Dross and Nickel in the Kettle
After I added nickel to my kettle, my dross problem seemed to get worse. What caused this?
Dross formation is one of the largest contributors to zinc loss in the galvanizing process. Dross particles form in the galvanizing kettle through the reaction between iron and zinc. Dross particles may either present themselves as floating particles at the top of the zinc bath, or dross which settles to the bottom of the kettle. Its formation can be largely controlled by monitoring the temperature of the bath and limiting the amount of iron particles or iron salts introduced into the bath.
Small iron particles left on the surface of the steel after pickling and fluxing may be introduced into the molten zinc bath. These small iron particles may actually be completely dissolved in the molten zinc bath and not create any dross if the dissolved content and temperature are at suitable levels. Dross formation can occur when the temperature of the molten zinc bath drops below the precipitation point of the dissolved iron. Iron particles will fall out of solution, trigger a diffusion reaction with the zinc, and create dross particles in the kettle. These particles cannot be simply removed by increasing the temperature of the bath to the previous levels. They must be removed through mechanical processes. Keeping the bath at a constant temperature and limiting the amount of iron dissolved in the bath with proper rinsing procedures, will decrease the amount of dross formed in the kettle.
However, even if the dissolved iron content and bath temperature are kept at constant levels, dross may still be formed after an elemental addition of nickel into the bath. This is due to the fact that the presence of nickel in the galvanizing bath decreases the solubility of iron in the kettle. The chart below, courtesy of Teck Metals Ltd., shows that as the concentration of nickel is increased in the galvanizing kettle, the solubility of iron decreases at a fixed temperature.
As an example, a molten zinc bath may be completely saturated or nearly saturated (meaning the bath has dissolved nearly all the iron it is capable of) of dissolved iron at 842 F. If these conditions were held constant with no other outside factors, there would be no new dross formation as all iron would remain in solution. If, however, through an elemental or alloy addition to the kettle the nickel concentration is raised from 0.020% to 0.055%, the solubility and the total amount of iron the molten zinc bath is capable of dissolving will be decreased. The zinc bath will now be completely saturated with iron. Since there is no decrease in the total amount of iron in the bath the only option for the excess iron is to particulate and fall out of solution. When this happens, the iron particles will react with the zinc and form dross. Typically, dross formation occurring after a nickel addition tends to settle near the bottom of the kettle in the form of bottom dross.
Typically, the formation of dross can be slowed by maintaining a constant kettle temperature, using an inhibitor in the pickling tanks, thoroughly rinsing after the different stages of cleaning, or other similar methods. But the chemistry of the galvanizing kettle will also have an impact on the solubility of iron, and therefore have an impact on dross formation. While the two may seem like unrelated issues, an increase of nickel concentration in the galvanizing kettle may have a large impact on the amount of bottom dross formed.
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