Galvanized coatings are generally bright and shiny but within a year will weather to a uniform dull gray appearance. The basic finish requirements of the galvanized coating are that it be relatively smooth, continuous, and free from gross surface imperfections. Smoothness is an ambiguous term; the product’s end use must be the determining factor in setting tolerances for smoothness. The galvanized coating is continuous and provides optimum corrosion protection.

Handling techniques for galvanizing may require the use of chain slings, wire or other holding devices to lower material into the galvanizing kettle if suitable lifting features are not available on the item. Chains, wires, and special jigs used to handle the items may leave a mark on the galvanized item. These marks are not detrimental to the coating, nor are they cause for rejection. If considered necessary, or expose bare steel, these areas can be easily touched up using the procedures described in ASTM A780.

The difference between the shiny luster and the dull luster. The appearance of the galvanized part does not make a difference in corrosion protection. The steel will be protected for the same amount of time.

Differences in the luster and color of galvanized coatings do not significantly affect corrosion resistance. The presence or absence of spangle has no effect on coating performance. The well-known spangled appearance found on galvanized products is a crystallization process dependent upon the zinc bath chemistry, the rate of cooling, the method of pickling, the steel chemistry, and the thickness of the coating. Dull gray or patchy matte gray galvanized coatings give a service life equal to bright or spangled coatings since the service life depends on the zinc coating thickness. Variations in coating appearance or finish are important only if they will affect the intended use of the article. The primary function of the galvanized coating is corrosion protection.

The HDG Coating

The coating that develops during the galvanizing process is metallurgically bonded to the steel – virtually becoming a part of the steel itself.  During the reaction in the kettle, the zinc interacts with the iron in the steel to form a series of zinc-iron alloy layers.  The photomicrograph below is a cross section of the galvanized steel coating, showing a typical microstructure comprised of three alloy layers and a layer of pure metallic zinc. 

The thin Gamma layer composed of an alloy that is 75% zinc and 25% iron
The Delta layer composed of an alloy that is 90% zinc and 10% iron
The Zeta layer composed of an alloy that is 94% zinc and 6% iron
The outer Eta layer that is composed of pure zinc

In addition to the chemistry of each layer, the figure identifies the hardness of each layer expressed as a Diamond Pyramid Number (DPN).  The DPN is a progressive measure of hardness; the higher the number, the greater the hardness. Typically, the Gamma, Delta and Zeta layers are harder than the underlying steel. The hardness of these inner layers provides exceptional protection against coating damage by abrasion. The Eta layer is quite ductile, providing the coating with some impact resistance. The galvanized coating is adherent to the underlying steel on the order of several thousand pounds per square inch (psi). Other coatings typically offer adhesion rated at several hundred psi at best. Hardness, ductility and adherence combine to provide the galvanized coating with unmatched protection against damage caused by rough handling during transportation to and/or at the job site, as well as in service. The toughness of the galvanized coating is extremely important since barrier protection is dependent upon the integrity of the coating.

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How Long Does HDG Last?

Hot-dip galvanizing (HDG) is often used to protect steel from corrosion in some of the harshest environments imaginable, yet it provides maintenance-free longevity for decades. The corrosion resistance of hot-dip galvanizing varies according to its surroundings, but generally corrodes at a rate of 1/30 of bare steel in the same environment. Measurements of the actual consumption rate of the coating during the first few years of service provide good data for projecting a conservative estimate for the remaining life to first maintenance, because as zinc corrosion products build on the surface, which in most environments are adherent and fairly insoluble, the corrosion rate often slows as time progresses.

Galvanized steel can be used in almost any environment and will be able to withstand the corrosive elements and the longevity of the steel will not be affected

The corrosion resistance of zinc coatings is determined primarily by the thickness of the coating but varies with the severity of environmental conditions.  Each environment affects hot-dip galvanizing differently based on a unique set of corrosion variables.  The predictability of the lifetime of a coating is important for planning and budgeting for required maintenance.

Whether exposed in the atmosphere, subjected to blazing UV rays, snow, and/or other elements, submerged in water, embedded in soil or concrete, or various other environments, hot-dip galvanized steel can withstand the different corrosive elements and fulfill the intended design life.

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Why Specify Galvanizing?

Whether an artful sculpture glinting under the sun or a sturdy bridge arcing over the waves of a rushing river, galvanized structures can be seen standing strong and corrosion-free across the continent. For more than 100 years hot-dip galvanizing (HDG) has been a mainstay of North American industry. 

Hot-dip galvanizing is used throughout various markets to provide steel with unmatched protection from the ravages of corrosion.  Myriad applications of steel products in the harshest environments benefit from the use of hot-dip galvanizing.

The uses of hot-dip galvanized steel continue to evolve, and new markets are emerging all the time.  Similarly, the decision to galvanize has matured beyond the customary corrosion protection to an array of other reasons.  Though corrosion resistance is inherent any time HDG is utilized, more and more specifiers select hot-dip galvanized steel for other reasons including lowest initial cost, durability, longevity, availability, versatility, sustainability, and aesthetics.

Facts About  Hot Dip Galvanizing--Source: American Galvanizing Association
Using zinc to protect steel from corrosion (hot-dip

galvanizing) is a 150-year-old practice! Corrosion is

caused by the inherent tendency of metals, when

subjected to air and moisture, to revert to their original

earthly forms, usually an ore state. They do this

through a chemical or electrochemical reaction with

the environment.

Galvanizer’s kettles are set at temperatures ranging between 815 F and 850 F (435 C to 454 C).

A galvanizer knows a piece of steel should be immersed for a specific amount of time in order for the metallurgical reaction between zinc and iron to reach completion. The completion of the metallurgical reaction is observed when bubbling of the molten zinc in the kettle stops. At this point, the galvanizing is complete and the steel is removed from the kettle to cool.

Galvanizers can hot-dip galvanize a piece of steel that is larger than the kettle dimensions; it’s called progressive dipping.

Zinc seals the underlying steel from contact with its environment. If the steel is exposed to the elements due to mechanical damage, the surrounding zinc corrodes sacrificially, protecting the underlying steel from corrosive attack.

The zinc coating on galvanized steel is uniform: inside, outside, corners and edges.
The hot-dip galvanized reinforcing steel bond with concrete is at least as great as the bond of bare steel to concrete.

When the Brooklyn Bridge was built, over 14,500 miles of hot-dip galvanized wire were used for its four main cables. Over 100 years later when the bridge underwent massive rehabilitation, the hot-dip galvanized wire was in excellent condition. Hot-dip galvanized steel lasts longer today than it did 20 years ago. Because of environmental laws, our air is cleaner and less contaminated with corrosive emissions.

A reddish-brown staining infrequently develops on the surface of a newly galvanized piece of steel that is comprised entirely of intermetallic layers. The steel is not rusting; there is just a very small amount of iron in the zinc-iron alloy layers that is oxidizing, causing the staining to occur. This does not cause any adverse effects on the corrosion performance of the galvanized steel.

Corrosion annually costs the US economy 3.2 percent of the gross national product, over $423 billion. Indirect costs to the public could raise the percentage as much as 5-11 percent. Some indirect costs of corrosion are: lost productivity due to traffic delays, accidents caused by corroded hand and guardrails, excessive use of nature’s raw materials and energy to replace corroded steel.

Based on a study by NACE International (The Corrosion Society), members of Congress, and the Department of Transportation (DOT), better corrosion management can be achieved using preventive strategies at every level of involvement (owner, operator, user, government, Federal regulators, and general public).


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