1.How does the electrochemical protection mechanism of the coating work?
Potential difference synergistic effect of multi-metals
Zinc-aluminum-magnesium coating is usually composed of three metals: zinc, aluminum and magnesium, and the electrode potentials of the three are different: the standard electrode potential of zinc is - 0.76V, aluminum is - 1.66V, and magnesium is - 2.37V (relative to the standard hydrogen electrode).
Magnesium has the lowest potential and becomes the "sacrificial anode" in the coating. When a micro-battery is formed on the surface of the coating, magnesium loses electrons first and is corroded, thereby protecting zinc and aluminum with higher potentials, as well as the base metal.
Uniform sacrificial anode effect
Compared with traditional zinc coating, the addition of magnesium in zinc-aluminum-magnesium coating makes the corrosion current distribution more uniform. Even if the coating is partially damaged, the surrounding magnesium and zinc can still form protection through electrochemical action to delay the corrosion of the substrate.
2.What is the physical barrier function of corrosion products?
Dense and stable corrosion product film
When the zinc-aluminum-magnesium coating comes into contact with water, oxygen, salt, etc. in the environment, the following reactions will occur: magnesium reacts with water to form magnesium hydroxide, aluminum to form aluminum hydroxide, and zinc to form zinc hydroxide.
These hydroxides will further react with carbon dioxide in the air to form complexes such as basic zinc carbonate and basic magnesium carbonate.
These corrosion products are intertwined to form a dense, water-insoluble, poorly conductive film that is tightly attached to the surface of the coating, preventing moisture and oxygen from penetrating, like a "physical barrier" to isolate the corrosive medium.
Special role of aluminum element
Aluminum can form amorphous aluminum hydroxide colloid in corrosion products, filling the pores of the corrosion product film, improving the density and adhesion of the film, and further enhancing the barrier effect.
3.What are the microstructural advantages of the coating?
Microstructure of multiphase alloys
The microstructure of zinc-aluminum-magnesium coatings is usually composed of zinc-rich phase, aluminum-rich phase and magnesium-zinc-aluminum intermetallic compounds. This multiphase structure enables the coating to form a complex micro-battery network during the corrosion process, slowing down the corrosion rate of a single phase.
Uniform distribution of magnesium
Magnesium is dispersed in the coating in the form of fine particles or solid solution, ensuring the continuity and uniformity of the sacrificial anode effect and avoiding accelerated local corrosion.
4.What are the advantages compared with traditional coatings?
The corrosion resistance of zinc-aluminum-magnesium coating is 5-10 times that of traditional zinc coating, and it has significant advantages in harsh environments such as oceans and industrial atmospheres.
The galvanized layer mainly relies on the sacrificial anode protection of zinc, the corrosion products are loose and the corrosion resistance is limited.
The corrosion resistance of zinc-aluminum coating is better than that of zinc coating, but when the magnesium content is insufficient, the density of the corrosion product film is poor.
5.What are the typical application scenarios?
Marine environment: offshore wind power towers, port facilities, etc., resist salt spray corrosion.
Industrial atmosphere: chemical plants, power plants, etc., resist acidic gas (such as SO₂) and dust erosion.
Construction field: roof steel plates and steel structures, which remain corrosion-resistant even after long-term exposure to rain and ultraviolet rays.