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How to Choose the Right Coating Thickness

If the GalvInfo inquiries list is any indication, the industry is a little confused on which coating thickness to use, and when.

While it’s impossible for us to give you an exact answer, reading through the prompts below can help you begin to determine the correct coating for your specific end use.

1. Understand the lingo

While your first reaction may be to refer to the amount of coating on steel sheet as its thickness, your supplier probably refers to thickness as coating weight, or in rarer cases, coating mass.

That’s because while “the thickness of the zinc coating largely determines its ultimate life, it is not used directly to specify the amount of coating,” according to GalvInfo.

So, what is used to specify the amount of coating?

When ordering steel sheet with zinc coating, coating weights are defined by a set of ASTM specifications.

As part of those standards, you may have heard common coating weight designations that use descriptors like G40, G60 and G90.

GalvInfo explains: “The ‘G’ means the coating is galvanize (zinc) and the numbers refer to the weight of the zinc on the surface of the steel sheet in inch-pound units.”

Let’s look at an example of how the numbers come about:

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.

When applied evenly on both sides of the sheet, a G90 coating designates that there is a minimum of 0.45 ounces on each square foot of surface. This weight is often determined by a triple spot test (TST).

Here’s something else you should consider: different coating weights add different thicknesses to your base metal thickness.

For example, a G90 coating weight adds .0015 thousands to the base metal thickness, while a G60 adds .001 thousands, and G40 adds .00067 thousands.

Thickness is easier to grasp than weight, but it’s still important to identify these subtle difference between the coatings because they could equate to years of service life.

2. Understand the environment

As you may imagine, different conditions cause corrosion faster than others.

Time of wetness, for example, is a major contributor to faster corrosion rates. For that reason, a piece of galvanized steel sheet in dry Colorado air would theoretically corrode slower than that same sheet in the wet Florida marshlands.

Other variables, including temperature and air impurities can also have dramatic effects on how the zinc will corrode over time.

For this reason, it’s imperative that you do your due diligence in understanding the conditions your steel will be up against before ordering.

3. Don’t just go for the heaviest coating weight

Based on what we’ve just learned, it may be tempting to order the heaviest coating weight and call it a day. However, that’s not the best course of action.

For starters, more zinc on the sheet means higher costs, so a coating weight of G40 would be cheaper than a coating weight of G90.

If your application doesn’t need a G90 coating, purchasing one can put unwarranted strain in your budget.

Further, zinc coatings corrode slowly, so the difference of a decade or more may not be a consideration when you’re talking about coatings that can withstand 95+ years in certain environments.

4. Determine your deal breakers

While it’s true that zinc coatings can last a lifetime, it’s also important that you consider what failure looks like for you.

For example, if you’re using galvanized sheet on a roof, failure could be the appearance of any amount of red rust.

Conversely, for applications where aesthetics aren’t a consideration, a small amount of red rust may be acceptable if it doesn’t interfere with performance.

Outlining these deal breaking failures upfront can save you headaches over the product life of your sheet.

5. Determine the product life

Speaking of product life, it’s one of the most important aspects to consider when choosing your coating weight.

We’ve already learned that corrosion rates can vary considerably depending on environments, but let’s say that you want to double the life of your sheet for a specific application.

In that case, GalvInfo says that twice the coating weight is required:

“For example, a G60 coating weight will exhibit approximately twice the life of a G30 coating weight, and a G90 coating will exhibit 50% longer life than your G60.”

Again, in many cases, we could be discussing the difference of many decades, so it’s always important to look at the full scope of your project before committing to a specific coating weight.

For more information on coating weights, check out our longer form technical paper: Understanding Zinc-Based Coating Weights: G30, G40, G60 and G90.

Have questions on an individual project? Contact GalvInfo at info@galvinfo.com.

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