ASTM E2338-22

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2338 − 17 E2338 − 22
Standard Practice for
Characterization of Coatings Using Conformable Eddy
Current Sensors without Coating Reference Standards
This standard is issued under the fixed designation E2338; 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.
1. 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 ) or nonmagnetic (μ = μ ) substrates. This In addition to discrete coatings
0 0
on 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). alloys) and to assess the condition of other layered media such as joints
(for example, lap joints and skin panels over structural supports). 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 Practices D7091 and E376 and the following test methods
issued by ASTM: B244 and E1004. Guidance for the use of conformable eddy current sensor arrays is provided in Guide E2884.
1.3 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical
conversions to inch-pound units that are provided for information only and are not considered standard.
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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 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.
2. Referenced Documents
2.1 ASTM Standards:
B244 Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on
Nonmagnetic Basis Metals with Eddy-Current Instruments
D7091 Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals
and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals
This practice is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.07 on Electromagnetic
Method.
Current edition approved Nov. 1, 2017June 1, 2022. Published November 2017June 2022. Originally approved in 2004. Last previous edition approved in 20112017 as
E2338 - 11.E2338 – 17. DOI: 10.1520/E2338-17.10.1520/E2338-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2338 − 22
E376 Practice for Measuring Coating Thickness by Magnetic-Field or Eddy Current (Electromagnetic) Testing Methods
E543 Specification for Agencies Performing Nondestructive Testing
E1004 Test Method for Determining Electrical Conductivity Using the Electromagnetic (Eddy Current) Method
E1316 Terminology for Nondestructive Examinations
E2884 Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays
NOTE 1—See Appendix X1.
2.2 ASNT Documents:
SNT-TC-1A Recommended Practice for Personnel Qualification and Certification In Nondestructive Testing
ANSI/ASNT-CP-189 Standard for Qualification and Certification of NDT Personnel
2.3 AIA Standard:
NAS 410 Certification and Qualification of Nondestructive Testing Personnel
NOTE 1—See Appendix X1.
2.4 ISO Standards:
ISO 9712 Non-destructive Testing—Qualification and Certification of NDT Personnel
3. Terminology
3.1 Definitions—For definitions of terms relating to this practice, refer to Terminology E1316. The following definitions are
specific to the conformable sensors:, including Section C on Electromagnetic Testing, and also Guide
3.1.1 conformable—refers to an ability of sensors or sensor arrays to conform to nonplanar surfaces without any significant effects
on the measurement results.
3.1.2 lift-off—normal distance from the conformable sensor winding plane to the top of the first conducting layer of the part under
examination.E2884
3.1.3 model for sensor response—a relation between the response of the sensor (for example, transimpedance magnitude and phase
or real and imaginary parts) to properties of interest, for example, electrical conductivity, magnetic permeability, lift-off, and
conductive coating thickness, etc. These model responses may be obtained from database tables and may be analysis-based or
empirical.
3.1.4 depth of sensitivity—depth to which sensor response to features or properties of interest, for example, coating thickness
variations, exceeds a noise threshold.
3.1.5 spatial half-wavelength—spacing between the center of adjacent primary (drive) winding segments with current flow in
opposite directions; this spacing affects the depth of sensitivity. Spatial wavelength equals two times this spacing. A single turn
conformable circular coil has an approximate spatial wavelength of twice the coil diameter.
3.1.6 insulating shims—conformable insulating foils used to measure effects of small lift-off excursions on sensor response.
3.1.1 air standardization—standardization, n—an adjustment of the instrument with the sensor in air, that is, at least one spatial
wavelength away from any conductive or magnetic objects, to match theresponse to air or another insulating material to match a
model for the sensor response. Measurements on conductive materials after air standardization should provide absolute electrical
properties and lift-off values. The performance can be verified on certified reference standards over the frequency range of
interest.response in air.
3.1.1.1 Discussion—
It is generally sufficient for the sensor to be placed at least one spatial wavelength away from any conductive or magnetic object
to provide the equivalent of response in air alone. Measurements on conductive materials after air standardization should provide
absolute electrical properties and lift-off values. The performance can be verified on certified reference standards over the
frequency range of interest.
3.1.2 reference substrate standardization—performance verification, coated part, n—an adjustment of the instrument to an
appropriate reference substrate standard. The adjustment is to remove offsets between the model for the sensor response and at least
two reference substrate measurements (for example, two measurements with different lift-offs at the same position on the standard).
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These standards should have a known electrical conductivity that is essentially uniform with depth and should have essentially the
same electrical conductivity and magnetic permeability as the substrate in the components being characterized.a measurement of
coating electrical conductivity or thickness, or both, on a coated reference part with known properties to confirm that the coating
electrical conductivity or thickness, or both, are within specified tolerances for the application.
3.1.2.1 Discussion—
Performance verification is a quality control procedure that does not represent standardization and should be documented in the
report (see Section 9).
3.1.3 performance verification, uncoated part—part, n—a measurement of electrical conductivity performed on a reference part
with known properties to confirm that the electrical conductivity variation with frequency is within specified tolerances for the
application. When a reference standardization is performed, reference parts used for standardization should not be used for
performance verification. These variations should be documented in the report (see Section 9). Performance verification is a quality
control procedure recommended prior to or during measurements after standardization.
3.1.3.1 Discussion—
When a reference standardization is performed, reference parts used for standardization should not be used for performance
verification. These variations should be documented in the report (see Section 9). Performance verification is a quality control
procedure recommended prior to or during measurements after standardization.
3.1.10 performance verification, coated part—a measurement of coating electrical conductivity and/or thickness on a coated
reference part with known properties to confirm that the coating electrical conductivity and/or thickness are within specified
tolerances for the application. Performance verification is a quality control procedure that does not represent standardization and
should be documented in the report (see Section 9).
3.1.4 process-affected zone—zone, n—a region near the surface with depth less than the a spatial half wavelength that can be
represented by a conductivity that is different than that the conductivity of the base material, that is, substrate.material.
3.1.4.1 Discussion—
In some cases, the process affected zone near the surface of a material can be modeled as a coating on a substrate.
3.1.5 reference substrate standardization, n—an adjustment of the instrument response to an appropriate reference substrate
standard.
3.1.5.1 Discussion—
The adjustment is to remove offsets between the model for the sensor response and at least two reference substrate measurements
(for example, two measurements with different lift-offs at the same position on the standard). These standards should have a known
electrical conductivity that is essentially uniform with depth and should have essentially the same electrical conductivity and
magnetic permeability as the substrate in the components being characterized.
3.1.6 sensor footprint—footprint, n—area of the sensor face placed against the material under examination.
4. Significance and Use
4.1 Conformable Eddy Current Sensors—Conformable, eddy current sensors can be used on both flat and curved surfaces,
including fillets, cylindrical surfaces, etc. When used with models for predicting the sensor response and appropriate algorithms,
these sensors can measure variations in physical properties, such as electrical conductivity and/oror magnetic permeability, or both,
as well as thickness of conductive coatings on any substrate and nonconductive coatings on conductive substrates or on a
conducting coating. These property variations can be used to detect and characterize heterogeneous regions within the conductive
coatings, for example, regions of locally higher porosity.
4.2 Sensors and Sensor Arrays—Depending on the application, either a single-sensing element sensor or a sensor array can be used
for coating characterization. A sensor array would provide provides a better capability to map spatial variations in coating thickness
and/or conductivity or conductivity, or both (reflecting, for example, porosity variations)variations), and provideprovides better
throughput for scanning large areas. The size of the sensor footprint and the size and number of sensing elements within an array
depend on the application requirements and constraints, and the nonconductive (for example, ceramic) coating thickness.
4.3 Coating Thickness Range—The conductive coating thickness range over which a sensor performs best depends on the
difference between the electrical conductivity of the substrate and conductive coating and available frequency range. For example,
a specific sensor geometry with a specific frequency range for impedance measurements may provide acceptable performance for
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an MCrAlY coating over a nickel-alloy substrate for a relatively wide range of conductive coating thickness, for example, from
75 to 400 μm (0.003 to 0.016 in.). Yet, for another conductive coating-substrate combination, this range may be 10 to 100 μm
(0.0004 to 0.004 in.). The coating characterization performance may also depend on the thickness of a nonconductive topcoat. For
any coating system, performance verification on representative coated specimens is critical to establishing the range of optimum
performance. For nonconductive coatings, such as ceramic coatings, the thickness measurement range increases with an increase
of the spatial wavelength of the sensor (for example, thicker coatings can be measured with larger sensor winding spatial
wavelength). For nonconductive coatings, when roughness of the coating may have a significant effect on the thickness
measurement, independent measurements of the nonconductive coating roughness, for example, by profilometry, may provide a
correction for the roughness effects.
4.4 Process-Affected Zone—For some processes, for example, shot peening, the process-affected zone can be represented by an
effective layer thickness and conductivity. These values can in turn be used to assess process quality. A strong correlation must be
demonstrated between these “effective coating” properties and process quality.
4.5 Three-Unknown Algorithm—Use of multi-frequencymultiple-frequency impedance measurements and a three-unknown
algorithm permits independent determination of three unknowns: (1) thickness of conductive nonmagnetic coatings, (2)
conductivity of conductive nonmagnetic coatings, and (3) lift-off that provides a measure of the nonconductive coating thickness.
4.6 Accuracy—Depending on the material properties and frequency range, there is an optimal measurement performance range for
each coating system. The instrument, its air standardization and/oror reference substrate standardization, or both, and its operation
permit the coating thickness to be determined within 615 % of its true thickness for coating thickness within the optimal range
and within 630 % outside the optimal range. Better performance may be required for some applications.
4.7 Databases for Sensor Response—Databases of sensor responses may be used to represent the model response for the sensor.
These databases may be based upon physical models or empirical relations. The databases list expected sensor responses (for
example, the real and imaginary parts or the magnitude and phase of the complex transimpedance between the sense element and
drive winding) over relevant ranges in the properties of interest. Example properties for a coated substrate material are the magnetic
permeability or electrical conductivity of the substrate, or both, the electrical conductivity and thickness of the coating, and the
lift-off. The ranges of the property values within the databases should span the expected property ranges for the material system
to be examined.
5. Interferences
5.1 Thickness of Coating—The precision of a measurement can change with coating thickness. The thickness of a coating should
be less than the maximum depth of sensitivity. Ideally, the depth of sensitivity at the highest frequency should be less than the
conductive coating thickness, while the depth of sensitivity at the lowest frequency should be significantly greater than the
conductive coating thickness. The number of frequencies used in the selected frequency range should be sufficient to provide a
reliable representation of the frequency-response shape.
5.2 Thickness of Substrate—The thickness of the substrate should be larger than the depth of sensitivity at the lowest frequency.
Otherwise, this thickness must be known and accounted for in the model for the sensor response.
5.3 Magnetic Permeability and Electrical Conductivity of Base Metal (Substrate)—The magnetic permeability and electrical
conductivity of the substrate can affect the measurement and must be known prior to coating characterization unless they can be
determined independently on a coated part. When the substrate properties vary spatially, this variation must be determined as part
of the coating characterization on a noncoated part that preferably has the same thermal history as the coated parts. Original
uncoated parts may have significantly different microstructure than heat treated coated substrates. Uncoated colder regions of
otherwise coated parts may have different properties than the coated substrate due to changes during coating and heat treatment,
and, thus, may or may not be reasonably representative of the substrate under the coating. In the case these variations are consistent
from component to component, a reference standard essentially equivalent to the actual substrate must be used. Differences
between the actual substrate values at any coating measurement location and the values assumed for property estimation, for
example, in the sensor response model, may produce errors in coating property estimates.
5.4 Electrical Conductivity of Coating—The precision of a measurement can change with the electrical conductivity of the coating.
The electrical conductivity of the coating should be substantially different from the conductivity of the substrate. For a
nonmagnetic coating on a nonmagnetic substrate, if the electrical conductivities are essentially the same, reliable coating thickness
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measurements cannot be obtained since the coating and substrate are electromagnetically indistinguishable. The electrical
conductivity of the coating should also be large enough for sufficient eddy currents to be induced to affect the sensor response.
5.5 Edge Effect—Examination methods may be sensitive to abrupt surface changes of specimens or parts. Therefore,
measurements made too near an edge (see 8.5.1) or inside corner may not be valid or may be insufficiently accurate unless the
instrument is used with a procedure that specifically addresses such a measurement. Edge-effect correction procedures must either
account for edge effects in the property estimation algorithm (for example, in the sensor response model) or incorporate careful
standardization on reference parts with fixtures to control sensor position relative to the edge.
5.6 Curvature of Examination Surface—For surfaces with a single radius of curvature (for example, cylindrical or conical), the
radius of curvature should be large compared to the sensor half-wavelength. In the case of a double curvature, at least one of the
radii should significantly exceed the sensor footprint and the other radius should be at least comparable to the sensor footprint,
unless customized sensors are designed to match the double curvature. Performance verification tests should be run to verify lift-off
sensitivity using insulating shims.
5.7 Instrument Stability—Drift and noise in the instrumentation can cause inaccuracies in the measurement. Restandardization and
performance verifications on at least one uncoated and one to two coated reference parts should be performed as needed to maintain
required performance levels.
5.8 Surface Roughness Including That of Base Metal—Since a rough surface may make single measurements inaccurate, a greater
number of measurements will provide an average value that is more truly representative of the overall coating thickness. These
repeat measurements should be performed in a “pick-and-place” mode, completely removing the sensor from the surface between
measurements. Coating surface roughness also may result in overestimated ceramic layer thickness or any other nonconducting
coating thickness since the probe may rest on peaks.
5.9 Directionality of Base-Metal Properties—Measurements may be sensitive to anisotropy of the base metal due to the fabrication
process, for example, rolling, directional solidification, single-crystal growth, etc. It is essential to keep the alignment of
sensor/probe consistent throughout the standardization step and measurements on a given part and from part to part.
5.10 Residual Magnetism in Base Metal—Residual magnetism in coating/substrate may affect accuracy of measurement.
5.11 Residual Stress—Directional stress variations for magnetizable substr
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