Isabella Flores
Stainless steel is a cost-effective and long-lasting material, often used in architecture, transportation, medical equipment, energy, off-shore oil, pipes, and aerospace. It is commonly coated with paint for decorative purposes, or to provide additional protection against corrosion. The thickness of paint on stainless steel can be measured using a paint thickness gauge, also known as a dry film thickness (DFT) gauge. This is a critical measurement in the coatings industry, as it can be used to evaluate the expected life, appearance, and performance of the coating. Paint thickness gauges can be used to obtain accurate and repeatable coating thickness readings, and there are a variety of options available depending on the type of material being measured.
Characteristics and Values of Checking DFT of Paint on Stainless Steel
| Characteristics | Values |
|---|---|
| Purpose of Coating Stainless Steel | Protection against wear and tear caused by weather and other factors |
| Importance of Measuring Coating Thickness | Determines coating's expected life, product's appearance, performance, and compliance with standards |
| Types of Measurement | Destructive (cutting to substrate) and Non-destructive magnetic, magnetic induction, eddy current |
| Common Non-destructive Measurement Tools | Hand-held coating thickness gauges, PosiTector 6000, PosiTest DFT, Elcometer 456, Elcometer 311 |
| Factors Affecting Measurement | Type of material, shape, surface finish, magnetic properties, conductivity |
| Tips for Accurate Measurement | Check zero on uncoated part, adjust if necessary, grip probe properly, avoid calibrating |
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What You'll Learn
Use a coating thickness gauge
The varying magnetic properties of stainless steel make measuring paint thickness particularly challenging, so using the right equipment with the correct settings is critical.
A coating thickness gauge (also referred to as a paint meter) is used to measure dry film thickness. Dry film thickness (DFT) can be measured using two methods:
- Destructive thickness measurement: The coating is cut to the substrate using a cutter.
- Non-destructive coating thickness measurement: Techniques that do not damage the coating or the substrate, such as magnetic, magnetic induction, and eddy current thickness measurement methods.
The non-destructive method can be used on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium. Electromagnetic induction is used for non-magnetic coatings on ferrous substrates such as steel, while the eddy current principle is used for non-conductive coatings on non-ferrous metal substrates.
Combination gauges, which combine magnetic and eddy-current technologies, provide accurate coating thickness measurements on nearly all families and grades of stainless steel, including magnetic, non-magnetic, and partially magnetic stainless steels. They first detect whether there is any magnetism present in the substrate. If so, measuring is done using the magnetic method; if not, then the eddy-current method is used.
To use a coating thickness gauge, follow these steps:
- Check for a repeatable zero on the uncoated part and perform any necessary adjustments.
- Measure the coated part.
- Ensure that the probe is properly adjusted for substrate characteristics, including mass, metallurgy, curvature, temperature, and roughness.
Modern coating thickness measurement gauges are simple and non-destructive. Examples include DeFelsko's PosiTector 6000 FN and Elcometer's 415.
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Calibrate the gauge to zero
Calibration is critical to maintaining your coating thickness gauge and ensuring continuous accuracy in measurement. Calibration ensures proper procedural inspection and accuracy as per industry standards.
To calibrate the gauge to zero, you must first check for a repeatable zero on an uncoated part and adjust if necessary. This is known as the zero method or the single-point method. It is the simplest calibration method as the gauge automatically regulates the calibration when it is confirmed to have complete contact with the uncoated surface to be measured.
For instance, when measuring the thickness of paint on stainless steel, you would first check for zero on the uncoated steel and adjust the gauge if necessary before measuring the coated part. This ensures that the probe is properly adjusted for substrate characteristics, including mass, metallurgy, curvature, temperature, and roughness.
It is important to note that the calibration of coating thickness gauges will be affected by the type of material, the shape, and the surface finish of the metal substrate being tested. For example, the varying magnetic properties of stainless steel make measuring paint thickness particularly challenging, so using the right equipment with the correct settings is critical. Thus, it is important to choose a calibration method that is suitable for the specific substrate and coating being measured.
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Use the correct settings
When checking the dry film thickness (DFT) of paint on stainless steel, it is important to use the correct settings on your chosen device. The varying magnetic properties of stainless steel make paint thickness measurement particularly challenging, so using the right equipment with the correct settings is critical.
The first step is to choose the right device. There are many options available, such as the PosiTest DFT, PosiTector 6000 F, and PosiTector 200 B1. These devices can measure paint thickness on a variety of metals, including stainless steel. They use different principles to measure thickness, such as magnetic, eddy current, and ultrasonic methods. The choice of device and setting depends on the type of material being measured. For example, magnetic methods are used for ferrous metals like steel, while eddy current methods are used for non-ferrous metals like aluminium.
Once you have selected the appropriate device, it is important to calibrate it correctly. Calibration can be affected by the type of material, shape, and surface finish of the metal substrate being tested. Most coating thickness gauges have features that allow you to set the gauge according to the work being carried out, maximising the accuracy of the readings. For example, the PosiTector 6000 FN features an N-Lock (Non-Ferrous lock) mode, which uses the eddy current principle of measurement for non-ferrous metals. This mode can be particularly useful when dealing with partially magnetic stainless steels, as it temporarily turns the substrate into a non-magnetic one, allowing for accurate readings.
It is also important to consider the pressure applied when using a probe to measure paint thickness. Constant pressure mechanisms built into modern electronic DFT gauges ensure the probe settles perpendicular to the surface and eliminates operator pressure from influencing the result. Holding the probe improperly can reduce its life and accuracy. Therefore, it is recommended to grip the sliding probe sleeve with your fingers low to the surface for the best results.
Additionally, it is worth noting that access to the uncoated substrate is ideal when taking measurements. This allows you to check for a repeatable zero on the uncoated part and perform any necessary adjustments. However, if access to the uncoated substrate is not possible, there are correction values provided by ISO 19840 that can be subtracted from DFT readings to account for this.
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Measure on an uncoated part
The varying magnetic properties of stainless steel make measuring paint thickness a challenging task. Therefore, it is critical to use the right equipment with the correct settings. The first step in measuring the thickness of a coating on stainless steel is to check for a repeatable zero on the uncoated part and perform an adjustment if needed. This is known as the "golden rule" and ensures that the probe is properly adjusted for substrate characteristics, including mass, metallurgy, curvature, temperature, and roughness.
Combination gauges, which combine magnetic and eddy-current technologies, provide accurate coating thickness measurements on nearly all families and grades of stainless steel, including magnetic, non-magnetic, and partially magnetic stainless steels. They first detect whether there is any magnetism present in the substrate. If so, measuring is done using the magnetic method; if not, then the eddy-current method is used. For example, the PosiTector 6000 FN instrument features N-Lock (non-ferrous lock) mode, which causes the gauge to exclusively use eddy-current measurement. This mode is particularly useful when dealing with partially magnetic stainless steel, as it temporarily turns the substrate into a non-magnetic one, allowing for accurate and repeatable coating thickness readings.
Another method for measuring coating thickness on stainless steel is to use a handheld coating thickness gauge, such as those offered by DeFelsko. These gauges non-destructively measure the thickness of coatings applied to all metal substrates, including stainless steel. They are ideal for use in a variety of industries, including architecture, transportation, medical, energy, and aerospace.
It is important to note that the specific type of stainless steel, whether magnetic, non-magnetic, or partially magnetic, will impact the choice of measurement technique and equipment. For instance, ferritic and martensitic stainless steels are magnetic, while austenitic stainless steel is generally non-magnetic but can become partially magnetic when cold-worked.
By following these steps and choosing the appropriate equipment for the specific type of stainless steel, one can effectively measure the thickness of a coating on an uncoated part of stainless steel.
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Use non-destructive testing devices
The varying magnetic properties of stainless steel make measuring paint thickness particularly challenging. Therefore, the correct non-destructive testing device must be chosen to ensure accurate measurements.
Types of non-destructive testing devices
- Magnetic gauging devices are used to measure the thickness of non-magnetic coatings on magnetic stainless steel. DeFelsko's PosiTest DFT is a magnetic pull-off thickness gage (also known as a banana gauge) that can be used to measure the thickness of non-magnetic coatings like paint, enamel, and galvanizing on steel.
- Electronic gauging devices are used to measure the change in magnetic flux density at the surface of a magnetic probe as it nears a steel surface. The magnitude of the flux density at the probe surface is directly related to the distance from the steel substrate. DeFelsko's PosiTector 6000 is a fully electronic coating thickness gauge that uses magnetic and eddy current principles to measure coating thickness on both ferrous and non-ferrous metals.
- Ultrasonic gauging devices are used to measure the total thickness of the paint system. DeFelsko's PosiTector 200 is an ultrasonic coating thickness gauge that can measure the thickness of paint on steel, aluminium, and other metals.
- Hall effect gauges work on almost every material, including plastics, non-ferrous metals, glass, wood, and certain non-magnetic stainless steel alloys. The Olympus Magna-Mike 8600 is a popular hall effect gauge model that provides reliable thickness measurements on non-ferrous materials.
Factors to consider when choosing a non-destructive testing device
- The grade and magnetic properties of the stainless steel: Austenitic stainless steel, for example, is generally non-magnetic, but it can become partially magnetic when cold-worked.
- The type of coating: Is it a magnetic or non-magnetic coating? This will determine the type of gauging device needed.
- The shape and surface finish of the metal substrate: The calibration of coating thickness gauges will be affected by the type of material, the shape, and the surface finish of the metal substrate being tested.
Best practices for using non-destructive testing devices
- Calibrate the device according to the manufacturer's instructions before starting the test.
- Take multiple readings at various locations on the coated surface to obtain comprehensive data.
- Calculate the average thickness and compare it to the specified range to determine if the coating application meets the requirements.
- Identify the specific grade of stainless steel being used and its magnetic properties.
- Place the probe flush with the coating surface during measurement and lift it completely from the surface between readings.
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