ASTM B504-90(2002)
(Test Method)Standard Test Method for Measurement of Thickness of Metallic Coatings by the Coulometric Method
Standard Test Method for Measurement of Thickness of Metallic Coatings by the Coulometric Method
SCOPE
1.1 This test method covers the determination of the thickness of metallic coatings by the coulometric method, also known as the anodic solution or electrochemical stripping method.
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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Endorsed by American
Designation:B504–90(Reapproved 2002) Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
Standard Test Method for
Measurement of Thickness of Metallic Coatings by the
Coulometric Method
This standard is issued under the fixed designation B 504; 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.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 3.4 Commercial instruments using this principle are avail-
able. The method is rapid and versatile, but destructive to the
1.1 This test method covers the determination of the thick-
coating. In general, its range is considered to be between 0.75
ness of metallic coatings by the coulometric method, also
and 50 µm. Chromium, gold, tin, and other coatings can be
known as the anodic solution or electrochemical stripping
measured down to 0.075 µm.
method.
1.2 This standard does not purport to address all of the
4. Significance and Use
safety concerns, if any, associated with its use. It is the
4.1 Measurement of the thickness of a coating is essential to
responsibility of the user of this standard to establish appro-
assessing its utility and cost.
priate safety and health practices and determine the applica-
4.2 The coulometric method destroys the coating over a
bility of regulatory limitations prior to use.
verysmall(about0.1cm )testarea.Thereforeitsuseislimited
2. Referenced Documents to applications where a bare spot at the test area is acceptable
or the test piece may be destroyed.
2.1 ISO Standard:
ISO 2177 Metallic Coatings—Measurement of Coating
5. Factors Affecting the Accuracy of the Method
Thickness—Coulometric Method by Anodic Dissolution
5.1 Composition of Electrolytes—Electrolytes used for cou-
3. Summary of Test Method lometric thickness measurements must permit the coating
metal to dissolve at a constant anodic-current efficiency (pref-
3.1 Thethicknessofthecoatingisdeterminedbymeasuring
erably 100 %); they must have a negligible spontaneous
the quantity of electricity (coulombs) required to dissolve the
chemical effect on the coating metal and must so differentiate
coating anodically from a known and accurately defined area.
electrochemically between the coating and the substrate that a
3.2 As commonly practiced, the method employs a small
suitably sharp and large voltage change occurs at the end point
metal cell which is filled with an appropriate electrolyte. The
of the test.
test specimen serves as the bottom of the cell and an insulating
5.1.1 Electrolytes furnished with commercial instruments
gasket between the cell and the specimen defines the test area
may be presumed to meet these requirements; others must be
(about 0.1 cm ). With the test specimen as anode and the cell
evaluated before use by testing standards having known
as cathode, a constant direct current is passed through the cell
thicknesses. Appendix X1 lists some electrolytes and coating-
until the coating has dissolved, at which time a sudden change
substrate combinations that have been used with some instru-
in voltage occurs.
ments.
3.3 The thickness of the coating may be calculated from the
5.2 Current Variation—For coulometric instruments em-
quantity of electricity used (current multiplied by time), the
ploying the constant-current technique, variation of the current
area, the electrochemical equivalent of the coating metal, the
during a test will result in errors. For instruments using a
anodic-current efficiency, and the density of the coating.
current-time integrator, variation of the current during a test
Alternatively, the equipment may be calibrated against stan-
will not result in error unless the current change is such as to
dards with known coating thicknesses.
displace the anodic current density beyond the range of
constant or 100 % anodic-current efficiency.
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic
5.3 AreaVariation—Theaccuracyofthethicknessmeasure-
and Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10on
ment will not be better than the accuracy with which the test
General Test Methods.
Current edition approved Feb. 23, 1990. Published April 1990. Originally area is defined or known. Typically, this test area is defined by
published as B 504 – 70. Last previous edition B 504 – 89.
a flexible, insulating gasket. Area variation is usually mini-
Available from American National Standards Institute, 11 W. 42nd St., 13th
mized by using as large an area as practical and by using a
Floor, New York, NY 10036.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B504
constant pressure device. If excessive pressure is applied to 6.3 Calibration of Nondirect-Reading Instruments:
such a gasket, the test area may be altered undesirably. 6.3.1 Nondirect-reading instruments shall be calibrated
5.4 Agitation—In most, but not all, coulometric thickness against standards having a known coating thickness by using a
measurements, a relatively high anodic-current density is calibration constant, C, calculated as follows:
employedtoshortenthetesttime.Itisthennecessarytoagitate
C 5 coating thickness of the standards/instrument reading (1)
the electrolyte to maintain a constant anodic-current efficiency.
6.3.2 The instrument shall be adjusted so that where stan-
Where agitation is required, insufficient agitation may result in
dards having known coating thicknesses are tested, the correct
polarization of the specimen, thereby causing a premature and
thickness is obtained by multiplying the instrument reading by
false endpoint.
the calibration constant, C.
5.5 Alloying Between Coatings and Metallic Substrates—
6.4 Thickness Standards—The thickness standards shall
The measurement of a coating thickness by the coulometric
consist of the same type of coating and substrate as the
method implicitly assumes that a sharply defined interface
specimens to be measured, and they shall have an accuracy of
exists between the coating and the substrate. If an alloy layer
65 % or better.
exists between the coating and the substrate as, for example, in
the case of coatings applied by hot dipping, the coulometric
7. Procedure for Making Measurements
end-point may occur at some point within the alloy layer, thus
7.1 If commercial equipment is used, the manufacturer’s
giving a high value of the thickness of the unalloyed coating.
instructions shall be followed insofar as they are compatible
5.6 Purity of Coating—Impurities or additives that code-
with this test method.
positwiththecoatingmaychangetheeffectiveelectrochemical
7.2 The test surface shall be cleaned of all foreign material
equivalent of the coating and also change the anodic current
that might affect the measurement.
efficiency.
5.6.1 Alloy Coating—Variations in the composition of alloy NOTE 1—Certain nickel deposits, frequently dull nickel, may exhi
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