
ingcellasisdoneonmanycommercialcoulometrictesters.Ifthisisdone,
recordertimebaseis30mm/min,andthedeplatingareais0.08
secure the cell and gasket to the test piece as in 6.3 and 6.4 but do not
cm , it would take 19.2 s to deplate 2.5 µm of nickel.The chart
insert the electrode assembly. Fill the cell with a common test stripping
would travel 9.6 mm.Ageneral equation that may be used is as
solution for chromium (Test Method B504) and hook up only the cell and
follows:
test piece to the power supply. Apply the current until all the chromium
hasbeenremoved.Adenseblanketofbubblesonthesurfaceofthesample
SL A I
~ !~ !~ !
indicates that all the chromium is removed. Remove the stripping solution
5 T (1)
0.303 S
~ !
fromthecellwithoutmovingordisturbingthesealofthegaskettothetest
surface. Wash the cell three times with purified water (Type IV or better
where:
as specified in Specification D1193) and once with the step test solution.
Proceed to 6.5. SL = chart scan length, mm,
S = chart speed, mm/min,
6.3 Positionthetestspecimeninasecurehorizontalposition
I = cell current, mA,
sothatthechromium-strippednickelsurfaceisdirectlybeneath
A = deplating area, cm,
the cell gasket.
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
simplify the above procedure.
The criterion is that there be no leakage of the electrolyte. If
leakage does occur, discontinue test and start a new one.
7. Factors Affecting the Accuracy of the Method
6.5 Fill the coulometric deplating cell to the appropriate
7.1 Excessive Metal Build-Up in Coulometric Deplating
level with the step test solution making sure that no air is
Cell—Excessive buildup of deposited nickel or the formation
trapped within the solution.
of “whiskers” on the inside of the coulometric deplating cell
6.6 Lower the reference electrode assembly into the coulo-
(cathode), especially near the gasket hole, can cause erratic
metric deplating cell, if necessary. The positioning of the
results and produce “noisy” curves. When buildup is observed,
reference electrode should be such that the distance from the
remove it completely according to the manufacturer’s instruc-
end of the electrode to the test specimen is reproducible to
tions or as follows:
within 1 mm and be held constant throughout the test.
7.1.1 If a metallic cell is used as a cathode:
NOTE 6—The insertion depth of the electrolyte agitation tube which
7.1.1.1 Ream with a round, fine file. (Adrill or reamer may
includes the reference electrode is important and should always be the
be used.)
same. The difference of potential rather than the absolute potential is the
7.1.1.2 Soak for 15 to 20 s in a solution of four parts
important measurement.
concentratedsulfuricacidandonepartconcentratednitricacid.
6.7 Check all electrical connections. Make sure all connec-
If 316 stainless steel is used for the cell, it may be soaked
...
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