Author: Lorella Angelini, Angelini Consulting Services, LLC
Steel corrosion tops the list of bridge preservation problems. It shortens the service life of bridges and can pose a safety issue. Once corrosion has started, it is very difficult, almost impossible, to stop it. In the US 40% of carbon steel is used to replace corroded steel at a cost of $425 B corresponding to approx. 2.5% of the GDP. This cost does take into consideration the additional cost related to suspending or limiting the use of steel damaged infrastructures.
Galvanized steel is being increasingly adopted by DOTs as a mean to protect structural steel from corrosion. To know more about this growing technology I spoke with Kevin Irving, National Marketing Specialist with AZZ Metal Coatings.
Kevin, who is based in the Chicago area, has more than 25 years of experience in the Hot-Dip Galvanizing (HDG) industry. He has many qualifications, such as being a certified coating inspector by the NACE International and a certified presenter of the American Galvanizers Association (AGA). He currently serves as a National Board Member for the Chemical Coaters Association International (CCAI) and as a member of the TRB Bridge Preservation Committee. He is industry director for the Western and Midwest TSP2 Bridge Preservation Partnerships.
What are the benefits of HDG for bridge preservation?
HDG increases bridge safety while reducing maintenance costs. It consists of applying a protective zinc coating to steel thereby preventing rusting and deterioration of the steel. Since zinc corrodes up to 30 times slower than steel, galvanizing dramatically reduces steel corrosion. Zinc adheres to steel through the metallurgical bond provided by HDG.
What are major differences between galvanizing and other technologies for steel protection, such as metalizing and painting?
Galvanizing, metalizing and zinc-rich paints all provide barrier protection to the underlying steel. What separates galvanizing from the other two technologies is the fact that galvanizing provides a metallurgical bond between zinc and steel, creating three zinc-iron alloy layers and a nearly pure zinc outer layer. Instead, metalizing and paints provide a mechanical, not a metallurgical bond, between coating and steel. The mechanical bond is generally considered as a less performing bond when compared to the metallurgical.
All three technologies provide cathodic protection to the underlying steel although some types of zinc-rich paint provide less protection than others. If the coating gets scratched thereby exposing the steel, the zinc will preferentially corrode to protect the exposed area. HDG has the advantage of preventing scratches in the first place. In fact, the zinc-iron alloy layers created during galvanizing are harder than the base steel, making HDG much more abrasion resistant than other coatings.
Can HDG be considered an innovative technology?
HDG is a very good technology with an exceptional track record but it cannot be considered as an innovative technology. Some applications like the I-69 bridge northbound lanes near Indianapolis, the Stearns Bayou Bridge in Grand Haven, Michigan and the Ford County Bridge in Illinois were completed almost 50 years ago. These three bridges represent a success story since no maintenance has been needed to date and another 30 years of service life should be expected for all of them.
Continuous-Galvanized Rebar (CGR) per ASTM A1094 produces a galvanized rebar with a pure zinc alloy coating structure. Although the continuous galvanizing process has been used for decades on galvanized automotive steel, its application to rebar is new. The CGR process allows long lengths of rebar sizes to be stored for fabrication and then formed without cracking, peeling or flaking, and without requiring special equipment for forming.
The CGR coating passivates faster and corrodes at a slower rate than traditional HDG coatings. The CGR process is automated and faster, resulting in quicker lead times, repeatable quality and consistency in the coating.
CGR can also be dual-coated with epoxy to produce a duplex coating according to ASTM A1055 specification. The additional epoxy barrier over CGR provides an affordable belt and suspenders approach for most extreme environments.
Is galvanizing an expensive technology?
According to Philip G. Rahrig, executive director for the AGA, the cost for HDG is approximately $1.76 per square foot. This cost should be slightly less for large tonnage jobs.
What are the fields of application of galvanizing technology?
The technology is widely used for steel structural elements, such as beams and rebars in reinforced concrete. It is also used for guard rails, street signs and posts.
Could you briefly describe the galvanizing plant process for a steel beam, for example?
In the HDG process, the steel beam comes to the galvanizing plant after being manufactured and prior to service. The first step of the process entails an inspection to check that no paint, mill markings and heavy grease are present on the steel surface. After the inspection the beam goes to a caustic degreasing bath to remove any grease, grime, oil, or dirt. Once rinsed in a water bath, the beam is immersed in an acid tank or pickling bath so as to remove of any mill scale or rust oxide. This is also followed by water rinsing.
After cleaning, the steel beam is immersed in a flux tank. The flux is a combination of zinc, ammonium and chloride. At 160 degrees Fahrenheit, the flux bath deposits a light coating on the steel, which keeps it from oxidizing. The flux bath also preheats the steel. In the next and final step of the process the beam is submerged in a galvanizing kettle. This bath must have a minimum of 98-percent pure zinc to meet ASTM A123 requirements.
The galvanizing kettle is heated to a temperature ranging from 830 to 840 degrees Fahrenheit, at which point the zinc is in a liquid state. The steel beam remains in the zinc bath until the steel reaches the bath temperature. When the beam comes out from the kettle, the zinc coating must have a minimum thickness of 3.9 mils for steel more than 1/4-inch thick (ASTM A123).
It usually takes approximately three hours to complete the process from beginning to end. The steel is typically immersed in the molten zinc in the galvanizing kettle for less than 10 minutes. There are currently more than 75 plants in the United States that can provide galvanizing for steel elements for bridge construction.
What are the challenges in the HDG process and the steps to ensure it is properly done?
It is extremely important to check placement and number of vent and drain holes prior to the galvanizing bath. This ensures safety during immersion of the steel in the molten zinc by allowing pressure to escape from air and moisture present in internal sections of fabrications. It also allows the molten zinc to contact and form the HDG coating on all internal and external surfaces. Vent and drain hole requirements are outlined in ASTM A385.
It is also preferred that the piece is completely fabricated before being galvanized so as to avoid welding after the galvanizing process. If welding is needed after HDG, the zinc must be burnt off with a torch or ground off until there are sparks in order to be sure that the steel substrate is reached. Upon completion of welding, it is necessary to restore corrosion protection on the area where the galvanizing was removed and on the weld.
It is good practice to communicate with the galvanizer prior to design and fabrication. This provides the best opportunity to address design details that could affect the formation of the HDG coating.
Can you share a case study?
The Stearns Bayou Bridge in Grand Haven, Michigan, was galvanized and installed in 1966. The steel is in very good conditions without having required any maintenance from the time of construction.
This county bridge is 430 feet long with a 30-foot clear roadway and a 5-foot walkway along each side. The majority of the steelwork is 6 feet above a freshwater river in a rural location. The bridge experiences light to moderate traffic and the entire bridge is subject to winter salting.
After 51 years following the construction, all the beams and diagrams are in very good shape showing no signs of rusting or staining despite the fact that the bridge is subjected to snow and deicing salts. The average zinc coating thickness is 5.4 mils, which indicates that HDG will continue to provide corrosion protection for decades in this environment. All bolted connections also show no signs of rust.
However, in five places on the tubular rails on the top of the bridge there are signs that the zinc has been completely consumed thus exposing the metal substrate. The preservation process for these areas simply requires applying a zinc rich paint over the exposed steel. The rest of the rails should be power washed with some type of salt mediation and painted over without sandblasting on a completely dry surface. Standard specifications (ASTM D6386 and SSPC Guide 19) must be used as guidance for the paint application.
AZZ Metal Coatings https://www.azz.com/metalcoatings
You Tube video from AGA: “Hot-Dip Galvanizing: Protecting Steel For Generations” https://www.youtube.com/watch?v=kwCyq06aatA
You Tube video: “AZZ Galvanizing Overview”
You Tube video: “AZZ Galvanizing Workflow”