Core Principles of Zn-Al-Mg Alloy Coating
Date: 2025-9-28
Category: Metallurgical encyclopedia terms
Views: 156
I.Core Definition: What is it?
Zn-Al-Mg coating is a type of ternary or multi-component alloy coated steel sheet developed on the basis of traditional hot-dip galvanizing (GI). In addition to zinc, a certain proportion of aluminum (Al), magnesium (Mg), and a small amount of other elements are added to the coating metal bath, forming this alloy coating.
It is not a simple mixture, but an alloy with a specific microstructure formed through precise metallurgical formulas and process control. Its performance is far superior to that of traditional galvanized sheets.
II.Core Advantages: Why is it better?
The outstanding performance of Zn-Al-Mg coating stems from the clever synergistic effect among aluminum, magnesium, and zinc, which is mainly reflected in the following three aspects:
1.Barrier Protection and Sacrificial Protection: Aluminum can form a very dense and stable aluminum oxide (Al2O3) film on the coating surface, acting like a solid shield to effectively block the intrusion of corrosive factors such as moisture and oxygen—this is known as "barrier protection". Magnesium, with more active chemical properties than zinc, will be preferentially corroded when the coating is scratched and the substrate is exposed at the cut. It forms protective corrosion products covering the cut surface, "sacrificing" itself to protect the underlying zinc layer and steel substrate, thus providing stronger "sacrificial anode protection".
2.Self-Healing Ability: When the coating is damaged, the corrosion products of zinc, aluminum, and magnesium (such as zinc hydroxide, zinc carbonate, magnesium hydroxide, etc.) will quickly accumulate at the damaged area, forming a dense and insoluble protective film to automatically "repair" the damaged area and prevent further expansion of corrosion. This enables Zn-Al-Mg steel sheets to require no additional anti-corrosion treatment after cutting and punching.
3.Formation of Reinforced Phases: In the microstructure of the coating, magnesium and zinc form high-hardness MgZn2 intermetallic compounds. This makes the Zn-Al-Mg coating harder and more wear-resistant than pure zinc coatings, allowing it to better resist scratches during production, transportation, and installation.
III.Main Types (Classified by Composition)
According to different application requirements, Zn-Al-Mg has developed different composition systems, mainly classified based on the content of aluminum:
1.Low-Al Zn-Al-Mg: The aluminum content is usually around 1%-3.5%. This formula achieves the optimal balance among corrosion resistance, processability, and cost. It is currently the most widely used type, commonly applied in photovoltaic brackets, building roof panels, and livestock breeding equipment.
2.Medium-Al Zn-Al-Mg: The aluminum content is approximately 5%-11%. Its corrosion resistance is further enhanced, making it suitable for harsher environments such as coastal areas or around chemical plants.
3.High-Al Zn-Al-Mg: Based on 55% Al-Zn alloy (e.g., Galvalume), approximately 1.5%-2% magnesium is added. It is characterized by good high-temperature oxidation resistance and an appearance closer to aluminum. It is mostly used in the construction field with special requirements for heat resistance or appearance.
In addition, by adding trace elements such as molybdenum (Mo) and chromium (Cr) to the traditional Zn-Al-Mg system, the fluidity of the molten bath can be improved, the uniformity of the alloy molten bath can be better ensured, which helps solve the problem of easy segregation of the alloy and makes the coating structure more uniform and stable. Adding zirconium (Zr) can significantly increase the nucleation rate of the alloy, refine the microstructure of the hot-dip Zn-Al-Mg alloy, thereby improving its corrosion resistance and scratch resistance.
IV. Key Control Points of the Production Process
To obtain Zn-Al-Mg coatings with excellent performance, multiple links in the production process need to be precisely controlled:
Pretreatment and Annealing: The steel strip must undergo strict cleaning and pickling to ensure a clean surface, and then undergo recrystallization annealing in an annealing furnace filled with protective gas to prepare for hot-dip coating.
Hot-Dip Coating and Air Knife: The steel strip is immersed in a Zn-Al-Mg alloy bath with strictly controlled temperature (e.g., 440-460℃). Immediately after being pulled out, it is blown with high-speed air flow (air knife) to precisely control the thickness and uniformity of the coating.
Cooling Control (Most Critical): The cooling rate of the steel strip after leaving the zinc pot is crucial. Rapid cooling (e.g., cooling to room temperature at a rate greater than 30℃/s) can promote the formation of fine zinc-rich dendrites and uniformly distributed Zn/MgZn2 eutectic structure in the coating, which is the key to achieving excellent corrosion resistance.
Ⅴ.Main Application Fields
With its outstanding performance, Zn-Al-Mg alloy coating materials are rapidly replacing traditional galvanized products in multiple fields:
Photovoltaic Industry: Solar photovoltaic brackets require weather resistance of more than 25 years, and the super corrosion resistance and self-healing ability of Zn-Al-Mg materials make them an ideal choice.
Construction Industry: Used in roof panels and wall panels of large industrial plants and warehousing logistics parks, it can significantly extend the building maintenance cycle and reduce the whole-life cycle cost.
Automotive Manufacturing: Applied in automotive internal structural parts and chassis components, leveraging its good corrosion resistance and processability.
Home Appliances and General Industry: Used in washing machine inner drums, microwave oven casings, and electrical cabinets.
Through ingenious alloy design, Zn-Al-Mg technology achieves the effect of 1+1+1 > 3 and becomes a representative of high-performance anti-corrosion steel materials. Its excellent corrosion resistance, self-healing ability, wear resistance, and processability enable it to play an important role in sustainable development-oriented fields such as photovoltaic power generation and green buildings.
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