ASTM B504-90(1997)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: B 504 – 90 (Reapproved 1997) Endorsed by American
Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
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
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 able. The method is rapid and versatile, but destructive to the
coating. In general, its range is considered to be between 0.75
1.1 This test method covers the determination of the thick-
and 50 μm. Chromium, gold, tin, and other coatings can be
ness of metallic coatings by the coulometric method, also
measured down to 0.075 μm.
known as the anodic solution or electrochemical stripping
method.
4. Significance and Use
1.2 This standard does not purport to address all of the
4.1 Measurement of the thickness of a coating is essential to
safety concerns, if any, associated with its use. It is the
assessing its utility and cost.
responsibility of the user of this standard to establish appro-
4.2 The coulometric method destroys the coating over a
priate safety and health practices and determine the applica-
very small (about 0.1 cm ) test area. Therefore its use is limited
bility of regulatory limitations prior to use.
to applications where a bare spot at the test area is acceptable
2. Referenced Documents or the test piece may be destroyed.
2.1 ISO Standard:
5. Factors Affecting the Accuracy of the Method
ISO 2177 Metallic Coatings—Measurement of Coating
2 5.1 Composition of Electrolytes—Electrolytes used for cou-
Thickness—Coulometric Method by Anodic Dissolution
lometric thickness measurements must permit the coating
3. Summary of Test Method metal to dissolve at a constant anodic-current efficiency (pref-
erably 100 %); they must have a negligible spontaneous
3.1 The thickness of the coating is determined by measuring
chemical effect on the coating metal and must so differentiate
the quantity of electricity (coulombs) required to dissolve the
electrochemically between the coating and the substrate that a
coating anodically from a known and accurately defined area.
suitably sharp and large voltage change occurs at the end point
3.2 As commonly practiced, the method employs a small
of the test.
metal cell which is filled with an appropriate electrolyte. The
5.1.1 Electrolytes furnished with commercial instruments
test specimen serves as the bottom of the cell and an insulating
may be presumed to meet these requirements; others must be
gasket between the cell and the specimen defines the test area
evaluated before use by testing standards having known
(about 0.1 cm ). With the test specimen as anode and the cell
thicknesses. Appendix X1 lists some electrolytes and coating-
as cathode, a constant direct current is passed through the cell
substrate combinations that have been used with some instru-
until the coating has dissolved, at which time a sudden change
ments.
in voltage occurs.
5.2 Current Variation—For coulometric instruments em-
3.3 The thickness of the coating may be calculated from the
ploying the constant-current technique, variation of the current
quantity of electricity used (current multiplied by time), the
during a test will result in errors. For instruments using a
area, the electrochemical equivalent of the coating metal, the
current-time integrator, variation of the current during a test
anodic-current efficiency, and the density of the coating.
will not result in error unless the current change is such as to
Alternatively, the equipment may be calibrated against stan-
displace the anodic current density beyond the range of
dards with known coating thicknesses.
constant or 100 % anodic-current efficiency.
3.4 Commercial instruments using this principle are avail-
5.3 Area Variation—The accuracy of the thickness measure-
ment will not be better than the accuracy with which the test
This test method is under the jurisdiction of ASTM Committee B-8 on Metallic
area is defined or known. Typically, this test area is defined by
and Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10on
General Test Methods. a flexible, insulating gasket. Area variation is usually mini-
Current edition approved Feb. 23, 1990. Published April 1990. Originally
mized by using as large an area as practical and by using a
published as B 504 – 70. Last previous edition B 504 – 89.
constant pressure device. If excessive pressure is applied to
Available from American National Standards Institute, 11 W. 42nd St., 13th
such a gasket, the test area may be altered undesirably.
Floor, New York, NY 10036.
B 504
5.4 Agitation—In most, but not all, coulometric thickness 6.3.1 Nondirect-reading instruments shall be calibrated
measurements, a relatively high anodic-current density is against standards having a known coating thickness by using a
employed to shorten the test time. It is then necessary to agitate calibration constant, C, calculated as follows:
the electrolyte to maintain a constant anodic-current efficiency.
C 5 coating thickness of the standards/instrument reading (1)
Where agitation is required, insufficient agitation may result in
6.3.2 The instrument shall be adjusted so that where stan-
polarization of the specimen, thereby causing a premature and
dards having known coating thicknesses are tested, the correct
false endpoint.
thickness is obtained by multiplying the instrument reading by
5.5 Alloying Between Coatings and Metallic Substrates—
the calibration constant, C.
The measurement of a coating thickness by the coulometric
6.4 Thickness Standards—The thickness standards shall
method implicitly assumes that a sharply defined interface
consist of the same type of coating and substrate as the
exists between the coating and the substrate. If an alloy layer
specimens to be measured, and they shall have an accuracy of
exists between the coating and the substrate as, for example, in
65 % or better.
the case of coatings applied by hot dipping, the coulometric
end-point may occur at some point within the alloy layer, thus
7. Procedure for Making Measurements
giving a high value of the thickness of the unalloyed coating.
7.1 If commercial equipment is used, the manufacturer’s
5.6 Purity of Coating—Impurities or additives that code-
instructions shall be followed insofar as they are compatible
posit with the coating may change the effective electrochemical
with this test method.
equivalent of the coating and also change the anodic current
7.2 The test surface shall be cleaned of all fore
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