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