ASTM B244-97

Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments

SCOPE
1.1 This test method covers the use of eddy-current instruments for the nondestructive measurement of the thickness of a nonconductive coating on a nonmagnetic basis metal.  
1.2 This test method is particularly useful for measuring the thickness of an anodic coating on aluminum alloys. Chemical conversion coatings are too thin to be measured by this method.

General Information

• Status: Historical
• Publication Date: 09-Apr-1997
• Technical Committee: B08 - Metallic and Inorganic Coatings
• Drafting Committee: B08.10 - Test Methods

Relations

• Replaced By: ASTM B244-97(2002) - Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments
• Effective Date: 10-Apr-1997
• Referred By: ASTM E2338-22 - Standard Practice for Characterization of Coatings Using Conformable Eddy Current Sensors without Coating Reference Standards
• Effective Date: 10-Apr-1997
• Referred By: ASTM B177/B177M-11(2021) - Standard Guide for Engineering Chromium Electroplating
• Effective Date: 10-Apr-1997
• Referred By: ASTM B580-79(2019) - Standard Specification for Anodic Oxide Coatings on Aluminum
• Effective Date: 10-Apr-1997
• Referred By: ASTM E376-19 - Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy Current (Electromagnetic) Testing Methods
• Effective Date: 10-Apr-1997
• Referred By: ASTM B893-23 - Standard Specification for Hard-Coat Anodizing of Magnesium for Engineering Applications
• Effective Date: 10-Apr-1997
• Referred By: ASTM B659-90(2021) - Standard Guide for Measuring Thickness of Metallic and Inorganic Coatings
• Effective Date: 10-Apr-1997

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.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 244 – 97 An American National Standard
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 Anodic Coatings on
Aluminum and of Other Nonconductive Coatings on
Nonmagnetic Basis Metals with Eddy-Current Instruments
This standard is issued under the fixed designation B 244; 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 coil to basis metal spacing, can be calibrated to indicate the
thickness of coatings.
1.1 This test method covers the use of eddy-current instru-
ments for the nondestructive measurement of the thickness of
5. Factors Affecting the Measuring Accuracy
a nonconductive coating on a nonmagnetic basis metal.
5.1 Inherent in the test method is a measuring uncertainty
1.2 This test method is particularly useful for measuring the
that, for thin coatings, is constant and independent of the
thickness of an anodic coating on aluminum alloys. Chemical
coating thickness, and, for a single measurement, not less than
conversion coatings are too thin to be measured by this test
0.5 μm; for thicknesses greater than about 25 μm, this
method.
uncertainty is proportional to the coating thickness.
2. Referenced Documents 5.2 Electrical Properties of the Basis Metal—Eddy-current
measurements are affected by the electrical conductivity of the
2.1 ASTM Standards:
basis metal, which, itself, is often affected by heat treatments.
B 499 Test Method for Measurement of Coating Thick-
5.3 Basis-Metal Thickness—For each measurement, there is
nesses by the Magnetic Method: Nonmagnetic Coatings on
2 a critical thickness of the basis metal above which the
Magnetic Basis Metals
measurements will not be affected by an increase in that
3. Significance and Use thickness. Its value should be determined experimentally, if not
supplied by the manufacturer of the measuring instrument,
3.1 The thickness of a coating is often critical to its
since it depends on both the measuring frequency of the probe
performance. This eddy-current method is nondestructive and
system and the electrical conductivity of the basis metal.
is suitable for measuring the thickness of anodic coatings on
5.3.1 General Rule—For a given measuring frequency, the
aluminum, as well as the thickness of most nonconductive
higher the conductivity of the basis metal, the smaller its
coatings on nonmagnetic basis metals.
critical thickness. For a given basis metal, the higher the
3.2 This test method requires that the conductivity of the
measuring frequency, the smaller the critical thickness of the
substrate be the same in the calibration standard and in the
basis metal.
coated article to be measured.
5.4 Edge Effect—This method is sensitive to abrupt changes
4. Apparatus
in the surface contour of the test specimen. Therefore, mea-
surements made too near an edge or inside corner will not be
4.1 The coating thickness shall be determined with an
valid unless the instrument is specifically calibrated for such a
electromagnetic instrument that measures the changes in ap-
measurement.
parent impedance of the coil inducing the eddy currents into
the basis metal. The design of the instrument shall be such that
variations in apparent impedance, produced by variations in
Boonton Metal Film Gage—Type 255-A, Boonton Radio Corp., Boonton, NJ
07005 (This instrument is no longer manufactured, but many are still in use.)
Dermitron Thickness Tester, Unit Process Assemblies, Inc., 53-15 37th Ave.,
This method is under the jurisdiction of ASTM Committee B-8 on Metallic and
Woodside NY 11377
Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
Filmeter, American Instrument Co., Silver Spring, MD 20910
General Test Methods.
Elcotector—MK-II, Ferro Engineering Div. of Ferro Corp., 4150 East 56th St.,
Current edition approved April 10, 1997. Published March 1998. Originally Cleveland, OH 44105; also available from East Lancashire Chemical Co., Ltd.,
published as B 244 – 49 T. Last previous edition B 244 – 79 (1993). Fairfield, Manchester, England.
Annual Book of ASTM Standards, Vol 02.05. Accuratest Mark II, Coloral Products, Ltd., Steward St., Birmingham 18,
The following instruments have been found satisfactory for this purpose: England.
Permascope—Type EC-8-Ty, Twin City Testing Corp., P. O. Box 552, Isometer—Type 2–080, Institute, Dr. Forster, Reutlingen/Wurtt. Grathwohl-
Tonawanda, NY 14150 strasse 4, West Germany.
B 244
5.5 Curvature—Measurements are affected by the curvature similar material to make the readings independent of the basis
of the test specimen. The influence of curvature varies consid- metal thickness. A way to determine if the basis metal
erably with the make and type of instrument, but always thickness exceeds the critical thickness is to make measure-
becomes more pronounced as the radius of curvature de- ments before and after backing up the basis metal with copper
creases. or aluminum at least 3 mm thick. If there is no difference
5.6 Surface Roughness: between the readings, the critical thickness is exceeded.
5.6.1 Measurements are influenced by the surface topogra- 6.5 If the test specimen is soft and thin, it is subject to
phy of the substrate and the coating, and a rough surface will indentation by the probe. Because of this, and despite the use
give individual instrument readings that will vary from point to of special probes or fixtures, measurements on such specimens
point. In this case, it is necessary to make many readings at are sometimes impossible to make.
different pos
...