5 Steel Spangle Myths You Need to Know
MYTH #1: All hot-dip galvanized steel sheet made today has a visible spangle.
FALSE! Actually, galvanized sheet now has fewer spangles than ever before.
But that doesn’t always mean this was false. In fact, in the years leading up to the 90’s, visible spangle helped buyers confirm they were buying good product. Although this practice was highly inaccurate, buyers thought more spangle meant more zinc.
Now that technology has improved, spangle is no longer a foolproof way to identify how much zinc is on the strip’s surface – and therefore, how much corrosion protection – the product will offer. You can thank changes in the zinc refining process and in the galvanizing process and lead restrictions (1992) for the shift.
When you’re buying galvanized, here’s an important thing to keep in mind: today, the smaller the spangle and the smoother the surface, the better the corrosion resistance.
MYTH #2: Spangle is caused by exclusively by zinc.
FALSE! While it’s true that you won’t get a spangle unless you run sheets through a zinc bath, the most common reason for a well-defined spangle is the presence of lead in the coating.
To be clear, we’re talking about the most common type of spangle (called dendritic growth pattern), which means the spangle that looks like snowflakes or trees.
GalvInfo tells us that lead is a common impurity in zinc, and the varying distribution of lead particles across the surface of the coating defines whether it’s dull, or shiny and spangled.
In recent years, technology has been able to refine the process to remove the iron and lead, meaning less spangle and purer coatings.
MYTH #3: Spangle develops when molten zinc is cooled below its melting point.
TRUE! Spangles develop when the molten zinc on the sheet is cooled below the melting point of zinc.
Once the temperature drops to approximately 787°F, the atoms in the liquid zinc begin to position themselves into an ordered arrangement by process of solidification or crystallization.
In the case of zinc, the crystals form in hexagonal symmetry, creating that six-fold, snowflake shape we’ve come to recognize.
MYTH #4: Spangle can be controlled to a specific size and compliance.
FALSE! Because spangle size is affected by factors including zinc chemistry, cooling rate and the smoothness of the substrate, it’s incredibly difficult to control the formation of spangling.
This is also the reason that galvanized spangle is broken up into three categories: regular spangle, minimized spangle and spangle free.
MYTH #5: Buyers can order by spangle size.
FALSE! Generally is it not possible to order by spangle size.
Because spangle sizes vary greatly from producer to producer and different spangles are not available in all parts of the world, it can be difficult to obtain steel with specific spangle requirements.
Keep in mind that as more and more lead is removed from the coating process, spangle will become less visible to the naked eye.
To learn more about spangle, take a look at GalvInfo note, The Spangle on Hot-Dip Galvanized Steel Sheet.