1. What are the main methods for measuring the coating thickness of galvanized color-coated steel coils?
There are two main categories of coating thickness measurement: non-destructive testing and destructive testing. The most commonly used method is the magnetic thickness gauge method. In practical applications, there are four main methods:
**Magnetic Thickness Gauge Method:** This is the most commonly used and fastest non-destructive method. It is based on the principle of electromagnetic induction and is suitable for color-coated steel sheets with magnetic materials such as steel as the substrate.
**Micrometer Method:** This is a destructive method. It calculates the coating thickness by measuring the thickness difference of the substrate before and after removing the coating. It is suitable for color-coated steel sheets with various substrates.
**Metallurgical Microscopy Method:** This is the most precise destructive method and is often used as an arbitration test. It involves preparing a cross-sectional sample and directly measuring the coating thickness under a microscope.
2. How to accurately measure the coating thickness of galvanized steel sheets?
A: For galvanized color-coated coils, the key to measuring coating thickness is to distinguish between the coating and the zinc layer, as common magnetic thickness gauges measure the total thickness of the non-magnetic layer. The standard procedure is as follows:
Step 1: Measure the total thickness. Using a magnetic thickness gauge, measure the total thickness of the "galvanized layer + coating" at five different locations more than 25mm from the edge of the sample.
Step 2: Measure the zinc layer thickness. Carefully remove the coating using a paint remover that does not corrode the zinc layer. Measure again at the same locations to obtain the zinc layer thickness.
Step 3: Calculation. Subtract the zinc layer thickness from the total thickness to obtain the coating thickness.
3. What standards should be followed when measuring coating thickness? How should measurement points be selected?
A: In China, the main national standard followed is GB/T 13448-2019 "Test Methods for Color-Coated Steel Sheets and Strips". Internationally, there are also corresponding standards such as ISO 2178 and ASTM standards.
To ensure the representativeness and accuracy of the measurement, the following guidelines should be followed when selecting measurement points:
Avoid edges: Measurement points should be at least 25mm away from the edge of the sample, as the coating may be uneven or damaged at the edge.
Multiple measurements: According to the standard requirements, measurements should be taken at five different locations evenly distributed on the sample surface, and the arithmetic mean of these readings should be taken as the final result.
Spacing requirements: The distance between single-point measurements should not be less than 25mm.
Sample flatness: The surface of the sample to be tested should be flat, free of oil stains, damage, and burrs on the edges, and must be flat and without bending to ensure that the probe can fit tightly.
4. How to Choose Measuring Instruments? What are the differences between magnetic thickness gauges and magnetic-eddy current thickness gauges?
A: The choice of thickness gauge mainly depends on the substrate material of the color-coated steel sheet being measured. The main differences between magnetic thickness gauges and magnetic-eddy current thickness gauges are:
Instrument Type | Working Principle | Applicable Scope
Magnetic Thickness Gauge | Utilizes the principle of electromagnetic induction, measuring changes in magnetic reluctance. Only applicable to magnetic substrates, such as cold-rolled steel sheets and galvanized steel sheets.
Magnetic-Eddy Current Thickness Gauge | Combines the principles of magnetic induction (measuring magnetic substrates) and eddy current (measuring non-magnetic metal coatings). Has a wider range of applications, capable of measuring both magnetic substrates and non-magnetic metal substrates such as aluminum and copper sheets, and can simultaneously distinguish between plating and coating.
5. What are the precautions and influencing factors during the measurement process?
A: To ensure measurement accuracy, the following points should be noted:
Instrument Calibration: Before measurement, the instrument must be calibrated using a standard piece with a thickness similar to the coating to be measured. For magnetic thickness gauges, zeroing is also required on an uncoated substrate of the same material.
Probe Contact: During measurement, the probe should be perpendicular and tightly fitted to the coating surface. Record the reading only after it has stabilized.
Surface Cleaning: Ensure the sample surface and probe are clean to avoid dust, oil, etc., affecting the measurement results.
Environmental Influence: Avoid measurements in strong magnetic or electric fields as much as possible. For high-precision measurements, it is recommended to conduct the measurements in a standard environment with a temperature of 23℃±2℃ and a relative humidity of 50%±5%.
Special Coatings: When the coating contains metallic fillers, it may affect the results of the eddy current probe; this factor needs to be considered.