ASTM E376-96

Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Test Methods

SCOPE
1.1 This practice covers the use of magnetic- and eddy-current-type thickness instruments (gages) for nondestructive thickness measurement of a coating on a metal substrate.  
1.2 More specific uses of these instruments are covered by the following test methods issued by ASTM: Test Methods B 244, B 499, B 530, D 1186, D 1400, and G 12.  
1.3 The values stated in SI units are to be regarded as standard. The inch-pound units in parentheses are for information only and may be approximate.
1.4 Measurements made in accordance with this test method will be in compliance with the requirements of ISO International Standard 2178 as printed in 1982.  
1.5 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.

General Information

• Status: Historical
• Publication Date: 09-Dec-1996
• ICS: 17.040.20 - Properties of surfaces
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.07 - Electromagnetic Method

Relations

• Replaced By: ASTM E376-03 - Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Examination Methods
• Effective Date: 10-Mar-2003
• Referred By: ASTM D7803-19 - Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating
• Effective Date: 10-Dec-1996
• Referred By: ASTM D6386-22 - Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting
• Effective Date: 10-Dec-1996
• Referred By: ASTM A1060/A1060M-22 - Standard Specification for Zinc-Coated (Galvanized) Steel Welded Wire Reinforcement, Plain and Deformed, for Concrete
• Effective Date: 10-Dec-1996
• Referred By: ASTM G33-99(2020) - Standard Practice for Recording Data from Atmospheric Corrosion Tests of Metallic-Coated Steel Specimens
• Effective Date: 10-Dec-1996
• Referred By: ASTM F1941/F1941M-16 - Standard Specification for Electrodeposited Coatings on Mechanical Fasteners, Inch and Metric
• Effective Date: 10-Dec-1996
• Referred By: ASTM A1094/A1094M-20 - Standard Specification for Continuous Hot-Dip Galvanized Steel Bars for Concrete Reinforcement
• Effective Date: 10-Dec-1996
• Referred By: ASTM F3019/F3019M-19 - Standard Specification for Chromium Free Zinc-Flake Composite, with or without Integral Lubricant, Corrosion Protective Coatings for Fasteners
• Effective Date: 10-Dec-1996
• Referred By: ASTM G198-17 - Standard Test Method for Determining the Relative Corrosion Performance of Driven Fasteners in Contact with Treated Wood
• Effective Date: 10-Dec-1996
• Referred By: ASTM A865/A865M-06(2017) - Standard Specification for Threaded Couplings, Steel, Black or Zinc-Coated (Galvanized) Welded or Seamless, for Use in Steel Pipe Joints
• Effective Date: 10-Dec-1996
• Referred By: ASTM F1428-23 - Standard Specification for Aluminum Particle-Filled Basecoat/Organic or Inorganic Topcoat, Corrosion Protective Coatings for Fasteners
• Effective Date: 10-Dec-1996
• Referred By: ASTM F1043-18(2022) - Standard Specification for Strength and Protective Coatings on Steel Industrial Fence Framework
• Effective Date: 10-Dec-1996
• Referred By: ASTM A767/A767M-19 - Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement
• Effective Date: 10-Dec-1996
• Referred By: ASTM F2329/F2329M-15 - Standard Specification for Zinc Coating, Hot-Dip, Requirements for Application to Carbon and Alloy Steel Bolts, Screws, Washers, Nuts, and Special Threaded Fasteners
• Effective Date: 10-Dec-1996
• Referred By: ASTM E543-21 - Standard Specification for Agencies Performing Nondestructive Testing
• Effective Date: 10-Dec-1996

Standards Content (Sample)

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 Methods
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
Dry Film Thickness of Nonconductive Coatings Applied to
1.1 This practice covers the use of magnetic- and eddy-
a Nonferrous Metal Base
current-type thickness instruments (gages) for nondestructive
E 1316 Terminology for Nondestructive Examinations
thickness measurement of a coating on a metal substrate.
G 12 Test Method for Nondestructive Measurement of Film
1.2 More specific uses of these instruments are covered by
Thickness of Pipeline Coatings on Steel
the following test methods issued by ASTM: Test Methods
2.2 International Standard:
B 244, B 499, B 530, D 1186, D 1400, and G 12.
ISO 2178 Nonmagnetic Coatings on Magnetic Substrate—
1.3 The values stated in SI units are to be regarded as
Measurement of Coating Thickness—Magnetic
standard. The inch-pound units in parentheses are for informa-
Method
tion only and may be approximate.
1.4 Measurements made in accordance with this test method
NOTE 1—See Appendix X1.
will be in compliance with the requirements of ISO Interna-
3. Terminology
tional Standard 2178 as printed in 1982.
1.5 This standard does not purport to address all of the
3.1 Definitions—Definitions of terms relating to electro-
safety concerns, if any, associated with its use. It is the
magnetic testing are given in Terminology E 1316.
responsibility of the user of this standard to establish appro-
4. 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
and materials. The limitations of a particular instrument are
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
Instruments
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 Metals
a magnetic or nonmagnetic substrate.
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 Substrates
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 Base
1 4
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde- Annual Book of ASTM Standards, Vol 03.03.
structive Testing and is the direct responsibility of Subcommittee E07.07 on Annual Book of ASTM Standards, Vol 06.02.
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.
e1 7
published as E 376 – 89. Last previous edition E 376 – 89 (1994) Autocatalytically deposited nickel-phosphorus alloys containing more than 8 %
Annual Book of ASTM Standards, Vol 02.05. phosphorus are sufficiently nonmagnetic to be measured by this method, as long as
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.
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.
E376–96
5. Interferences 5.11 Cleanness of Probe and Test Surface—Measurements
are sensitive to foreign material that prevents intimate contact
5.1 Thickness of Coating—The precision of a measurement
between probe and coating surface.
changes with coating thickness depending on method used and
5.12 Pressure of Probe—Instrument readings can be sensi-
instrument design. Generally, the precision is a percentage of
tive to pressure with which probe is applied to test surface.
the coating thickness except at the lower end of the ranges
5.13 Probe Position—Some magnetic-type gages are sensi-
where it is a fixed thickness.
tive to position of probe relative to the earth. For example,
5.2 Magnetic Properties of Basis Metal—Magnetic thick-
operation of gage in a horizontal or upside-down position may
ness gages are affected by variations of the magnetic properties
require a new calibration or may be impossible.
of the basis metal. For practical purposes, magnetic variations
5.14 Temperature—Eddy-current instruments may be af-
in low-carbon 1005-1020 steel may be considered to be
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-
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
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
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
netic thickness gages for measurement of nonmagnetic coat-
the instrument is specifically calibrated for such a measure-
ings. Nonconductive plastic foils can be used to calibrate
1 1
ment. The effect usually extends 3 to 13 mm ( ⁄8 to ⁄2 in.) from
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
6.2.1.2 Resilient foils should not be used if there is possi-
diameter.
bility that the instrument probe will cause a change in thickness
5.6 Curvature of Test Surface—Thickness measurements
reading. Use of two or more foils on top of each other should
are sensitive to curvature of the specimen. This sensitivity
be avoided unless flexibility of thin foils is required for a
varies considerably between instruments and becomes more
curved surface.
pronounced with increasing curvature.
6.2.2 Coated Substrates Standards—Calibration standards
5.7 Smoothness of Surface, Including That of Basis Metal—
consist of coatings of known thickness permanently bonded to
Since a rough surface may make single measurements inaccu-
the substrate material.
rate, a greater number of measurements will provide an
6.3 Thicknesses of calibration standards should bracket and
average value that is more t
...