Practical Coating Thickness Measurement Overview Presented by: Paul Lomax, Fischer Technology, Inc.

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Practical Coating Thickness Measurement
OverviewPresented by Paul Lomax, Fischer
Technology, Inc.
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Learning Objectives

• Test Methods
• Test methods available for coating thickness
  measurement
• Working knowledge of the basic theory of common
  test methods
• Best practices
• Factors that influence coating thickness
  measurement
• Instrument and probe selection criteria
• Instrument repeatability and minimum
  specification limits
• Evaluating the results of coating thickness
  
• Data transfer to inspection reports

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Part 1 Common Coating Thickness Test Methods and
Gages

• Magnetic Induction Method
• Eddy Current Method
• Type II Electronic Coating Thickness Gages
• Best Practices

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Magnetic Induction Method Basic Theory

• The magnetic field of the probe interacts with
  the steel substrate
• The nearer the probe to the substrate the higher
  the magnification of the magnetic field and vice
  versa

• The changes of the magnetic field induce a
  voltage U in the measuring coil dependent on the
  distance of the probe from the ferrous (steel)
  base

• The instrument translates this signal into a
  coating thickness value

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Magnetic Induction Method
Main Areas of Application

• Non-ferromagnetic coatings on ferromagnetic
  substrate material

• Paint, enamel, epoxy powder coating, plastic on
  steel or iron

• Electroplated coatings such as chromium, zinc,
  copper or aluminum on steel or iron

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Magnetic Induction Method

• Advantages
• Non-destructive
• Relatively low cost
• Easy to operate
• Accurate and repeatable thickness readings
• Instantaneous, digital thickness display
• Available in bench top and hand-held models
• Limitations
• Not recommended for coatings under 0.0001 (2.5
  microns)

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Eddy Current Method Basic Theory

• A high-frequency magnetic field induces Eddy
  currents into the conductive substrate material

Excitation current

• The magnitude of these Eddy currents depends on
  the distance between the coil and the substrate
  material

Measurement signal Uf(th))

• The measurement signal is derived from the
  reflected impedance change in the probe coil as a
  function of the Eddy currents generated in the
  substrate material

- Non-conducting, Non-magnetic coating material
Induced eddy currents
Electrically conducting nonferrous metal
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Eddy Current Method Basic Theory
Main Areas of Application
Excitation current

• Non-conductive, non-magnetic coatings applied to
  a non-ferrous substrate

Measurement signal Uf(th))

• Paint, enamel, epoxy, powder coating, plastic on
  aluminum, stainless steel, copper, brass, tin
  etc.
• Anodize over aluminum

th
Induced eddy currents
Electrically conducting nonferrous metal
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Eddy Current Method

• Advantages
• Non-destructive
• Relatively low cost
• Easy to operate
• Accurate and repeatable thickness readings
• Instantaneous, digital thickness display
• Available in bench top and hand-held models
  available
• Limitations
• Not recommended for coatings under 0.0001 (2.5
  microns)

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Coating Thickness Test Methods
Magnetic Induction Method (EN ISO 2178)
Eddy Current Method (EN ISO 2360)
(ASTM D 7091)
Measurement signal U f(d)
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Type II Electronic Dry Film Thickness Gages

• DFT Gage Types
• Integrated Probes
• Separate Interchangeable probes
• Basic
• Memory
• Ferrous
• Non-Ferrous
• Dual Ferrous and Non-Ferrous
• Measurement Strategies
• SSPC-PA2 Capabilities
• IMO PSPC Capabilities

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Coating Thickness Probes

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Duplex Measurement Multi Layer Coatings
Example 1 Application e.g., ELO-Zn, thin
hot-dip-Zn
Coating 1-2 mils Zinc coating .2.4
mils Steel substrate
Example 2 Application e.g., thick hot-dip-Zn
Paint coating 3 5 mil Pure zinc
coating 3 8 mil Zinc iron diffusion zone
(non-magnetic) Steel substrate
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Terminology Related to Coating Thickness
Measurement

• Calibration
• Normalization
• Verification of Gage Accuracy
• Adjustment

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Calibration

• Calibration of coating thickness gages is
  performed by the equipment manufacturer, an
  authorized agent, or by an authorized, trained
  calibration laboratory in a controlled
  environment using a documented process. The
  outcome of the calibration process is to
  restore/realign the gage to meet/exceed the
  manufacturers stated accuracy
• Source ASTM D7091

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Verification of Accuracy

• Obtaining measurements on coating thickness
  standards, comprising of at least one thickness
  value close to the expected coating thickness,
  prior to gage use for the purpose of determining
  the ability of the coating thickness gage to
  produce thickness results within the gage
  manufacturers stated accuracy
• Source ASTM D7091

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Verification of Accuracy
GAGE IDENTIFICATION FMP40 25.06.08 CALIBRATION
25.06.08 1618 Appl.No.3 ProbeFD10 ISO/NF th
.0.000 mil s0.010 mil Iso/NF 0.94
mil th.0.93 mil s0.009 mil Iso/NF 2.80
mil th.2.78 mil s0.012 mil
Uncoated base material
Calibration Standard 1
Calibration Standard 2

• Verification of accuracy should be done on a
  regular basis such as beginning and end of each
  shift
• Keeping a record of an instruments verification
  of accuracy is good business practice

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Normalizing and Adjustment

• A smooth surface zero plate or preferably an
  uncoated substrate similar to the substrate that
  will be coated can be used to normalize a Type II
  coating thickness gage

• If necessary adjustments can often times be made
  on electronic (Type II) coating thickness gages
  using certified foils on a specific surface

• Using certified mylar foils is important for
  optimizing a gage and monitoring film thickness

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Normalizing and Adjustment
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Part 1 Test Method Summary

• Magnetic Induction and Eddy Current are common
  test methods incorporated in Type II electronic
  coating thickness gages
• Magnetic Induction Gages measure coatings over
  steel or iron (ferrous substrates)
• Eddy Current Gages measure coatings over
  aluminum, stainless, steel and other (non-ferrous
  substrates)
• Best practices include a record of the
  verification of gage accuracy along with an
  understanding of terminology such as calibration,
  normalization, adjustment

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Factors that Influence Coating Thickness
Measurement

• Shape of the part to be measured
• Substrate material and coating material
• Instrument properties
• Measurement practice of the operator
• External influences

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Factors that Influence Coating Thickness
Curvature
Normalization and Adjustment
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Factors that Influence Coating Thickness
Curvature Example

• Different curvature radi in one part

Anodic coating thnom 10 µm
Meas. location



Meas. location 3


Meas. location 1
Meas. location 2
Meas. location 4
_
_
_
_
Readings (N5)

x

s

x

s

x

s

x

s

Standard Probe

9.2

0.4

52.1

0.76

22.3

0.85

61.9

1.4


9.8

0.25

10.2

0.52

10.4

0.65

10.5

0.59

Compensated Probe

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Factors that influence Coating Thickness Size of
the Measurement Area

• Magnetic field reaches beyond the measurement
  area
• Hand placement will lead to greater measurement
  data spread
• A minimum area must be available
• Consult manufacturers probe data sheets to
  determine specific capabilities

th nom
th meas gt th nom
Normalization
Spread
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Factors that Influence Coating Thickness Field
Penetration Depth

• Magnetic field reaches through!
• Measurement error due to insufficient substrate
  material thickness
• Measurement spread due to fluctuating substrate
  material thickness

th nom
th meas gt th nom
Normalization
Spread
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Factors that Influence Tilting of Probe

• Making sure that the probe tip is perpendicular
  to the substrate will help ensure that the
  measurement is taken properly

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Perpendicular Probe Placement
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Coating Thickness Probes

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Influence of the Substrate Material
Permeability
Magnetic induction measurement method
th
thmeas
th
thmeas
thmeas
th
µr2 gt µr1
µr1
µr3 lt µr1
Substrate material 1
Substrate material 2
Substrate material 3
Normalization
Examples Hard or soft magnetic steel, hardened
surface
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Influence of Roughness Reduction
With the magnetic induction method due to two-tip
probes (or larger probe tip, respectively)
With eddy current due to larger probe tip
Low measurement data spread due to resting on
roughness peaks
Low measurement data spread due to integration
via roughness profile
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Surface Roughness Factor Reduction

• The effects of substrate roughness and the
  roughness of coatings can be reduced by utilizing
  two-tip probes
• A pre-inspection scan of the coating can also be
  accomplished quickly

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Influence of the Substrate Material -
Conductivity
th
thmeas
thmeas
th
thmeas
th
Non-Ferrous Substrate material 1
Non-Ferrous Substrate material 2
Non-Ferrous Substrate material 3
Normalization
Recommendation Normalize on the respective
substrate material unless instrumentation is
conductivity compensated.
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Coating Thickness Probes

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Part 2 Factors and Probe Selection Summary

• Factors including curvature, edge effect,
  permeability, penetration depth, and roughness
  effect coating thickness measurement
• Probe selection criteria including performance
  specifications in relation to the above mentioned
  factors are available from manufacturers of
  coating thickness instruments
• Just because a probe is capable of measuring
  doesnt mean it is ideally suited for the
  application

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Part 3 Measuring According to SSPC-PA2 and
Documenting Results
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Spot Mean Calculation

• Low cost DFT Gages offer instant spot mean
  calculations. Typically the mean of three gauge
  readings are recorded in accordance with SSPC-PA2

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Efficiency in Coating Thickness Measurement

• Naming applications reduces the likelihood of
  documentation errors

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Measuring and Documenting Inspection Reports
According to SSPC-PA2
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Measuring and Documenting Inspection Reports
According to SSPC-PA2
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Measuring and Documenting Inspection Reports
According to SSPC-PA2
Tolerances set and automatic monitoring 80-120
rule
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Measuring and Documenting Inspection Reports
According to SSPC-PA2
Number of spot readings per area
Overall Summary
Summary per spot
Individual readings per spot
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Hand Writing or Typing Previously Required to
Complete Forms
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User Completes Form on the DFT Instrument
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Defining Locations, Visual Guidance and Sequence
of Measurements
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Data Communication

• Common Data Communication Methods
• Bluetooth
• USB Port
• RS-232

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Data Transfer to PC

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Readings Transferred From Unit to DFT Log
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Part 3 Summary Measuring According to SSPC-PA2
and Documenting Results

• Most Type II electronic gages now offer
  measurement specification guidance such as
  SSPC-PA2
• Visual guidance and measurement sequencing
  allows for inspection plans to be followed in the
  field by using hand held dry film thickness
  instrumentation
• Technology advancements yield reduction in
  costs, reduction in administrative time and
  reduction in errors