How To Verify The Quality of Zn-al-mg Steel Products Through Salt Spray Test?

As users' requirements for material life increase, research and update of coating products are also in progress. Since the 1990s, zn-al-mg steel company has successively developed zinc-aluminum-magnesium coating products that are more corrosion-resistant than traditional zinc-aluminum alloy coatings.

1. What is zn-al-mg steel?

Zinc-aluminum-magnesium is a coated steel plate that adds a certain amount of Al and Mg elements to hot-dip galvanized coating or hot-dip aluminum-zinc coating. It has high corrosion resistance and trimming protection properties. Experimental and outdoor exposure test results show that increasing the appropriate Al and Mg content can significantly improve corrosion resistance. Zinc-based corrosion products added with Mg can cover the cut surface and improve the corrosion resistance of the cut edge. In recent years, zinc-aluminum-magnesium coating products have attracted the attention of countries around the world because it can replace hot-dip galvanizing products, improve performance while reducing costs, and has a high cost performance. However, during use, zinc-aluminum-magnesium coating products may lose their anti-corrosion effect due to corrosion in different environments. In order to evaluate the corrosion resistance in different environments, the salt spray test is widely used.

2. How to verify the quality of zn-al-mg steel products through salt spray test?

In the salt spray test, in order to compare the protective lifespan of different surface treatments, the corrosion resistance of metal materials is evaluated by simulating high-salt environments such as the ocean.

During the test, zinc-aluminum-magnesium coating products and pure zinc coating products were put into the test environment, and the corrosion conditions on the surface were observed by recording the moments when "white" and "red" rust stains appeared on the product surface. The test results show that in the salt spray test, the zn-al-mg steel products first turned black, then white rust appeared, and finally red rust appeared.

(1) Melanosis

The passivation layer is slightly damaged and the surface layer of the coating begins to oxidize. Galvanized steel sheets with poor corrosion resistance oxidize quickly and will quickly pass through the blackening stage and white rust will occur.

(2) White rust

It is because the passivation film and oxide film are destroyed and water enters, or the density of the passivation film is poor. The water penetrates and reacts with the coating to form oxidized substances. The passivation layer of the zinc-aluminum-magnesium coating product is damaged in a large area, and the coating reacts with moisture to form corrosion products.

(3) Red rust

When the iron matrix corrodes and the zinc layer loses its sacrificial protection, an electrochemical reaction occurs between the substrate and water:

4Fe+6H2O=4(FeOH)3

2(FeOH)3=Fe2O3+3H2O

After neutral salt spray corrosion for 42 days, the surfaces of the zinc-aluminum-magnesium coating and the pure zn-al-mg steel sheet sections were covered with a layer of oxide. When the corrosion section was further enlarged, we found that the surface of the pure zinc coating had been corroded into pits of varying sizes, with a maximum depth of 81um, while the surface of the zn-al-mg steel sheet was still relatively smooth, with a corrosion depth of only 20um.

The time for red rust to appear on zinc-aluminum-magnesium-coated products is more than four times longer than that of pure zinc-coated products. The corrosion product on the surface of zinc-aluminum-magnesium coating is basic zinc chloride, which is denser than the oxide produced by pure zinc coating. It can delay the formation of zinc oxide, thereby protecting the substrate from further corrosion.

Although the corrosion environment in the salt spray test is different from the actual environment, it can clearly compare the protection time of different surface treatments. Therefore, zinc-aluminum-magnesium coating products have better corrosion resistance and service life than pure zinc coating products.