ASTM B504-90(2023)

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: B504 − 90 (Reapproved 2023) Endorsed by American
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 B504; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope (about 0.1 cm ). With the test specimen as anode and the cell
as cathode, a constant direct current is passed through the cell
1.1 This test method covers the determination of the thick-
until the coating has dissolved, at which time a sudden change
ness of metallic coatings by the coulometric method, also
in voltage occurs.
known as the anodic solution or electrochemical stripping
method. 3.3 The thickness of the coating may be calculated from the
quantity of electricity used (current multiplied by time), the
1.2 This standard does not purport to address all of the
area, the electrochemical equivalent of the coating metal, the
safety concerns, if any, associated with its use. It is the
anodic-current efficiency, and the density of the coating.
responsibility of the user of this standard to establish appro-
Alternatively, the equipment may be calibrated against stan-
priate safety, health, and environmental practices and deter-
dards with known coating thicknesses.
mine the applicability of regulatory limitations prior to use.
1.3 This international standard was developed in accor- 3.4 Commercial instruments using this principle are avail-
dance with internationally recognized principles on standard-
able. The method is rapid and versatile, but destructive to the
ization established in the Decision on Principles for the coating. In general, its range is considered to be between
Development of International Standards, Guides and Recom- 0.75 μm and 50 μm. Chromium, gold, tin, and other coatings
mendations issued by the World Trade Organization Technical can be measured down to 0.075 μm.
Barriers to Trade (TBT) Committee.
4. Significance and Use
2. Referenced Documents
4.1 Measurement of the thickness of a coating is essential to
assessing its utility and cost.
2.1 ISO Standard:
4.2 The coulometric method destroys the coating over a
ISO 2177 Metallic Coatings—Measurement of Coating
very small (about 0.1 cm ) test area. Therefore its use is limited
Thickness—Coulometric Method by Anodic Dissolution
to applications where a bare spot at the test area is acceptable
3. Summary of Test Method
or the test piece may be destroyed.
3.1 The thickness of the coating is determined by measuring
5. Factors Affecting the Accuracy of the Method
the quantity of electricity (coulombs) required to dissolve the
5.1 Composition of Electrolytes—Electrolytes used for cou-
coating anodically from a known and accurately defined area.
lometric thickness measurements must permit the coating
3.2 As commonly practiced, the method employs a small
metal to dissolve at a constant anodic-current efficiency (pref-
metal cell which is filled with an appropriate electrolyte. The
erably 100 %); they must have a negligible spontaneous
test specimen serves as the bottom of the cell and an insulating
chemical effect on the coating metal and must so differentiate
gasket between the cell and the specimen defines the test area
electrochemically between the coating and the substrate that a
suitably sharp and large voltage change occurs at the end point
of the test.
This test method is under the jurisdiction of ASTM Committee B08 on Metallic
5.1.1 Electrolytes furnished with commercial instruments
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
may be presumed to meet these requirements; others must be
Test Methods.
Current edition approved May 1, 2023. Published June 2023. Originally
evaluated before use by testing standards having known
approved in 1970. Last previous edition approved in 2017 as B504 – 90 (2017).
thicknesses. Appendix X1 lists some electrolytes and coating-
DOI: 10.1520/B0504-90R23.
substrate combinations that have been used with some instru-
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. ments.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B504 − 90 (2023)
5.2 Current Variation—For coulometric instruments em- 6. Calibration of Equipment
ploying the constant-current technique, variation of the current
6.1 The equipment shall be calibrated by means of standards
during a test will result in errors. For instruments using a
having known coating thicknesses. If commercial equipment is
current-time integrator, variation of the current during a test
used, the manufacturer’s instructions shall be followed insofar
will not result in error unless the current change is such as to
as they are compatible with this test method.
displace the anodic current density beyond the range of
6.2 Calibration of Direct-Reading Instruments—Direct-
constant or 100 % anodic-current efficiency.
reading instruments shall be calibrated against standards hav-
5.3 Area Variation—The accuracy of the thickness measure-
ing known coating thicknesses, and adjusted to produce correct
ment will not be better than the accuracy with which the test
readings corresponding to the coating thicknesses of the
area is defined or known. Typically, this test area is defined by
standard.
a flexible, insulating gasket. Area variation is usually mini-
6.3 Calibration of Nondirect-Reading Instruments:
mized by using as large an area as practical and by using a
6.3.1 Nondirect-reading instruments shall be calibrated
constant pressure device. If excessive pressure is applied to
against standards having a known coating thickness by using a
such a gasket, the test area may be altered undesirably.
calibration constant, C, calculated as follows:
5.4 Agitation—In most, but not all, coulometric thickness
C 5 coating thickness of the standards/instrument reading (1)
measurements, a relatively high anodic-current density is
employed to shorten the test time. It is then necessary to agitate
6.3.2 The instrument shall be adjusted so that where stan-
the electrolyte to maintain a constant anodic-current efficiency.
dards having known coating thicknesses are tested, the correct
Where agitation is required, insufficient agitation may result in
thickness is obtained by multiplying the instrument reading by
polarization of the specimen, thereby causing a premature and
the calibration constant, C.
false endpoint.
6.4 Thickness Standards—The thickness standards shall
5.5 Alloying Between Coatings and Metallic Substrates— consist of the same type of coating and substrate as the
The measurement of a coating thickness by the coulometric
specimens to be measured, and they shall have an accuracy of
method implicitly assumes that a sharply defined interface 65 % or better.
exists between the coating and the substrate. If an alloy layer
7. Procedure for Making Measurements
exists between the coating and the substrate as, for example, in
7.1 If commercial equipment is used, the manufacturer’s
the case of coating
...