1.What are the core test indicators?
Resistivity (ρ): Measured in Ω・mm²/m (or μΩ・cm). The lower the value, the better the conductivity. (Pure zinc has a resistivity of approximately 5.9 μΩ・cm, while a mild steel substrate has a resistivity of approximately 10 μΩ・cm. The overall resistivity of galvanized coils is slightly higher than that of pure zinc due to the influence of the bonding between the substrate and the coating.)
Conductivity (σ): The reciprocal of resistivity (σ = 1/ρ), often expressed in "% IACS" (International Annealed Copper Standard). (Pure copper is defined as having a conductivity of 100% IACS, while pure zinc is approximately 28% IACS.)
2.What are the commonly used testing methods?
Four-probe method (most commonly used, highly accurate)
Principle: Four equally spaced probes contact the surface of the galvanized coil. A constant current (I) is passed through the two outer probes. The voltage (V) between the two inner probes is measured, and the resistivity is calculated using the formula.
DC Two-Electrode Method (suitable for evaluating overall conductivity)
Principle: A DC voltage is applied to the sample between two electrodes, and the current is measured. Ohm's law is used to calculate the resistance, which is then converted to conductivity (taking sample size into account).
Eddy Current Method (Non-contact, Suitable for Online Testing)
Principle: Eddy currents generated by alternating current are induced on the metal surface. The degree of eddy current decay reflects the material's conductivity (the higher the conductivity, the slower the eddy current decay).
3.What are the requirements for sample status?
The surface must be clean (free of oil, scale, or dirt). Otherwise, contact resistance will cause the measured value to be inaccurate.
Avoid bending or stress deformation of the sample (stress can affect the metal lattice, causing conductivity fluctuations).
4.What is the impact of coating and substrate?
Galvanized coils are a composite structure of a steel substrate and a zinc coating. Test results reflect overall conductivity (zinc has a higher conductivity than steel, and the thicker the coating, the higher the overall conductivity).
To test the conductivity of the coating alone, the stripping method (testing after stripping the coating) or the thin-film four-probe method (for thin coatings, the probe spacing should be less than the coating thickness) can be used.
5.What influence do environmental factors have?
Temperature: The conductivity of metal decreases as the temperature increases (for example, the temperature coefficient of zinc is about 0.004/°C). Therefore, the test should be conducted in a constant temperature environment (20±2°C).
Humidity: High humidity may cause surface oxidation. It is recommended to test in a dry environment (humidity < 60%).