ASTM B499-09(2021)e1

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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Designation: B499 − 09 (Reapproved 2021) Endorsed by American
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 B499; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
ε NOTE—Editorial corrections were made throughout in May 2021.
1. Scope 1.6 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 This test method covers the use of magnetic instruments
ization established in the Decision on Principles for the
for the nondestructive measurement of the thickness of non-
Development of International Standards, Guides and Recom-
magnetic coatings over ferrous or other magnetic base metals.
mendations issued by the World Trade Organization Technical
It is intended to supplement manufacturers’instructions for the
Barriers to Trade (TBT) Committee.
operation of the instruments and is not intended to replace
them.
2. Referenced Documents
NOTE 1—Autocatalytically deposited nickel-phosphorus alloys contain-
2.1 ASTM Standards:
ing more than 8 % phosphorus are sufficiently nonmagnetic to be
B530 Test Method for Measurement of Coating Thicknesses
measured by this test method, as long as the measurement is made prior
by the Magnetic Method: Electrodeposited Nickel Coat-
to any heat treatment.
ings on Magnetic and Nonmagnetic Substrates
1.2 These instruments measure either the magnetic attrac-
2.2 International Standard:
tionbetweenamagnetandthebasismetal,asinfluencedbythe
ISO 2178 Non-Magnetic Coatings on Magnetic Substrate—
presence of the coating (categorized as “magnetic pull-off”), or
Measurement of Coating Thickness—Magnetic Method
the change in magnetic-flux density within the probe (catego-
rizedas“electronic”).Theseinstrumentscannotdistinguishthe
3. Terminology
thickness of individual layers. They can only measure the
cumulative thickness of all layers beneath the probe down to
3.1 Definitions of Terms Specific to This Standard:
the base metal.
3.1.1 accuracy, n—the measure of the magnitude of error
between the result of a measurement and the true thickness of
1.3 Measurementsmadeinaccordancewiththistestmethod
the item being measured.
will be in compliance with the requirements of ISO Interna-
tional Standard 2178. 3.1.2 adjustment, n—the physical act of aligning a instru-
ment’s thickness readings to match those of a known thickness
1.4 Units—The values stated in SI units are to be regarded
sample (removal of bias), in order to improve the accuracy of
as the standard. The values given in parentheses are for
the instrument on a specific surface or within a specific portion
information only.
of its measurement range. An adjustment will affect the
1.5 This standard does not purport to address all of the
outcome of subsequent readings.
safety concerns, if any, associated with its use. It is the
3.1.3 calibration, n—the high-level, controlled and docu-
responsibility of the user of this standard to establish appro-
mented process of obtaining measurements on traceable cali-
priate safety, health, and environmental practices and deter-
bration standards over the full operating range of the
mine the applicability of regulatory limitations prior to use.
1 2
ThistestmethodisunderthejurisdictionofASTMCommitteeB08onMetallic For referenced ASTM standards, visit the ASTM website, www.astm.org, or
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.10 on contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Test Methods. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved April 1, 2021. Published May 2021. Originally the ASTM website.
approvedin1969.Lastpreviouseditionapprovedin2014asB499 – 09(2014).DOI: Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
10.1520/B0499-09R21E01. 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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B499 − 09 (2021)
instrument, then making the necessary instrument adjustments 6.2 Coating Thickness Standards, with assigned values
(as required) to correct any out-of-tolerance conditions. traceable to a National Metrology Institution. They may be
3.1.3.1 Discussion—Calibration of coating thickness instru- coated or plated steel plates, or may be foils or shims of flat,
ments is performed by the equipment manufacturer, an autho- non-metallic sheet (typically polyester).
rized agent, or by an authorized, trained calibration laboratory
in a controlled environment using a documented process. The 7. Calibration and Standardization
outcome of the calibration process is to restore/realign the
7.1 Calibration of coating thickness instruments is per-
instrument to meet/exceed the manufacturer’s stated accuracy.
formed by the equipment manufacturer, an authorized agent, or
3.1.4 reference standard, n—aspecimenofknownthickness
by an authorized, trained calibration laboratory in a controlled
used to verify the accuracy of a coating thickness measuring
environment using a documented process. A Certificate of
instrument.
Calibration showing traceability to a National Metrology
Institution can be issued. There is no standard time interval for
3.1.5 verification of accuracy, n—obtaining measurements
re-calibration, nor is one absolutely required, but a calibration
on a reference standard prior to instrument use for the purpose
interval can be established based on experience and the work
ofdeterminingtheabilityofthecoatingthicknessinstrumentto
environment. A one-year calibration interval is a typical
produce reliable values, compared to the combined instrument
frequency suggested by many instrument manufacturers.
manufacturer’s stated accuracy and the stated accuracy of the
reference standard.
7.2 Before use, each instrument’s calibration accuracy shall
be verified in accordance with the instructions of the
4. Summary of Test Method
manufacturer, employing suitable thickness standards and, if
4.1 Magneticpull-offinstrumentsemployanattractionprin-
necessary, any deficiencies found shall be corrected.
ciple and a stationary magnetic field. These mechanical instru-
7.3 During use, calibration accuracy shall be verified at
ments measure the force required to pull a permanent magnet
frequent intervals, at least once a day.Attention shall be given
from a coated magnetic metal substrate. The magnetic force of
to the factors listed in Section 8 and to the procedures
attraction to the substrate beneath the coating is opposed by a
described in Section 9.
spring or coil. Tension is applied to the spring/coil until the
7.4 Coating thickness standards of known thickness are
magnetic attraction to the magnetic substrate is overcome. The
available either as shims, foils, or as coated specimens.
instrument must be placed directly on the coated surface to
7.4.1 Foils:
obtain a measurement. The force holding the permanent
magnet to the magnetic base is inversely proportional to the
NOTE 2—In the following paragraphs, the use of the word “foil’’ will
thickness of the coating layer(s) between the magnet and the
imply a nonmagnetic metallic or nonmetallic foil or shim.
magnetic base. For example, a thin coating applied to a ferrous
7.4.1.1 Because of the difficulty of ensuring adequate
substrate will require greater spring tension to pull the magnet
contact, foils are generally not recommended for the
off than will a thicker coating, since the magnet is closer to the
calibration, verification of accuracy, and adjustment of mag-
ferrous substrate with the thinner coating. This inverse rela-
netic pull-off instruments but they are suitable in some circum-
tionship is reflected on the nonlinear instrument scale.
stances provided the necessary precautions are taken.They can
4.2 Electronic instruments measure a change in magnetic
normally be used with other types of instruments.
flux density within the probe to produce a coating thickness
7.4.1.2 Foils are advantageous on curved surfaces and are
measurement.Theinstrumentprobemustbeplaceddirectly(in
more readily available than coated standards. To prevent
a perpendicular position) on the coated surface to obtain a
measurement errors, it is necessary to ensure that intimate
measurement. These instruments determine the effect on the
contactisestablishedbetweenfoilandsubstrate.Resilientfoils
magnetic field generated by the probe due to the proximity to
should be avoided to prevent indentation errors. Only nonfer-
the substrate.
rous metal foils should be used for thicknesses less than 15 µm
(0.6 mil). Foils are subject to wear and indentation and,
5. Significance and Use
therefore,shouldbereplacedfrequently.Wornfoilsshallnotbe
5.1 The thickness of a coating is often critical to its
used.
performance. For most nonferrous coatings on steel, the
7.4.2 Coated Standards— These calibration standards con-
magnetic method is reliable for measuring coating thickness
sist of nonconductive coatings of known, uniform thickness
nondestructively and is suitable for specification acceptance
permanently bonded to the substrate material.
testing and SPC/SQC applications.
7.4.3 The coating thickness of the standards used shall
bracket the user’s highest and lowest coating thickness mea-
5.2 This test method should not be used to determine the
surement requirement. Standards suitable for many applica-
thickness of electrodeposited nickel coatings on steel. Test
tionsofthetestmethodarecommerciallyavailableandmaybe
Method B530 is suitable for that determination.
used provided the certified values are traceable to a National
Metrology Institution.
6. Apparatus
6.1 Coating Thickness Instrument, based on magnetic 7.5 Insomecasesthecalibrationoftheinstrumentshouldbe
principles, commercially available, suitable to measure coating checked by rotating the probe in increments of 90° (see 8.1.8
thickness accurately. and 8.1.9).
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B499 − 09 (2021)
7.6 The basis-metal thickness for the test and the calibration scatter in measurements. Therefore, it is necessary, on a rough
adjustment shall be the same if the critical thickness, defined in or scratched surface, to make a greater number of measure-
8.1.3, is not exceeded. It is often possible to back up the basis ments at different positions to obtain an average value that is
metal of the standard or of the test specimen with a sufficient representative of the mean coating thickness. If the basis metal
thickness of similar material to make the readings independent
is rough, it may also be necessary to check, and adjust if
of the basis-metal thickness. necessary, the zero of the instrument at several positions on a
portion of the uncoated, rough, basis metal.
7.7 If the curvature of the coating to be measured is such as
8.1.7 Direction of Mechanical Working of the Basis Metal—
to preclude calibration adjustment on a flat surface, the
Measurements made by an instrument having a two-pole probe
curvature of the coated standard, or of the substrate on which
or an unevenly worn single-pole probe may be influenced by
the foil is placed, shall be the same.
the direction in which the magnetic basis metal has been
8. Factors Affecting the Measuring Accuracy
subjected to mechanical working (such as rolling), the reading
changing with the orientation of the probe on the surface.
8.1 The following factors affect the accuracy of a coating
thickness measurement:
8.1.8 Residual Magnetism—Residual magnetism in the ba-
8.1.1 Coating Thickness—Inherent to the test method is a sis metal affects the measurements made by instruments that
measurementuncertaintythat,forthincoatings,isconstantand
employ a stationary magnetic field. Its influence on measure-
independent of the coating thickness. The magnitude of this ments made by instruments employing an alternating magnetic
measurement uncertainty is primarily a function of test piece
field is much smaller.
surface finish (see 8.1.6 on surface roughness). For thicknesses
8.1.9 Stray Magnetic Fi
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