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G30, G40, G60 AND G90: UNDERSTANDING ZINC-BASED COATING WEIGHTS

Zinc-coated, or hot-dipped galvanized, steel is a widely used material in numerous applications, thanks

to its corrosion-resistance, strength and relatively inexpensive cost. Zinc offers protection to the steel by

forming a barrier against the elements, as well as sacrificing itself to prevent steel corrosion.

The amount of zinc coating applied over top the core steel for corrosion protection varies depending

upon end-use and specification requirements, and can be controlled and measured to industry standards.

Zinc coating weights are listed in the U.S. measurement system in ounces per square foot (oz/ft2), and in

the metric system as grams per square meter (g/m2). In both systems, the amount of zinc measured is the

total average for both sides of the core steel.

Common coating weights for galvanized steel are G30, G40, G60 and G90. It is important to note that

these coating weights designate a total of 0.30, 0.40, 0.60, and 0.90 oz/ft2, respectively. While these

are very common galvanized coating weights used in the construction and appliance industries, as well

as other industries, zinc coatings weights can sometimes exceed G300, or 3.00 oz/ft2. A typical use for

a coating weight of that caliber could include highway guardrails.

The corrosion resistance, and thus, the service life, is proportionate to the amount of zinc coating on

the steel. Hence, the service life of a G90 will generally be 3 times that of a G30 and 1.5 times that of a

G60 in the same environment. However, the service life of any galvanized steel product is dependent

upon the environment in which it is exposed.

It is extremely important to be aware of which zinc coating weights are appropriate for specific ap-

plications. If the zinc coating weight applied is less than required for its end use, early corrosion or

shortened product service life may occur. If the zinc coating weight is more than necessary, there are

no negative performance ramifications, but the total project cost could be higher than necessary.

Steel is one of the most widely used materials in the world, forming a variety of structural, functional and

aesthetic products from skyscrapers to HVAC units to high-end art. The relatively low cost, high strength

and formability of steel makes it an appealing option for countless applications.

Unfortunately, bare steel has a tendency to corrode. According to Galvinfo.com, regarded by many

in the industry as the leader in zinc-coated steel sheet information, corrosion is an electrochemical

process that, in the case of steel, oxidizes the iron in the steel and causes the sheet to thin over time.

The oxygen and steel react, causing rust to form. Rust not only creates an unattractive appearance, but

also threatens the stability and integrity of the material, which could ultimately lead to product failure.

EXECUTIVE SUMMARY

INTRODUCTION:

GALVANIZED STEEL

The International Zinc Association estimates that steel corrosion can cost an industrialized countrys

economy up to 4 percent of GDP each year. Not to mention, a corroded product can lead to expensive

repairs or replacements for businesses, or to negative reviews for the product manufacturer.

Luckily, there are a number of methods employed to protect steel from corrosion. Steel can be inter-

nally alloyed (creating stainless steel) or painted, but one of the most effective and economical options

is zinc coating, or galvanizing, the steel. Galvanized steel is a proven, economical and effective way to

protect bare steel with a surface that is more resilient than paint on its own, although paint can still be

applied as a secondary layer of protection over the coated steel.

Zinc is a naturally-occurring element present in rock, soil, water, the biosphere, plants, animals and

humans. More than half of the 12 million tons of zinc produced annually worldwide is used for galvaniz-

ing steel to protect it from corrosion, according to the International Zinc Association. Zinc is beneficial

as a steel protectant because it is formable, corrosion resistant and recyclable.

Zinc-coated steel, or galvanized steel, is protected from rust in two ways as a physical barrier and by

cathodic protection. As a physical barrier, zinc coating blocks moisture and oxygen from reaching the

steel underneath. Zincs inherent corrosion resistance is a result of the elements ability to form dense,

adherent corrosion by-products, which leads to a rate of corrosion considerably lower than iron-based

materials up to 10 to 100 times slower, according to the American Galvanizers Association (AGA), a

not-for-profit trade association dedicated to serving the needs of specifiers, architects, engineers,

contractors, fabricators, and after-fabrication hot-dip galvanizers throughout North America.

The by-products, known as zinc patina, serve as an additional barrier protection for the steel. The bar-

rier coat keeps moisture, salt, other liquids and corrosive atmospheres away from the steel.

Cathodic protection references zincs ability to protect steel by electrochemically sacrificing itself.

Zinc is able to do this because it is a less noble metal than steel. Therefore, when steel is exposed

through a scratch or defect, the steel itself cannot corrode adjacent to a zinc coating only the sur-

rounding zinc will corrode.

For a metal to be noble, it means that the metal is resistant to corrosion and oxidations in environ-

ments where moisture is present. The Galvanic Series determines the nobility of metals and displays

them according to their resistance to corrosion.

Properties and Characteristics

of Zinc Coating on Steel

MA JESTICSTEEL .COM HOW TO BE A BETTER STEEL BUYER 2

Potential (VOLTS)Galvanic Series Chart

More noble metals are the least susceptible to corrosion. As you can see in the chart, platinum, gold

and silver are the most noble metals, and are not vulnerable to corrosive attack. However, steel is less

noble, which means oxygen and water causes steel to red rust or oxidize.

When two metals or more are in contact with each other, the less noble will oxidize before the more

noble, says Allen Garrett, Director of Business Development at Majestic Steel, a steel solutions ser-

vice center headquartered in Cleveland, Ohio. Moreover, the further apart on the Galvanic Series the

metals are, the quicker the less noble metal will oxidize.

Magnesium, zinc, cadmium and aluminum are the metals that are less noble than steel, and therefore

will sacrifice themselves for the core steel in cathodic protection.


However, not all of these metals are conducive to high-volume commercial coating for a few different

reasons:

1. Magnesium: expensive and explosive if handled incorrectly, making it difficult to work with.

2. Zinc: less expensive than magnesium with a relatively low melting point and is fairly safe.

3. Cadmium: extremely expensive.

4. Aluminum: good for certain steel coating applications, most commonly when the final product is

exposed to heat.

Therefore, historically zinc and aluminum are the two metals most often used to coat steel to protect

it from corrosion.

Lest you think things are set in their ways, however, recent developments have started to include small

percentages of magnesium into metallic coatings just one reminder that innovation can occur even

after years of standard protocol.

Zinc coating thickness alone can be misleading when evaluating zinc applied to steel using different

processes, according to the AGA. Therefore, the typical designation used to consistently evaluate the

protection value of the zinc is coating weight. The coating weight determines the coating life in con-

junction with time, atmospheric conditions and environmental circumstances. Humidity, the presence

of water, oxygen, sulfates, temperature, chlorides and nitrates can all dramatically affect the rate of

corrosion for zinc coatings, according to Galvinfo.com.

Galvanized steel is given designations that indicate the coating weight, specified by the American

Society for Testing and Materials, also known as ASTM International. ASTMs job as a governing body

determines the amount of zinc needed on the strip based upon a customers request in order to meet

end-use requirements.

As outlined earlier, common designations include G30, G40, G60, and G90, though zinc coating

weights span the gamut and range all the way up to G360. ATSM specifies that the coating bath chem-

istry for galvanized be no less than 99 percent zinc, and contain between 0.05-0.25 percent aluminum

to promote adhesion.

The G means the coating is galvanic zinc while the numbers are in reference to the weight of zinc

on the surface of the steel in ounces per square foot, or oz/ft2 units.

How Zinc Coating Weights are

Determined for Galvanized Steel


Buying Blanket

or Direct Trade

So for G90 steel, when tested with a triple spot test (testing at one edge, the center of the sheet, and

the other edge), one square foot of sheet steel coated with zinc on both sides should have an average

minimum coating weight of .9 ounces. Assuming the coating is applied evenly, this means there is .45

ounces on each side of the sheet steel.

The table shows the amount of zinc applied by designation in both the U.S. and corresponding equiva-

lent metric measurement systems. This shows the thickness in mils (0.001 of an inch), and also in

microns, as well as the metric equivalents.

ASTM sets the minimum coating weights for each coating designation, which are determined accord-

ing to specified testing methods, called a triple spot test and a single-spot test.


A triple-spot test is used for samples from product more than 18 in width, and it takes the average

of three checks per side one from the middle and one at least 2 from each edge. The total of the

average coating weights of each side must be equal to or greater than the specified minimum for that

designation.

For G90, the total of the per-side average coating weights must be 0.90 oz/ft2. But because the coat-

ing will not be exactly equal on both sides, there is also a minimum allowance for the triple spot one-

side average. For G90, this is 0.32 oz/ft2. There is also some variation in coating weights across the

width of the product, so the test requires that no single spot be too far below the specified minimum.

The single-spot minimum considers the total of the weights from each side in one location instead of

the average. Again for G90, the single-spot minimum is 0.80 oz/ft2. There are also times when the

product is less than 18 wide, and a triple spot test is not possible. In this case, the single-spot test is

performed, and the specified minimums for each designation must be met as well.

What does this mean in terms of corrosion resistance?

Remember that the coating weight determines the coating life which is the basis for corrosion re-

sistance for steel. Depending on the conditions to which the steel is exposed, the coating life varies.

The corrosion resistance is in linear proportion to the amount of coating on the strip, says Jeff Reall,

Technical Advisor at Majestic Steel. As you increase the coating weight, you increase the service life of

that product.

Generally speaking, if a G30-coated steel product lasts five years until it starts to red rust or corrode,

then a G60 will last 10 years and a G90 will last 15 years. In other words, a G60 coating has twice the

thickness of a G30 coating, and the life of the product in a given environment is approximately twice

as long. Using these assumptions, a G90 coating is nearly 50 percent thicker than G60 and can be ex-

pected to perform 50 percent better, according to Galvinfo.com.

If you add the top and the bottom, the total zinc thickness on the G90 is about .0015 inches about half

the thickness of a hair. Therefore, a G30 adds .0005 inches 1/16 the thickness of a hair.

There are several ways that zinc can be applied to steel, the most common of which is called continu-

ous hot-dipped galvanizing. In this process, coils of flat-rolled steel sheet are unwound and welded

end-to-end to produce a continuous strip of steel that will run through the galvanizing line. The steel is

cleaned with an alkaline detergent, annealed and then passed through a molten zinc bath.

Applying Zinc Coating to Steel


To control the coating to the desired thickness, air knives are used. These air knives are operated by

gas pressure and held at designated heights, angles and distances to the strip in order to achieve

the appropriate coating weight. Either air or nitrogen can be used. Nitrogen has been proven to limit

oxidation and improve surface smoothness, so it is often used in air knives for applications that require

high surface quality. The fluid process by which excess zinc is removed is called jet stripping.

Once these processes take place, the steel is inspected to make sure it meets requirements. To en-

sure accuracy, x-ray gauges are used to measure the steel after the coating is complete, providing

closed-loop feedback for the automated coating thickness control system.

When purchasing steel, it is important to specify the correct coating weight on the purchase order

to ensure optimal performance based on the environmental conditions to which the steel will be ex-

posed. Other factors to consider when choosing the proper coating weight are the total cost of the

materials and the appearance of the final product.

If an improperly thin coating weight is used, early corrosion and a shortened service life for the prod-

uct may occur. On the other hand, using a thicker coating weight than needed could prove to be an

expensive mistake. For these reasons, contractors and manufacturers should be fully aware of which

coating weight is appropriate for their given application and environment.

Often, people arent sure which coating weight they need for their application, and sometimes the

higher coating weight isnt the better one, says Stephen Wolf, Business Analyst at Majestic Steel.

Air Knives and Coating Line

Using Correct Coating Weights

in Dif ferent Applications


Theoretical Nominal

Weight or Theoretical

Minimum Weight

What is the desired product life given the exposure to corrosion that the steel is going to face? In

agricultural applications that will be exposed to Mother Nature and will likely be assaulted by rain,

sun, wind and other environmental factors, the steel should have a higher coating weight. Conversely,

something inside the home may be able to pass with a lesser coating weight because its not exposed

to the elements all day. Well discuss anomalies to this example in just a moment.

Is appearance a factor for the final product? On basic galvanized product, the general rule of thumb

is that the higher the coating weight, the less consistent the surface finish of the coating. That doesnt

mean that corrosion-resistance is impacted in any way; its just that the material can sometimes be

less appealing visually, with small blemishes and inconsistencies. For that reason, material with a G30

or G60 coating weight will probably have a more consistent coating than a G90 or higher.

One way to obtain a more consistent finish, regardless of coating weight, is to order the material as

extra-smooth also known as skin-passed or temper-passed. When metal-coated steels are ordered

extra-smooth, the coated steel is run through a single-stand rolling mill after the coating is applied.

The rolls in the mill remove or mask any blemishes in the coating, creating the desired appearance.

This process also provides the added benefits of more consistent mechanical properties, better shape,

and a resistance to fluting. However, the buyer should know that the surface of the coating would be

duller and less light reflective than non-temper passed galvanized.

What is the total cost of ownership? Coating weights such as G90 and above are more expensive than

G30, G40 and G60. However, weve learned that in the same environment, the G90-coated steel will last

longer than a lesser coating weight.

There are applications where G30 and G40 are more appropriate, but it is important to consider the total

cost of ownership over the expected life of the product, says Wolf. Its like anything in life you pay a

little more for something nicer, assuming its going to last you longer.

Its important to note that there is no guarantee on any coating weight as far as corrosion resistance in

any specific environment. To better prepare and understand the different applications, environments

can be broken into general categories such as industrial, marine, agricultural and rural.


In an industrial environment, acid, soot and other pollutants come out of exhaust stacks, settle on

steel products and attack. According to the AGA, industrial environments are typically the most ag-

gressive in terms of corrosion. Most outdoor applications in city or urban areas can be classified as

moderately industrial.

In a marine environment, anywhere within approximately three miles of a coastal situation may fall

victim to airborne salt spray or sodium chloride that also can aggressively attack steel. In these cir-

cumstances, higher corrosion resistance may be necessary, depending on the actual application. If a

long service life is required, then a higher coating weight should be used.

Agricultural environments expose steel to fertilizers that go airborne, crop dustings and other factors

that can attack the product. Steel applications such as feeding bins, grain elevator and animal pens

must take into account the chemical and environmental corrosion threats that the product will face. In

agricultural applications, galvanized steel is preferable to aluminum materials such as aluminized or

Galvalume, because certain elements inherently present in animal waste attack aluminum very aggres-

sively. Therefore, applications such as animal containment structures or anything that may come in con-

tact with animals or animal waste would be better suited to galvanized steel over aluminum-containing

metallic coated steel.

Rural environments are much less aggressive than other environments, due to the relatively low levels

of sulfur and other emissions. For this reason, a lighter coating weight can often be used.

Each environment creates its own situation, but there are three basic factors that enable corrosion:

oxygen, water and the metal itself. If its dry, indoors and/or shielded from aggressive elements, me-

tallic coating weights allow for a longer service life. In more humid, outdoor applications, the service

life will be shortened. So, as a general rule, the higher the coating weight, the more service life the

product will exhibit in each of these settings.

Unpainted galvanized steel used in outdoor applications commonly use the G90 coating weight. G90

is also recommended for indoor applications where there may be dampness due to condensation,

though a G40 or G60 coating is usually sufficient for indoor uses that remain relatively dry, according

to Galvinfo.com.

In an interior environment where its environmentally controlled, you dont necessarily need a signifi-

cant amount of sacrificial corrosion protection to shield the core steel, so you would typically specify

a lower metallic coating weight, Garrett says. But in the environments where you have an electrolyte

like near marine environments where theres salt, or interior pool environments where chlorine is

present those electrolytes can help to accelerate corrosion.

INDUSTRIAL

MARINE

AGRICULTURE

RURAL


For example, the HVAC industry is one of the largest consumers of galvanized steel. For typical HVAC

applications, G30, G40 and G60 coating weights can be used in ductwork pipes, elbows and connec-

tions. Because of the thinness of the end use product, lighter coating weights are acceptable when

the environment is cool and dry. Ideally in the ductwork itself the main runs of the rectangular part

of the duct G90 coating weight should be used.

In addition, if the application is outdoors, such as a rooftop unit, or in a corrosive-inducing environ-

ment, such as an indoor pool, a higher metallic coating weight may be necessary. The Sheet Metal

and Air Conditioning Contractors National Association (SMACNA) drive these specifications. The

contractor for each job must follow SMACNAs standards to ensure proper coating weights for each

individual product.

Using the proper zinc coating weight is important for several major reasons. An application or prod-

uct that has a coating weight that is too low will likely produce a shortened product life span, as well

as the potential for unsightly rusting and structural damage. More importantly, safety is a huge factor

to consider in regards to load-bearing or architectural applications failing due to improper coating

weight specifications.

With commercial air ducts, people are breathing in air that comes through the system. If the

product corrodes earlier than expected, rust or mold may enter the facility, creating a signif icant

health concern when people breathe it in. Spending a little more on a higher-quality product like

G90, or installing antimicrobial-coated steel can help to maintain healthier atmospheres.

Load-bearing and structural applications pose obvious threats if there is product failure. Structural

applications in highly-trafficked areas may rust or corrode earlier than expected, collapse and cause

injury or worse if the incorrect coating weights are used. This type of situation is a huge liability for

manufacturers and can lead to what is known as a tear-out.

Tim Quinn, Sales Advisor at Majestic Steel, recalls a situation in which a tear-out took place due to im-

proper coating weights being used in an HVAC application. A building inspector at a major Ivy League

university performed a micrometer test on the material used in ductwork. The duct had already been

hung, with drywall covering it. Once it was determined that the ductwork did not meet code, it had

to be pulled out.

The money spent on the original product, the time spent to fabricate that product, the time and

money to transport the product to the job site and the labor to hang the ductwork, then the cost to

tear it out and completely reinstall it ended up wasted because the product wasnt up to code. Us-

ing the wrong coating weight for the project was not only a quality issue, but also posed safety and

environmental concerns.

How Using The Wrong Coating

Can Compromise Your End Use


You can have product failure in the field, short of the products intended service life, which creates

a big problem there, Reall says. For example, government paid-for facilities typically require G90

coating weight minimums for rectangular ducts in HVAC applications. If you get into a situation

where somebody finds that theres an inferior coating weight on the project, it creates quite an em-

barrassment for everyone involved.

These are just a few illustrations of why it is important to educate oneself on zinc coating weight

specifications and how they apply to particular applications.

Galvanized steel is a versatile, strong and affordable material that can be used in a myriad of applica-

tions. The zinc coating protects the steel from corrosion elements, providing a barrier and sacrificing

itself so the core steel may exhibit a longer service life.

Here are a few action items wed recommend taking to ensure the proper coating weights are used for

each application:

1. Do your research. To get the most out of your galvanized steel, its important to put education

before all else. Be aware that multiple coating weights exist, and each one is best suited to certain

environments and applications.

For more specific details on applications and end use, consult the sound educational outlets weve

outlined in this technical paper: Galvinfo.com, ASTM International, and SMACNA.

2. Talk to your supplier. Your supplier should know every detail of the steel they provide. From the

coating weight to the chemical makeup to common end use applications, the supplier can guide and

advise for best practices and usage to ensure optimal results.

3. Consider testing. If there is any doubt or question as to whether or not a product, structure or com-

ponent is the right coating weight, there are spot tests or lab surveys that can be conducted to verify

the material. Once the correct coating weight is determined, testing can confirm that the galvanized

steel in use is in fact the appropriate material.

Follow these steps to gain an understanding of proper zinc coating weights and specifications, and to

help ensure ideal service life and quality of your steel products.

ROADMAP FOR SUCCESS


DISCL AIMER THE MATERIAL CONTAINED IN THIS WHITEPAPER IS FOR INFORMATIONAL PURPOSES ONLY

AND IS PROVIDED AS IS WITH NO WARRANTIES, INCLUDING ANY WARRANTY OF MERCHANT-

ABILITY, TITLE OR FITNESS FOR A PARTICULAR PURPOSE. NO LICENSE TO ANY INTELLECTUAL

PROPERTY IS GRANTED HEREIN.


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