The appearance of general galvanized zinc coatings is largely a function of the chemistry of the steel being galvanized. The bulk of steel galvanized in the general galvanizing process comes from fabricators who purchase it from steel service centers and thus the chemistry is either not known or not specifically selected. Silicon and phosphorous of certain percentages in the steel yield the customary shiny silver coating and other percentages result in matte gray coatings. Regardless, after a short period of time (approximately 6 months or less in most atmospheric exposure conditions) the galvanized steel will look the same, i.e. matte gray. It is important to note, the corrosion protection is the same for bright and dull coatings of the same thickness.
Continuously galvanized steel can have a number of different appearances, custom to the order.
Typically, it exhibits a spangle (flake look) and is bright and shiny. However, passivation coatings
can be applied to dull the coating in preparation for painting or other value-added processes.
Corrosion protection is linearly related to the zinc coating thickness, i.e. the thicker the zinc coating, the longer lasting the corrosion protection of the steel. (See Time to First Maintenance chart below.) General galvanized steel coating thickness is largely a function of the available iron in the steel - the more mass, the more iron and the thicker the coating. Elements such as silicon and phosphorous in certain ranges can also promote thicker coatings. General galvanized steel coatings have three zinc-iron alloy layers, all harder than the steel itself, and a ductile, pure zinc outer layer. Continuous sheet is largely pure zinc, with little alloy layer and thin relative to general galvanized coatings. Regulating the speed of the steel as it passes through the molten zinc bath and the force of the air knife as the steel exits the bath controls whether the coating is a G60 or a G210.
All general galvanized coatings are produced the same way, in a > 98% pure zinc bath and have no specific coating designation. However, various steel products are galvanized with specific coating thickness minimums per ASTM) A123, A153, A767, ISO 1461, CSA G164, or AASHTO
specifications. Continuously galvanized sheet steel is most commonly produced to ASTM A653 and
designated as a G120, G185, G210, etc. coating.
General galvanized steel is used primarily for corrosion protection in exterior applications (utility poles, sign structures, boat trailers, agricultural equipment, guardrail) but may also be used for indoor applications (swimming pools, pulp and paper plants).
Continuously galvanized sheet steel zinc coatings are so thin they do not provide long term
corrosion protection on their own for exterior applications. They are usually painted for use
in electrical cabinets, automotive body panels, or building façades. Primary interior uses for
continuously galvanized sheet steel include appliances, almost always painted, HVAC duct, and
electrical junction boxes.
Any steel piece (≥ 14 gauge rollforming) or weldment that can fit into the general galvanizing
bath can be galvanized. Furthermore, structural steel members up to 1.5 times the length of the
galvanizing bath can be galvanized via a process called progressive dipping.
Sheet steel is always galvanized coil to coil. Once the coil has cooled, the steel is uncoiled and
either slit into narrower width coils or cut to length in sheets commonly available in sizes such as
48” x 96” and 60” x 120.” Some manufacturers stamp parts directly from the coated coil.
The general galvanized coating is comprised of four distinct coating layers, three of which are
alloys of zinc and iron and harder than the substrate steel. The Gamma alloy layer at the steel-coating interface has a Diamond Pyramid Number (DPN) hardness of 250 compared to the steel DPN of
159. The Delta and Zeta layers above the Gamma layer have a DPN of 244 and 179, respectively.
Only the pure zinc outer layer (Eta) is softer than the steel with a DPN of 70. All this means the
galvanized coating can withstand the abrasive forces applied during the process of driving posts
into the ground, during shipping, and erection.
Continuously galvanized sheet is a 100% pure zinc coating with minimal alloy layers. This coating is
subject to abrasive damage during rough handling and pile driving.
Because the general galvanized coating is a complete immersion process and applied after
all welding, shearing, flame-cutting, drilling, and hole punching, all surfaces are protected from
corrosion by zinc, including the interior of tubing and pipe.
Continuously galvanized sheet is formed, punched, drilled, cut, and sheared after the zinc is
applied, leaving bare edges exposed to corrosion. These bare, corroding surfaces accelerate the
corrosion of the zinc surrounding the exposed area and shorten the durability of the overall coating.