Steel is an abundant, efficient building material that provides specifiers design freedom. However, for projects exposed to the atmosphere and other harsh environments, it is critical to coat the steel for cor-rosion protection. Often large construction projects target a 50-100 year design life, highlighting the need for durable, long-lasting corrosion protection. Hot-dip galvanizing (HDG) provides three levels of corro-sion resistance to steel: barrier protection, cathodic protection, and the zinc patina.
Hot-dip galvanized coatings isolate the steel from the environment, similar to other coatings. As long as the barrier is intact, corrosion will not occur. Once the barrier is breached, the steel can be attacked, leading to rust. Two important properties to consider with barrier protection are adhesion to the base metal and abrasion resistance, as this will lead to a stronger barrier.
Hot-dip galvanizing’s second level of corrosion resistance is cathodic protection. Zinc is anodic to steel, and therefore will preferentially corrode to protect the underlying metal. Zinc’s sacrificial action will also offer protection where small areas of steel may be exposed due to cut edges, drill holes, scratches or surface abrasion. In fact, even if the HDG coating is damaged to the point bare steel is exposed (up to about ¼ inch in the maximum diameter) no corrosion will begin until all the surrounding zinc is consumed. For instances where a larger area were exposed, the zinc would keep the corrosion isolated to that area, rather than allow spreading across the part.
The final key to hot-dip galvanizing’s corrosion protection is the development of the zinc patina. Zinc, like all metals, corrodes when exposed to the atmosphere. During exposure to moisture and free flowing air (natural wet/dry cycles in the environment), a series of zinc corrosion byproducts will form on the galva-nized coating’s surface. The formation of these byproducts (zinc oxide, zinc hydroxide, and zinc car-bonate) develop into the zinc patina, which acts as an additional passive, non-soluble barrier for the hot-dip galvanized coating. Once developed, the zinc patina will slow the corrosion rate of the zinc to about 1/30th the rate of steel in the same environment. Thus, a relatively thin coating of zinc can last much long-er in the environment to protect a much thicker piece of steel.
Another aspect of hot-dip galvanizing is proven durability. Hot-dip galvanized steel has been specified extensively ro-chemical, industrial, power/utility, and bridge/highway projects because of its unmatched durability in these harsh vironments. 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), sion resistant intermetallic layers. The photomicrograph (below) 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 photomicrograph also shows the hardness of each of the layers as a Diamond Pyramid Number (DPN), you can see three intermetallic layers are harder than the base steel, while the eta layer has ductility which makes damaging the coating very difficult.
Hot-dip galvanizing’s abrasion resistance provides unmatched protection against damage caused by rough handling ing 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 integrity of the coating.
Uniform Protection & Complete Coverage
During the metallurgical diffusion reaction in the galvaniz-ing kettle, the galvanized coating grows perpendicular to all surfaces. Therefore, the coating is naturally as thick on corners and edges as flat surfaces. Since coating damage commonly occurs at edges, added protection at these junc-tures is important. Brush- or spray-applied coatings have a natural tendency to thin at corners and edges leaving the part prone to attack. The uniform protection of hot-dip gal-vanized steel leaves no weak points for accelerated corro-sion.
Furthermore, hot-dip galvanizing is a total immersion process, meaning the steel is fully submerged into cleaning solutions and the molten zinc, coating all interior and exterior surfaces. This complete coverage ensures even the insides of hollow and tubular structures and the threads of fasteners are coated. As corrosion tends to occur at an increased rate on the inside of hollow structures where humidity and condensation occur, interior coverage is very beneficial. Hollow structures that are painted have no corrosion protection on the inside. Fully coating fasteners is equally important as they are utilized at con-nection points which are critical to structural integrity.
Longevity in Atmosphere: Time to First Maintenance
Hot-dip galvanized steel is often utilized in some of the harshest environments imaginable, yet it pro-vides 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
Whether exposed in the atmosphere, subjected to blazing UV rays, snow, and/or other elements, sub-merged in water, or embedded in soil or concrete, hot-dip galvanized steel can withstand the different cor-rosive elements and fulfill the intended design life. More information about hot-dip galvanized steel’s lon-gevity 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 galva-nized 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 predic-tor of HDG’s longevity.
The performance of atmospherically exposed hot-dip galvanized steel depends on five main factors: tem-perature, 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 deter-mining the corrosion protection hot-dip galvanized (zinc) coatings can provide in certain atmospheric con-ditions.
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 parame-ters 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. Accord-ing 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 mini-mum requirement. Therefore, using the TFM chart, hot-dip galvanized structural steel (>1/4-inch thick) pro-vides 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. 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.
Efficiency & Safety
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.
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 piec-es 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 dldamage or even total failure during seismic activity.
Aesthetics are important to nearly every construction project. Whether an attractive, artfully designed sculp-ture or architecturally exposed steel element, bridge, bus station, or other infrastructure element, galvanized steel offers design flexibility, and an attractive, natural gray finish.
The natural, matte gray finish does not suit every project of every specifier – as sometimes color is preferred or needed for branding, safety marking, etc. However, by specifying a duplex system – galvanizing your pro-ject and then painting or powder coating over to the desired color ensures you don’t have to sacrifice the cor-rosion protection benefits and extended maintenance of HDG steel.
Duplex systems provide more benefit than just aesthetic options. The combination of hot-dip galvanized steel and paint or powder coating provide a synergistic effect. The paint/powder extends the life of the coating by providing an additional barrier coating to the zinc layers, while the galvanized steel prolongs the life of the paint coating by preventing underfilm corrosion and peeling.
The result of the two coatings working in synergy is extended corrosion protection. The service life of a du-plex system is 1.5 to 2.3 times the sum of the individual systems. For example, if the life of the galvanized coating in a particular environment is 70 years, and the expected life of the paint is 10 years, the expected life of the duplex system would be at least 120 years (1.5 x (70+10)).
This extended service life assumes no maintenance will be done to keep the paint or powder coating in-tact. Realistically, if someone invests the premium cost upfront for a duplex system, they likely plan to keep the color on the structure. So, in practical terms, the synergistic effect of utilizing a duplex system is the extended maintenance cycle it provides. With hot-dip galvanized steel as a “primer,” the time to first maintenance of the paint or powder coating is extended 1.5 to 2.0 times what it would be for bare steel. The AGA’s publication Duplex Systems: Painting Over Hot-Dip Galvanized Steel and instructional DVD guide Preparing HDG Steel for Paint provide more information on specifying duplex systems.
A significant aspect of striving for a more sustainable built environment is to evaluate the materials used in construction. In particular, it is important to evaluate the content and availability of the raw materials as well as the impact of processing them.
When it comes to hot-dip galvanizing, it’s important to remember the primary components (zinc and steel/iron ore) are both natural elements (4th (iron) and 24th (zinc) in order of abundance) which means we are not introducing harmful, man-made elements into the environment. In fact, both iron and zinc are essential to life and healthy reproduction.
Adding to the natural abundance and necessity to life, both zinc and steel are 100% recyclable without the loss of any chemical or physical properties. This means they can be multi-cycled (used as zinc/steel over and over again) rather than down-cycled into a lesser product – making hot-dip galvanized steel a truly infinitely renewable resource. Furthermore, both steel and zinc have high reclamation rates, which means recycling them is not merely a talking point, it is done as a common practice. Approximately 80% of zinc available for recycling is reclaimed, and for steel, the most recycled material in the world, it is nearly 100% (98%+).
Finally, the maintenance-free longevity provided by galvanized steel significantly reduces or eliminates the need of more natural resources and output of emissions throughout its service life.
The AGA has produced an industry-wide, 3rd party verified Environmental Product Declaration (EPD) in accordance with the LEED v4 standard. The EPD covers hot-rolled sections, plate, and hollow structural sections (HSS). The study was conducted by thinkstep, Inc. and verified by UL Environment, and can be found on our website at https://galvanizeit.org/hot-dip-galvanizing/is-galvanizing-sustainable/hdg-and-leed/environmental-product-declaration-epd. By: https://galvanizeit.org/hot-dip-galvanized-steel-bridges-online-seminar/why-hdg-bridges/sustainability-galvanizing-is-green www.steeloretank.com