When hot dip galvanizing, the zinc-iron alloy layer is metallurgically bonded to the steel substrate and the zinc layer, which is stronger than the combination of paint and steel.
On the one hand, zinc has a more negative electrode potential than iron. When zinc and iron constitute a micro-battery, zinc is the anode and iron is the cathode. When corroded, zinc dissolves and iron is not damaged. When the galvanized layer has small cracks or damage, zinc will prevent the rust of steel from continuing to crack or damage in the form of a sacrificial anode, which is the main feature of the galvanized layer superior to other coatings.
On the other hand, the zinc layer has a shielding effect on steel. Zinc forms a shielding layer on the surface of the steel substrate in the atmosphere or other environments. The corrosion process of zinc increases its ability to form a barrier layer because zinc reacts with oxygen, carbon dioxide, moisture, etc. in the air to form a corrosion-resistant byproduct zinc carbonate film. This basic zinc carbonate film is hard and insoluble, and becomes an effective shielding protective layer. It seals the corrosion of iron in the external environment, slows down the corrosion of zinc, and reduces the corrosion rate of steel to 1/25.