ASTM B499-96(2002)
(Test Method)Standard Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals
Standard Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals
SIGNIFICANCE AND USE
The thickness of a coating is often critical to its performance. For most nonferrous coatings on steel, the magnetic method is reliable for measuring coating thickness nondestructively and is suitable for specification acceptance testing and SPC/SQC applications. The test method requires that the magnetic properties of the substrate used during the calibration be the same as that of the test specimen.
SCOPE
1.1 This test method covers the use of magnetic instruments for the nondestructive measurement of the thickness of nonmagnetic coatings over ferrous or other magnetic base metals.
Note 1—Autocatalytically deposited nickel-phosphorus alloys containing more than 8 % phosphorus are sufficiently nonmagnetic to be measured by this test method, as long as the measurement is made prior to any heat treatment.
1.2 These instruments measure either the magnetic attraction between a magnet and the basis metal, as influenced by the presence of the coating, or the reluctance of a magnetic-flux path passing through the coating and the basis metal.
1.3 Measurements made in accordance with this test method will be in compliance with the requirements of ISO International Standard 2178 as printed in 1982.
1.4 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.
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Endorsed by American
Designation:B499–96 (Reapproved 2002) Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
Standard Test Method for
Measurement of Coating Thicknesses by the Magnetic
Method: Nonmagnetic Coatings on Magnetic Basis Metals
This standard is issued under the fixed designation B 499; 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 testing and SPC/SQC applications. The test method requires
that the magnetic properties of the substrate used during the
1.1 This test method covers the use of magnetic instruments
calibration be the same as that of the test specimen.
for the nondestructive measurement of the thickness of non-
magnetic coatings over ferrous or other magnetic base metals.
4. Factors Affecting the Measuring Accuracy
NOTE 1—Autocatalyticallydepositednickel-phosphorusalloyscontain-
4.1 The following factors affect the accuracy of a coating
ing more than 8 % phosphorus are sufficiently nonmagnetic to be
thickness measurement:
measured by this test method, as long as the measurement is made prior
4.1.1 Coating Thickness—Inherent to the test method is a
to any heat treatment.
measurementuncertaintythat,forthincoatings,isconstantand
1.2 These instruments measure either the magnetic attrac-
independent of the coating thickness. The magnitude of this
tionbetweenamagnetandthebasismetal,asinfluencedbythe
measurement uncertainty is primarily a function of test piece
presence of the coating, or the reluctance of a magnetic-flux
surface finish (see 4.1.6 on surface roughness). For thicknesses
path passing through the coating and the basis metal.
greater than about 25µ m, this uncertainty is proportional to the
1.3 Measurementsmadeinaccordancewiththistestmethod
coating thickness.
will be in compliance with the requirements of ISO Interna-
4.1.2 Magnetic Properties of the Basis Metal—Magnetic
tional Standard 2178 as printed in 1982.
thickness measurements are affected by variations in the
1.4 This standard does not purport to address all of the
magnetic properties of the basis metal. (For practical purposes,
safety concerns, if any, associated with its use. It is the
magnetic variations in low-carbon steel can often be consid-
responsibility of the user of this standard to establish appro-
ered to be insignificant. To avoid the influences of severe or
priate safety and health practices and determine the applica-
localized heat treatments and cold working, the instrument
bility of regulatory limitations prior to use.
should be calibrated using a calibration standard having a basis
metal with the same magnetic properties as that of the test
2. Referenced Documents
specimen or, preferably and if available, with a sample of the
2.1 International Standard:
part to be tested before application of the coating.)
ISO 2178 Non-Magnetic Coatings on Magnetic Substrate—
4.1.3 Basis Metal Thickness—For each instrument, there is
Measurement of Coating Thickness—Magnetic Method
a critical thickness of the basis metal above which the
measurements will not be affected by an increase in the
3. Significance and Use
thickness of the basis metal. Since it depends on the instrument
3.1 The thickness of a coating is often critical to its
probe (Note 2) and the nature of the basis metal, its value
performance. For most nonferrous coatings on steel, the
should be determined experimentally if not supplied by the
magnetic method is reliable for measuring coating thickness
manufacturer.
nondestructively and is suitable for specification acceptance
NOTE 2—In this method “instrument probe” will also include the term
“magnet.”
This test method is under the jurisdiction ofASTM Committee B08 on Metallic
4.1.4 Edge Effects—The method is sensitive to abrupt
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on
changes in the surface contour of the test specimen. Therefore,
General Test Methods.
Current edition approved April 10, 1996. Published June 1996. Originally
measurements made too near an edge or inside corner will not
published as B 499 – 69. Last previous edition B 499 – 88. Replaces portions of
bevalidunlesstheinstrumentisspecificallycalibratedforsuch
A 219.
2 a measurement. The effect may extend to about 20 mm from
Available from American National Standards Institute, 11 W. 42nd St., 13th
Floor, New York, NY 10036. the discontinuity, depending on the instrument.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B499
4.1.5 Curvature—The measurements are affected by the 5. Calibration of Instruments
curvature of the test specimen. The influence of curvature
5.1 Before use, each instrument shall be calibrated in
varies considerably with the make and type of instrument but
accordance with the instructions of the manufacturer, employ-
always becomes more pronounced as the radius of curvature
ing suitable thickness standards.
decreases. Instruments with two-pole probes may also produce 5.2 During use, the calibration shall be checked at frequent
different readings if the poles are aligned in planes parallel or intervals, at least once a day. Attention shall be given to the
factors listed in Section 4 and to the procedures described in
perpendicular to the axis of a cylindrical surface. A similar
Section 5.
effect can occur with a single-pole probe if the tip is unevenly
5.3 Calibration standards of known thickness are available
worn.
either as shims or foils or as coated specimens.
4.1.6 Surface Roughness—Measurements are influenced by
5.3.1 Calibration Foils:
the surface topography of the basis metal and coating. Surface
roughnessbecomessignificantwhenthedegreeofroughnessis NOTE 3—In the following paragraphs, the use of the word “foil’’ will
imply a nonmagnetic metallic or nonmetallic foil or shim.
greater than 10 % of the coating thickness, causing increased
scatter in measurements. Therefore, it is necessary, on a rough 5.3.1.1 Because of the difficulty of ensuring adequate con-
or scratched surface, to make a greater number of measure- tact, foils are generally not recommended for the calibration of
instruments based on the principle of magnetic attraction but
ments at different positions to obtain an average value that is
they are suitable in some circumstances provided the necessary
representative of the mean coating thickness. If the basis metal
precautions are taken. They can normally be used for the
is rough, it may also be necessary to check the zero of the
calibration of other types of instruments.
instrument at several positions on a portion of the uncoated,
5.3.1.2 Foils are advantageous for calibration on curved
rough, basis metal.
surfaces and are more readily available than coated standards.
4.1.7 DirectionofMechanicalWorkingoftheBasisMetal—
To prevent measurement errors it is necessary to ensure that
Measurements made by an instrument having a two-pole probe
intimate contact is established between foil and substrate.
or an unevenly worn single-pole probe may be influenced by
Resilient foils should be avoided to prevent indentation errors.
the direction in which the magnetic basis metal has been
Only nonferrous metal foils should be used for thicknesses less
subjected to mechanical working (such as rolling), the reading
than 15 µm. Calibration foils are subject to wear and indenta-
changing with the orientation of the probe on the surface.
tion and, therefore, should be replaced frequently. Worn foils
4.1.8 Residual Magnetism—Residual magnetism in the ba-
shall not be used to calibrate the instrument.
sis metal affects the measurements made by instruments which
5.3.2 Coated Standards—These calibration standards con-
employ a stationary magnetic field. Its influence on measure- sist of coatings of known, uniform thickness permanently
ments made by reluctance instruments employing an alternat- bonded to the substrate material.
5.4 The basis metal of the calibration standards shall have
ing magnetic field is much smaller.
magnetic properties similar to those of the basis metal of the
4.1.9 Stray Magnetic Fields—Strong stray magnetic fields,
coated test specimen.To confirm their suitability, a comparison
such as are produced by various types of electrical equipment,
of the readings obtained with the basis metal of the bare
canseriouslyinterferewiththeoperationofmagneticthickness
standard and that of the test specimen is recommended.
instruments.
5.5 Insomecasesthecalibrationoftheinstrumentshouldbe
4.1.10 Foreign Particles—Magnetic instruments of all
checked by rotating the probe in increments of 90° (see 4.1.7
types must make physical contact with the test surface and are,
and 4.1.8).
therefore, sensitive to foreign material that prevents intimate
5.6 The basis-metal thickness for the test and the calibration
contact between probe and coating surface. Both the test
shall be the same if the critical thickness, defined in 4.1.3, is
surface and instrument probe should be kept free of foreign
not exceeded. It is often possible to back up the basis metal of
material.
the standard or of the test specimen with a sufficient thickness
of simi
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