Chemical Requirements for Zinc Melt

Chemical specifications for commercial zinc according to ASTM Standard B6 are as follows:

Chemical Composition, %
Grade	Color	Lead	Iron, max	Cadmium, max	Aluminium, max	Copper, max	Tin, max	Total Non-Zinc, max	Zinc, min by diference
Special High Grade (Z13001)	yellow	0.003 max	0.003	0.003	0.002	0.002	0.001	0.010	99.990
High Grade (Z15001)	Green	0.03 max	0.02	0.02	0.01	…	…	0.10	99.90
Prime Western (Z19001)	Black	0.5-1.4	0.20	0.20	0.01	0.20	…	2.0	98.0

The presence of elements other than zinc has a significant effect on the properties of the hot-dip galvanized product as well as the quality of zinc coating. The deliberate addition of specific elements to the molten zinc bath can improve the quality and lifetime of the coating, while certain impurities and contaminants have a detrimental effect on the properties of the galvanized steel.

This section is dedicated to a number of elements commonly found during the hot-dip galvanizing of steel products along with a brief illustration of the effect of each element on the quality of galvanizing coating.

Iron can enter the molten zinc bath once the iron/steel components are immersed in the melt. The reactions taking place between zinc and iron are the primary phenomena responsible for the formation of galvanizing coating. If the concentration of iron in the bath exceeds a critical value, the bottom dross is formed, which is a porous precipitate at the bottom of the kettle and generally contains ~20-60 % zinc in metallic form. The bottom dross needs to be treated in order to recover the metallic zinc.

The presence of around 0.1 % cadmium in zinc ingot is inevitable. Cadmium is advantageous in prohibiting the oxidation of molten zinc and forces the insoluble iron particles to stick together and form agglomerates, which is beneficial in preventing the dross from floating inside the bath.

Lead can be present in the zinc melt to up to 1.4 %. The presence of lead in the molten zinc bath can decrease the surface tension of the melt. This is especially advantageous for the extra melt to drip easily from the galvanized surface once the component is withdrawn from the bath; otherwise, visible drainage spikes, also known as zinc tear drops, will be left at the surface of the galvanized product.

The maximum concentration of copper in molten zinc bath is 0.5 %. The solubility of copper in zinc at 450 ^oC (temperature of the melt) is 2 % based on Cu-Zn phase diagram. If the concentration of Cu exceeds this value, it can cause embrittlement of the galvanizing coating. The high concentration of copper in molten zinc bath can be observed as dark red particles accumulated at the surface of the melt.

The presence of magnesium in zinc bath is generally beneficial in enhancing the corrosion resistance of the zinc protective coating. However, a high concentration of this element in the melt produces too much zinc ash, which increases zinc consumption and reduces the efficiency of the process.

Tin is present in very small concentrations of ~ 0.001 % in the zinc melt. Tin in proper concentration can increase the fluidity of zinc melt, and reduce the zinc solidification time, hence preventing peeling. Too much tin in the melt, however, tends to increase the coating embrittlement.

The presence of antimony in the molten bath produces lustrous zinc coatings. The critical concentration of antimony is 0.1 %, above which antimony trioxide is formed in the zinc coating causing embrittlement.

Aluminium enhances the malleability of the galvanizing coating and facilitates the dripping of extra zinc melt from the galvanized surface after withdrawal from the bath. Aluminium forms a protective oxide interlayer between the zinc coating and the steel surface, which prevents the Zn-Fe intermetallic phases from forming and prevents the overgrowth of zinc coating. The critical concentration of aluminium in molten zinc bath is 0.005 %, above which the risk of peeling increases.