1、 Corrosion resistance of zinc, aluminum and magnesium and self repairing principle of scratched incision
First of all, theoretical analysis. ZAM plate has unique corrosion resistance, especially the notch protection performance. It can be divided into two cases: the plane without scratch and the two cases with incision and scratch.
1 when there is no scratch on ZAM surface
The joint action of magnesium and aluminum in the coating will produce a dense oxidation protection film with high adhesion on the surface of the coating in long-term use, which will isolate the coating from the external corrosion environment, play the role of isolation and corrosion prevention, and prevent the coating from corrosion. Magnesium is a more active metal than aluminum. It is very easy to be oxidized in air. The oxide film formed by solid magnesium is also dense, which can effectively protect the internal structure and prevent further oxidation. Due to the difference between magnesium and aluminum, the uniform and fine distribution of magnesium in the coating makes the oxide film cover the whole coating surface and protect all the structures in the coating.
When the coating is scratched or cut
The magnesium dissolved from the coating can form a fine and dense oxide film containing magnesium on the incision section, which can cover the steel substrate exposed at the scratch or the incision, so as to improve its corrosion resistance. In this case, the sacrificial anti-corrosion effect of zinc and the isolation anti-corrosion effect of aluminum and magnesium can also be brought into play. On this basis, because magnesium is very active, it can reduce the loose oxide on the steel plate surface and replace it with dense magnesium oxide, thus protecting the exposed steel base.
In the new cut position of ZAM plate, the exposed steel base will be oxidized in corrosive environment, and red rust will appear at the initial stage. However, with the passage of time, magnesium, zinc and aluminum dissolved from the coating will change the red rust into gray black oxide film, and the microstructure is relatively dense, which protects the fracture site from being oxidized.
The incision exposure test shows that the fracture surface appears red at the beginning of the test, but the red color will not develop seriously. After 3 years, the original red color turns to gray black. After testing, the oxide film on the fracture surface contains zinc, magnesium, aluminum, oxygen, iron, chlorine, sulfur and other complex components.