1.How do the electrical conductivity of zinc compare to that of steel?
The conductivity of zinc is slightly higher than that of low-carbon steel (about 30%-70% higher), but much lower than good conductors such as copper and aluminum.
Principle: The conductivity of metals is determined by the concentration and mobility of free electrons. The number of valence electrons of zinc (2) is close to that of iron (2-3), but the crystal structure of zinc (close-packed hexagonal) has slightly lower electron scattering, so the conductivity is slightly higher.
2.What is the specific effect of the zinc coating on conductivity?
Thin coating: such as electroplating zinc, the resistance of the zinc layer can be ignored.
Thick coating (>50μm): such as hot-dip galvanizing, the resistance of the zinc layer will increase slightly, but it is usually still much lower than the contact resistance (such as bolt connection resistance) and can be ignored in practical applications.
3.What influence does the coating structure and surface condition have on conductivity?
Density: A uniform and dense zinc layer has more stable conductivity; if there are pores or oxide layers in the coating, the contact resistance will increase significantly.
Surface treatment: If passivation is performed after galvanizing, the passivation film is an insulating layer, which will greatly reduce the surface conductivity.
4.What effect does the contact scenario have on conductivity?
Direct conduction: When current passes through the inside of the galvanized layer, the conductivity of zinc is better than that of steel, so the overall conductivity is slightly improved (such as galvanized wire).
Contact interface conduction: If the current needs to pass through the interface between the coating and the substrate or the coating and other metal contact surfaces, factors such as interface oxidation and rough coating will increase the contact resistance (such as the need to polish the contact surface when bolting).
5.What are the conductivity optimization and precautions?
Process control ◦ Avoid over-passivation: Chromate passivation film thickness should be < 1μm, or use conductive passivation (such as silane treatment).
Control coating porosity: Hot-dip galvanizing can reduce porosity and improve conductivity through alloying treatment (such as Fe-Zn alloy layer).
Application Design
Conductive contact surface treatment: Grind the surface of the galvanized layer to expose the metal zinc; or apply conductive paste (such as silver-containing silicone grease) to reduce contact resistance.
Electrochemical compatibility: When galvanized steel contacts metals such as copper and aluminum, insulating gaskets or anti-corrosion coatings are required to avoid galvanic corrosion that causes increased interface resistance.
Test method
Contact resistance test: Use the four-probe method or micro-ohmmeter to measure the surface or interface resistance of the coating, and refer to GB/T 21388-2008 for the standard.