How to measure the zinc coating thickness on SGCC hot-dip galvanized steel sheets?
Measuring the zinc coating thickness on SGCC hot-dip galvanized steel sheets must comply with relevant standards (such as Japanese JIS G 3302 and international ISO 1460). Common methods include destructive and non-destructive testing, as detailed below:
I. Non-destructive Testing (commonly used, does not damage the coating)
1. Magnetic Thickness Measurement (Most Common)
Principle: Utilizes the difference in magnetic permeability between the zinc coating (non-magnetic) and the base steel sheet (magnetic). The change in the magnetic field generated by the probe is used to measure the coating thickness.
Applications: Suitable for non-magnetic coatings (zinc coatings) on magnetic substrates (such as steel sheets). It is simple, quick, and can be tested on-site.
Key Points:
Before testing, calibrate the instrument with a standard thickness block to ensure accuracy.
Select multiple measurement points evenly distributed across the steel sheet surface (usually at least five, avoiding edges and defects) and take the average value as the zinc coating thickness.
Advantages: Non-destructive, efficient, and suitable for batch testing, it is the most commonly used method in industry. 2. Eddy Current Thickness Measurement
Principle: Eddy currents generated by high-frequency alternating currents form a magnetic field in the coating. Changes in coating thickness cause changes in eddy current intensity, which is used to calculate thickness.
Applicable Scenario: Suitable for conductive coatings on non-magnetic substrates. However, since the SGCC substrate is magnetic steel, it is rarely used alone. Sometimes it is combined with magnetic methods for specialized applications.
II. Destructive Testing (accurate but damaging to the product, used for sampling or arbitration)
1. Gravimetric Method (Weight Difference Method)
Principle: The zinc coating thickness is calculated based on the weight difference before and after coating.
Formula: Zinc coating thickness (μm) = (coating weight g/㎡) ÷ zinc density (7.14 g/cm³) × 1000
Steps:
Cut a sample of a certain area (e.g., 100 mm x 100 mm), remove surface oil, and weigh it (W1).
Completely remove the zinc coating using a chemical method (such as a stripping solution), clean and dry, and weigh again (W2). Calculate the coating weight: (W1 - W2) ÷ Area (m2), then convert to thickness.
Advantages: Highly accurate, and one of the arbitration methods specified in the standard. However, it damages the specimen and is only used for random sampling.
2. Metallographic Method (Cross-Section Microscopy)
Principle: Cut a cross section of the specimen, grind, polish, and etch it. Observe the coating cross section under a microscope to directly measure the zinc coating thickness.
Key Points:
The cross section must be perpendicular to the steel plate surface to ensure measurement accuracy.
Measure multiple locations under the microscope (e.g., 10 points) and take the average.
Advantages: Simultaneously observes the bonding between the coating and the substrate, making it suitable for analyzing coating uniformity. However, the procedure is complex and inefficient.
3. Anodic Dissolution Coulometric Method
Principle: The zinc coating is electrolytically dissolved in a specific electrolyte. The amount of electricity consumed during the dissolution process is used to calculate the coating weight (Faraday's law) and then convert it to thickness. Application Scenarios: Extremely accurate, often used for precise laboratory measurements or calibration with standard samples. However, the equipment is complex and time-consuming.
III. Testing Standards and Precautions
Standards to be Followed: Zinc coating testing on SGCC galvanized steel sheets must comply with JIS G 3302 (Japanese Standard) or ISO 1460 (International Standard), which clearly stipulates sampling methods, the number of measurement points, and the allowable error.
Precautions:
Non-destructive testing requires regular calibration of the instrument with standard blocks to avoid errors.
Coating thickness uniformity is required, and single-point measurements must be within the standard's allowable range (for example, JIS stipulates that the average zinc coating thickness must meet the standard, and the minimum single-point thickness must be no less than 80% of the specified value).
In summary, magnetic thickness measurement is often used in industrial production for rapid batch testing, while gravimetric and metallographic methods are primarily used for precise laboratory verification or for arbitration of quality disputes.