ASTM B244-97(2002)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Endorsed by American
Designation:B244–97 (Reapproved 2002) Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
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 parent impedance of the coil inducing the eddy currents into
thebasismetal. Thedesignoftheinstrumentshallbesuchthat
1.1 This test method covers the use of eddy-current instru-
variations in apparent impedance, produced by variations in
ments for the nondestructive measurement of the thickness of
coil to basis metal spacing, can be calibrated to indicate the
a nonconductive coating on a nonmagnetic basis metal.
thickness of coatings.
1.2 This test method is particularly useful for measuring the
thickness of an anodic coating on aluminum alloys. Chemical
5. Factors Affecting the Measuring Accuracy
conversion coatings are too thin to be measured by this test
5.1 Inherent in the test method is a measuring uncertainty
method.
that, for thin coatings, is constant and independent of the
2. Referenced Documents coating thickness, and, for a single measurement, not less than
0.5 µm; for thicknesses greater than about 25 µm, this
2.1 ASTM Standards:
uncertainty is proportional to the coating thickness.
B 499 Test Method for Measurement of Coating Thick-
5.2 Electrical Properties of the Basis Metal—Eddy-current
nessesbytheMagneticMethod:NonmagneticCoatingson
measurements are affected by the electrical conductivity of the
Magnetic Basis Metals
basis metal, which itself is often affected by heat treatments.
3. Significance and Use
5.3 Basis-Metal Thickness—For each measurement, there is
a critical thickness of the basis metal above which the
3.1 The thickness of a coating is often critical to its
measurements will not be affected by an increase in that
performance. This eddy-current method is nondestructive and
thickness.Itsvalueshouldbedeterminedexperimentally,ifnot
is suitable for measuring the thickness of anodic coatings on
supplied by the manufacturer of the measuring instrument,
aluminum, as well as the thickness of most nonconductive
since it depends on both the measuring frequency of the probe
coatings on nonmagnetic basis metals.
system and the electrical conductivity of the basis metal.
3.2 This test method requires that the conductivity of the
substrate be the same in the calibration standard and in the
coated article to be measured.
4. Apparatus
The following instruments have been found satisfactory for this purpose:
Permascope—Type EC-8-Ty, Twin City Testing Corp., P.O. Box 552,
4.1 The coating thickness shall be determined with an
Tonawanda, NY 14150
electromagnetic instrument that measures the changes in ap-
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.,
ThismethodisunderthejurisdictionofASTMCommitteeB08onMetallicand 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.,
approved in 1949. Last previous edition approved in 1993 as B 244 – 79 (1993). Fairfield, Manchester, England.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Accuratest Mark II, Coloral Products, Ltd., Steward St., Birmingham 18,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM England.
Standards volume information, refer to the standard’s Document Summary page on Isometer—Type 2–080, Institute, Dr. Forster, Reutlingen/Wurtt. Grathwohl-
the ASTM website. strasse 4, West Germany.
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B244–97 (2002)
5.3.1 General Rule—For a given measuring frequency, the 6.2.2 Coated Standards—These calibration standards con-
higher the conductivity of the basis metal, the smaller its sist of nonconductive coatings of known, uniform thickness
critical thickness. For a given basis metal, the higher the permanently bonded to the substrate material.
measuring frequency, the smaller the critical thickness of the
6.3 The basis metal of the calibration standards shall have
basis metal.
electrical properties similar to those of the basis metal of the
5.4 EdgeEffect—This method is sensitive to abrupt changes
coated test specimen.To confirm their suitability, a comparison
in the surface contour of the test specimen. Therefore, mea-
of the readings obtained with the basis metal of the bare
surements made too near an edge or inside corner will not be
standard and that of the test specimen is recommended.
valid unless the instrument is specifically calibrated for such a
6.4 The basis metal thickness for the test and the calibration
measurement.
shall be the same if the critical thickness, defined in 4.3, is not
5.5 Curvature—Measurements are affected by the curvature
exceeded. When possible, back up the basis metal of the
of the test specimen. The influence of curvature varies consid-
standard or of the test specimen with a sufficient thickness of
erably with the make and type of instrument, but always
similar material to make the readings independent of the basis
becomes more pronounced as the radius of curvature de-
metal thickness. A way to determine if the basis metal
creases.
thickness exceeds the critical thickness is to make measure-
5.6 Surface Roughness:
ments before and af
...