What’s the Difference Between Coating Thickness and Weight?
Often the industry uses coating weight and coating thickness interchangeably, especially when ordering zinc-coated sheets and coils.
But in fact, weight and thickness are two distinct entities, and it’s valuable to understand the difference between the two to ensure you’re accurately ordering from your supplier.
Let’s start with coating weight for HDG, which is defined by standards set by ASTM (a governing body that defines technical standards for coatings).
Hot-dip galvanized product is designated by descriptors you’ve probably heard thrown around pretty frequently: G40, G60, G90, etc.
In these cases, the “G” means the coating is galvanized (zinc) and the numbers refer to the weight of the zinc on the surface of the steel sheet. The weight is signified in inch-pound units.
So, for example, if a sheet has a coating weight of G90, that means there’s an average minimum of 0.90 ounces of zinc per square foot, or 0.45 ounces evenly applied per side. Pretty easy to understand: 0.45 x 2 (both sides) = 0.90. To find this measurement, a simple, and inexpensive triple spot test is used.
So now we know that weight in ounces gives us guidelines for the coating weight, but as Galvinfo points out, most of us care more about how thick the coating is.
After all, it’s the thickness of the coating that will wear away over time and cause corrosion to our base metal, right?
Here’s why we don’t refer to coating thickness instead of weight:
It’s too hard to measure.
Wait, what? We can almost hear the collective shouting: “It’s too hard, and that’s why we don’t do it?”
Before you get all riled up, let’s look at that same G90 coating and describe it in thickness instead of weight.
A G90 coating contributes about 1.6 mils (0.0016 inches, or roughly 42 microns) to the total thickness of the sheet, according to Galvinfo. “For a coating equally applied to both sides of the sheet, this means there is about 0.0008 inches (21 microns) of zinc on each surface,” they add.
So, to determine the thickness accurately, we’d need a gauge capable of accurately reading to the nearest ten-thousandth of an inch. As you can imagine, that’s no easy feat.
But let’s say that you really, really want to measure thickness instead of weight so you can sleep soundly at night.
Luckily for you, there’s a technique that uses x-ray devices that is incredibly precise. However, Galvinfo notes that it requires considerable expertise to operate, and once the thickness is measured on-line, it’s usually converted to the more familiar coating weight designation anyway.
So, now you know: while thickness is technically the most accurate way to measure coatings exactly, it’s incredibly difficult, expensive and labor intensive, which is why the industry has adopted coating weights as the global standard.