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E: yuantai@yuantai-steel.com
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Views: 117 Author: Site Editor Publish Time: 2024-07-22 Origin: Site
Zinc-aluminum-magnesium strip steel is increasingly used in photovoltaic brackets and other scenarios due to its good corrosion resistance and edge protection.
However, in actual production and use, it is found that zinc-aluminum-magnesium strip steel sometimes has local black spots, which affects the appearance and also causes customers to doubt whether it affects corrosion resistance.
Let's discuss what causes the black spots on zinc-aluminum-magnesium strip steel? Will it affect product performance?
As we all know, zinc-aluminum-magnesium strip steel contains magnesium, which is an element that is easily oxidized. If the coating is exposed to the air, oxidation will occur, making the coating look slightly milky white. The coating that is not in contact with the air should look silvery white, not black!
We tested the surface of zinc-aluminum-magnesium strip steel with the help of chemical methods and found that the surface oxide of the light black coating is weakly acidic, and the surface oxide of the normal light milky white coating is weakly alkaline.
Why does this happen? Through the elimination method, it was found that the main reasons for the appearance of black spots on zinc-aluminum-magnesium strip steel are the following three points:
(1) Oil stains and oxides on the substrate surface
If there are impurities such as oil stains and oxides on the substrate surface, black spots will be generated in zinc-aluminum-magnesium galvanizing. Because the coating cannot adhere evenly to these impurities to form a passivation layer, the current vector is abnormal and the current density increases, which will form defects such as black spots and shedding in these places.
(2) Incomplete pickling
Insufficient pickling will cause more oxides, impurities and other substances on the surface of the substrate, and the zinc-aluminum-magnesium galvanizing produced is prone to black spots.
(1) Surface defects of the material
During the manufacturing process of the substrate, if there are defects such as running lines, cracks, and oxide accumulation on the surface, it will cause local oxidation during zinc-aluminum-magnesium galvanizing, causing black spots and shedding of the galvanized layer.
(2) Internal defects of the material
When there are inclusions, holes, etc. inside the material, oxidation is likely to occur during the zinc-aluminum-magnesium galvanizing process, causing the coating to shear off and black spots.
If the electrolyte composition, concentration, temperature, etc. do not meet the requirements during zinc-aluminum-magnesium zinc plating, defects in the zinc-aluminum-magnesium coating will occur, resulting in the generation of black spots.
The above are the main reasons for the generation of black spots in zinc-aluminum-magnesium strip steel, but due to the single testing method, there is a lack of data support for whether the black spots in zinc-aluminum-magnesium strip steel affect the corrosion resistance.
In order to further analyze the cause of the defect and whether it affects the corrosion resistance, and then control the defect, we conducted neutral salt spray tests on black spot samples and normal samples respectively.
Through the neutral salt spray test, we divided the black spot samples and normal samples into four conditions: 0h, 500h, 2000h and 4000h. The corrosion resistance of the black spot samples of zinc-aluminum-magnesium strip steel is the same as that of the normal samples, and is not affected by the black spots. It also verifies that the corrosion resistance of the black spot area will not deteriorate. The black spot samples have very good corrosion resistance like the normal samples, and do not affect the product performance, but only the appearance.
