We all know that galvanizing steel is a process that contains mostly zinc. But often, we overlook a key component of the galvanizing process: aluminum.
That’s right. Small amounts of aluminum are present in molten zinc baths and serve several important functions.
Let’s take a look at why aluminum is integral to properly galvanized steel.
FACT #1: Aluminum ensures optimum coating adhesion is achieved
Don’t let anyone tell you otherwise – this is by far and away the number one reason that aluminum is used in zinc coatings.
Because even small amounts of aluminum help to create an alloy with the iron in the steel substrate, aluminum is a key ingredient in strengthening the bond between the steel and the zinc. We’ll get into more details on how this happens as we move along.
FACT #2: Despite common belief, aluminum isn’t present for corrosion protection
If you ask around, many people will tell you that aluminum helps keep the substrate safe. That’s false, as aluminum’s influence on corrosion behavior is generally insignificant.
While it does help the alloy bond (which protects the steel), aluminum in and of itself is not responsible for substrate corrosion protection.
FACT #3: It improves formability
Without aluminum in the bath, the alloys are very hard and brittle instead of soft and pliable. By adding even tiny amounts of aluminum into the bath – sometimes as little as 0.15% – the alloy layer thins, which in turn slows the zinc’s reaction rate.
And what does a thinner layer mean? More malleable sheets. So, now you know: aluminum is to thank for why we can shape galvanized into everything from fenders to roofing panels.
FACT #4: It has a strong affinity for iron
You might be wondering why aluminum has such a profound impact, even though it occurs in such small amounts? That’s because aluminum loves iron – well, at least more than it loves steel substrate. As quickly as 0.15 seconds after the steel enters the coating bath, an aluminum-iron, not a zinc-iron compound is created.
Essentially what this means is that the growth rate of the alloy layer slows long enough to thwart a zinc-iron reaction. When that happens, it takes zinc longer to break through the aluminum-iron barrier, which results in a thinner alloy layer.
And, according to GalvInfo, a thinner layer “allows the sheet to be bent or formed without the alloy layer cracking and resulting in loss of coating adhesion.“
To learn more about aluminum’s role in zinc coating, take a look at GalvInfo note, The Role of Aluminum in Continuous Hot-Dip Galvanizing