The magnetic nature of galvanized sheets depends on the properties of their base materials. The zinc coating itself is a non-ferromagnetic material. The hot-dip galvanizing process is carried out at 460°C, forming a pure zinc layer 5-25μm thick on the surface of the low-carbon steel substrate. The magnetic permeability of this coating is close to the vacuum value (relative magnetic permeability ≈1.000022), and it cannot be adsorbed by magnets. However, if the base material is SPCC cold-rolled carbon steel (with a carbon content of 0.12%), its body-centered cubic lattice structure will retain strong ferromagnetism, with a saturation magnetization of 2.1 teslas. Even after galvanization, it can still adsorb permanent magnets with a magnetic force of up to 800 gauss. Tests conducted by General Motors’ laboratory in 2023 showed that the magnetic flux density of 0.7mm thick galvanized sheet used for car doors remained at 1.8T, meeting the 15N adsorption force requirement of electromagnetic door locks.
When the substrate is galvanized with austenitic stainless steel (such as SUS304), the material is completely non-magnetic. Because its nickel-chromium alloy (18%Cr+8%Ni) forms a face-centered cubic crystal, its magnetic permeability is only 1.003-1.005μ, and its residual magnetic intensity is less than 0.005 Tesla. In 2022, Tesla’s Berlin factory adopted this process to produce charging pile shells and successfully passed the IEC 61000 electromagnetic compatibility test, with a radiation interference attenuation rate of 98%. In contrast, the hysteresis loss of electrical steel (such as 50W470) galvanized sheets is controlled at 2.3W/kg (@1.5T/50Hz), which is only an increase of 0.15W/kg compared with the ungalvanized same model, and the efficiency loss is less than 5%.
The galvanizing process parameters significantly affect the magnetic performance. In the continuous hot-dip galvanizing line (CGL), a ±10°C fluctuation in the temperature of the zinc pot will change the thickness of the Fe-Zn alloy layer (the growth rate of the δ phase layer reaches 0.2μm/s), resulting in a magnetic permeability deviation of ±3%. The 2021 case of Baosteel confirmed that when galvanizing automotive steel sheets with DC04+Z, the holding time was shortened by 15 seconds (to 3 minutes), reducing the iron loss value from 4.1W/kg to 3.8W/kg (@1.0T/400Hz). However, for high-strength steel that has been galvanized (such as DP980) and then subjected to hot stamping, the martensitic transformation causes the coercive force to suddenly increase from 80A/m to 1200A/m, raising the difficulty of magnetization by approximately 85%.
The special zinc alloy coating can regulate the electromagnetic properties. The aluminum-magnesium-zinc coating (ZM, containing 4%Al+3%Mg) enhances the electromagnetic shielding performance to 70dB in the 2.4GHz frequency band, which is 15dB higher than that of the pure zinc coating. Huawei’s base station antenna covers for 2023 are made of 0.5mm thick ZM galvanized steel plates, reducing the magnetic field interference of base station antennas by up to 92% and lowering the bit error rate to the 10^-6 level. Nanostructured galvanized sheets (with a grain size of 50nm) have a 25% reduction in high-frequency eddy current loss due to the refinement of magnetic domains, making them suitable for the heat dissipation shells of 5G millimeter-wave communication equipment.
In practical applications, strict monitoring of residual magnetism is required. Galvanized sheets for construction (such as ASTM A653 G90) must pass residual magnetism testing (threshold <0.5mT) to avoid interfering with medical MRI equipment (magnetic field intensity 1.5-3T). The accident analysis of the University of Tokyo Hospital in 2022 shows that a domestic galvanized air duct had a local residual magnetism of 1.2mT due to impurities in the substrate, resulting in an MRI image distortion rate of 17%. At present, mainstream steel mills provide magnetic permeability test reports for galvanized sheets (with an error of ±1%), which comply with the ISO 2178 international standard to ensure electromagnetic compatibility safety. The Galvanized plate plays a key role in industrial scenarios.