Fasteners are mechanical parts widely used in various machinery, equipment, vehicles, ships, railways, bridges, buildings, structures, tools, instruments, chemical equipment, meters, and supplies. Their primary function is to connect and secure components, ensuring the normal operation and safety of various equipment. Therefore, the quality requirements for fasteners are very high, necessitating professional methods to test their quality.

The determination of the thickness of the zinc coating on fasteners can be carried out through various methods, among which the weighing method, metallographic microscope method, and magnetic method are commonly used.
The metallographic testing method is one of the most common methods for measuring zinc coating thickness. The process involves: first, polishing the sample mechanically, then observing its metallographic structure under a macro microscope. By measuring the thickness of the zinc layer, the thickness of the base material, and the distribution of the zinc layer on the base material, the thickness of the zinc coating is calculated.
The basic principle is to select a representative area on the fastener to be tested. After appropriate sample preparation, a calibrated eyepiece micrometer or a high-precision scale is used under a metallographic microscope with image magnification to measure the width of the cross-section of the coating, which is the thickness of the coating. This method is simple and intuitive, with high measurement accuracy. With the development of optical microscopes toward digital photoelectric image display, digital microscope image detection technology has begun to be applied in the field of coating thickness measurement. High-resolution CCD and CMOS cameras are used to convert the microscope's optical images into a computer, and then specialized measurement software systems are employed for image measurement, improving both the accuracy and efficiency of measurements.