Addition of hot dip galvanizing aluminum
1.In hot dip galvanizing, a small amount of aluminum is often added to the zinc bath, which can play the following roles: 1) to improve the brightness of the coating; 2) to reduce the oxidation of zinc on the surface of zinc solution and reduce the production of zinc ash. However, if the aluminum content in zinc bath is not well controlled, not only can not improve the quality of products, but will cause the surface of the coating particles, and even leakage plating defects.
2.The influence on the brightness of the coating. A small amount of aluminum in the zinc bath can make the surface of the coating pure zinc. As A1 in zinc bath is selectively oxidized, a continuous A12O3 film is formed on the surface, which provides a physical barrier to the zinc layer and prevents zinc from being oxidized by air, thus maintaining the metallic luster of zinc. The results show that the brightness of the coating will be improved when the content of A1 in zinc bath is 0.001%-0.005%; but the brightness of the coating will become unstable when the content of A1 in zinc bath is 0.001%-0.005%; when the content of A1 in zinc bath exceeds 0.005%, the brightness of the coating will increase steadily with the increase of aluminum content. The brightness of the coating will not increase or even decrease slowly after A1 exceeds 0.02%. Therefore, in the production of hot dip galvanizing, in order to improve the brightness of the coating, aluminum can be added to the zinc bath to make the A1 content of 0.005% ~ 0.02%.
3.Influence on zinc slag and zinc ash. When the A1 content in zinc bath is 0.005%~0.02%, the A1-rich scum which floats on the surface and zinc bath will not be formed. At the same time, the zinc ash will be reduced. This is due to the formation of a protective layer of A12O3 on the surface of zinc bath, which prevents zinc from being oxidized to form zinc ash.
4.The effect of Galvanizing auxiliaries. Ammonium zinc oxide is usually used as an assistant for hot dip galvanizing. A1C13 is formed by the reaction of A1 with NH4C1 in ammonium zinc oxide, which weakens or invalidates the assistant effect.
5.The way to add aluminum in zinc bath. The melting point of aluminum is 658.7 Centigrade, and the dissolution rate of aluminum is very slow at 450 Centigrade galvanizing temperature. At the same time, aluminum is easy to be oxidized in the air and forms a compact A12O3 protective layer on the surface, which prevents aluminum from dissolving and diffusing in the zinc bath. Therefore, it is not advisable to add aluminum ingot directly in zinc bath, because the surface oxide film of aluminum ingot makes it difficult to dissolve aluminum in zinc bath. Once dissolved, it is easy to cause excessive local aluminum content in zinc bath, so it is easy to form scum which may cause local plating aid failure and leak plating. Therefore, the addition of aluminum in zinc bath should adopt the method of adding Zn-A1 master alloy, the content of A1 in this master alloy is generally 4%-10%.
6.In order for aluminum to mix quickly and fully, the alloy must be loaded into the container with holes and put into the bottom of the zinc pot. In addition, aluminum is often added to compensate for loss (due to oxidation and depletion of the coating), and the amount and number of additions must be determined according to the results of each zinc bath. In order to prevent excessive aluminum, the principle of "less frequent" should be adopted at the same time, such as adding two times per class. In order to quickly determine the content of zinc, a small amount of ammonium chloride grains can be sprayed on the surface of slightly oxidized zinc. When A1 is lower than 0.007%, the oxide film is dissolved and the grains move freely, but when Al content is higher, the grains stop on the surface and evaporate gradually.