Galvanized VS. Galvannealed Steel

2020-11-01

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What is Galvanneal?

Galvanneal is similar to other galvanized steels. It is perhaps most similar to hot-dip galvanizing. In order to create Galvanneal steel, the material is run through a liquid bath of zinc alloy. This zinc adheres to the surface without any large changes to the metallurgical properties of either the zinc layer the steel. This is essentially the hot-dip galvanizing process. For the galvannealing process, the galvanized steel is then passed through a low pressure, high volume air knife which blows excesses coating off of the steel before it solidifies. This leaves the steel with a thinner zinc coating compared to standard galvanized steel. The material is then placed into a furnace it remains until the steel hits an annealing temperature for a determined amount of time. This allows the zinc the surface of the steel to alloy with one another. The Galvannealed steel is then cut into specified dimensions.

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What is the difference between Galvanized Galvannealed Steel?

They both are zinc coated at the mill are designed for rust corrosion resistance. Both are passed through a hot dip coating process; however Galvannealed steel goes through an additional annealing process, which induces diffusion alloying between the molten zinc coating the steel. This vastly improves the formability paint adhesion of the Galvannealed material but leaves it with a duller matte surface compared to the spangle finish achieved with just Galvanized.

They both fall under ASTM A 653/A 653M. The Galvannealed is designated with an “A” (A40, A60); Galvanized designated with a “G” (G40, G60). The number call out for both is the weight of the coating; G60 A60 is 0.60 ounces per square foot (resulting in an ‘average’ thickness of about 0.0005 inches).

So the question is: When is it worth the additional cost to create the zinc-iron alloy coating? The general answer is if the parts will be formed significantly during manufacturing; / if the are to have a painted/powdercoated finish.

Advantages Disadvantages of Galvanneal

There are advantages to using Galvannealed steel over uncoated steel other coated materials. Galvanneal is primarily designed to be painted. The matte finish of the zinc coating is much more absorbent than standard galvanized steel which allows paints to adhere to the surface far better. Galvannealed steel also has a zinc coating that is harder than many other types of galvanized steels. This increases its resistance to scratching other types of coating damage that could expose the steel underneath to the environment. The formability weldability of the Galvanneal coating is typically better than other types of galvanized steel. Using Galvanneal will also provide much better corrosion resistance than uncoated carbon steels.

A disadvantage of Galvanneal is that it does offer the same level corrosion resistance as galvanized steel due to the thinner coating.

Common Applications of Galvannealed Steel

Applications of Galvanneal vary widely because many different types of steel can be Galvannealed. For instance, a thin low carbon steel may be Galvannealed made into a bracket on an automobile frame, while a thick high-strength low-alloy steel may be Galvannealed used in the structure of a building. Galvannealing can be performed on steels that are meant for forming, deep drawing, high-tensile stress applications, marine applications, welding operations, many other activities.