When deciding on which steel to spec for your next project, you may want to look at either galvanized or galvannealed steel. While not appropriate for many applications, if you’re considering an external application where rust is a factor, galvanized and/or galvannealed merit consideration. It’s important to understand the uses of each as well as the differences. Both provide significant rust and corrosion protection, but there are material differences between the two types of zinc coated steel that should be understood.
The process of galvanization involves steel sheet being immersed in molten zinc at 850 °F, through which a zinc layer is bonded to the steel substrate at a molecular level. Zinc protects the steel from oxidation when it is exposed to a corrosive environment, creating a protective layer at the base. While there are many applications wherein galvanized steel is the proper material, common uses include HVAC ducts, wrought iron gates, roofing, body parts of vehicles, safety barriers, balconies, and building framework. Galvanized steel has a spangled appearance, but is very durable and can withstand salt and the elements, which is useful in outdoor applications.
The Galvannealing process is very similar to that of galvanizing, but with galvannealing, the steel substrate is heated to 1050 °F. At this temperature, more iron is drawn out of the steel where it bonds with the zinc to form an alloy coating that is lower in zinc and higher in iron, than that of galvanized. This creates a stronger surface that allows for better weld (and paint) adhesion. Additionally, galvannealed steel has a scratch resistant surface and a low-luster, dull, matte finish which does not require a primer for paint, unlike galvanized steel. Galvannealed steel is often found in industries where long, reliable service life is required, such as the automotive, architectural, electric equipment, and signage industries.
So how do you know when to use each of these two materials in your application? When choosing between galvanized or galvannealed, consider these issues:
1.) Is significant welding involved? Due to the additional iron in the coating layer, galvannealed steel offers more weldability as compared to a galvanize coating.
2.) Is your product going to remain unpainted? If the answer is ‘yes’ then galvanized steel is almost certainly a better choice. Unpainted galvannealed steel will exhibit a reddish-orange appearance (the increased concentration of iron at the source leads to accelerated oxidation) when exposed to moisture. In almost all cases, this discoloration is something procurement professionals would look to avoid.
Now that you know when to use either of the coatings, let’s better understand how galvanized and galvannealed steel is spec’ed.
As discussed above, galvanize and galvanneal differ in the zinc coating/bonding process primarily due to difference in contact and exposure temperature. But for both galvanized and galvannealed materials, steel suppliers are able to adjust the thickness of the zinc coating to provide higher or lower density zinc penetration. For galvannealed steel, Type A40 is the minimum thickness of zinc coating in common use. Where the “A” indicates “Anneal” and the ‘40’ indicates that there is a nominal weight of .40 ounces of zinc/iron coating per square foot. Therefore, A60 galvannealed steel has a relatively thicker zinc/iron coating with .60 ounces of zinc/iron coating per square foot. As the thickness of the coating layer increases, it becomes relatively easier to weld and paint, but this thickness tends to correlate with cost from the mill so it is wise to not over spec your galvannealed requirements.
As with galvanized steel, the materials are denoted with a letter/number combination, but this time a “G” is used to signify ‘Galvanized’ while the number represents the same nominal weight (in ounces) per square foot. So G40 and G60 are both galvanized steel with .40 and .60 ounces of zinc per square foot respectively.
Based on the environment your product will be exposed to, it’s important to make sure you specify both the steel type as well as the coated zinc thickness to ensure the best performance of the material over the life time of your application.
Experienced fabricators can work with you and your engineering team to make sure that you’re using the best material for your application. Contact your supplier to discuss how you can optimize the best lifetime value of your purchasing decisions.