The Spangle on Hot-Dip Galvanized Steel Sheet
Rev 2.1 Jul 2016

Lead is a metal found in most zinc-containing ores, and this refining process carried it through as an impurity in the zinc. From the early days of galvanizing its spangled appearance was therefore a natural and expected result. It easily identified steel as being galvanized, so much so that if a spangle wasn’t visible, the steel was not considered to be galvanized. This was a problem in some markets when zero-spangle, hot-dip galvanize was first introduced, and even helped to reinforce the misconception by many users that spangled galvanize was “better” than non-spangled product.

The first galvanize coatings contained as much as 1% lead. Increasingly over the past 50 years, the presence of such high lead levels has become less and less common in galvanized steel sheet, at least in North America, Europe, and Japan. Typical concentrations of lead in most hot-dip galvanized sheet made up to about 25 years ago was usually less than 0.15%, and sometimes as low as 0.03 to 0.05%. Even this lower amount of lead is still sufficient to develop the dendritic growth behaviour during coating solidification that results in a spangle.

As there is now much more concern about the environmental and health hazards of lead, some galvanized sheet manufacturers that wish to market a spangled product have established practices that use lead-free zinc, but add a small amount of antimony to the zinc coating bath. Antimony influences spangle formation in a similar fashion to lead. The final result is a relatively smooth, visibly spangled coating. Typically, the amount of antimony in the coating bath is about 0.03 to 0.10%.

Years ago, the production of zinc from zinc-containing ores was changed to an electrolytic recovery process. In this method of zinc production, the refined zinc is very pure, with lead being excluded. This change was in place at the time many users of galvanized sheet, especially those desiring a high quality finish after painting, such as the automotive and appliance industries, needed a zero spangle coating. Removing the lead gave them the product they desired. The lead level in zinc used to produce zero spangle product is a maximum of 0.007% (70 ppm), and often less than 0.005% (50 ppm).

The combination of high processing speeds and low coating weights allows producers to use lead-free coating baths, avoid the development of spangles, and still attain a ripple-free coating. Improved gas-wiping technology and practices have also helped in producing smoother coatings.

PLB

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