Galvanized steel is corrosive to all metals except lead, tin, zinc and aluminum. Although it does not last infinitely, galvanized steel is the unparalleled corrosion-resistant metal. It is worth noting however, that applying a protective coating such as paint to galvanized steel will alleviate the problems caused by corrosion of the protective zinc coating.
Galvanized steel has been used for almost 2,000 years because of its unrivaled ability to last a very long time and resist rust. Hot dipped galvanized steel and electroplated galvanized steel are made using different methods and their zinc galvanized coatings corrode completely differently. Learn about these galvanizing processes (here) and how zinc corrosion varies between them (here) . Yes, galvanized steel resistance to rust corrosion depends largely on the type and thickness of the protective galvanized zinc coating, but the type of corrosive environment is also a critical factor. Factors that rust and corrode galvanized steel:
So how long does it take for a handy new galvanized steel bucket to rust and corrode into a useless heap of metal? It takes a long time. A galvanized steel bucket (produced with any method) can last practically forever if it's gently used and kept dry and out of the rain. But for those galvanized buckets and tubs destined to become garden planters, landscape decorations, animal feeders, and farm water buckets corrosion is inevitable. Galvanized steel intended for prolonged outdoor use should be hot-dipped galvanized steel; which commonly lasts for about 70 years in many different environments.
Table 1 below predicts how long galvanized steel will last based on a 30 month corrosion study of environmental factors like wetness, humidity, and air pollutants in 2004.
Source: Journal of Materials in Civil Engineering 2004 (11)
The corrosion resistance of zinc coatings is determined primarily by the type and thickness of the coating but, varies with the severity of environmental conditions exposed to (as in the table above). Hot dipped galvanized zinc coating resistance to corrosion depends primarily on a protective film (patina) formed on its surface.
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The type of zinc galvanization and how that process controls the way in which the galvanized steel corrodes must be understood first. The environments, elements, and conditions that any given type of galvanized steel is exposed to, nevertheless, indeed determines how long it will last before corrosion.
A 1926 study of galvanized steel corrosion in industrial, rural and sea regions found:
The handy chart below (from American Galvanizers Association) illustrates how long galvanized steel will last before corroded areas should be maintained to prevent further deterioration. Want to learn how to refinish galvanized steel? Click here. Put another way, this chart shows how long it takes for galvanized steel to rust in different environments.
The thicker the zinc coating the longer galvanized steel will last without corrosion. The thickness of zinc is displayed along the horizontal axis (8). As in the chart below and noted in the 1926 study, for each location the corrosion rate is essentially constant with time (9).
Chart 1: Time to first sign of corrosion in various environments
The environments below are listed from the most corrosive to the least corrosive:
Industrial Environments:
Tropical Marine Environments
Temperate Marine Environments
Suburban environments
Rural Environments
Elements and conditions:
Air
Sulphur Dioxide (SO2) is the most significant atmospheric air pollutant. The presence of SO2 in the atmosphere largely regulates the atmospheric corrosion rate of zinc. When acids - with a pH below seven - attack and corrode a galvanized zinc coating, the pH decreases and the rate of corrosion increases. In industrial locations mists and dews have been observed having a pH as low as three. It is rational, therefore, to attribute the greater corrosivity of industrial atmospheres to the acid-forming SO2 pollution contained within them (9).
Results of a galvanized zinc metal corrosion potential study published in 2015 found the highest corrosion impact from SO2, dust, humidity and CO2. Concentrations of these pollutants were highest values in winter; when fossil fuel combustion increases. The presence of chlorides in air during also influenced the rate of corrosion (10).
Soil
Just as the acidity of the atmosphere influences the rate of corrosion, so too does the acidity of the soil. The zinc coating of hot-dipped galvanized steel will last in the harshest soil is 35 to 50 years and in less corrosive soil 75 years or more.
Temperature
Although humidity affects corrosion, temperature itself has less of an impact. Galvanized zinc coatings respond well in extreme cold and hot temperatures. There are no significant differences in corrosion rate in temperatures below -40 F for hot-dip galvanized coatings. In higher temperatures the zinc can be impacted. For long-term continuous exposure the maximum recommended temperature is 392 F, according to a publication by American Galvanizers (8). Temperatures this high can cause the outer zinc layer to peel away from the zinc-alloy layers. Although the remaining zinc-alloy layers will provide corrosion protection to the steel, protection will last for less time than if the outer free zinc layer remained intact.
Because the applications of steel are many, hot-dip galvanizing will continue to be called upon to ensure long-lasting and maintenance-free corrosion protection.
Hot-dip galvanized steel has been around since 1742. In the nearly-280 years since a French chemist named Paul Jacques Moulin first described the galvanizing process, fabricators and construction engineers all over the world have gathered a tremendous amount of evidence on the durability of galvanized steel. But the million-dollar question — or depending on the size of your project, maybe the $10 million or $100 million question — is how long does galvanized steel last?
ASTM International, formerly known as the American Society for Testing and Materials, has a precise answer for this critical question.
Time to First Maintenance, or TFM, is defined as the appearance of rust on 5 percent of a galvanized steel surface. At this point, 95 percent of the steel surface is still covered with zinc but preventive maintenance is necessary to prevent deterioration of the structure.
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TFM varies by thickness of the coating of zinc put on the steel during the galvanizing process and by the severity of pollution in the atmosphere to which the galvanized steel is exposed. Thinner zinc offers less protection. Industrial air pollution destroys zinc faster, but not very fast.
ASTM Standard A123 requires any galvanized steel more than 1/4-inch thick to be coated with at least 3.9 mm of zinc. According to the Time to First Maintenance Chart, steel treated with this thickness of zinc will reach TFM only after decades of service, depending on environment. (Here “environment” refers to relative air quality.)
TFM for standard galvanized steel in an industrial environment is 72 to 73 years. Example of an industrial environment include Newark, Atlanta, and Houston.
TFM for standard galvanized steel in a tropical marine environment is 75 to 78 years. Examples of a tropical marine environment include Miami and Mazatlan.
TFM for standard galvanized steel in a temperate marine environment is 86 years. Examples of temperate marine environments are Seattle and Charleston, South Carolina.
TFM for standard galvanized steel in a suburban environment is 97 years. Examples of a suburban environment include Minneapolis and Orlando.
TFM for standard galvanized steel in a rural environment is over 100 years. Examples of a rural environment include Fargo, North Dakota and Las Cruces, New Mexico.
In locations with minimal air pollution, galvanized steel will not need maintenance for a full century or more! And because the Time to First Maintenance Chart is based on data collected since the 1920s, before major improvements in air quality, these longevity estimates for galvanized steel used for construction in the twenty-first century are likely to be conservative.
Galvanized steel is a construction material for the truly long term. The longevity of galvanized steel is so well documented that financial planners can build expected TFM into their return on investment models for different sites and different real estate costs. The longevity of galvanized steel may give construction in a high-pollution location an acceptable ROI, but galvanized steel may justify long-term ROI at low-pollution sites that financial planners can find spectacular.
Don’t be misled by claims of manufacturers using other protective processes that their products last as long as galvanized steel. These claims are based on short-term tests, usually involving exposure of the steel to salt spray, that don’t run long enough to capture data about the three ways galvanized steel is protected against corrosion:
. Zinc provides a molecularly dense protective coating over steel. Paint develops pin holes that leave it susceptible to underfilm corrosion. Zinc maintains a continuous coating that extends the life of the steel beneath it by 25 to 40 times.
. Rusting is an electrochemical process. Metals donate electrons to oxygen to form oxides. Iron, for example, donates electrons to oxygen to form iron oxide, or rust, that is, unless an electrical current keeps it from interacting with iron. When zinc is in physical contact with iron, it generates a tiny electrical current that interferes with the corrosion process. Zinc protects against water, acids, and salt.
. Even zinc slowly corrodes when it is exposed to air. The interaction of zinc and the oxygen in the air forms a thin layer of gel-like, slippery, viscous zinc hydroxide. As the zinc hydroxide dries out, it interacts with the carbon dioxide in the air to form a stone-like compound known as zinc carbonate. Crystals of zinc carbonate, or patina, offer yet another layer of protection for the steel beneath them.
Salt spray tests keep steel constantly wet so the protective patina never forms, at least for the duration of the test. In real-world applications, the protective layer of patina appears only after six to twelve months of exposure to the weather.
If you are looking for a competitively priced construction material that you can use to build or fabricate for generations to come, choose galvanized steel. There is no longer-lasting steel for your construction and fabrication needs.
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