Durability & Longevity
Another aspect of hot-dip galvanizing is proven durability. Hot-dip galvanized steel has been specified extensively in petro-chemical, industrial, power/utility, and bridge/highway projects because of its unmatched durability in these harsh environments. Hot-dip galvanizing remains durable thanks to its abrasion resistance, uniform protection, and complete coverage.
A unique characteristic of the hot-dip galvanized coating is the development of metallurgically bonded (~3,600 psi), abrasion resistant intermetallic layers. This photomicrograph is a cross-section of a galvanized steel coating showing the three intermetallic layers (Gamma, Delta, and Zeta) and top layer of pure zinc (Eta). During the galvanizing process, these layers develop naturally during a metallurgical reaction between the iron in the steel and zinc in the kettle. As the photomicrograph also shows the hardness of each of the layers as a Diamond Pyramid Number (DPN), you can see the three intermetallic layers are harder than the base steel, while the eta layer has ductility which makes damaging the HDG coating very difficult.
Hot-dip galvanizing’s abrasion resistance provides unmatched protection against damage caused by rough handling during transport and erection, as well as in service. Other coatings with lower bond strengths (300-600 psi) can be easily damaged during shipment and construction, weakening their effectiveness, as barrier protection is dependent upon the integrity of the coating.
Longevity in Atmosphere: Time to First Maintenance
Hot-dip galvanized steel is often utilized 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 surrounding environments, 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.
Whether exposed in the atmosphere, subjected to blazing UV rays, snow, and/or other elements, submerged in water, or embedded in soil or concrete, hot-dip galvanized steel can withstand the different corrosive elements and fulfill the intended design life. More information about hot-dip galvanized steel’s longevity can be found in the AGA’s publication Performance of Hot-Dip Galvanized Steel Products.
The most common exposure environment for hot-dip galvanized steel is atmospheric. As hot-dip galvanized steel is exposed to the atmosphere, the zinc interacts with free flowing air and moisture to develop the zinc patina. The zinc patina is critical to the longevity of galvanized steel in the atmosphere; and thus, accelerated, salt-spray tests that do not mimic real world exposure conditions are not an accurate predictor of HDG’s longevity.
The performance of atmospherically exposed hot-dip galvanized steel depends on five main factors: temperature, humidity, rainfall, sulfur dioxide (pollution) concentration in the air, and air salinity. None of these factors can be singled out as the main contributor to zinc corrosion, but they all play a role in determining the corrosion protection hot-dip galvanized (zinc) coatings can provide in certain atmospheric conditions.
For nearly a century, independent and industry testing of samples in five environments (industrial, rural, suburban, tropical marine, and temperate marine) have yielded real-world performance data for hot-dip galvanized steel. Using this real-world corrosion data, statistical methods, and neural network technology, Dr. Gregory Zhang of Teck Metals Ltd. Developed the Zinc Coating Life Predictor (ZCLP) to estimate the life of hot-dip galvanized coatings in atmospheric conditions. Using the ZCLP, you can input specific parameters for any environment and get an estimated time to first maintenance (TFM) for the galvanized coating
Time to first maintenance is defined as 5% rusting of the base steel surface, which means 95% of the zinc coating is still intact, and an initial maintenance is recommended to extend the life of the structure. According to ASTM A123, the governing specification for hot-dip galvanizing, steel ¼-inch thick or greater must have at least 3.9 mils of zinc on the surface, but more often than not, there will be greater than the minimum requirement. Therefore, using the TFM chart, hot-dip galvanized structural steel (>1/4-inch thick) provides 72-73 years of life to first maintenance even in the most corrosive atmosphere, industrial.
Availability & Versatility
Hot-dip galvanized (HDG) steel is versatile and readily available. A wide variety of shapes and sizes ranging from small nuts, bolts, and fasteners to larger structural pieces, to even the most intricately detailed artistic pieces, can be galvanized. Because of the total immersion process, even complex fabrications can be coated entirely for corrosion protection.
Many corrosion protection methods depend on proper temperature and humidity conditions for correct application. However, because hot-dip galvanizing is a factory controlled process, it can be accomplished 24/7/365 rain or shine. Zinc solidifies upon withdrawal from the bath, so there are no delays for curing; and galvanized steel could realistically be galvanized, shipped to the site, and erected on the same day. On the other hand, if the galvanized material does not need to be installed immediately, it is easily stored outside, as UV rays do not degrade the coating’s integrity.
As North America continues to strive for sustainable development, the same thinking should be applied to the materials used in construction. Another characteristic of zinc, making it ideally suited for this job is its abundance. The main materials used in the galvanizing process, zinc and steel are common; in fact, zinc is the 27th most abundant material in the earth’s crust, and iron is the 4th. Zinc and steel are also both 100% recyclable without the loss of any chemical or physical properties – steel is the most recycled material in the world.
Zinc is a natural element found in air, soil, and water. Approximately 5.8 million tons of zinc are cycled through the atmosphere annually through natural phenomena. Zinc is also common and essential to life. Zinc is found in a number of products we use daily such as cosmetics, tires, cold remedies, baby creams to prevent diaper rash, treatments for sunburns, and sunscreens. In fact, zinc oxide blocks more UV rays than any other single ingredient used in sunscreen. Furthermore, we all require zinc to live as it helps with regular functions such as vision, reproduction, digestion, and breathing.
With galvanized steel, you can do more with less. Because of the high strength-to-weight ratio, specifying projects with steel saves materials and energy. In fact, on average one ton of steel provides the same strength as 8 tons of concrete, and according to the World Steel Association, the strength-to-weight ratio minimizes substructure costs and can also save money on transportation and handling.
In addition to the abundance and quick turnaround time, galvanized steel also facilitates the expansion of existing structures due to its flexibility and stockability. Galvanized steel members are easy to retrofit and expand by welding, bolting, and/or splicing of existing vertical and horizontal structural elements and adding reinforcement. Adding to existing structures allows maximum efficiency of available space. Steel members are lighter than many other materials but still provide great strength, so expansion within the same footprint is possible.
Furthermore, because of hot-dip galvanized steel’s durable, maintenance-free nature, elements can be stockpiled outside for years without compromising the zinc coating and its corrosion protection. The zinc coating of HDG is unaffected by UV rays, extreme temperatures, rain, snow, or humidity, so with proper storage it is simple to keep an inventory on-hand for quick replacement in an emergency. Stocking frequently used parts until they are needed saves time, and if bought when prices are low, saves money.
Structural safety and stability are critically important to the integrity of steel construction, and cannot be maintained if the structure has been weakened by the ravages of corrosion. Hot-dip galvanized steel pieces which remain corrosion resistant for decades preserve the structural integrity of steel construction and protect against disaster.
One area of safety where hot-dip galvanized steel provides advantages is in areas of seismic activity. Steel elements are more ductile and lighter, reducing the inertia effects of seismic loading. Able to bend within reason without breaking, the tensile strength of hot-dip galvanized steel can protect structures from damage or even total failure during seismic activity.
Durability & Longevity Case Study
Bridges of Stark County-Stark County, OH; 1970s
The Bridges of Stark County Ohio are galvanized bridges and have been for 40 years. In the early 1970s, Rich Larocco was a young bridge engineer for Stark County, just out of college. At that time, 110 of Stark Countys bridges were deficient and 50% of the bridge budget was used to repaint bridges. Rich worked with the former owner of the galvanizing facility to change those numbers. Rich decided rather than continue to repaint bridges over and over, he would dismantle the bridge. He had the decks and beams removed. The beams were sandblasted and galvanized. Then the entire recycled bridge was reinstalled. After that they never repainted an old bridge or painted a new bridge even if the long beams or large trusses had to be progressively dipped in the galvanizing plant. There are 442 Bridges in Stark County, 335 of which are maintained by the county. Many are short span low clearance steel bridges over creeks and streams in rural areas, making an ideal environment for hot-dip galvanized bridges.