NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
An American National Standard
Designation: E 376 – 96
Standard Practice for
Measuring Coating Thickness by Magnetic-Field or Eddy-
Current (Electromagnetic) Test Methods1
This standard is issued under the fixed designation E 376; 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 D 1400 Test Method for Nondestructive Measurement of
1.1 This practice covers the use of magnetic- and eddy- Dry Film Thickness of Nonconductive Coatings Applied to
current-type thickness instruments (gages) for nondestructive a Nonferrous Metal Base3
thickness measurement of a coating on a metal substrate. E 1316 Terminology for Nondestructive Examinations4
1.2 More specific uses of these instruments are covered by G 12 Test Method for Nondestructive Measurement of Film
the following test methods issued by ASTM: Test Methods Thickness of Pipeline Coatings on Steel5
B 244, B 499, B 530, D 1186, D 1400, and G 12. 2.2 International Standard:
1.3 The values stated in SI units are to be regarded as ISO 2178 Nonmagnetic Coatings on Magnetic Substrate—
standard. The inch-pound units in parentheses are for informa- Measurement of Coating Thickness—Magnetic
tion only and may be approximate. Method6
iTeh Standards
1.4 Measurements made in accordance with this test method
will be in compliance with the requirements of ISO Interna-
tional Standard 2178 as printed in 1982. 3. Terminology
NOTE 1—See Appendix X1.
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1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.1 Definitions—Definitions of terms relating to electro-
magnetic testing are given in Terminology E 1316.
Document 4.Preview
responsibility of the user of this standard to establish appro-
Significance and Use
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. 4.1 General—No presently available thickness gage is ap-
plicable to all combinations of coating-substrate thicknesses
2. Referenced Documents ASTM E376-96
and materials. The limitations of a particular instrument are
/catalog/standards/astm/f635ccfb-b7e8-4408-ace2-a898c41ebbc3/astm-e376-96
2.1 ASTM Standards: generally delineated by its manufacturer.
B 244 Test Method for Measurement of Thickness of An- 4.2 Magnetic—Magnetic-type gages measure either mag-
odic Coatings on Aluminum and of Other Nonconductive netic attraction between a magnet and a coating or its substrate,
Coatings on Nonmagnetic Basis Metals with Eddy-Current or reluctance of a magnetic flux path passing through the
Instruments2 coating and substrate. These gages are designed to measure
B 499 Test Method for Measurement of Coating Thick- thickness of a nonmagnetic coating on a magnetic substrate.
nesses by the Magnetic Method: Nonmagnetic Coatings on Some of them will also measure thickness of nickel coatings on
Magnetic Basis Metals2 a magnetic or nonmagnetic substrate.7
B 530 Test Method for Measurement of Coating Thick- 4.3 Eddy Current—Eddy-current-type thickness gages are
nesses by the Magnetic Method: Electrodeposited Nickel electronic instruments that measure variations in impedance of
Coatings on Magnetic and Nonmagnetic Substrates2 an eddy-current inducing coil caused by coating thickness
D 1186 Test Methods for Nondestructive Measurement of variations. They can only be used if the electrical conductivity
Dry Film Thickness of Nonmagnetic Coatings Applied to of the coating differs significantly from that of the substrate.
a Ferrous Base3
1 4
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde- Annual Book of ASTM Standards, Vol 03.03.
5
structive Testing and is the direct responsibility of Subcommittee E07.07 on Annual Book of ASTM Standards, Vol 06.02.
6
Electromagnetic Method. Available from American National Standards Institute, 11 West 42nd Street,
Current edition approved Dec. 10,1996. Published February 1997. Originally 13th Floor, New York, NY 10036.
published as E 376 – 89. Last previous edition E 376 – 89 (1994)e1 7
Autocatalytically deposited nickel-phosphorus alloys containing more than 8 %
2
Annual Book of ASTM Standards, Vol 02.05. phosphorus are sufficiently nonmagnetic to be measured by this method, as long as
3
Annual Book of ASTM Standards, Vol 06.01. the measurement is made prior to any heat treatment.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
1
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 376 – 96
5. Interferences 5.11 Cleanness of Probe and Test Surface—Measurements
5.1 Thickness of Coating—The precision of a measurement are sensitive to foreign material that prevents intimate contact
changes with coating thickness depending on method used and between probe and coating surface.
instrument design. Generally, the precision is a percentage of 5.12 Pressure of Probe—Instrument readings can be sensi-
the coating thickness except at the lower end of the ranges tive to pressure with which probe is applied to test surface.
where it is a fixed thickness. 5.13 Probe Position—Some magnetic-type gages are sensi-
5.2 Magnetic Properties of Basis Metal—Magnetic thick- tive to position of probe relative to the earth. For example,
ness gages are affected by variations of the magnetic properties operation of gage in a horizontal or upside-down position may
of the basis metal. For practical purposes, magnetic variations require a new calibration or may be impossible.
in low-carbon 1005-1020 steel may be considered to be 5.14 Temperature—Eddy-current instruments may be af-
insignificant. To avoid the influences of severe or localized heat fected by temperature variations.
treatments and cold working, the instrument should be cali-
brated using a calibration standard having a base metal with the 6. Calibration and Standardization
same magnetic properties as that of the test specimen or, 6.1 Each instrument should be calibrated in accordance with
preferably and if available, with a sample of the part to be the manufacturer’s instructions before use by employing suit-
tested before application of the coating. able thickness standards. Calibration should be checked at
5.3 Thickness of Substrate—For each method there is an frequent intervals during use. Attention should be given to
effective depth of penetration of field created by the instrument Section 5 and Section 7.
probe. This is the critical depth or thickness beyond which the 6.2 Calibration standards of uniform thickness are available
instrument will no longer be affected by increase or substrate in either of two types, foil or coated substrate, as supplied or
thickness. Since it depends on the instrument probe and recommended by the manufacturer of the instrument. There are
substrate, it should be determined experimentally. instances, however, where calibration standards are made by
5.4 Structure and Composition of Coating and Substrate— other than instrument manufacturers.
Eddy-current instruments are sensitive to variations of struc-
iTeh Standards
6.2.1 Calibration Foils (Shims)—Calibration foil is placed
ture, composition, and other factors affecting electrical con-
on the surface of uncoated basis metal when calibrating the
ductivity and magnetic permeability of the coating and
instrument. Foils are advantageous for calibrating on curved
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substrate. For example, such instruments are sensitive to
surfaces and are often more readily available than a coated
differences between: (1) aluminum alloys, (2) chromium coat-
standard. To prevent measurement errors due to poor contact
ings deposited at different temperatures, and (3) organic
Document Preview
between foil and substrate, make sure of intimate contact
coatings containing variable amounts of metallic pigments.
between them. Calibration foils are subject to indentation and
5.5 Edge Effect—All test methods are sensitive to abrupt should, therefore, be replaced when damaged.
surface changes of test specimens; therefore, measurements
6.2.1.1 Nonmagnetic foils may be used to calibrate mag-
made too near an edge or inside corner will not be valid unless
ASTM E376-96
netic thickness gages for measurement of nonmagnetic coat-
the instrument is specifically calibrated for such a measure-
/catalog/standards/astm/f635ccfb-b7e8-4408-ace2-a898c41ebbc3/astm-e376-96
ment. The effect usually extends 3 to 13 mm (1⁄8 to 1⁄2 in.) from
ings. Nonconductive plastic foils can be used to calibrate
eddy-current instruments for measurement of nonconductive
the discontinuity, depending on method probe configuration,
coatings.
and instrument. Edge effect is usually a function of coil
diameter. 6.2.1.2 Resilient foils should not be used if there is possi-
5.6 Curvature of Test Surface—Thickness measurements bility that the instrument probe will cause a change in thickness
are sensitive to curvature of the specimen. This sensitivity reading. Use of two or more foils on top of each other should
varies considerably between instruments and becomes more be avoided unless flexibility of thin foils is required for a
pronounced with increasing curvature. curved surface.
5.7 Smoothness of Surface, Including That of Basis Metal— 6.2.2 Coated Substrates Standards—Calibration standards
Since a rough surface may make single measurements inaccu- consist of coatings of known thickness permanently bonded to
rate, a greater number of measurements will provide an the substrate material.
average value that is more truly representative of the overall 6.3 Thicknesses of calibration standards should bracket and
coating thickness. Roughness also may cause certain instru- be as close as possible to the coating thickness being measured.
ments to read high since their probes may rest on peaks. 6.4 For magnetic instruments, calibration standards should
5.8 Direction of Rolling of Basis Metal—Instruments with have the same magnetic properties as the coated test specimen.
two pole pieces may be sensitive to direction of rolling of the 6.5 For eddy-current instruments, the calibration standard
basis metal; that is, gage readings may change depending on should have the same electrical and magnetic properties as
alignment of pole pieces with surface of specimen under test. those of coated test specimen being measured (see 5.4).
5.9 Residual Magnetism in Basis Metal—Residual magne- 6.6 To determine calibration validity, a reading on a bare
tism in basis metal may affect readings of magnetic- and specimen identical in magnetic and electrical properties to that
eddy-current-type instruments. of the test specimen substrate is recommended.
5.10 Stray Magnetic Fields—Strong magnetic fields, as 6.7 If the coating process is changed, the calibration may no
from arc welding, can seriously interfere with operations of longer be valid, especially for magnetic coatings and eddy-
certain thickness gages. current gages (see 5.4).
