Aluminum Concentration in the Zinc Melt
How do I add aluminum or adjust aluminum concentration in the zinc melt using brightener bars?
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
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. 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). ASTM B860 lists the recommended varieties of brightener bars and maximum impurity levels. Of the available brightener bar compositions, Type B-2 (90/10, Zn/Al alloy) is most often used in the industry since it adds the maximum amount of aluminum to the bath at a melting temperature which most closely coincides with the melting temperature of the feed zinc. It is not recommended to use pure or scrap aluminum due to the concern of undesired and uncontrolled amounts of impurities such as iron, copper, chrome, manganese, and magnesium found in common aluminum alloys. Also, because pure and scrap aluminum are less dense than zinc and have a melting temperature (1220F) higher than a typical zinc bath, much-wasted aluminum will float to the top and there will likely be issues distributing the aluminum consistently within the bath.
Target and Maximum Recommended Aluminum Levels:
The target aluminum concentration for the dry galvanizing method (separate flux tank) is 0.002%0.004%. Concentrations of aluminum above the maximum recommended value of 0.007% can cause defects such as bare/black spots in the galvanized coatings. The target aluminum concentration for the wet galvanizing method (flux layer floating on the zinc) is 0.001% 0.002%. Concentrations of aluminum above the maximum recommended value of 0.002% can react with the zinc-ammonium chloride flux used in wet galvanizing.
Approximating the Minimum Quantity of Brightener Bars:
The following variables must be recorded or calculated in order to approximate the minimum quantity of brightener bars required to achieve target Al concentrations:
- Density (?zn) of the zinc, acquired by the manufacturer (lbs/ft3)
- Volume (i.e. Length x Width x Height) of the melted zinc (ft3)
- Current Al Concentration % (0% for new kettles)
- Target Al Concentration %
- Average weight of 90/10 bars
Then the following calculations1 must be performed in order:
Because aluminum is less dense than zinc, it is likely that some aluminum will rise and not be distributed within the zinc melt. As a result, the calculated quantity of Zn/Al brighter bars is the absolute minimum required to achieve the target level of aluminum. The percentage of undistributed aluminum varies from kettle to kettle, and the galvanizers experience will determine any amount of additional aluminum required. When building up the aluminum concentration within the bath, do not attempt to go over the target value. Distribute the minimum required quantity of brightener bars within the bath and take samples from the bath for analysis by the zinc supplier. More frequent sampling may be required when building up the initial aluminum concentration within the bath until the aluminum level has stabilized.
Distributing Brightener Bars in the Zinc Bath:
New kettles should be filled with molten zinc before adding alloying elements to the bath. 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. This is preferably achieved by placing brightener bars underneath a zinc bar make-up load, lifting the loads together, placing in the kettle, and moving slowly back and forth down the length of the kettle until the entire load is melted. Aluminum additions should be performed in small amounts to avoid major fluctuations in concentration and allow for even distribution. A typical aluminum addition frequency is 1-2 bars per shift.
1: The calculations assume the amount of zinc blocks placed on top of the brightener bars to assist with even distribution is negligible when calculating the quantity of bars required to achieve target aluminum levels in the zinc melt.
© 2020 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.