ASTM B504-90(2007)

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Endorsed by American
Designation:B504–90(Reapproved 2007) 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 Department of Defense.
1. Scope Alternatively, the equipment may be calibrated against stan-
dards with known coating thicknesses.
1.1 This test method covers the determination of the thick-
3.4 Commercial instruments using this principle are avail-
ness of metallic coatings by the coulometric method, also
able. The method is rapid and versatile, but destructive to the
known as the anodic solution or electrochemical stripping
coating. In general, its range is considered to be between 0.75
method.
and 50 µm. Chromium, gold, tin, and other coatings can be
1.2 This standard does not purport to address all of the
measured down to 0.075 µm.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Significance and Use
priate safety and health practices and determine the applica-
4.1 Measurement of the thickness of a coating is essential to
bility of regulatory limitations prior to use.
assessing its utility and cost.
2. Referenced Documents 4.2 The coulometric method destroys the coating over a
verysmall(about0.1cm )testarea.Thereforeitsuseislimited
2.1 ISO Standard:
to applications where a bare spot at the test area is acceptable
ISO 2177 Metallic Coatings—Measurement of Coating
or the test piece may be destroyed.
Thickness—Coulometric Method by Anodic Dissolution
5. Factors Affecting the Accuracy of the Method
3. Summary of Test Method
5.1 Composition of Electrolytes—Electrolytes used for cou-
3.1 The thickness of the coating is determined by measuring
lometric thickness measurements must permit the coating
the quantity of electricity (coulombs) required to dissolve the
metal to dissolve at a constant anodic-current efficiency (pref-
coating anodically from a known and accurately defined area.
erably 100 %); they must have a negligible spontaneous
3.2 As commonly practiced, the method employs a small
chemical effect on the coating metal and must so differentiate
metal cell which is filled with an appropriate electrolyte. The
electrochemically between the coating and the substrate that a
test specimen serves as the bottom of the cell and an insulating
suitably sharp and large voltage change occurs at the end point
gasket between the cell and the specimen defines the test area
of the test.
(about 0.1 cm ). With the test specimen as anode and the cell
5.1.1 Electrolytes furnished with commercial instruments
as cathode, a constant direct current is passed through the cell
may be presumed to meet these requirements; others must be
until the coating has dissolved, at which time a sudden change
evaluated before use by testing standards having known
in voltage occurs.
thicknesses. Appendix X1 lists some electrolytes and coating-
3.3 The thickness of the coating may be calculated from the
substrate combinations that have been used with some instru-
quantity of electricity used (current multiplied by time), the
ments.
area, the electrochemical equivalent of the coating metal, the
5.2 Current Variation—For coulometric instruments em-
anodic-current efficiency, and the density of the coating.
ploying the constant-current technique, variation of the current
during a test will result in errors. For instruments using a
current-time integrator, variation of the current during a test
This test method is under the jurisdiction ofASTM Committee B08 on Metallic
will not result in error unless the current change is such as to
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
displace the anodic current density beyond the range of
Test Methods.
Current edition approved March 1, 2007. Published March 2007. Originally
constant or 100 % anodic-current efficiency.
approved in 1970. Last previous edition approved in 2002 as B504 – 90 (2002).
5.3 AreaVariation—Theaccuracyofthethicknessmeasure-
DOI: 10.1520/B0504-90R07.
2 ment will not be better than the accuracy with which the test
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. area is defined or known. Typically, this test area is defined by
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B504–90 (2007)
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
6.3.2 The instrument shall be adjusted so that where stan-
employedtoshortenthetesttime.Itisthennecessarytoagitate
dards having known coating thicknesses are tested, the correct
the electrolyte to maintain a constant anodic-current efficiency.
thickness is obtained by multiplying the instrument reading by
Where agitation is required, insufficient agitation may result in
the calibration constant, C.
polarization of the specimen, thereby causing a premature and
6.4 Thickness Standards—The thickness standards shall
false endpoint.
consist of the same type of coating and substrate as the
5.5 Alloying Between Coatings and Metallic Substrates—
specimens to be measured, and they shall have an accuracy of
The measurement of a coating thickness by the coulometric
65 % or better.
method implicitly assumes that a sharply defined interface
exists between the coating and the substrate. If an alloy layer
exists between the coating and the substrate as, for example, in 7. Procedure for Making Measurements
the case of coatings applied by hot dipping, the coulometric
7.1 If commercial equipment is used, the manufacturer’s
end-point may occur at some point within the alloy layer, thus
instructions shall be followed insofar as they are compatible
giving a high value of the thickness of the unalloyed coating.
with this test method.
5.6 Purity of Coating—Impurities or additives that code-
7.2 The test surface shall be cleaned of all foreign material
positwiththecoatingmaychangetheeffectiveelectrochemical
that might affect the measurement.
equivalent of the coating and also change the anodic current
NOTE 1—Certain nickel deposits, frequently dull nickel, may
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