Practical Coating Thickness Measurement Overview

Presented by: Paul Lomax, Fischer Technology, Inc.

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

Part 1: Common Coating Thickness Test Methods and Gages

•Magnetic Induction Method •Eddy Current Method

•Type II Electronic Coating Thickness Gages

•Best Practices

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

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

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)

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

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

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)

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 signalU = f(d)

(ASTM D 7091)

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

Coating Thickness Probes

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

Terminology Related to Coating Thickness Measurement

Calibration

Normalization

Verification of Gage Accuracy

Adjustment

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

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

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

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

Normalizing and Adjustment

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

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

th

Flat surface

Thcvx > th

Convex curvature

Normalization and Adjustment

Concave curvature

thccv < th

Factors that Influence Coating Thickness Curvature

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

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

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

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

Perpendicular Probe Placement

Coating Thickness Probes

Substrate material 2

Normalization

Substrate material 1

Substrate material 3

th < thmeas th > thmeas

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

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

•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

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

Coating Thickness Probes

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

Part 3: Measuring According to SSPC-PA2 and Documenting Results

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

Measuring and Documenting Inspection Reports According to SSPC-PA2

Measuring and Documenting Inspection Reports According to SSPC-PA2

Tolerances set and automatic monitoring 80%-120% rule

Measuring and Documenting Inspection Reports According to SSPC-PA2

Overall Summary Number of spot readings per area

Summary per spot

Measuring and Documenting Inspection Reports According to SSPC-PA2

Individual readings per spot

Hand Writing or Typing Previously Required to Complete Forms

User Completes Form on the DFT Instrument

Defining Locations, Visual Guidance and Sequence of Measurements

Common Data Communication Methods

•Bluetooth®

•USB Port

•RS-232

Data Communication

Data Transfer to PC

Readings Transferred From Unit to DFT Log

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