Effects of Adding Pure or Scrap Aluminum to the Zinc Bath
I have heard of galvanizers using scrap aluminum plate or pure aluminum to add to the zinc bath as an alternative to purchasing Zn/Al brightener bars to change the concentration. Are there any adverse effects caused by the addition of scrap or pure aluminum in the zinc bath?
Aluminum is a very common bath additive and is typically added to the kettle at concentrations between 0.001% - 0.005% to:
- Increase brightness of the galvanized coating
- Reduce zinc ash formation
- Improve zinc flow/drainage
- Reduce spangle size
- Control coating overgrowth on high-silicon steels
However, it is important to note the aluminum content of the zinc melt for the wet galvanizing process must not exceed 0.002%. This is because aluminum can react with the zinc-ammonium chloride flux used in wet galvanizing. Higher concentrations of aluminum than the maximum recommended value can cause defects such as bare/black spots in the galvanized coatings. For the dry process, this maximum value is higher (0.007%) since the reaction between the aluminum and the top flux (forming aluminum chloride) is not an issue.
For adjusting the aluminum content in the zinc-bath to acquire these benefits, the aluminum must be evenly distributed until the desired concentration is reached.
Because pure aluminum is less dense than zinc and has a melting temperature (1220F) higher than a typical zinc bath, wasted aluminum will float to the top of the zinc melt and there will likely be issues distributing the aluminum consistently within the bath. In order to avoid sections of the zinc melt containing a higher aluminum concentration than desired, pure aluminum should be carefully distributed and well mixed within the bath as it dissolves.
It is not recommended to use scrap aluminum due to the additional concern of undesired and uncontrolled amounts of impurities such as iron, copper, chrome, manganese, and magnesium found in common aluminum alloys.
In order to best distribute aluminum and avoid bare spots, it is recommended to add aluminum to the zinc bath in the form of Zn/Al alloy bars (brightener bars) because the lower melting points of Zn/Al alloys more closely coincide with the zinc bath temperature. Zn/Al brightener bars should not be floated on top, but instead plunged beneath the surface of the bath to the bottom and mixed well to promote even distribution as the aluminum rises. ASTM B860 lists the recommended varieties of brightener bars (4, 5 or 10% Al content) and also the maximum allowable impurity contents.
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