practical coating thickness measurement overview presented by: paul lomax, fischer technology, inc
TRANSCRIPT
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Practical Coating Thickness Measurement Overview
Presented 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
•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
Main Areas of Application
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Magnetic Induction Method
Advantages:Non-destructiveRelatively low costEasy to operateAccurate and repeatable thickness readingsInstantaneous, digital thickness displayAvailable in bench top and hand-held models
Limitations:Not recommended for coatings under 0.0001” (2.5 microns)
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Excitation current
Measurement signal U=f(th))
- Non-conducting, Non-magnetic coating material
Induced eddy currents
Electrically conducting nonferrous metal
•A high-frequency magnetic field induces Eddy currents into the conductive substrate material
•The magnitude of these Eddy currents depends on the distance between the coil and the substrate material
•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
Eddy Current Method Basic Theory
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Excitation current
Measurement signal U=f(th))
Induced eddy currents
Electrically conducting nonferrous metal
th
Main Areas of Application:
Eddy Current MethodBasic Theory
•Non-conductive, non-magnetic coatings applied to a non-ferrous substrate
•Paint, enamel, epoxy, powder coating, plastic on aluminum, stainless steel, copper, brass, tin etc.
•Anodize over aluminum
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Eddy Current Method
Advantages:Non-destructiveRelatively low costEasy to operateAccurate and repeatable thickness readingsInstantaneous, digital thickness displayAvailable 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)
Excitation current
I~
Steel/iron substrate material
th
Excitation current
I~
th
Electrically conducting non-ferrous metal
Measurement signal
U = f(d)
(ASTM D 7091)
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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 1Application: e.g., ELO-Zn, thin hot-dip-Zn
Example 2Application: e.g., thick hot-dip-Zn
Coating: 1-2 milsZinc coating: .2–.4 milsSteel substrate
Paint coating: 3 – 5 milPure 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 manufacturer’s 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 manufacturer’s stated accuracy
•Source ASTM D7091
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Verification of Accuracy
•Verification of accuracy should be done on a regular basis such as beginning and end of each shift
•Keeping a record of an instrument’s verification of accuracy is good business practice
GAGE IDENTIFICATION FMP40 25.06.08CALIBRATION 25.06.08 16:18Appl.No.:3 Probe:FD10
ISO/NFth.=0.000 mil s=0.010 mil
Iso/NF: 0.94 milth.=0.93 mil s=0.009 mil
Iso/NF: 2.80 milth.=2.78 mil s=0.012 mil
Uncoated base material
Calibration Standard #1
Calibration Standard #2
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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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th
Flat surface
Thcvx > th
Convex curvature
Normalization and Adjustment
Concave curvature
thccv < th
Factors that Influence Coating Thickness Curvature
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x = mean value, s = standard deviation_
_ _ _ _
•Different curvature radi in one part
Meas. location Meas. location 1 Meas. location 2 Meas. location 3 Meas. location 4
Readings (N=5) 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
Compensated Probe 9.8 0.25 10.2 0.52 10.4 0.65 10.5 0.59
Anodic coating:thnom = 10 µm
Factors that Influence Coating Thickness Curvature Example
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Magnetic field reaches beyond the measurement area
Hand placement will lead
to greater measurement data spread
A minimum area must be available
Consult manufacturer’s probe data sheets to determine specific capabilitiesNormalization th meas > th nom
Spread
th nom
Factors that influence Coating Thickness Size of the Measurement Area
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Magnetic field reaches through!
Measurement error due to insufficient substrate material thickness
Measurement spread due to fluctuating substrate material thickness
Normalization
th meas > th nom
Spread
th nom
Factors that Influence Coating ThicknessField Penetration Depth
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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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Substrate material 2
Normalization
Substrate material 1
Substrate material 3
th < thmeas th > th
meas
Magnetic induction measurement method
Examples: Hard or soft magnetic steel, hardened surface
Influence of the Substrate Material: Permeability
µr2 > µr1
th
thmeasthmeas
µr1
th
th
µr3 < µr1
thmeas
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With eddy current due to larger probe tip
With the magnetic induction method due to two-tip probes (or larger probe tip, respectively)
Low measurement data spread due to resting on roughness peaks
Low measurement data spread due to integration via roughness profile
Influence of Roughness – Reduction
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•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
Surface Roughness Factor Reduction
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Non-FerrousSubstrate material 2
Normalization
Non-FerrousSubstrate material 1
Non-FerrousSubstrate material 3
Recommendation: Normalize on the respective substrate material unless instrumentation is conductivity compensated.
Influence of the Substrate Material - Conductivity
th
thmeas thmeasth
th
thmeas
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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 doesn’t 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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Tolerances set and automatic monitoring 80%-120% rule
Measuring and Documenting Inspection Reports According to SSPC-PA2
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Overall Summary Number of spot readings per area
Summary per spot
Measuring and Documenting Inspection Reports According to SSPC-PA2
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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Common Data Communication Methods
•Bluetooth®
•USB Port
•RS-232
Data Communication
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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