ASTM E2338-04
(Practice)Standard Practice for Characterization of Coatings Using Conformable Eddy-Current Sensors without Coating Reference Standards
Standard Practice for Characterization of Coatings Using Conformable Eddy-Current Sensors without Coating Reference Standards
SCOPE
1.1 This practice covers the use of conformable eddy-current sensors for nondestructive characterization of coatings without standardization on coated reference parts. It includes the following: (1) thickness measurement of a conductive coating on a conductive substrate, (2) detection and characterization of local regions of increased porosity of a conductive coating, and (3) measurement of thickness for nonconductive coatings on a conductive substrate or on a conductive coating. This practice includes only nonmagnetic coatings on either magnetic μ = μ 0) or nonmagnetic (μ = μ0) substrates. This practice can also be used to measure the effective thickness of a process-affected zone (for example, shot peened layer for aluminum alloys, alpha case for titanium alloys). For specific types of coated parts, the user may need a more specific procedure tailored to a specific application.
1.2 Specific uses of conventional eddy-current sensors are covered by the following test methods issued by ASTM: Test Methods B 244, D 1186, D 1400, E 376, E 1004, and G 12.
1.3 The values stated in SI units are to be regarded as standard. The inch-pound units are provided for information.
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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Designation:E2338–04
Standard Practice for
Characterization of Coatings Using Conformable Eddy-
Current Sensors without Coating Reference Standards
This standard is issued under the fixed designation E 2338; 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.
1. Scope D 1186 Test Methods for Nondestructive Measurement of
Dry Film Thickness of Nonmagnetic Coatings Applied to
1.1 This practice covers the use of conformable eddy-
a Ferrous Base
current sensors for nondestructive characterization of coatings
D 1400 Test Method for Nondestructive Measurement of
without standardization on coated reference parts. It includes
DryFilmThicknessofNonconductiveCoatingsAppliedto
the following: (1) thickness measurement of a conductive
a Nonferrous Metal Base
coating on a conductive substrate, (2) detection and character-
E 376 Practice for Measuring Coating Thickness by
ization of local regions of increased porosity of a conductive
Magnetic-Field or Eddy-Current Electromagnetic Methods
coating, and (3) measurement of thickness for nonconductive
E 543 Practice for Agencies Performing Nondestructive
coatings on a conductive substrate or on a conductive coating.
Testing
This practice includes only nonmagnetic coatings on either
E 1004 Test Method for Electromagnetic (Eddy-Current)
magnetic (µ fi µ ) or nonmagnetic (µ = µ ) substrates. This
0 0
Measurements of Electrical Conductivity
practice can also be used to measure the effective thickness of
E 1316 Terminology for Nondestructive Examinations
a process-affected zone (for example, shot peened layer for
G 12 Test Method for Nondestructive Measurement of Film
aluminum alloys, alpha case for titanium alloys). For specific
Thickness of Pipeline Coatings on Steel
types of coated parts, the user may need a more specific
2.2 ASNT Documents:
procedure tailored to a specific application.
SNT-TC-1A Recommended Practice for Personnel Qualifi-
1.2 Specific uses of conventional eddy-current sensors are
cation and Certification In Nondestructive Testing
covered by the following test methods issued by ASTM: Test
ANSI/ASNT-CP-189 Standard for Qualification and Certi-
Methods B 244, D 1186, D 1400, E 376, E 1004, and G 12.
fication of NDT Personnel
1.3 The values stated in SI units are to be regarded as
2.3 AIA Standard:
standard. The inch-pound units are provided for information.
NAS 410 Certification and Qualification of Nondestructive
1.4 This standard does not purport to address all of the
Testing Personnel
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
NOTE 1—See Appendix X1.
priate safety and health practices and determine the applica-
3. Terminology
bility of regulatory limitations prior to use.
3.1 Definitions—Definitionsoftermsrelatingtoelectromag-
2. Referenced Documents
netic examination are given in Terminology E 1316. The
2.1 ASTM Standards:
following definitions are specific to the conformable sensors:
B 244 Test Method for Measurement of Thickness of An-
3.1.1 conformable—refers to an ability of sensors or sensor
odic Coatings on Aluminum and of Other Nonconductive
arraystoconformtononplanarsurfaceswithoutanysignificant
CoatingsonNonmagneticBasisMetalswithEddy-Current
effects on the measurement results.
Instruments
3.1.2 lift-off—normal distance from the conformable sensor
windingplanetothetopofthefirstconductinglayerofthepart
under examination.
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.07 on
Electromagnetic Methods.
Current edition approved January 1, 2004. Published February 2004.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from The American Society for Nondestructive Testing 1711 Arlin-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM gate Lane, PO Box 28518, Columbus, OH 43228-0518.
Standards volume information, refer to the standard’s Document Summary page on AvailablefromAerospaceIndustriesAssociationofAmerica,Inc.,1250EyeSt.
the ASTM website. NW, Washington, D.C. 20005. (Replacement standard for MIL-STD-410.)
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2338–04
3.1.3 model for sensor response—a relation between the 3.1.11 process-affected zone—aregionnearthesurfacewith
response of the sensor (for example, transimpedance magni- depth less than the half wavelength that can be represented by
a conductivity that is different than that of the base material,
tude and phase or real and imaginary parts) to properties of
that is, substrate.
interest, for example, electrical conductivity, magnetic perme-
3.1.12 sensor footprint—area of the sensor face placed
ability, lift-off, and conductive coating thickness, etc. These
against the material under examination.
model responses may be obtained from database tables and
may be analysis-based or empirical.
4. Significance and Use
3.1.4 depth of sensitivity—depth to which sensor response
4.1 Conformable Eddy-Current Sensors—Conformable,
to features or properties of interest, for example, coating
eddy-current sensors can be used on both flat and curved
thickness variations, exceeds a noise threshold.
surfaces, including fillets, cylindrical surfaces, etc. When used
3.1.5 spatial half-wavelength—spacing between the center
with models for predicting the sensor response and appropriate
ofadjacentprimary(drive)windingsegmentswithcurrentflow
algorithms, these sensors can measure variations in physical
in opposite directions; this spacing affects the depth of sensi-
properties, such as electrical conductivity and/or magnetic
tivity. Spatial wavelength equals two times this spacing. A
permeability, as well as thickness of conductive coatings on
single turn conformable circular coil has an approximate
any substrate and nonconductive coatings on conductive sub-
spatial wavelength of twice the coil diameter.
strates or on a conducting coating. These property variations
3.1.6 insulating shims—conformable insulating foils used
can be used to detect and characterize heterogeneous regions
to measure effects of small lift-off excursions on sensor
within the conductive coatings, for example, regions of locally
response. higher porosity.
4.2 Sensors and Sensor Arrays—Depending on the applica-
3.1.7 air standardization—an adjustment of the instrument
tion, either a single-sensing element sensor or a sensor array
with the sensor in air, that is, at least one spatial wavelength
can be used for coating characterization.Asensor array would
away from any conductive or magnetic objects, to match the
provide a better capability to map spatial variations in coating
model for the sensor response. Measurements on conductive
thickness and/or conductivity (reflecting, for example, porosity
materials after air standardization should provide absolute
variations) and provide better throughput for scanning large
electricalpropertiesandlift-offvalues.Theperformancecanbe
areas. The size of the sensor footprint and the size and number
verified on certified reference standards over the frequency
of sensing elements within an array depend on the application
range of interest.
requirements and constraints, and the nonconductive (for
3.1.8 reference substrate standardization—an adjustment of
example, ceramic) coating thickness.
the instrument to an appropriate reference substrate standard.
4.3 Coating Thickness Range—The conductive coating
The adjustment is to remove offsets between the model for the
thickness range over which a sensor performs best depends on
sensor response and at least two reference substrate measure-
the difference between the electrical conductivity of the sub-
ments (for example, two measurements with different lift-offs
strate and conductive coating and available frequency range.
at the same position on the standard). These standards should
For example, a specific sensor geometry with a specific
haveaknownelectricalconductivitythatisessentiallyuniform
frequency range for impedance measurements may provide
with depth and should have essentially the same electrical
acceptable performance for an MCrAlY coating over a nickel-
conductivity and magnetic permeability as the substrate in the alloy substrate for a relatively wide range of conductive
components being characterized. coating thickness, for example, from 75 to 400 µm [0.003 to
0.016 in.]. Yet, for another conductive coating-substrate com-
3.1.9 performance verification, uncoated part—a measure-
bination, this range may be 10 to 100 µm [0.0004 to 0.004 in.].
ment of electrical conductivity performed on a reference part
The coating characterization performance may also depend on
with known properties to confirm that the electrical conduc-
the thickness of a nonconductive topcoat. For any coating
tivityvariationwithfrequencyiswithinspecifiedtolerancesfor
system, performance verification on representative coated
theapplication.Whenareferencestandardizationisperformed,
specimens is critical to establishing the range of optimum
reference parts used for standardization should not be used for
performance. For nonconductive, for example, ceramic, coat-
performance verification. These variations should be docu-
ings the thickness measurement range increases with an
mented in the report (see Section 9). Performance verification
increase of the spatial wavelength of the sensor (for example,
is a quality control procedure recommended prior to or during
thicker coatings can be measured with larger sensor winding
measurements after standardization.
spatial wavelength). For nonconductive coatings, when rough-
3.1.10 performance verification, coated part—a measure-
ness of the coating may have a significant effect on the
ment of coating electrical conductivity and/or thickness on a
thickness measurement, independent measurements of the
coated reference part with known properties to confirm that the
nonconductive coating roughness, for example, by profilom-
coating electrical conductivity and/or thickness are within
etry may provide a correction for the roughness effects.
specified tolerances for the application. Performance verifica-
4.4 Process-Affected Zone—For some processes, for ex-
tion is a quality control procedure that does not represent
ample, shot peening, the process-affected zone can be repre-
standardization and should be documented in the report (see
sented by an effective layer thickness and conductivity. These
Section 9). values can in turn be used to assess process quality. A strong
E2338–04
correlation must be demonstrated between these “effective corner may not be valid or may be insufficiently accurate
coating” properties and process quality. unless the instrument is used with a procedure that specifically
4.5 Three-Unknown Algorithm—Use of multi-frequency
addresses such a measurement. Edge-effect correction proce-
impedance measurements and a three-unknown algorithm
dures must either account for edge effects in the property
permits independent determination of three unknowns: (1)
estimation algorithm (for example, in the sensor response
thicknessofconductivenonmagneticcoatings, (2)conductivity
model) or incorporate careful standardization on reference
of conductive nonmagnetic coatings, and (3) lift-off that
parts with fixtures to control sensor position relative to the
provides a measure of the nonconductive coating thickness.
edge.
5.6 Curvature of Examination Surface—For surfaces with a
5. Interferences
single radius of curvature (for example, cylindrical or conical),
5.1 Thickness of Coating—The precision of a measurement
the radius of curvature should be large compared to the sensor
can change with coating thickness. The thickness of a coating
half-wavelength. In the case of a double curvature, at least one
should be less than the maximum depth of sensitivity. Ideally,
of the radii should significantly exceed the sensor footprint and
the depth of sensitivity at the highest frequency should be less
the other radius should be at least comparable to the sensor
than the conductive coating thickness, while the depth of
footprint, unless customized sensors are designed to match the
sensitivity at the lowest frequency should be significantly
double curvature. Performance verification tests should be run
greater than the conductive coating thickness. The number of
to verify lift-off sensitivity using insulating shims.
frequencies used in the selected frequency range should be
sufficient to provide a reliable representation of the frequency- 5.7 Instrument Stability—Drift and noise in the instrumen-
response shape. tation can cause inaccuracies in the measurement. Restandard-
5.2 Thickness of Substrate—The thickness of the substrate ization and performance verifications on at least one uncoated
should be larger than the depth of sensitivity at the lowest and one to two coated reference parts should be performed as
frequency. Otherwise, this thickness must be known and
needed to maintain required performance levels.
accounted for in the model for the sensor response.
5.8 Surface Roughness Including That of Base Metal—
5.3 Magnetic Permeability and Electrical Conductivity of
Since a rough surface may make single measurements inaccu-
Base Metal (Substrate)—The magnetic permeability and elec-
rate, a greater number of measurements will provide an
trical conductivity of the substrate can affect the measurement
average value that is more truly representative of the overall
and must be known prior to coating characterization unless
coating thickness. These repeat measurements should be per-
they can be determined independently on a coated part. When
formed in a “pick-and-place” mode, completely removing the
the substrate properties vary spatially, this variation must be
sensor from the surface between measurements. Coating sur-
determined as part of the coating characterization on a non-
face roughness also may result in overestimated ceramic layer
coated part that preferably has the same thermal history as the
thickness or any other nonconducting coating thickness since
coated parts. Original uncoated parts may have significantly
the probe may rest on peaks.
different microstructure than heat treated coated substrates.
5.9 Directionality of Base-Metal Properties—
Uncoated colder regions of otherwise coated parts may have
Measurementsmaybesensitivetoanisotropyofthebasemetal
different properties than the
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